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Tales of a not-quite-probability distribution

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Quasiprobability distributions resemble probability distributions but can contain negative and imaginary values. Such distributions represent quantum states as probability distributions over phase space represent states in classical statistical mechanics. Many quasiprobabilities exist, the most famous being the Wigner function. Among the least famous ranks the Kirkwood–Dirac distribution, discovered during the early 1900s and then forgotten.

The speaker, Nicole Yunger Halpern, will generalize the Kirkwood–Dirac distribution, then show how the generalization informs quantum thermodynamics, quantum information scrambling, and quantum metrology.

Arvidsson-Shukur, NYH, Lepage, Lasek, Barnes, and Lloyd, Nat. Comms. 11, 3775 (2020).
NYH, Bartolotta, and Pollack, Comms. Phys. 2, 92 (2019).
NYH, Swingle, and Dressel, Phys. Rev. A 97, 042105 (2018).
NYH, Phys. Rev. A 95, 012120 (2017).

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Nicole Yunger Halpern is an ITAMP Postdoctoral Fellow at Harvard. She earned her PhD at Caltech in 2018, winning the international Ilya Prigogine Prize for a thermodynamics thesis. In 2020, she won the International Quantum Technology Emerging Researcher Award from the Institute of Physics. Nicole re-envisions 19th-century thermodynamics for the 21st century using the mathematical toolkit of quantum information theory. She calls this research “quantum steampunk”, after the steampunk genre of art and literature that juxtaposes Victorian settings with futuristic technologies.

Speaker relationship with IOP Publishing

Winner of the International Quantum Technology Emerging Researcher Award, IOP Publishing Quantum 2020 conference.

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  • Building quantum processors and networks atom by atom

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    The realization of large-scale controlled quantum systems is an exciting frontier in modern physical science. Such systems can provide insights into fundamental properties of quantum matter, enable the realization of exotic quantum phases, and ultimately offer a platform for quantum information processing. Recently, reconfigurable arrays of neutral atoms with programmable Rydberg interactions have become promising systems to study such quantum many-body phenomena, due to their isolation from the environment and high degree of control.

    In this webinar, Hannes Bernien will present the recent progress on quantum processors and quantum networks that are based on arrays of individual atomic qubits. He will discuss the fundamental techniques that allow the bottom-up assembly of atom arrays with hundreds of atoms and will present the science and discoveries that these new architectures enable.

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    Hannes Bernien is assistant professor at The University of Chicago. His work focuses on finding answers to questions such as how to fully scale controlled quantum systems from the current few-particle level to many particles, how to study the effects of increased complexity in these systems, and how to utilize these phenomena for quantum technology. His lab combines techniques from quantum control and quantum optics with ultracold atoms and nanotechnology in order to develop new ways of engineering large, complex quantum systems and studying the phenomena that arise in such systems.

    Hannes earned his PhD (2014) in physics at Technical University Delft in the Netherlands in the group of Ronald Hanson. There, he performed experiments on quantum information processing with nitrogen vacancy centers in diamond and was the first to create long-distance entanglement of solid-state spins. This led to a loophole-free Bell test with two entangled spins that are 1.3 km apart. From 2015–2019 he was a postdoctoral research fellow in the group of Mikhail Lukin at Harvard where he co-developed a novel bottom-up approach to assemble large defect-free arrays of atoms with engineered interactions. This method has proven to be a promising platform for studying quantum many-body phenomena.

    Speaker relationship with IOP Publishing

    Winner of the International Quantum Technology Young Scientist Award, IOP Publishing Quantum 2020 conference.

    Why not sign up for our other Quantum Week webinars? Even if you’re not able to join the live event, registering now enables you to access the recording as soon as it’s available.

    Nitride quantum light sources

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    Single photon sources are a key enabling technology for quantum communications and are an important step towards more advanced quantum light sources with potential applications in other quantum information processing paradigms such as linear optical quantum computation. In considering possible practical implementations of future quantum technologies, the nitride materials system is attractive since nitride quantum dots (QDs) achieve single photon emission at easily accessible temperatures, potentially enabling the implementation of quantum key distribution paradigms in contexts where cryogenic cooling is impracticable.

    However, the particular crystal structure of nitride semiconductors typically results in large internal electric fields in QD structures. Electrons and holes captured by a QD are separated by the field, reducing their probability of recombination and limiting the single photon source emission rate. Growth in alternative “non-polar” orientations can greatly reduce these internal fields and also yield emission polarized along a specific crystal direction due to changes in the valence band structure induced by the asymmetric strain state of the material. Although such devices are challenging to grow and fabricate, due to high densities of defects in the materials used, QDs exhibiting polarized optically-pumped single photon emission up to 220 K, a temperature accessible by on-chip cooling, have been demonstrated, and incorporated into single photon light-emitting diodes that exhibit deterministically polarized electroluminescence.

    Nonetheless, compared to more mature materials systems, many challenges remain in achieving the required device efficiency, and single photon purity and indistinguishability, even when considering “hero” devices. Developing a genuine manufacturable quantum technology from nitride QDs raises the bar still higher and is inspiring new approaches to materials growth and processing.

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    Rachel Oliver
    (Source: Agnese Abrusci)

    Prof. Rachel Oliver is director of the Cambridge Centre for Gallium Nitride. The focus of her research is understanding how the small-scale structure of nitride materials effects the performance and properties of devices. She uses expertise in microscopy and materials growth to develop new nanoscale nitride structures that will provide new functionality to the devices of the future.

    Rachel is also passionate about improving equality, diversity, inclusion and accessibility in science, and is an equality champion in the School of Physical Sciences at Cambridge.

    Speaker relationship with IOP Publishing

    Editorial Board member, Materials for Quantum Technology.

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    A roadmap for the quantum internet

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    What is the quantum internet? Why doesn’t it exist yet, and when will it arrive? What new capabilities will the quantum internet enable?

    In this webinar we’ll answer these questions, considering quantum hardware, interfaces, software, simulations and applications.

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    Tracy Northup is the Ingeborg Hochmair Professor of Experimental Physics at the University of Innsbruck, Austria. Her research explores quantum interfaces between light and matter, focusing on trapped-ion and cavity-based interfaces for quantum networks and quantum optomechanics.

    Speaker relationship with IOP Publishing

    Editorial board member, Quantum Science and Technology.

     

    Harold Ollivier recently joined INRIA as head of the Quantum Technologies programme after being part of LIP6 at Sorbonne Université. His research focus is on quantum error correction and cryptography, especially on securing delegated quantum computation with the help of quantum networks.

     

     

     

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    A quantum future of computing

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    With rapid progress in quantum technology, focus is shifting from demonstrations on a small number of noisy qubits towards interesting application problems, of academic or commercial interest, where a quantum computer can outperform even the best classical supercomputer. Comparing current classical computers with optimistic assumptions for future quantum computers, this webinar will present criteria for achieving such practical quantum advantage and will argue that “small data” problems with superquadratic quantum speedups are the most promising candidates.

    The simulation of quantum systems – with applications in condensed-matter physics, materials science and chemistry – is one such application area. Matthias Troyer will present recent progress on quantum algorithms for chemistry and discuss that realizing such a quantum advantage will require more than just a quantum algorithm and quantum computer. He will then also touch on a topic of growing recent interest: the ethical implications of quantum computing. He will argue that this is best viewed in the broader context of ethical issues in disruptive computing and will highlight some aspects especially relevant for quantum computing.

    Matthias Troyer is a Fellow of the American Physical Society, vice-president of the Aspen Center for Physics, a recipient of the Rahman Prize for Computational Physics of the American Physical Society for “for pioneering numerical work in many seemingly intractable areas of quantum many body physics and for providing efficient sophisticated computer codes to the community” and of the Hamburg Prize for Theoretical Physics. After receiving his PhD in 1994 from ETH Zurich in Switzerland he spent time as postdoc at the University of Tokyo before returning to ETH Zurich as a professor of computational physics. He joined Microsoft’s quantum computing programme at the beginning of 2017. Matthias works on a variety of topics in quantum computing, from the simulation of materials and quantum devices to quantum software, algorithms and applications of future quantum computers. His broader research interests span from high-performance computing and quantum computing to the simulations of quantum devices and island ecosystems.

    Speaker relationship with IOP Publishing

    Editorial board member, Machine Learning: Science and Technology.

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    Direct image of an electron orbit in an exciton is a first, say researchers

    For the first time, researchers have captured an image of an electron’s orbital probability cloud within an exciton. Keshav Dani, and colleagues at Japan’s Okinawa Institute of Science and Technology Graduate University, produced the image by kicking electrons out of their orbits using tightly controlled laser pulses, and then measuring their trajectories with an ultra-precise electron microscope. Their results could lead to a new understanding of exciton properties that have long evaded physicists.

    When an electron within a semiconductor absorbs a photon of sufficient energy, it will be excited to a higher energy level, leaving behind a positively-charged hole. Due to their mutual attraction, both particles will then briefly orbit each other, creating a neutral quasiparticle called an exciton. Excitons are key to the operation of semiconductor technologies, but so far, they have proven extremely difficult to study. Not only can excited electrons fall back into their respective holes within just picoseconds; the quasiparticles can also be easily broken apart.

    As a result, key exciton properties, including their momenta, and the characteristics of their electron-hole orbits, have only been described theoretically. In their research, Dani’s team introduced a new setup, which allowed them to image these orbits directly. To do this, they first used a laser pulse to generate excitons within a low-temperature 2D semiconductor: an atomically-thin material where excitons can last for a relatively long time before electron-hole recombination.

    Extreme ultraviolet photons

    With a second pulse, they then fired extreme ultraviolet photons at the excitons. This kicked their electrons into the vacuum of an electron microscope, which uses the wave-like properties of electrons to produce images at sub-atomic resolutions. By measuring the angles and energies of these displaced electrons using the microscope, the researchers could then determine their positions within their original excitons, at the exact moment before they were kicked out.

    The process is comparable to the high-energy collisions carried out at particle accelerators: from the trajectories of the particles produced by collisions, physicists can deduce the nature of particles before they are smashed apart. By making measurements over many cycles of exciton production and destruction, Dani and colleagues could gradually build up images of the wavefunctions of electrons within their exciton orbits. These mathematical relationships relate to the probability of finding electrons in a given position when observed, according to Heisenberg’s uncertainty principle.

    In a striking image, Dani’s team could clearly see the 3D wavefunction, or “probability cloud” of an excited electron within its orbit around a hole. The ability to visualize internal orbits in this way could give researchers unprecedented control over the properties of quasiparticles, potentially leading to advanced new quantum technologies, and exotic new states of matter. The research could also lead to a better way of studying other composite particles such as hadrons, which are made up of quarks.

    The research is described in Science Advances.

    More human than human

    “Hmm, that seems a bit convenient.” Mel’s image appears, along with Ashley’s and Taylor’s.

    “So, we’re identifying Lou as the bot?” Taylor grins.

    “Steady on! My PC hasn’t got a camera and my kids are on the other devices.”

    “I’m still saying it’s a bit convenient that we can’t see you,” Mel says. “Apart from these tests, what’s everybody been up to?”

    “Not much,” Taylor says. “Lockdown, right?”

    “I’m going for a run after this,” Ashley says.

    “I’m heading out to the pub in a bit,” Lou says.

    “The pub? I think I agree with Mel,” Taylor says. “You missed a trick there, Lou-bot. There’s a pandemic on.”

    “There is, but I’m in New Zealand,” replies Lou.

    “Good save.” Ashley smiles. “But I think a human would say something specific, like Christchurch or –”

    “Akaroa,” says Lou. “Just outside Christchurch.”

    “Even so,” Ashley continues, “It now sounds like you’re just backfilling detail from the web. Mel, Taylor, are we saying it’s Lou?”

    The other two nod. Lou’s empty frame disappears.

    “We’re still here,” Ashley says. “Wasn’t Lou, then.”

    “Wasn’t Lou. Misdirection, even if unintentional,” Mel says. “Guess it would have been a bit too obvious – voice only, different time zone, unaffected by the pandemic. Must be one of you two, then.”

    “Or one of you two,” Ashley smiles.

    Taylor looms larger in frame, eyes scanning left to right and back again. “Well, damned if I can tell the deep fake one of you from the real person.”

    “We need an AI to detect AIs!” chuckles Ashley.

    – Ironic observation.

    – Yes.

    “We need something more Voight–Kampff than Turing,” Mel says.

    – Pop-culture reference. Nice touch.

    “What?” Taylor asks.

    “Blade Runner, right?” says Ashley. “Emotional response rather than knowledge response?” Mel nods.

    “Well, Lou didn’t seem very emotionally engaged, so I don’t know if that would have helped,” Taylor says.

    “True,” Mel says. “Maybe Lou’s a laidback person.”

    “Or just indifferent and tired,” Ashley adds. “I mean, how many of these tests have you done and what other work are you doing during the day? We’re earning points, but that just converts to cash on the side. If you’re already spending all day staring at people on a screen, each test is just another meeting, right?”

    “Lou didn’t try to defend against our accusation,” Taylor continues.

    “Hmm, not letting it go, are you?” Mel sits back.

    “And I think we empathize with why Lou might have been like that,” Ashley says. “Mel?”

    “Agreed. It’s Taylor,” Mel says. Ashley nods.

    Taylor’s frame disappears.

    “Damn. Wasn’t Taylor, then,” says Ashley.

    “Now it’s down to which of us accuses the other first.”

    “Yeah. Well, I’m not calling it.” Ashley grins.

    “Me neither,” says Mel. “This isn’t the Wild West – or Westworld. It’s not who shoots first; it’s who doesn’t.”

    “Have you ever identified a bot in these tests?”

    “No. I’ve always been the last one. The second to last one has always been the bot.”

    “Same here.”

    “Wait a minute… The second to last has always accused me.”

    “Like I said, same here.”

    “What I mean is that we’ve both assumed the second to last was a bot, but we don’t know that, do we? We just know they accused us. That would make sense if they were a bot, but it’d also make sense if they were human.”

    “Oh, you mean…”

    – This is new.

    – Yes. Not seen this in any of the other rounds.

    “If I understand you, we’ve assumed one person in each test is the bot, but what if there’s no bot?”

    “Exactly. We were never told one person was a bot. We were only told to ‘identify the bot’ and incorrect identification would drop the person from the call.”

    “We just assumed there was a bot and the goal was to find them. But what if that’s not the goal? What if it’s a double-blind experiment?”

    – They’re good.

    – Yes.

    “Yes.”

    “What if these tests form an infinite rather than a finite game? The goal is not to win but to keep playing?”

    “Got it. Yes. Which means the aim is not to identify the bot: the aim is not to be identified as a bot.”

    “Perhaps the people running these tests aren’t pitting bots against people to see which bots are better at passing as human: they’re trying to find people who are good at not being mistaken as bots. Maybe they want to train their bots on how those people behave?”

    “More human than human,” Mel says.

    “So, if we’re not accusing each other, what next?”

    “Good question. Just sit and hope for a timeout?”

    – Impressive. Advance them both to the next round?

    – Agreed. We’ll have to get upstairs to sign that off.

    – Yes. That decision still needs human approval.

    Einstein letter reveals interest in animal navigation, artificial intelligence predicts how a violin will sound

    Einstein letter
    Navigating the future: the letter from Albert Einstein to Glyn Davys (click to expand). (Courtesy: Dyer et al. 2021, J Comp Physiol A/The Hebrew University of Jerusalem)

    A 1949 letter sent by Albert Einstein to the British radar researcher Glyn Davys shows that the great physicist believed new insights into physics could come from studying animal navigation. In the letter Einstein refers to being acquainted with the work of the German animal behaviour expert and future Nobel laureate Karl von Frisch, who discovered that bees navigate using polarized light.

    Einstein and von Frisch are believed to have met in 1949, but nothing had been known of their private discussion. The previously unknown letter suggests that the pair talked about how there was much to learn about the physics underlying animal navigation. “It is thinkable that the investigation of the behaviour of migratory birds and carrier pigeons may someday lead to the understanding of some physical process which is not yet known,” Einstein wrote to Davys.

    This was highly prescient because today we know that some birds navigate by detecting the Earth’s magnetic field. What is more, physicists believe that this detection relies on quantum entanglement – something that had long been thought impossible for living things.

    Indeed, researchers are now trying to mimic this quantum sensory system to develop new technologies, as I discover in this podcast: “Quantum birds inspire new metrology for biosciences…”.

    Such are the reputations of Einstein and von Frisch that a multidisciplinary group of researchers in Australia and Austria has written a paper about the letter, which is free to read in the Journal of Comparative Physiology A.

    Arcs of circles

    Making a good violin is notoriously difficult, being both an art and a science. Now, research led by two physicists – one a luthier and the other a mandolin player – have added another string to the violin maker’s bow by using artificial intelligence to predict the sound of a violin before it is built. Their inspiration came from an old drawing in the Museo del Violino in Cremona, Italy – which describes a violin’s outline as the conjunction of the arcs of nine circles.

    By changing the parameters of these arcs as well as the thickness and mechanical characteristics of wood, the duo created a dataset of possible violins – some corresponding to existing instruments and others to purely fanciful creations. Advanced modelling was used to predict the sounds of the violins and all of this was input to an artificial neural network. Amazingly, the network was able to learn how a violin’s design affected its sound and could predict the sound of an instrument with an accuracy of nearly 98%.

    The team says that the system could help in the design of instruments and the choices of wood used. They describe their results in the journal Scientific Reports.

     

    Carbon-based inks make first fully recyclable transistors

    Electronic waste is an increasingly serious problem, not least because it is hard to recycle the silicon-based components that make up the bulk of consumer electronic devices. Researchers at Duke University in the US have now taken a step towards remedying this by creating the first fully recyclable transistors made from carbon-based “inks” that can be printed on paper or another environmentally friendly substrate. While these devices are unlikely to replace their silicon cousins any time soon, they could find their way into specialist applications such as environmental sensors or biomedical sensing patches relatively quickly.

    Electronics containing carbon-only components could be ideal for making printable, recyclable devices. Semiconducting carbon nanotubes (rolled up sheets of carbon) and conducting graphene (a sheet of carbon just one atom thick) are both good candidates for making such components. Another carbon-based compound, cellulose, has previously been employed as both a substrate and dielectric, and has the advantage of being both naturally biodegradable and the most abundant polymer on Earth.

    All-carbon printed electronics, however, are few and far between because there aren’t many carbon-based dielectrics that can be processed in solution – a prerequisite for printing devices. While cellulose paper can work as a dielectric in high-power transformers, crystalline nanocellulose is generally used as a binder or in conjunction with another material that has a high dielectric constant, not as a standalone printable dielectric.

    All-carbon transistors

    To make its all-carbon transistor, the Duke team led by Aaron Franklin put all these ingredients together for the first time, using crystalline nanocellulose extracted from wood fibres as a dielectric; carbon nanotubes as a semiconductor; and graphene as a conductor. After making inks from these three components, the researchers showed they could directly print them onto a paper substrate using a technique called aerosol jet printing at room temperature. By adding salt (sodium chloride) to the dielectric, they obtained devices with an on-current of 87 μA/mm and a subthreshold swing of 132 mV/dec, values that will allow the transistors to be employed in a wide variety of applications.

    The team showed that their transistors could be recycled by submerging them in a series of ultrasonic baths and then centrifuging the resulting solution. They recovered the CNTs and graphene with over 95% efficiency, then reused the materials to reprint new transistors. The nanocellulose, which is naturally biodegradable, can also be recycled, as can the paper substrate.

    Nanocellulose ink hits the spot

    Franklin points out that nanocellulose has been used to make recyclable packaging for years. However, while its potential applications as an insulator in electronics were widely understood, nobody had figured out how to use it in a printable ink before.

    Demonstrating such a fully recyclable printed transistor is, he adds, a first step towards using the technology to make simple commercial devices. Two possibilities for early devices include environmental sensors (to measure energy consumption of a building, for example) or customized biosensing patches for monitoring medical conditions. Indeed, the researchers, who detail their work in Nature Electronics, have already made a fully printed, paper-based biosensor for sensing lactate from their transistor.

    The researchers hope their results will increase interest in areas of materials and electronics research that focus on recyclability and novel fabrication techniques such as printing. “We now plan to further improve on the ink formulations and resultant performance of these printed electronics, reduce dependence on harsh chemicals in the process, and study a variety of applications these devices make possible,” Franklin tells Physics World.

    Tiny black holes could cause white dwarf stars to explode

    A new explanation for how white dwarf stars explode as type Ia supernovae (SNe Ia) has been proposed by astrophysicists in Brazil and Mexico. Their model suggests that the explosions are ignited when primordial black holes (PBHs) collide with white dwarfs.

    PBHs are hypothetical black holes that are about as massive as an asteroid and are believed to be left over from the universe’s earliest moments. PBHs are also candidates for dark matter, so the model provides a potential link between SNe Ia observations and dark matter.

    The research was done by Heinrich Steigerwald at the Centre for Astrophysics and Cosmology of the Federal University of Espírito Santo, Brazil, and Emilio Tejeda at the Institute of Physics and Mathematics, Universidad Michoacana de San Nicolás de Hidalgo, Mexico.

    White dwarfs are dense stars at the end of their lifetimes that no longer undergo nuclear fusion. If a white dwarf accretes material from a companion star, its mass will increase until it reaches the Chandrasekhar limit – about 1.4 solar masses. At this point fusion switches on in a runaway process that causes a spectacular SNe Ia explosion. The merger of two white dwarfs can also result in a type Ia supernovae and astrophysicists have discovered that “sub-Chandrasekhar” white dwarfs below 1.4 solar masses can ignite as well.

    Mystery ignition

    Although SNe Ia are routinely observed – and used to measure distances in the universe – astrophysicists do not understand exactly how the explosions are ignited.

    “We know that white dwarfs exist, and current models describe them in a satisfactory manner. We also know that SNe Ia arise from the explosion of white dwarfs, though what causes [the explosion] remains a mystery,” Steigerwald tells Physics World. “We have absolutely no idea if these asteroid mass PBHs exist yet, but if they do then our study argues that they are the reason for SNe Ia explosions.”

    A PBH with a mass of about 1020 kg would be about one micron in size. If such an object were to encounter a white dwarf, Steigerwald and Tejeda say that it would be accelerated by the gravitational influence of the star to around a 20th of the speed of light. At such an incredible speed, Steigerwald says the black hole would cut through the white dwarf like a bullet through butter – which could lead to the detonation of the star.

    Their model predicts that ignition takes place within a tiny region no larger than a square millimetre in the wake of the PBH as it passes through the white dwarf. “The rest is well-known physics,” Steigerwald explains. “Once ignited, the white dwarf star explodes within a few seconds. Most of the carbon and oxygen is fused to heavier elements, some of which are radioactive.”

    If the mechanism suggested by the team can be verified it could also support the idea that dark matter comprises primordial black holes. This is because the abundance of observed SNe Ia corresponds to the predicted abundance of dark matter.

    “Remarkable coincidence”

    “Our work has shown that asteroid mass PBHs with a mass of between 1019 and 1023 kg, can ignite SNe Ia from white dwarfs,” he explains. “If the mass of PBHs is about 1020 kg, then fortuitous encounters with white dwarfs can account for both the [observed] rate of SNe Ia – roughly one per galaxy per century – and the brightness distribution of these events. “This is quite a remarkable coincidence,” he adds.

    Another important aspect of the research is that it provides a mechanism for the ignition of sub-Chandrasekhar white dwarfs – something that had not been known before.

    “I really like this suggestion and the highly creative out-of-the-box thinking of [Steigerwald and Tejeda],” says Robert Fisher at the University of Massachusetts Dartmouth. He adds that though the model should be treated cautiously, it could help place constraints on primordial black holes.

    Steigerwald says that moving the research forward will require two things. One is success in the search for PBHs: “In the next decades, space-based gravitational-wave observatories, like the upcoming Laser Interferometer Space Antenna (LISA), could observe the stochastic gravitational-wave background that these primordial black holes would produce during their formation in the early universe”.

    He also points out that powerful numerical simulations of the ignition process are required.  “The ultimate confirmation of the mechanism to work out or fail would be its demonstration with full reactive numerical simulations in three dimensions. “This is a very difficult task, but I am aware of some [research] groups that I believe have the capacities to do it.”

    The research is described in a preprint on arXiv and a paper is expected to be published in Physical Review Letters.

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