Quantum spin liquid magnets are materials that cannot arrange their magnetic moments, or spins, in a regular, stable pattern because the spins interact in competing ways that cannot be simultaneously minimized. As a result, these “frustrated” spins constantly change direction, behaving like a liquid even at temperatures close to absolute zero. Such behaviour is predicted to give rise to many interesting physical phenomena, but despite great efforts in both experimental and theoretical studies, there is no well-recognized, real-world example of a frustrated magnet hosting a quantum spin liquid state.
In a recent paper published in Chinese Physics Letters, Sizhuo Yu, Yuan Gao, Bin-Bin Chen and Wei Li describe how machine-learning techniques can help us understand how quantum spin liquids behave, and thereby support experimentalists in their study of “candidate” materials that may (or may not) be quantum spin liquids. Here, they discuss their research and goals for the future.
What was the motivation of your research?
The study of strongly correlated quantum materials such as frustrated magnets is challenging yet important for condensed-matter physics as well as quantum science and technology. For example, magnetic quantum materials – especially frustrated magnets in two dimensions – can host highly entangled quantum states and emergent excitations called anyons that follow fractional statistics and can be used to store and manipulate quantum information in a robust, error-resistant manner. Highly frustrated magnets thus provide an intriguing platform for next-generation quantum technologies, including but not limited to topological quantum computation.
One key step in the search for quantum spin liquids in frustrated magnets is to determine the realistic description – the microscopic spin models – of the quantum magnets. These models are like a genome for quantum magnetism – they tell us what types of magnetism are possible. However, inferring the spin model from experimental measurements constitutes a notoriously hard inverse many-body problem. This hinders a precise understanding of spin-liquid candidate materials, including the most prominent ones like the triangular magnet YbMgGaO4, the Kitaev material α-RuCl3, and the Kagome magnet ZnCu3(OH)6Cl2, among others.
What did you do in the paper?
Inspired by recent successes in interdisciplinary research between quantum many-body calculations and machine learning, we proposed an efficient solution for this long-standing problem that greatly facilitates studies of intriguing quantum magnets. Our approach combines accurate many-body methods with highly efficient global optimizers (including the Bayesian and multi-restart auto-gradient strategies) so that we “learn” the effective spin model from the thermodynamic data measured in experiments. This method will not only interest theorists in the quantum many-body physics community, but also experimentalists working on quantum materials.
As part of our research, we also created and made public an open-source many-body calculation package, dubbed QMagen, that includes several state-of-the-art and original many-body methods we developed. Our hope is that such a package can be widely used in the studies of correlated quantum materials.
What was the paper’s most important finding?
We found that we could accurately reproduce the model parameters of a triangular quantum magnet, TmMgGaO4, as determined in a hand-tuned fitting of thermodynamics (see, for example, this paper from 2020), and we confirmed that these previously-determined parameters are located right in the only optimal regime that describes the compound in a rather large parameter space. More recently, we used this same combinative approach to pin down the effective spin model of α-RuCl3, which has interaction parameters that are much debated and quite challenging to determine otherwise. Using QMagen, we find very accurate modelling that convincingly reproduces major experimental observations such as those published earlier this year in Nature Communications.
With these exciting results, we are thrilled by how much computational resource we could save in tackling such tough problems with our approach – including some that are practically impossible to deal with using hand-tuned fittings – and by producing unbiased results free of personal preference.
Why is this research significant?
The framework we proposed creates new possibilities in the research of frustrated quantum magnets and correlated materials in general. With this methodology, it becomes possible to determine the magnetic interactions of 1D and 2D materials in a very efficient manner, which constitutes a very important step towards understanding these low-dimensional quantum magnets and lays down a solid foundation for their applications in future technologies.
We hope this method can contribute to the exploration of exotic states and phase transitions in frustrated quantum magnetism. We will apply this method to more cases of intriguing low-dimensional magnets and try to decode their accurate spin model descriptions – the magnetism genome. The ultimate goal is to establish a genome library for a vast family of quantum magnets. To achieve this, we plan to make continuous improvements and upgrades to our QMagen package.
Neutrinos are detected by observing the particle showers they create when they strike nuclei, but new research using electrons in place of neutrinos shows that the models used to reconstruct the energy of the incoming neutrinos from these showers usually give wrong answers. Researchers say the work highlights well-known gaps in the theory of neutrino-nucleus interactions, and that improving this theory is crucial if next-generation neutrino detectors such as the Deep Underground Neutrino Experiment (DUNE) in the US and Hyper-Kamiokande in Japan are to realize their full potential.
The study of neutrino oscillations continues to provide tantalizing hints of physics beyond the Standard Model of particle physics. The Standard Model did not originally predict that neutrinos have mass, but both accelerator experiments and astronomical observations have clearly demonstrated that neutrinos can periodically change flavour between electron, muon and tau neutrinos as they propagate. This neutrino oscillation is possible only if neutrinos have mass, thus changing the Standard Model.
Characterizing neutrino oscillations is therefore one of the highest priorities in physics today. Or Hen of the Massachusetts Institute of Technology offers an example. “If there is large violation of charge-parity symmetry in how neutrinos and antineutrinos interact, then under certain assumptions that can explain why we live in a matter-dominated universe,” he says. “We really need to measure whether neutrinos and antineutrinos oscillate slightly differently.”
Energy reconstruction
Neutrinos’ oscillation rates depend on their energy, so one needs to know this to characterize neutrino oscillations and search for any possible anomalies. Unfortunately, neutrinos are notoriously difficult to detect because of their extremely weak interaction with matter. The details of individual experiments vary, but they invariably use an immense volume of matter (Hyper-Kamiokande will use water; DUNE will use liquid argon) surrounded by sensors. When a neutrino interacts with a nucleus in the matter, the sensors pick up ejected particles and reconstruct the energy of the incident neutrino.
One problem is that this requires knowledge of how neutrinos interact with the atomic nuclei. “Reconstructing the energy of a neutrino is like looking at fireworks in the sky and trying to work out the energy that ignited the explosion just by seeing all the beautiful colours,” explains Adi Ashkenazi of Tel Aviv University in Israel; “There are lots of free parameters in the simulation.”
Individual detectors are usually calibrated with beams of neutrinos. However, neutrinos are only produced by particle decay – which is inherently random – so producing a mono-energetic neutrino beam is impossible. This makes calibrating the incoming flux at a detector at each energy in the detection bandwidth unfeasible. Researchers at the e4ν (electrons for neutrinos) collaboration, however, used the simple and surprising alternative of studying how nuclei interact with electrons. Monoenergetic electron beams are simple to generate using particle accelerators, and Hen says that, although the fundamental physics of the electron-nucleus interaction is different from that of the neutrino-nucleus interaction, the real difficulties in the simulations arise from interactions between protons and neutrons in the nucleus, and these are identical in both cases.
Simple version of neutrinos
“In essence, electrons are a simple version of neutrinos, so whatever you think you know about neutrinos, if that same model cannot explain electron data, it’s wrong,” says Hen.
The e4ν researchers therefore teamed up with the CLAS Collaboration, based in Virginia (Hen and Ashkenazi are members of both groups), to study scattering data from 1999 in which electrons of known energies were scattered off either carbon, helium or iron targets. They selected a subset of these in which the scattering was relatively simple – producing only one detected electron and one proton. In a paper in Nature, the researchers analyse the electron interactions as though they were neutrinos, using standard simulations to reconstruct the energy of the incident particle. For carbon, only 30-40% of the simulations estimate the energy to be within 5% of the actual beam energy. For iron, the proportion is only 20-25%.
“You shouldn’t be surprised that the models don’t agree very well with the data,” says Eric Zimmerman of the University of Colorado Boulder; “There’s been a wide variety of neutrino interaction models produced and they’ve had quite a lot of variation in their predictions…I think this work’s principal value is that this dataset will presumably go into making the models better, if it hasn’t already.”
“To anyone who’s paying attention, this should be no surprise,” agrees Daniel Cherdack of the University of Houston in Texas; “The real question is who’s paying attention and why?” Both Zimmerman and Cherdack believe current results from detectors are trustworthy because model uncertainties have been factored into the error calculations. However, Cherdack says that these uncertainties will need to get smaller if larger detectors are to discover smaller effects. “This is an important paper in that it’s highlighting part of the ditch-digging of neutrino physics that doesn’t make it to the front very often, and the fact that Nature is focusing on this is incredibly important because this is one of the things we really need to figure out to make DUNE a successful experiment.”
Quantum technology: on the cusp of a revolution. (Courtesy: IOP Publishing)
Physicists have long boasted of their success in “quantum 1.0” technology – semiconductor junctions, transistors, lasers and so on.
But the future will increasingly depend on “quantum 2.0” technology, which taps into phenomena like superposition and entanglement to permit everything from quantum computing and cryptography to quantum sensing, timing and imaging.
According to one estimate by Honeywell, in fact, such technology could be worth $1 trillion over the next three decades.
Philip Ball talks to researchers, business analysts and insiders at firms ranging from IBM to IonQ, while Michael Allen examines the commercial potential of quantum gravity sensors.
There are interviews with Ilena Wisby, chief executive of Oxford Quantum Circuits, as well as with the head of KETS Quantum Security, Chris Erven.
Careers editor Laura Hiscott is on hand to look at the job opportunities in the area, while James McKenzie hot foots it back from a quantum tech showcase that took place in London last month.
And if you need a guide to quantum computing to prime any wannabe investor, Hamish Johnston has the perfect book for you.
• A quantum promise – Freeke Heijman from Quantum Delta NL tells Martijn Boerkamp how the Netherlands is forging ahead in quantum technologies
• Call for “great observatory” to succeed Hubble – Michael Banks reveals the highlights of the long-awaited Astro2020 Decadal Survey, which will define the course of astronomy and astrophysics in the US and beyond over the next 10 years
• Towards quantum 2.0 – James McKenzie is excited about the prospects of firms that are developing technology based on seemingly esoteric fundamental quantum phenomena
• Quantum conundrum – While the technological applications of quantum mechanics are bright, its meaning remains opaque. Thankfully, as Robert P Crease explains, philosophers of science are working on it
• The quantum battleground – Yangyang Cheng examines a historical precedent for the emerging technological rivalry between the US and China
• Ethics in the quantum age – Mauritz Kop assesses the ethical principles we must all adopt so that the application of quantum technologies is equitable and safe
• Setting the scene for a quantum marketplace – As quantum computing makes its first forays from the lab to the real world, are the latest claims mere hype causing a bubble that will burst before the field finds its feet? Or are investors and researchers right to be enthusiastic about this burgeoning technological revolution? Philip Ball investigates the
successes and pitfalls of commercializing quantum information technology
• The key to our quantum future – Safeguarding our communications data and infrastructures will become a much harder task in a quantum-enabled future. KETS Quantum Security chief executive Chris Erven talks to Tushna Commissariat about how integrating quantum based systems into existing communication is key
• Sensing gravity, the quantum way – Devices that exploit the extreme sensitivity of quantum states are making their way out of the lab and into everything from construction and healthcare to seismology. Michael Allen learns more about the
technology that goes into building a quantum gravity sensor, and its multitude of uses in research and industry
• Quantum for all – Building a firm foundation Oxford Quantum Circuits chief executive llana Wisby talks to Tushna Commissariat about UK investments and innovation in quantum technology, and the potentially world-changing impact that it could have
• Quantum physics for investors – Hamish Johnston reviews Quantum Computing: How It Works and Why It Could Change the World by Amit Katwala, WIRED
• Dealing with deniers – Rachel Brazil reviews How to Talk to a Science Denier: Conversations with Flat Earthers, Climate Deniers, and Others Who Defy Reason by Lee McIntyre
• Brilliant polymath, troubled person – Andrew Robinson reviews The Man from the Future: the Visionary Life of John von Neumann by Ananyo Bhattacharya
• A tale of two scientists – Laura Hiscott reviews Flashes of Creation: George Gamow, Fred Hoyle, and the Great Big Bang Debate by Paul Halpern
• How to get ahead in quantum tech – The quantum industry is blossoming and has lots of new and exciting jobs that physicists are well placed to fill. Laura Hiscott talks to experts who have studied the quantum-tech jobs market about what’s on offer and what skills you’ll need to forge a successful career in this area
• Ask me anything – Careers advice from Farai Mazhandu, chief executive and co-founder of Bayete Quantum Technologies
• Winter wonder worlds – The winter holiday season comes but once a year – on Earth. Eleanor Spring takes a tour through some of the seasonal extremities experienced on distant worlds. A chilly Earth winter never looked so inviting!
Crystals all the way down: An artistic rendering of a time crystal. (Courtesy: Google Quantum AI)
Crystalline solids such as diamond have a hallmark property: their structure is periodic in space. For much of the past decade, physicists have wondered whether a similarly robust, repeating structure might also exist in time. By analogy with spatial crystals, this structure is known as a time crystal, and whereas diamonds may be forever, time crystals are both forever and forever changing.
Researchers have proposed several physical systems that could host time crystals, including platforms like nitrogen-vacancy (NV) centres and trapped ions. Most recently, a collaboration between researchers at QuTech, TU Delft and UC Berkeley demonstrated long-lived period-doubling oscillations in carbon-13 NV centres across a variety of initial conditions. However, each of these previous platforms lacked the full slate of capabilities to necessary to realize (and verify) a genuine time crystal.
Now, researchers at Google Quantum AI and Stanford University in the US have constructed a time crystal on Google’s Sycamore quantum processor, demonstrating that these exotic objects constitute their own distinct phase of matter. To do so, they ran a series of “experiments” on Sycamore, treating the computer as a laboratory to test whether their proposed time crystal met certain requirements. The result is the first to experimentally verify that a phase of matter can exist outside of thermal equilibrium. It also shows that even in today’s world of noisy intermediate scale quantum (NISQ) computing, quantum processors already have important implications for our understanding of physics.
Phases of matter
Phases of matter come in many varieties, from ubiquitous liquids and gases to quantum phases like superconductors and Bose–Einstein condensates that only crop up under extreme conditions. Regardless of their properties, all phases share a key quality: each one has some notion of order, a way of quantifying the patterning of particles within the collection.
Crystalline solids, for example, exhibit spatial ordering. When a crystal forms, a discrete set of points from the continuous expanse of space become the loci for its evenly spaced atoms. In so doing, the system breaks a well-known physical symmetry: as far as the fundamental laws of physics are concerned, no point in space should have priority over any other. This cosmic apathy is reflected in a set of symmetries, or transformations, under which these laws must remain invariant.
One such symmetry, known as continuous translation symmetry, implies that shifting a closed physical system by any amount in any spatial direction is inconsequential. During formation, crystals spontaneously break this continuous symmetry; in other words, they break it even though the system is not explicitly swayed towards any particular choice of points. The repeating pattern that remains displays only discrete translation symmetry: if every atom is shifted over by one interval, or unit cell, of the crystal, then the same set of spatial positions (apart from the edges) will retain their distinction.
Time crystals enter the fray
The fundamental forces of nature also regard all points in time with indifference. In other words, the laws of physics stay the same from second to second and day to day. This invariance is referred to as continuous time-translation symmetry. Just as crystalline solids disrupt the continuous translation symmetry of space, systems such as pendulums swinging to and fro and planets orbiting stars carve the infinite extent of time into regular, discrete intervals.
Unlike space, however, the passage of time is indelibly coloured by the second law of thermodynamics, which states that entropy – the amount of randomness or disorder in a closed system – cannot decrease over time. Simple systems, like a single pendulum or a planetary orbit, can exhibit stable, long-lived oscillations, as there are few ways for their constituent pieces to interact with each other and exchange energy. In contrast, oscillations quickly fall out of sync in systems with many degrees of freedom (such as a collection of coupled pendulums), as energy surges through every allowed avenue and the system explores many possible states.
In 2012 the physics Nobel laureate Frank Wilczek proposed that an extended phase of matter could exhibit regularity and ordering in time, while also spontaneously breaking time-translation symmetry. Drawing an analogy with the spatially ordered crystalline solid, Wilczek called this proposed phase a quantum time crystal. At that point, it was unclear whether such a phase could truly exist in the physical world, or whether entropy would extinguish all hope for time-crystalline order.
Wilczek’s original proposal involved the ground state, or lowest energy configuration, of a superconducting ring. By itself, this superconductor supports a steady current that flows without resistance. If an alternating pattern of electric charge known as a charge density wave is then superimposed onto this constant current, the ripples of charge would break down the continuous translation symmetry of space into a discrete translation symmetry. Combining the two, Wilczek hypothesized, would result in a stable many-body system with reduced (from continuous to discrete) time-translation symmetry.
Need for non-equilibrium phases
In 2014 theoretical physicists Haruki Watanabe and Masaki Oshikawa dealt a striking blow to Wilczek’s proposal, and to the prospect of time crystals. Their “no-go” theorem stated that any system in thermal equilibrium, including Wilzcek’s, could not be a time crystal. Instead, any time-crystalline phase would need to enter a strange new realm: non-equilibrium physics.
Time crystal theorist: Vedika Khemani is a condensed-matter theorist and associate professor of physics at Stanford University. (Courtesy: Vedika Khemani)
Around the same time, physicists at Princeton University, US and the Max Planck Institute in Dresden, Germany were working in earnest to define such phases. For years, other physicists had pursued similar research programs with little to show for it, but Vedika Khemani (now a professor at Stanford), Roderich Moessner, Shivaji Sondhi and colleagues succeeded, theoretically demonstrating a stable non-equilibrium phase. Their recipe contained two essential physical ingredients: periodic or Floquet driving, and many-body localization (MBL).
When a system is out of equilibrium, its dynamics must depend nontrivially on time, with the state of the system constantly evolving. Floquet driving evokes such dynamics by making the system interact with a laser (or microwave) pulse. The strength of coupling between the system and the laser is varied periodically, and this periodicity modifies the system’s temporal symmetry. While the fundamental forces don’t depend on time, the dynamical pressures generated by the coupled laser are cyclic. From the system’s point of view, time’s translational symmetry is now discrete rather than continuous, and its governing physics looks the same only at times separated by exact multiples of the Floquet driving period.
Reduction in symmetry alone, however, does not a time crystal make. Whereas the continuous translation symmetry breaking in a crystalline solid is spontaneous, symmetry breaking via Floquet driving is induced by coupling the system to the laser. Any time crystal constructed from a Floquet-driven system must therefore spontaneously break the discrete translation symmetry it inherits from the driving mechanism.
In Floquet-driven systems, this is typically done via a phenomenon known as period doubling, where the system falls into a regular cycle that takes twice as long as the period of the process that governs it. The phenomenon dates back to 1837, when Michael Faraday experimented with thin layers of liquid on top of oscillating piston-like platforms, and observed that the standing waves that formed in the liquid moved at half of the oscillator’s frequency. Period doubling does not guarantee a time crystal either. What it does do, when combined with a Floquet-driven many-body system, is to make time crystalline order possible – with one big caveat.
In most Floquet systems, the driving laser imparts energy to the system with every cycle. As the system absorbs this energy, both its temperature and entropy increase. Eventually entropic randomness overwhelms any information contained in spatial or temporal correlations, and the system no longer exhibits any order, crystalline or otherwise.
Many-body localization to the rescue
There is, however, one special class of physical system that overcomes this problem by skirting the encroach of entropy: systems exhibiting many-body localization (MBL). The term “localization” refers broadly to a set of phenomena in which particles or physical properties are confined in their motion. This confinement often stems from disorder. For example, in Anderson localization, named after the late physics Nobel laureate Phil Anderson, disorder in a crystal lattice immobilizes the electrons that would otherwise be free to travel from one atom in the lattice to another.
Whereas an ideal crystal comprises uniform atomic nuclei, leading to a regular lattice structure, in practice crystals can be dotted with defects – that is, with nuclei of different elements randomly interspersed in the lattice. These defects push and pull electrons in different directions, creating complex potential energy landscapes with deep, narrow trenches that act as traps.
Forever material: An artist’s rendering of a time crystal. (Courtesy: Matteo Ippoliti)
MBL, which Anderson also proposed, is a special type of disorder-induced localization that occurs in interacting quantum systems. When a many-body system is subjected to small quantities of randomness, these disturbances can spoil global symmetries like spatial translation invariance. However, once the degree of disorder exceeds a certain threshold, a set of new, local symmetries emerges, freezing particles in place.
Mired in strong many-body disorder, the particles in an MBL system have no way of absorbing energy. Correlations between particles are fixed from the start, and entropy stays the same, just barely satisfying the second law of thermodynamics. Crucially, the system never achieves thermal equilibrium.
Disorder begets order
In 2015 Khemani and colleagues showed that Floquet driving and MBL together make time crystals a distinct possibility, owing to the spontaneous symmetry breaking of the former and the entropy evasion of the latter. There is, however, one further ingredient. The key to combining Floquet driving and MBL to create a non-equilibrium phase of matter is a property of MBL known as eigenstate order.
In conventional phases of matter, physical quantities are measured and averaged over a thermal ensemble – a collection of states that captures the behaviour of the system at a fixed temperature. Individual states within that collection, known as eigenstates, are for the most part inaccessible and irrelevant. In MBL systems, on the other hand, where thermal equilibrium is never reached, these individual states are essential. For different initial conditions, the localizing randomness drives the system into different eigenstates.
Eigenstate order is also unique in that every eigenstate has a companion eigenstate. When time translation symmetry is broken, it is hard to define a consistent notion of energy. Instead, it makes sense to talk about the closely related concept of a quasi-energy, which is periodically defined, and behaves like a phase. Energies or quasi-energies can in theory be paired up in non-MBL systems; in practice, however, this partnership is sensitive to the slightest imperfections. In MBL systems, energies align not in spite of randomness but because of it. Even under imperfect conditions, this equality is exact – a rigidity that makes MBL well-suited for constructing time crystals.
Putting the pieces together
In their quest to define a non-equilibrium phase of matter, Khemani and her collaborators constructed a theoretical model that integrated Floquet driving and MBL. In this model, they envisioned a 1D chain of particles, each with its own quantum mechanical spin. Every particle in the chain is coupled to its neighbours and is also subject to its own local magnetic field.
Time crystal technicalities Left: In a discrete time crystal, the periodicity of the drive is further broken by the state of the system. Right: Constructing a genuine time crystalline phase requires interaction and disorder, as well as isolation from the outside world.
At equilibrium, even when all the magnetic fields and coupling strengths are uniform, this system can only exhibit magnetic phases. A ferromagnet, for instance, has all spins pointing either up or down, whereas in a paramagnet, each spin individually points up or down but the system shows no overall preference. When disorder is incorporated into the field strengths and couplings, the system hosts an additional, many-body localized phase, where the spins point in random directions. This phase, known as a spin glass, derives its name by analogy with glass, which is an amorphous solid composed of randomly located atoms.
Out of equilibrium, the model hosts a further two phases, one of which is time crystalline. In the scenario Khemani considered, the system is periodically driven with a laser that modulates the strength of the coupling between neighbouring spins, alternating below and above the disorder strength that induces many-body localization.
For a given initial configuration of the spins, the disorder first forces the system into a spin-glass eigenstate. Oscillating between weak and strong interactions, over the course of a single cycle the Floquet drive rotates all the spins 180 degrees into the companion eigenstate with paired quasi-energy. Another cycle rotates the spins by an additional 180 degrees, bringing the system back to the first eigenstate. Under the influence of the Floquet drive, the system endlessly oscillates between these two states, never heating up or gaining entropy.
This system of spins, eternally oscillating with twice the period of the Floquet drive, is known as a pi-spin glass (π-SG). Together, the spins robustly display spontaneous time-translation symmetry breaking in a many-body system with many degrees of freedom. “To get a stable non-equilibrium phase in a many-body system, you need MBL,” Khemani summarizes. “In the future, we might find other ways of generating time crystals, but in the current setting we have done so via Floquet driving and eigenstate order.”
Quantum computer as a laboratory
Over the past few years, time crystals have begun their journey from theoretical model to experimental reality. However, every prior experimental demonstration left something to be desired, both in terms of realization and verification. For one, platforms made out of NV centres or trapped ions lack some of the essential ingredients to construct a Floquet MBL system. To make matters worse, even if these platforms could host time-crystalline order, they would be unable to certify its presence. Demonstrating period doubling for a specific set of conditions is relatively easy in these systems; showing that it occurs for all initial conditions is far more challenging. As a result, prior experiments stopped far short of establishing the time crystal as a phase of matter.
Then, in 2020, Khemani and Matteo Ippoliti, a postdoctoral scholar in her group at Stanford, spotted an opportunity. While quantum processors can, like classical processors, run algorithms and traditional computations, they also offer something else: unprecedented programmatic control over the quantum world. With site-resolved measurements and tuneable interactions, Google’s Sycamore processor gives researchers the ability to systematically run “experiments” on exotic physical systems. “Up until recently, we’ve spent all of our time thinking about equilibrium physics, especially low-temperature properties,” Khemani says. “If you think of NISQ devices like Sycamore not as quantum computers but as experiments, new regimes of physics become accessible. Time crystals are one example of this.”
Welcome to the lab: Google’s Sycamore quantum processor. (Courtesy: Google Quantum AI / James Crawford)
In a recent paper published in Physical Review X, Ippoliti, Khemani, and co-authors outline a series of experiments on Sycamore to establish robust spatio-temporal order in Khemani’s original Floquet-MBL model. Taken together, these experiments would help fill in the gaps in argumentation left by previous experiments, providing the first unambiguous detection of a time crystalline phase in the laboratory. A preprint of this article caught the attention of Pedram Roushan, a researcher at Google Quantum AI, and the Google team and the Stanford researchers began putting the proposed experiments into action.
Establishing robustness
In the latest study, published in Nature, the researchers began by recreating the conditions in Khemani’s original Floquet-MBL model, ordering Sycamore’s qubits into a chain and coupling neighbouring qubits together with a microwave pulse that drives their evolution. Unlike previous experiments that have investigated spatio-temporal ordering, however, for Khemani and the Google team, simulating the system was just the beginning. In collaboration with lead authors Xiao Mi and Ippoliti, the Google Quantum AI team programmed the quantum processor and also rigorously tested the time crystal’s stability under various conditions.
One of the requirements the researchers checked is that period doubling occurs for all initial configurations of the quantum spins. Period doubling is seen for specific states in many systems, but time crystalline order demands that all states display this behaviour. To investigate this, the Google team and their collaborators turned to “quantum typicality” – the notion that random, highly entangled states reveal the system’s typical behaviour. In addition to sequentially sampling a large number of initial configurations to probe for outliers, they used scrambling circuits to generate a few entangled initial states, which they then subjected to Floquet evolution.
A second requirement for phases of matter is that order must survive as the system is made arbitrarily large. Although Sycamore offers only a relatively small number of qubits, the Google team was able to glean insights about infinitely large systems through a technique known as finite-size scaling. This technique, which the researchers exported from its original context in numerical studies of physical systems, uses measurements from systems of various small sizes to extrapolate trends for much larger systems.
Finally, any phase of matter must also exhibit stable ordering for arbitrarily long times. Of course, any real experiment, whether performed on a quantum computer or in a traditional laboratory, is confined to finite times. For Sycamore, the limiting time scales are set by the coherence times of the qubits, and the fidelity of operations performed by the processor. Nevertheless, through a series of control experiments, the researchers were able to distinguish decoherence in the quantum processor itself from the possibility of intrinsic dynamics (and increasing entropy) in the simulated system.
Together, these experiments bolster the case for the time crystal as a genuine non-equilibrium phase of matter. “None of these verification steps alone is definitive,” says Roushan, “but altogether, they provide a rather compelling set of evidence.”
Benchmarks for a burgeoning field
Khemani’s experiments with the Google team on Sycamore mark the first time all the requirements for a non-equilibrium phase of matter have been rigorously checked and verified. In doing so, they also lay vital groundwork for the use of NISQ devices in the study of non-equilibrium phenomena. Given how thoroughly the π-SG has been studied theoretically, the Sycamore results provide a practical benchmark for other quantum processor-based experiments. “We’ve only studied a tiny corner of possible physics,” Roushan says. “Quantum processors make entirely new physical regimes accessible and relevant. Our work should serve as a blueprint for these future explorations.”
RSNA 2021, the annual meeting of the Radiological Society of North America, brings together radiologists, radiation oncologists, medical physicists and related scientists to discuss the latest radiology research and developments. This year’s meeting is being held as a hybrid event, with more than 19,000 attendees expected to attend in-person in Chicago and another 4000 joining virtually. Here’s a small selection of the studies being presented at the conference this week, focusing on the latest innovative brain imaging research.
Mild to moderate COVID-19 during pregnancy doesn’t harm baby’s brain
Pregnant women appear to be more vulnerable to the SARS-CoV-2 virus, but little is known about the potential impact on an unborn child if the mother contracts COVID-19 during pregnancy. A study from Ludwig Maximilian University of Munich concludes that mild to moderate COVID-19 in pregnant women does not affect foetal brain development.
“Women infected with SARS-CoV-2 during pregnancy are concerned that the virus may affect the development of their unborn child, as is the case with some other viral infections,” explains senior author Sophia Stöcklein. “So far, although there are a few reports of vertical transmission to the foetus, the exact risk and impact remain largely unclear. The aim of our study was to fill this gap in knowledge regarding the impact of a maternal SARS-CoV-2 infection on foetal brain development.”
The researchers used foetal MRI to study 33 pregnant patients with COVID-19, acquiring T2-, T1- and diffusion-weighted brain images. The women were between 18 and 39 weeks into their pregnancies, with symptom onset at between four to 34 weeks.
Two expert radiologists evaluated the scans and quantitatively assessed structures of the brain stem and posterior fossa. They concluded that brain development – including brain surface and folding, cerebellar size and the size of all brain stem structures – was age-appropriate in all cases. There were no calcifications, signs of swelling or widening of the brain’s fluid-filled spaces.
Stöcklein cautions that the study only included mothers with mild to moderate symptoms and without hospitalization, emphasizing the importance of active protection against COVID-19 during pregnancy. The team plan to follow-up all newborns over the next five years, performing detailed neonatal assessment, and assessing neurological development.
Brain complications could affect over one in 100 hospitalized COVID-19 patients
A multi-institutional study has found that roughly one in 100 patients hospitalized with COVID-19 are likely to develop complications of the central nervous system. The retrospective study analysed nearly 40,000 hospitalized COVID-19 patients from seven US and four European university hospitals.
“Much has been written about the overall pulmonary problems related to COVID-19, but we do not often talk about the other organs that can be affected,” says lead author Scott Faro from Thomas Jefferson University. “Our study shows that central nervous system complications represent a significant cause of morbidity and mortality in this devastating pandemic.”
The patients in the study had an average age of 66 years old and many had comorbidities such as hypertension, cardiac disease and diabetes. The most common cause of hospital admission was confusion and altered mental status, followed by fever. Just over 10% of the cohort underwent neuroimaging; from these patients’ MRI or CT brain scans, the researchers identified 442 acute neuroimaging findings that were likely associated with COVID-19.
Brain complication: Haemorrhage seen in 56-year-old female with COVID-19 infection and no other significant past medical history. (Courtesy: Scott Faro/RSNA)
The overall incidence of central nervous system complications in all patients was 1.2%, suggesting that just over one in 100 patients admitted to hospital with COVID-19 will likely have a brain complication of some sort. The most common complication seen was ischemic stroke, followed by intracranial haemorrhage and then encephalitis (brain inflammation).
“It is important to know an accurate incidence of all the major central nervous system complications,” says Faro. “There should probably be a low threshold to order brain imaging for patients with COVID-19.”
MRI reveals how alcohol exposure impacts the foetal brain
Consuming alcohol during pregnancy can lead to foetal alcohol spectrum disorders, a range of conditions that may result in physical, behavioural and learning problems. In the first MRI-based study of pre-natal alcohol exposure, researchers have used MRI to identify early changes in the brain structure of foetuses exposed to alcohol.
Early changes: Pre-natal alcohol exposure leads to changes in foetal brain structures. (Courtesy: Marlene Stuempflen/RSNA)
“There are many post-natal studies on infants exposed to alcohol,” says Gregor Kasprian from the Medical University of Vienna. “We wanted to see how early it’s possible to find changes in the foetal brain as a result of alcohol exposure.”
Kasprian and colleagues studied 500 pregnant women referred for foetal MRI. Anonymous questionnaires revealed that 51 of the women had consumed alcohol during their pregnancy. The team analysed a final group of 24 foetuses with pre-natal alcohol exposure and a control group of 52 gender- and age-matched foetuses without alcohol exposure. At the time of imaging, the gestational age was between 20 and 37 weeks.
The researchers employed advanced post-processing techniques to generate super-resolution brain MR images and performed semi-automated atlas-based segmentation of the resulting images.
Analysis of 12 different foetal brain structures revealed increased volumes in the corpus collosum, the main connection between the brain’s two hemispheres, and decreased volumes in the periventricular zone, compared with controls. They note that this is the first time that a pre-natal imaging study has quantified these early alcohol-associated changes.
“It appears that alcohol exposure during pregnancy puts the brain on a path of development that diverges from a normal trajectory,” says Kasprian. “Foetal MRI is a very powerful tool to characterize brain development not only in genetic conditions, but also acquired conditions that result from exposure to toxic agents.”
Design, development and at-scale fabrication of “perfectly imperfect diamonds”, uniquely tailored for quantum metrology applications such as compact magnetometers, RF sensors, solid-state gyroscopes and room-temperature masers. That’s the mission for the quantum team at Element Six, which is applying its patents and know-how in chemical vapour deposition (CVD) to mass-produce quantum grades of single-crystal diamond containing deliberate and controlled levels of so-called nitrogen-vacancy (NV) spin centres.
Among the hundreds of different types of defects that can be found within the carbon lattice, the NV centre is especially interesting to scientists and technologists because it can be manipulated to provide an optical output that is sensitive to magnetic and radiofrequency signals at room temperature. This process, known as optically detected magnetic resonance (ODMR), is observed when measuring a change in fluorescence after shining green laser light on a single NV defect, or on an ensemble of them, in the presence of an applied microwave field – an interaction that provides the basis for a versatile solid-state platform with spin qubits that can be initialized and read out with long qubit lifetimes (up to seconds in certain circumstances).
Andrew Edmonds, a principal research scientist at Element Six, tells Physics World how the company’s DNV Series of synthetic diamond is being deployed by research and industrial end-users for a raft of emerging applications in quantum science and technology.
How does Element Six support wider commercialization efforts in quantum R&D?
I’ve been with Element Six for eight years and, along with my colleagues in the R&D and product development teams, work closely with academic and industrial customers to support a broad spectrum of quantum applications where synthetic diamond offers a unique value proposition. Collectively, our task is to transform early-stage applications into mainstream commercial opportunities.
Andrew Edmonds: “Collaboration is embedded in Element Six’s DNA.” (Courtesy: E6)
In a sense, what the Element Six quantum team is trying to do is make the “perfectly imperfect diamond”. Put another way: a synthetic diamond that contains a specific type of defect – the NV centre – without creating other defects that will have a detrimental effect on the material’s aggregate physical properties.
Advanced materials science and leading-edge fabrication: these are the key elements underpinning the innovation behind our DNV Series.
Element Six has just released DNV B14, the latest addition to its DNV Series of quantum-grade diamond. In what ways will this new product benefit your academic and industrial customers?
The DNV Series is designed to provide material with engineered NV centres that can be readily integrated into an industrial quantum device or a research experiment. Our new material, DNV B14, uses a uniform distribution of NV spin centres in a small diamond chip (3x3x0.5 mm) that can generate a much larger fluorescence signal compared with DNV B1, the first commercial product we launched as part of the DNV Series. The difference is evident in the physical appearance of the two materials: the DNV B1 crystal is pink under natural light while DNV B14 is a vivid purple (due to its 10x increase in NV defect concentration).
As such, the new material gives scientists and engineers greater flexibility when designing a quantum sensing device around the diamond, with some applications benefiting from a higher concentration of NV spin centres (typically 4.5 parts-per-million in DNV B14) and others better suited to the lower NV concentration offered by DNV B1 (around 300 parts-per-billion). It’s worth emphasizing, though, that while synthetic diamond forms an integral part of the quantum sensing device, the broader instrument package around the diamond is just as important.
So DNV B14 complements DNV B1 rather than supersedes it?
Correct. As a specialist materials supplier, we make a point of working closely with all our customers to understand their technical requirements at a granular level, ensuring they get the synthetic diamond best suited to their needs. That could mean detecting or tracking a magnetic field that is oscillating significantly in a known way – for which DNV B1 might be the optimum fit – or measuring small-scale perturbations in the Earth’s largely static magnetic field versus changing geography or location – for which DNV B14 might be a better option.
Are there more classes of synthetic diamond planned within the DNV Series?
The DNV Series is work-in-progress and our R&D roadmap includes plans to develop other classes of materials, characterized by NV centres in different concentrations and/or geometries, to address existing and nascent quantum sensing applications. Our priority right now is to lower diamond’s adoption barrier and seed early-stage quantum applications by giving researchers and industry the specialist materials they need to maximize scientific impact and commercial success.
Fundamental physics: A process known as optically detected magnetic resonance is observed in DNV Series diamond, manifesting as a change in fluorescence after shining green laser light on a single NV defect, or an ensemble of them, in the presence of an applied microwave field. Sample above measures 3x3x2 mm. (Courtesy: E6)
Down the line, for example, it’s likely that users will require innovative variations on the DNV Series offering – perhaps thin layers of high-NV synthetic diamond on top of a really pure bulk diamond with minimal defects and impurities. Heterogeneous materials like this will open up new lines of scientific enquiry and enable users to build novel quantum devices – for example, imaging microscopes capable of mapping magnetic fields with unprecedented spatial resolution and nanoscale sensitivity.
Right now, what does the addressable market look like for the DNV Series?
We have a large number of university groups among our customer base – scientists doing the fundamental research on NV centres in quantum-grade diamond. At the same time, there’s growing engagement from industry, spanning across quantum technology start-ups – the likes of Qnami, SB Quantum and Quantum Diamond Technologies – as well as established, diversified instrumentation manufacturers like Bosch and Thales. The priority at Element Six is to get DNV Series diamond into the hands of the brightest researchers and engineers to deliver on the scientific and commercial promise of next-generation quantum sensing devices.
It seems there’s no shortage of applications for DNV Series diamond?
Our engineered diamond has already supported significant breakthroughs in quantum R&D. In 2018, for example, scientists at Imperial College London utilized our engineered single-crystal material in the world’s first continuous-wave, room-temperature solid-state maser (the microwave equivalent of a laser). Elsewhere, a team at the University of Warwick, UK, has demonstrated the world’s most sensitive fibre-coupled diamond magnetometer, an instrument that has the potential to be miniaturized for applications in healthcare diagnostics.
In the US, meanwhile, the aerospace and defence manufacturer Lockheed Martin has demonstrated a magnetometer built around quantum diamond to measure the direction and strength of nearly imperceptible anomalies in the Earth’s magnetic field. Ultimately, this sort of approach could yield an alternative to satellite-based GPS navigation systems – one that does not rely on external signals that can be jammed.
What are the main engineering and manufacturing challenges facing customers using DNV Series diamond for quantum sensing applications?
Quantum diamond underpins a whole new range of applications, many of which have no analogues in existing materials. Customers for this disruptive technology are academics and industrialists who are approaching the engineering and manufacturing challenges in different ways. The academic community, for its part, is focused on pushing the limits of what can be done, leading to paradigm shifts in technology performance.
Industry, on the other hand, is all about taking the current state-of-the-art and figuring out how best to package and integrate DNV Series diamond into quantum sensing platforms. Think reliability, robustness, manufacturability, scalability and cost/performance ratio. Given that context, one of the big advantages of DNV Series diamond is that it delivers a baseline of known and repeatable performance – though ultimately the final result also depends on the quality of the instrument users build around the diamond.
As a specialist materials company, how does Element Six secure its talent pipeline – in particular, the recruitment of PhD-level quantum physicists, materials scientists and device engineers?
Collaboration is embedded in Element Six’s DNA. It is the company’s long-term commitment to academia and industry that feeds our talent pipeline. This is perhaps even more true in the quantum space, where we can leverage our many long-standing collaborations with academic groups to recruit new talent, while also using that network to help our commercial customers speed up innovation and get devices to market faster.
Fabrication of a double-layered skin model using human dermal fibroblasts (HDFs) and an immortal human keratinocyte cell line (HaCaTs). (Courtesy: Biofabrication 10.1088/1758-5090/ac2ef8)
Skin is the body’s first line of defence against toxins, radiation and harmful substances. It has at least six functions, regenerates itself approximately once each month, and consists of up to seven layers of tissue. It’s no wonder, then, that researchers and clinicians are interested in producing this remarkable organ in the lab so that it can be used to repair injuries caused by burns, surgery or disease.
While scientists can grow the epidermis, the outer layer of skin, in the lab, a major challenge for researchers today is growing functional, full thickness skin, which provides strength and flexibility and contains blood vessels. Researchers at the Intelligent Polymer Research Institute at the University of Wollongong in Australia are using 3D printing techniques to tackle this problem.
“There have been tremendous advances in biomaterials science and cell biology with respect to tissue engineering over the past two decades,” says Gordon Wallace, director of the Intelligent Polymer Research Institute. “Advances in 3D biofabrication enable us to converge knowledge in these two areas and to arrange existing materials in a way that dramatically enhances performance.”
Wallace is a senior author on a recent study applying 3D printing techniques to generate a skin-like structure that supports the growth of dermal fibroblasts found in the inner layers of skin. The study, published in the journal Biofabrication, presents a 3D printing platform that could be explored to engineer functional skin tissue.
“To facilitate regeneration of skin requires a 3D structure containing materials that can support development of appropriate cells through the provision of a composition and physical environment that promotes healing,” Wallace explains. “It was Ying’s insights and technical ability that brought this all to fruition.”
The team used a custom-designed ink for 3D printing the skin-like structures. Developed by the group last year, the ink is composed of catechol-hyaluronic acid (HACA), a polymer used in biological, stem cell and tissue engineering settings, and alginate, a compound found in the cell walls of brown algae that’s used in a variety of pharmaceuticals.
The HACA–alginate ink provides structural support with specific attributes while providing a balance between mechanical properties and cytocompatibility (it is not harmful to cells). The resulting printed hydrogel skin scaffold, verified using nuclear magnetic resonance and ultraviolent–visible spectroscopy, recovers after bending and has high elasticity and toughness.
“The importance of the elasticity is in providing an appropriate physical environment to facilitate cell proliferation, migration, reorganization and differentiation during the regeneration process,” says Zhilian Yue, another senior author on the Biofabrication study.
Flexible designs: The printed scaffolds were highly elastic and could recover after bending (top row); printed structures of various patterns (bottom row). (Courtesy: Biofabrication 10.1088/1758-5090/ac2ef8)
The multi-material scaffold, rather mimicking the elasticity of skin, encourages the development of tissue that can integrate with existing skin – the scaffold includes microchannels made from gelatin that can be used to mimic blood vessels and other vasculature and further facilitate cell growth – and that can be integrated with a cell-friendly matrix that promotes the growth of human dermal fibroblasts.
Introducing cells to scaffolds
With the skin-like scaffolds built, the researchers introduced the scaffolds to skin cells encapsulated in fibrin gel. They validated the self-assembly of fibrinogen, a soluble protein that’s converted to fibrin at wound sites in the presence of clotting enzymes, using scanning electron microscopy. The team’s initial analyses of the lab-grown skin also included measuring the thickness of the epidermal layer and analysing the differentiation of the epidermal layer and extracellular matrix deposition of the dermal layer using histology analyses and immunostaining.
“This work indicates that multicellular systems using skin as a model can be developed within a 3D structure comprised of commonly available biomaterial. These biomaterials work together to produce a tough and elastic hydrogel framework with built-in micro channels, to support cells in their specific environment as dictated by the targeted application,” Yue says.
Now, the researchers are working with their collaborators to determine the best way to deliver and use this multi-material skin regeneration platform in vivo, including tailoring its structure and composition to different types of injury.
The quantum physicist David Deutsch has won the 2021 Isaac Newton Medal and Prize for “founding the discipline named quantum computation and establishing quantum computation’s fundamental idea, now known as the ‘qubit’ or quantum bit”. Presented by the Institute of Physics (IOP), which publishes Physics World, the international award is given annually for “world-leading contributions to physics”.
Deutsch’s honour formed part of the IOP’s wider 2021 awards, which recognize everyone from early-career scientists and teachers to technicians and subject specialists. This year saw various changes to the IOP’s awards process, including self-nominations allowed for the first time and greater publicity to encourage a wider pool of applicants. Of those winners who chose to include data about their personal background, some 19% stem from a Black, Asian or minority ethnic group.
Born in Haifa, Israel, Deutsch studied physics at the University of Cambridge before doing a PhD at the University of Oxford. After several years at the University of Texas at Austin, he returned to Oxford, where he is currently based. Deutsch is also a founding member of the university’s Centre for Quantum Computation, which opened in 1998.
Deutsch has been awarded the Newton Medal and Prize thanks to his research in quantum theory. In 1985 Deutsch published his ground-breaking work that detailed the relationship between quantum theory and the universal quantum computer. Four years later he developed the theory of quantum computational gates and networks, which is today the basis of quantum-information science.
In the early 1990s Deutsch proved that a quantum computer would be able to solve problems that require exponentially more computational time on a classical computer due to its restricted modes of computation. His work opened the possibility that the properties of quantum mechanics could have tangible and useful applications in computing. Indeed, today there are several commercial quantum computers being developed by companies and governments worldwide.
The Isaac Newton Medal and Prize attracts an award of £1000 and is the only one of the IOP’s prizes that is open to physicists worldwide. Previous winners include Thomas Kibble, Deborah Jin and Ed Witten.“I am honoured and also very happy that the Institute of Physics recognizes the significance of quantum computation as a fundamental part of physics,” Deutsch told Physics World.
Rewarding excellence
The IOP has announced the winners of its other awards. Among them are Ian Chapman from the UK Atomic Energy Authority who receives the Richard Glazebrook Medal and Prize for outstanding leadership in fusion. “I’m honoured to receive this award on behalf of all the team at UKAEA,” says Chapman. “Realizing fusion energy is one of the biggest scientific and engineering grand challenges, but the rewards for success would be massive.”
Robert Crease from Stony Brook Univeristy in the US, meanwhile, receives the William Thomson, Lord Kelvin Medal and Prize for his 21 years writing Physics World’s Critical Point column, which describes key humanities concepts for scientists, and explains the significance of key scientific ideas for humanities scholars.
The annual IOP awards also recognize early-career scientists. Among them are Ying Lia Li from University College London, who receives the Clifford Paterson Medal and Prize for her work in quantum sensing as well as Rebecca Bowler from the University of Oxford who receives the Henry Moseley Medal and Prize for her research on the first galaxies in the universe.
“I warmly congratulate all of this year’s award winners. Each and every one of them has made a significant and positive impact in their profession, whether as a researcher, teacher, industrialist, technician or apprentice,” says IOP president Sheila Rowan. “Recent events have underlined the absolute necessity to encourage and reward our scientists and those who teach and encourage future generations. We rely on their dedication and innovation to improve many aspects of the lives of individuals and of our wider society.”
Sarah Bakewell, head of equality, diversity and inclusion at the IOP, says that nominations for next year’s award will be opening soon. “I urge you to have a chat with your teams or speak with your colleagues or even consider nominating yourself for our awards in the future,” adds Bakewell.
The full list of 2021 award winners is available here.
I had discussed all this many times with my good friend Bob, an amateur science buff. So, when Bob asked me one day: “what’s all the fuss about the measurement problem?” after he had read yet another article on the subject in the popular press, I braced myself. “You physicists like to talk a lot about it, but never explain what a single-particle detector is,” he said, smirking. Unsurprisingly, Bob was not satisfied with my answer involving projection operators and random stuff I recalled from school. “Tell me,” he said, “what is it that distinguishes a measuring from a non-measuring object.” It was right there I realized that I did not have a good understanding of what defines a QMD. Worst of all, I could not even answer Bob’s next question about how a particle distinguishes a Geiger counter from a chair.
I realized I could not answer a question about how a particle distinguishes a Geiger counter from a chair
I did not sleep well that night. I dreamed I was sitting on a huge chair in outer space while being pursued by a neutron. Preparing myself for an elastic collision and associated momentum exchange, I figured that, free of friction, the chair would experience a small change in its velocity that could nevertheless lead to a large modification of its position over a long period of time. I woke up wondering if this made the chair a QMD, and if the elastic interaction would collapse the neutron’s wavefunction. Early the next day, I was at the science library collecting every book I could find on particle detectors.
QMDs are macroscopic systems that can be prepared in non-thermal or thermodynamically unstable states, which are so close to a tipping point that an interaction with a single particle can lead to a massive change, visible to the bare eyes. As Bohr noted in his discussions with Einstein, measurements always involve an irreversible amplification, like an avalanche or chain reaction.
A classical-mechanics analogy for a quantum measuring device is a set of dominos that increase in size, so that the first one falling triggers an avalanche (Courtesy: Roberto Merlin)
A classical-mechanics example of a chain reaction is the domino sequence pictured, where the distances between dominos and their dimensions grow at a fixed rate α > 1. If a domino can knock down its larger neighbour, falling leads to exponential amplification, since the energy released by the nth domino when it tumbles is a factor of α4(n-1) larger than for the smallest one. The process is irreversible as mechanical energy is dissipated by friction between the dominos. Moreover, if the constant width-to-height ratio is w/h << 1, the energy needed to topple a domino is a factor of (w/h)2 /2 smaller than its gravitational potential energy when standing. Hence, the energy required to initiate the avalanche can be exceedingly small.
But back to QMDs. A literature survey shows that they divide into two main classes, depending on whether the detector’s pointer is initially in a macroscopic state of charge separation or a thermodynamically metastable phase. In the former, a measurement induces a transfer of charge, while in the latter it triggers a phase (or compound) transformation.
The first charge-transfer device to be developed was the Geiger counter, which became a practical instrument in 1928, and is still widely used to detect ionizing radiation. In these devices, individual α, β and γ particles generate charge avalanches when they ionize atoms of an inert gas filling a Geiger–Müller tube. For a brief instant, the electrically insulating gas becomes a conductor and the resulting current pulse signals the detection of a particle. Other charge-transfer devices include photomultipliers, avalanche photodiodes and the silicon detectors used in high-energy experiments.
Phase-transformation QMDs, on the other hand, include photographic plates, which operate by a chain-like chemical reaction, as well as cloud and bubble chambers, developed respectively by Charles Wilson in 1911 and Donald Glaser in 1952. In these chambers, a particle leaves tracks by colliding with and ionizing molecules, which then become nucleation sites for a transformation from an unstable state into a stable one – from supersaturated vapour into liquid droplets in a cloud chamber, and from superheated liquid into gas bubbles in a bubble chamber.
Another example of a phase-transformation QMD is the superconducting-nanowire single-photon detector, first reported by Gregory Gol’tsman and co-workers in 2001. Its key component is a thin, meandering superconducting wire carrying a near-critical current that sets the material at the verge of switching into the normal, non-superconducting state. The phase transition can be triggered locally by the absorption of a single photon.
Armed with the knowledge that the principal trait of a QMD is that it allows a quantum object to generate a chain reaction and, therefore, that a conventional chair could not possibly be a QMD, I arranged to meet Bob for lunch one week after my long day at the library. Of all the things I told him, he was most interested that each measurement increases the entropy of the universe.
“Well done,” Bob said. “You solved the measurement problem.” As I stared in bewilderment, he continued. “If measurements are irreversible, it is abundantly clear that wavefunctions must collapse to prevent the occurrence of superpositions involving states of different entropy.” In what seemed to be a thinly veiled attempt to annoy me, he added, “You surely know that such superpositions violate superselection rules.” I did not know that, and I was very surprised that Bob knew about superselection rules. “By the way,” he said, as I was getting ready to leave, “do you think collapses happened before the 20th century, when detectors were invented?”
Researchers have identified a quantum phase transition taking place in iron more than 1000 kilometres deep within the Earth’s mantle. This transition, known as a spin crossover, also occurs in nanomaterials used for recording information magnetically, meaning that the effect stretches from the macro- to the nanoscale.
Many of the physical processes that occur deep inside the Earth remain a mystery, especially those at depths of more than 660 km. This is because we only have access to seismic tomographic images of this region, explains Renata Wentzcovitch, a physicist at Columbia University in the US who studies materials under extreme conditions. To interpret these images, researchers need to calculate the seismic (acoustic) velocities in minerals at the pressures and temperatures that prevail in the Earth’s mantle. Such calculations can then be used to create 3D velocity maps and calculate the mineralogy and temperature of the regions being observed. In these maps, a phase transition such as a change in a mineral’s crystal structure usually produces a sharp velocity discontinuity.
Scientists have known for nearly two decades that a spin change occurs in an iron-rich mineral called ferropericlase (Fp), which is the second most abundant component of the Earth’s lower mantle. This change, or crossover, can occur when iron-bearing minerals are under pressure or exposed to high temperatures. During this process, the bulk modulus of Fp – a measure of its resistance to compression – drops as electrons in iron’s d-orbitals change from a high-spin to a low-spin state. The same spin transition is also exploited in magnetic recording applications, since the magnetic properties of materials just a few nanometres thick vary significantly when they are stretched or compressed.
Fp spin crossover signals deep within the Earth
Wentzcovitch and colleagues predicted in 2006 that the same effect effect should also occur in the Earth’s lower mantle, across a zone a thousand kilometres wide. Since then, her group and others have developed ways of modelling the spin crossover in Fp and another mineral, bridgmanite, which is the most abundant mineral in the Earth’s lower mantle. Using ab initio calculations based on density functional theory, they predicted the properties of these minerals during this quantum phase transition.
In the latest study, which is published in Nature Communications, the team identified Fp spin crossover signals deep within the Earth’s lower mantle (in the ~1400–2000 km depth range) by studying specific areas in regions where this mineral is expected to be abundant. They also identified similar signals below a depth of 1800 km.
Geophysicists have already used such results to simulate the effects of spin crossover on mantle convection. They have shown that spin crossover in iron can invigorate convection in the Earth’s mantle and speed up tectonic plate motion – meaning that this quantum phenomenon might possibly be linked to increases in the frequency of seismic events like earthquakes and volcanic eruptions.
The researchers are now developing more accurate simulation techniques to predict seismic velocities, particularly in regions rich in iron, at temperatures close to its melting point. The techniques developed in this and previous work can also be applied to materials such a multiferroics and ferroelectrics, in which electrons are strongly correlated, and to materials at high temperatures and pressures in general, they say.
