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QuTech launches a browser for the quantum Internet

The event I attended on Friday caught my attention for several reasons. Billed as the Quantum Network Explorer (QNE) Launch, it took place in the Hague, the Netherlands, not far from where I live. In a year that has seen so much exciting progress on quantum networks, I was curious to find out what it involved. But I was also drawn by the event’s name, which made it sound a lot like Microsoft’s all-but-defunct web browser, Internet Explorer.

Among Internet-using citizens of a certain age, the name “Internet Explorer” triggers one of two emotions: nostalgia or annoyance. For many (excluding Netscape fans from the 1990s and, of course, the CERN scientists who created the web in the first place), Explorer was their first experience with the World Wide Web. Over the past two decades, however, it has fallen well behind competing browsers such as Chrome and Firefox. So, from the perspective of the QNE launch, I was wondering: will this be the beginning of a new era, one that will eventually lead to us to take the quantum Internet for granted? Or will it fizzle out?

The event was hosted by QuTech, a collaboration between the TU Delft and TNO. From a broader perspective, it was part of the Europe-wide Quantum Internet Alliance, which has the long-term aim of enabling quantum communication applications between any two points on Earth. During the event, 50 people present on-site and around 400 viewing via livestreamed video watched the QuTech physicist Stephanie Wehner push the red button that officially launched QNE. Afterwards, we watched animations and listened to two discussion panels, containing members from Quantum Delta NL, SURF and RIPE NCC, that explained what the quantum Internet was and how QNE contributes to it.

QNE, it turns out, is a development platform based on the software stack that runs a quantum network. Because of the requirement for quantum entanglement, quantum networking software is very different from the software that runs classical networks – and complicated enough that most people who operate it have a PhD in physics. Although QNE has been developed with an actual quantum network in mind, for now, it is running on a classical simulation. In time, this simulation will be replaced with actual quantum hardware.

Is the quantum Internet finally here?

Importantly, though, QNE offers a user-friendly software interface that allows people to start developing applications for quantum networks (you can access it here). The idea is that anybody – with or without a PhD in physics – can start playing with the software and use it to develop next-generation quantum network applications. That way, QNE will train end-users to work with quantum networks, allow for new “killer apps” to come forward and attract new people (including high school students) to the field of quantum.

I believe it’s the right choice for developers to put this new technology in the hands of users, as it should help to accelerate progress. But will QNE lead to a new generation of quantum apps, inevitably named Qapps? Or will it lose ground to competing quantum browsers such as Qhrome or FirefoQs? Only time will tell, but thanks to projects like QNE, we can take at least take a step towards making such daydreams a reality. I guess the only thing left for me is to put my PhD in physics to good use…

This post was updated on 24 November 2021 to clarify the roles of the organizations involved.

Sticky-tape speckle patterns help characterize X-ray mirrors

A new X-ray imaging technique that uses sticky, or Scotch, tape as a diffuser to generate “speckle” patterns makes it possible to characterize strongly curved X-ray mirrors in two dimensions with nanoscale precision. The new technique could find use in super-precision metrology, while also aiding the development of next-generation X-ray mirrors for upgraded synchrotrons.

X-ray mirrors are routinely employed in synchrotron radiation facilities, X-ray free-electron lasers and X-ray telescopes. Such mirrors have become increasingly smooth in the past few years thanks to advanced polishing techniques and now boast “slope errors” smaller than 50 nanoradians rms (root mean square). While this is a boon for many applications, the downside is the mirrors are so smooth that existing metrology techniques can no longer be used to characterize them.

Speckle angular measurement

Researchers led by Kawal Sawhney, Hongchang Wang and Simone Moriconi from the Optics and Metrology group at the UK’s Diamond Light Source have now developed a new instrument and technique based on “speckle angular measurement” (SAM) that remedies this need. Indeed, in their work, which they describe in Light: Science & Applications the researchers show that they can push the angular precision of slope error measurements down to just 20 nanoradians rms thanks to an advanced sub-pixel tracking algorithm.

The SAM technique generates two-dimensional random intensity patterns (the speckles) by shining a laser through a “diffuser” made from good old sticky tape. Since the tape is see-through, the laser light can pass through it and generate the speckle patterns, with any variations in the slope of the mirror shifting these patterns. The researchers can then determine the exact value of these variations to the nanoradian level by precisely tracking the speckle shift with the algorithm they developed.

Unlike other techniques, the new method does not require high-precision optics and the samples do not need to be rotated during the experiments. The technique is also simple as it only requires Scotch tape.

Complement existing metrology methods

The researchers say the SAM instrument might also be used to measure toroidal, ellipsoidal and paraboloidal mirrors by raster scanning across the entire mirror surface. This type of technology would be straightforward to integrate into existing measurement probes, too, and thus complement other imaging methods. As well as characterizing synchrotron X-ray mirrors, it could have applications for characterizing freeform optics and extreme ultra-violet high-quality mirrors, with possible extensions to the inspection of biomedical and materials science samples.

The new nano-metrology technique could also aid the development of next-generation super-polished X-ray mirrors that are required for upgraded synchrotron facilities such as Diamond, the researchers say. “This novel instrument will enhance the capabilities of our state-of-the-art metrology laboratory at Diamond and enable us to metrology test the extremely high-quality X-ray mirrors required for use with the planned upgrade of Diamond to a low-emittance Diamond-II source,” Sawhney says. “Vendors of X-ray mirrors will also find this new instrument attractive as it will enable them to fabricate even better-quality optics than at present.”

Photon-counting CT promises a new era of medical imaging

**Transforming CT** Naeotom Alpha is the world’s first photon-counting CT scanner. (Courtesy: Siemens Healthineers)

Computed tomography, or CT, is a ubiquitous X-ray imaging technique used to perform more than 300 million medical imaging exams globally each year. Use of the technique continues to grow, with CT increasingly employed as a first-line diagnostic tool for conditions such as coronary artery disease, as well as moving into preventive care and early detection, such as lung cancer screening.

But there are still limitations to what conventional CT can achieve in the clinic. For starters, it’s not always possible to image all patients: medical implants, for example, can create image artefacts that inhibit diagnostic accuracy. There are also shortfalls with regard to image reproducibility and standardization, both essential for ongoing evaluation of disease progress. Finally, there’s a growing need for functional data alongside the standard anatomical CT images.

“These three shortcomings are the ones we had in mind when we started moving into a new era of how CT imaging can be provided,” said Philipp Fischer, head of CT at Siemens Healthineers. “This was the starting point for our thoughts around Naeotom Alpha.”

The culmination of more than 15 years of research, Naeotom Alpha is the world’s first photon-counting CT scanner. Photon-counting technology enables dramatic improvements in diagnostic imaging, including increased resolution and a reduction in radiation dose by up to 45% over conventional CT detectors. Naeotom Alpha is now cleared for clinical use in the USA and Europe, and Siemens Healthineers will be showcasing its new system at next week’s RSNA Annual Meeting.

Crystal breakthrough

Speaking at a media briefing, Fischer described the main differences between conventional and photon-counting CT technology. Conventional CT requires two conversion steps to turn photons into a medical image: X-rays are collected by a scintillator, which uses many photons together to generate an optical signal; a photodiode then converts this light into an electrical signal. The process is stable and reliable; but two-step conversion limits the dose efficiency and, with many photons contributing to the optical signal, the achievable resolution is limited.

“With photon-counting technology, we move from a two-step conversion process to a single one-step conversion of X-ray photons into an electrical current that generates the medical image,” Fischer explained, noting that removing a conversion step increases dose efficiency. “The large difference is that with this new technology, we are able to assess each and any single photon separately, and also assess the energy level of each and any photon.”

This new approach to image acquisition, however, required the development of an entirely new detector material: high-purity cadmium telluride (CdTe) crystals. CdTe crystals deliver the highest spatial resolution of any CT imaging system to date, said Fischer, enabling pixels nine times smaller than used in conventional CT, without any dose penalty.

The crystals also completely eliminate electronic noise from the detector, resulting in a higher signal-to-noise ratio and lower dose requirements. Another advantage is that, unlike conventional CT that often underrepresents low-energy photons, photon-counting technology uses all photons equally, which should be particularly beneficial for soft-tissue imaging. Finally, the detector’s intrinsic spectral sensitivity means that spectral data are available in every scan.

Clinical gains

Fischer shared some examples of how these CT advances could prove invaluable in the clinic. When diagnosing coronary disease, for instance, seven million invasive catheterization procedures are performed each year, half of which do not lead to therapy. CT angiography provides a non-invasive diagnostic alternative. But conventional CT scans suffer from image artefacts due to calcifications or implanted stents or pacemakers, precluding a large patient population from this approach.

The spectral imaging offered by photon-counting technology enables differentiation between calcifications, stents, vessel walls and contrast media. This allows the removal of unwanted data from the image, increasing diagnostic accuracy. “With photon-counting technology, we have the goal to make non-invasive coronary imaging available to all patients that could benefit from it,” said Fischer.

In oncology, meanwhile, Naeotom Alpha’s high precision is particularly beneficial. Cancer patients undergo diagnostic scanning and many follow-up exams to assess treatment response and disease progression. With conventional CT, however, the image acquisition parameters can influence the image itself, preventing accurate comparison of scans over time. Photon-counting CT, on the other hand, provides consistent signal quality, with stable Hounsfield unit values for each patient across every scan.

**Lung imaging** Photon-counting CT allows visualization of detailed structures (centre) with simultaneous functional imaging (right). For comparison, a conventional CT image is shown on the left. (Courtesy: J Ferda, University Hospital Plzen, Czech Republic)

Another target application is lung imaging, where multiple images are often required to achieve a meaningful diagnosis. “With conventional CT, you need to know in advance what you are looking for, and then design your imaging exam,” Fischer explained. “You cannot look for all the detail in one exam without compromising on patient dose, resolution or functional image information.” But with photon-counting technology, a single scan can provide both structural images and functional information on the perfusion of the lung.

Initial experience

The dual-source Naeotom Alpha scanner is already installed at 22 sites. One of the first installations was at the University Hospital Augsburg in Germany, where the system has been in routine clinical use since April 2021.

To date, the Naeotom Alpha has been used to scan more than 4000 patients, in areas including oncology, neurology, cardiology and musculoskeletal imaging. “We are very happy with the results so far,” said Thomas Kröncke, director of the hospital’s department of diagnostic and interventional radiology.

“The important thing to me is seeing better and seeing more, due to the intrinsic spectral separation that photon counting allows,” said Kröncke. “Spectral post-processing is always possible, and the considerable increase in spatial resolution and higher intrinsic iodine contrast make it possible to reduce not only the radiation but also the contrast material.”

Finger snaps rely on huge angular accelerations, fake fossils on Mars

Have you ever thought about what happens when you snap your fingers? Raghav Acharya, Elio Challita, Mark Ilton and Saad Bhamla have looked deeply into the physics of the finger snap and published a paper about their surprising findings in the Journal of the Royal Society Interface.

Based at the Georgia Institute of Technology and Harvey Mudd College in the US, the quartet has discovered that the motion of your finger during a snap undergoes the highest angular acceleration that the human body is known to be capable of. They came to this conclusion by studying snapping fingers using high-speed imaging, automated image processing and dynamic force sensors.

They found that a finger snap happens in about 7 ms, which is about one twentieth of the time it takes to blink an eye. The measured angular acceleration was 1.6 million degrees per second squared – which is about three times greater than the acceleration of the arm of a professional baseball pitcher. A pitcher, however, does develop a higher angular velocity than a finger snapper.

Latch and spring

When they are not snapping their fingers, the team studies the mechanisms used by a range of living organisms to store energy and then quickly release it. They say that finger snapping is an example of a latch-mediated spring-actuated system, which is used by some termites and ants to make snapping noises with their mandibles. The team also looked at the role friction plays in snapping by covering their fingers in various materials. When low-friction metal was used, the velocity of the finger dropped dramatically – illustrating the importance of friction in the snapping process. However, when high friction rubber was used, the velocity also dropped – suggesting that there is a “Goldilocks zone” of friction for snapping.

You can read more in “‘Oh, snap!’ A record-breaking motion at our fingertips”

False fossils

We know about organisms that lived on Earth a very long time ago because occasionally one of those plants or animals was fossilized and preserved for posterity. Scientists believe that Mars may have supported life several billion years ago, so looking for fossils on Mars seems like a reasonable thing to do. But what would Martian fossils look like and how could we tell them apart from structures in rock that were formed by non-living processes?

In the UK, Julie Cosmidis at the University of Oxford and Sean McMahon at the University of Edinburgh have done a review of non-biological geochemical processes that can create structures that look a lot like fossilized microbes. Writing in the Journal of the Geological Society, they identify dozens of processes and say that there could be many more.

These processes can create deposits that look like bacterial cells as well as carbon-based molecules that closely resemble the building blocks of life (see figure).

“We have been fooled by life-mimicking processes in the past,” says Cosmidis. “On many occasions, objects that looked like fossil microbes were described in ancient rocks on Earth and even in meteorites from Mars, but after deeper examination they turned out to have non-biological origins.”

McMahon adds, “For every type of fossil out there, there is at least one non-biological process that creates very similar things, so there is a real need to improve our understanding of how these form.”

Universal photonic quantum processor sets new size record

Scientists from QuiX Quantum and the adaptive quantum optics group at the University of Twente in the Netherlands have built the largest universal photonic quantum processor to date. The processor works by applying adjustable phase shifts to the optical signals going through its 12 modes and then merging the signals in adjustable proportions. The precision of its fabrication allows single photons to interfere as they propagate, making the processor capable of quantum operations – albeit not yet at a level that could outperform classical machines.

The new device takes in 12 input optical signals, processes them and outputs the result optically, all at the standard telecommunication wavelength. The device’s photonic configuration – that is, the phase shifts and the proportions of each signal being merged – determines the nature of the processing task, and users can reconfigure this by connecting it to an ordinary personal computer. In this way, the device can be programmed to perform any processing task realizable by a specific set of optical merging and phase shifting steps.

The photonic processor and its delicate engineering

To implement these steps, the device uses a series of optical components known as tuneable phase shifters and tuneable beam mergers. The latter consists of two beam mergers that combine pairs of input beams in equal proportions, plus a tuneable phase shifter. The key to making the system reconfigurable is thus to have full control over the phase shifters within the processor’s photonic circuit, where each of the phase shifters is a heater that induces a very well-tuned and specific change in the effective path length of the passing optical signals via a phenomenon known as the thermo-optic effect.

By satisfying a list of technical demands for the 12 modes, from high-quality microfabrication of the photonic waveguides (optical paths) to providing fast mechanisms for stabilizing the temperature of the photonic circuit, the team set a record for the number of on-chip modes with a programmable configuration that can process quantum optical inputs (such as single-photon ones). In other words, the probability of losing a single photon inside the processor is low, and furthermore, identical single photons injected to different inputs of the processor do not appear different at the output. These results have been published in Materials for Quantum Technology.

Characterizing the processor

To quantify the processor’s reconfigurability, the team changed the processor’s configuration and tested it using laser light (providing classical inputs) and photodetectors. By comparing the configuration obtained in this test with the desired one, they found the “amplitude fidelity” – a measure of similarity between different configurations – was about 93%, stretching to 98% for some target configurations.

Photo of the quantum photonic processor — **Well connected** A close-up of the processor’s core, including the photonic chip at its heart. (Courtesy: QuiX Quantum)

The researchers also evaluated the processor’s optical loss with the same input–output setup. They found that this was as low as 17% on average, although a significant amount of additional loss occurs at the input and output connectors. Finally, they characterized the processor’s ability to preserve the identical nature of single photons. The team did this by injecting two identical, single photons simultaneously and observing a phenomenon known as Hong–Ou–Mandel interference at the single-photon detectors connected to the outputs. They found that the on-chip interference has the same visibility as the off-chip interference of the two injected single photons – meaning that single photons at the chip’s output are as identical as they were at the input.

Next steps

Although this processor could, in principle, form the core of an efficient universal optical quantum computer, fabricating the other equipment needed for such a computer would be far more technically demanding. Nevertheless, there is one known computational problem for which a processor of this nature can outperform classical computers (a situation known as “quantum supremacy” or “quantum primacy”) without fancier equipment. This problem is known as boson sampling and it involves predicting the output of the processor itself in a special scenario.

To understand how boson sampling works, consider what happens if we inject some identical single photons into the processor. The photons propagate through the processor and appear at the outputs where they are detected by single-photon detectors. But which detectors will find a photon? This question is inherently impossible to answer. Even if the input and the configuration is exactly the same, different detectors will be activated at the output each time we run the experiment. Nonetheless, if we run the experiment many times, we can prepare statistical samples implying the probability of different detection events. The interesting point here, from a computational point of view, is that for a big enough number of modes, classical computers cannot efficiently prepare these statistical samples (or calculate the probability distribution function for the detection events).

In 2020, researchers led by Jian-Wei Pan and Chao-Yang Lu of the University of Science and Technology of China (USTC) demonstrated quantum advantage for a similar problem using their own photonic device. The USTC team’s device differs from the processor described in this study in one critical respect, however. “The authors of the 2020 paper use a static device for their proof-of-principle experiment,” explains Jelmer Renema, a physicist at QuiX Quantum and the University of Twente. “We build on that result and realize full reconfigurability.”

Renema goes on to explain that while the system he and his colleagues developed can run boson sampling experiments, “quantum supremacy doesn’t arise with 12 modes”. Nevertheless, he and other members of the research group, which is led by Pepijn Pinkse, are developing the processor. “We are working on improving the specifications of the system like reducing the optical loss and, furthermore, on increasing the number of modes. We expect to unveil a processor with 50 modes in 2022,” Renema tells Physics World.

Learned societies propose ‘International Year of Quantum Science and Technology’ in 2025

Physicists around the world are drawing up plans for a year-long celebration of quantum science and technology in 2025. The campaign is being led by the American Physical Society and the German Physical Society, which hope to persuade the United Nations (UN) to make 2025 the UNESCO International Year of Quantum Science and Technology. If approved, the year would involve workshops, conferences, festivals and activities in schools, co-ordinated by national nodes across the world.

UNESCO International Years are held to advance the UN’s goals by raising awareness of the year’s theme and its importance to society. Joe Niemela from the Abdus Salam International Centre for Theoretical Physics in Trieste, Italy, who is spearheading the plans, says it is crucial to include everyone in the year’s activities, from policy makers and diplomats to the general public and school students.

“It’s an opportunity for scientists to talk to a broader audience,” he told the Physics World Weekly podcast. “We don’t only need to develop technological solutions to global problems; we also need all people to understand how that technology improves our quality of life.”

Niemela adds that having an official UNESCO International Year is a valuable way to convince people of the importance of the theme. For example, it can empower teachers to ask for permission to run activities on the topic with their classes.

Those backing the event have chosen 2025 as it marks the centenary of Werner Heisenberg’s efforts to develop the mathematical formulation of quantum phenomena, allowing numerical calculations about quantum interactions to be made.

The proposal now has the support of about 30 academies of science and physical societies from six continents, including the Institute of Physics, which publishes Physics World. The International Union of Pure and Applied Physics (IUPAP) also endorsed the proposal during its 30^th general assembly last month. “That’s an important milestone for us,” says Niemela. “IUPAP is a recognized international scientific organization and a full member of the International Science Council, which is a strong partner of UNESCO.”

The next step towards official approval is to get the proposal on the agenda of the UNESCO executive board, which includes representatives of about 60 UN member states. This board meets twice a year, and Niemela and other physicists are aiming to have the proposal discussed at the meeting in autumn 2022. They will therefore be spending the next few months liaising with member-state delegations to find sponsors and co-supporters. The proposal would then have to be put on the agenda of the UN general assembly in autumn 2023 for final approval. “There’s much more to do,” says Niemela, “but we appreciate the support we’ve had so far.”

Physicists achieve fault-tolerant control of an error-corrected qubit

The quantum nature of qubits is a double-edged sword. While it could help quantum computers solve problems that are intractable on classical machines, it is also easily destroyed by noise arising from unintended interactions between qubits and their environment. To resolve this dilemma and create scalable, useful quantum computers, physicists are developing methods of correcting the errors that arise from this noise. Now, for the first time, researchers at the University of Maryland in the US have put one of these methods into practice by demonstrating fault-tolerant control of a single logical qubit – a key step towards fully error-corrected quantum computers.

To understand how this type of error correction works, think of the last time you corrected a typo. In doing so, you performed error correction on classical information. Because the meaning of a word is encoded in lots of letters, it doesn’t matter much if there is a mystake in one letter – you can still identify the intended word. Quantum error correction enables us to spot and correct typos in quamtum infornation in much the same way, by encoding the state of one logical qubit (the quantum word) within many physical qubits (the quantum letters). By performing specialized actions known as stabilizer measurements on these physical qubits, the system can then extract information about any errors that have occurred – crucially, without destroying the quantum information required for the computation. Based on this extracted information, the system can then apply the correct operations, or gates, to the physical qubits so that the overall state of the logical qubit is corrected, like replacing a letter to correct a word.

Error correction alone is not enough to enable scalable quantum computers, however. Spell-check would be counterproductive if it jumbled up other letters in the process of correcting one. Another essential condition for reliable quantum computers is that preparing the logical state, applying logic gates, detecting errors and correcting them must not introduce more errors into the system. In other words, these processes all need to be fault-tolerant, designed so that one error will not spread to cause more errors. This requirement is central to the task of building quantum computers that can solve useful problems.

Reduced error rates

In the latest work, which is published in Nature, researchers led by Laird Egan demonstrated the fault-tolerant control of a single logical qubit – including all the stages of preparation, logic gates and error correction. The qubits in this experiment consisted of ytterbium ions suspended above a radio-frequency Paul trap and controlled with individual laser beams. This is the hardware favoured by the quantum computing start-up IonQ, where Egan and some of his collaborators now work. The advantages of using trapped ions instead of the superconducting qubits favoured by many quantum computing firms include lower error rates and better connectivity between qubits, though there are challenges with scaling the technology.

Photo montage of Maryland team members and their quantum computer — **Remote access** The trapped-ion quantum computer (centre) and some of the Maryland team working from home, after setting up their system to run completely remotely due to the COVID-19 pandemic. (Courtesy: Laird Egan)

To demonstrate fault-tolerant control, the team used a 13-qubit encoding known as the Bacon–Shor code, with nine physical qubits to encode the logical state and four qubits for error correction. These 13 qubits were arranged in a single chain, with two extra qubits on either side to ensure uniform spacing. With this system, the researchers showed that they could control the states of the logical qubits in a fault-tolerant way and correct any single-qubit errors that occurred. The team also showed that the error rates in the logical qubit were lower than the corresponding error rates when using a non-fault-tolerant protocol.

Steps towards full fault-tolerance

Egan calls the team’s achievement “a really critical building block, and one that shows we are close to achieving the error threshold where logical qubits can outperform physical qubits”. He adds: “Nobody believes that you will be able to achieve this threshold without fault-tolerant error-correction protocols, and up until this work, no one had yet demonstrated fault-tolerant control of a logical qubit.”

To pass that error threshold, the team’s next goal is to maintain an error-free quantum state over time performing error correction repeatedly. To do this will require mid-circuit error detection, where the ions in the chain are physically moved apart so that some can be measured without affecting the others. “The hard part is when we put the chain back together, we need to make sure that ions did not heat up during their transport”, explains Egan, “and if they did, we need a way to cool them back down without destroying the quantum information”. The team has made progress towards this goal by showing, in other work, that cooling such ions is possible.

The researchers also hope to demonstrate fault-tolerant control between two qubits. To do this, they aim to implement a series of quantum operations known as a logical controlled-NOT gate, which flips the state of the second qubit conditional on the state of the first and is central to many other gates and algorithms. Egan is confident that the vision of fully fault-tolerant quantum control that outperforms physical qubits can be realized in ion traps in the near future. “Ion systems only need modest improvements to their gate fidelity, combined with mid-circuit detection, to really make this work in the next couple of years,” he says.

Clever aerodynamics makes owls silent hunters, why 2025 should be the International Year of Quantum Science and Technology

Owls mostly hunt at night when background noise levels can be low and potential prey have a better chance of hearing danger approaching. As a result, the birds have evolved structures on their wings that greatly reduce the noise owls make while flying. In this episode of the Physics World Weekly podcast, the engineer and owl expert Justin Jaworski talks about the aerodynamics of these wonderful creatures.

Quantum science and technology is a hot topic – and not just in the physics community, because new companies and investors are driving a boom in the nascent quantum industry. Now, a group of physicists is calling for the United Nations to declare 2025 the International Year of Quantum Science and Technology. Joe Niemela of the Abdus Salam International Centre for Theoretical Physics in Trieste is a leading proponent and makes the case for a quantum year on the podcast.

Also in this episode, we chat about a new multiscale X-ray imaging technique that has shed light on how COVID-19 affects the lungs and about a quantum effect called Pauli blocking, which has been seen in ultracold gases 30 years after it was first predicted.

Pauli blocking is spotted in ultracold fermionic gases

A manifestation of the Pauli exclusion principle in ultracold atomic gases has been spotted for the first time by three independent research groups. Called Pauli blocking, the effect was first predicted 30 years ago and occurs when fermionic atoms in a quantum gas are unable to make transitions to nearby quantum states.

In the three experiments, Pauli blocking reduced the ability of atoms to scatter light, making the gases more transparent. The effect could someday be used to improve technologies based on ultracold atoms such as optical clocks and quantum repeaters.

Famous principle

Particles with half-integer spin obey Fermi-Dirac statistics, which arises from symmetry considerations in quantum mechanics. Such particles are called fermions and they obey Pauli’s famous exclusion principle, which says that fermions cannot occupy the same quantum state in a system.

Electrons are fermions and the exclusion principle explains why individual electron orbitals in an atom can contain at most two electrons – one spin-up and the other spin-down. This gives us the varied properties of the different atoms in the periodic table. The exclusion principle is also at work in semiconductors and insulators, where electrons are unable to respond to the application of small electric fields because doing so would require moving to another quantum state – and in semiconductors and insulators, most nearby states are all full.

Degenerate gases

Many atoms are fermions and now physicists can chill some fermionic atomic gases to low enough temperatures that nearly all the low-lying quantum states are filled to what is called the Fermi level. Such dense, ultracold systems are said to be degenerate and the atoms are expected to behave in a similar way as electrons in a solid.

Instead of applying an external field to try to get the atoms to move, the three research teams fired photons of light at their degenerate Fermi gases. Normally when light is shone on an atomic gas, some of the photons will scatter from the atoms and the atoms will recoil, taking some energy and momentum from the photon.

However, in a degenerate Fermi gas of atoms, this recoil often requires a transition to a nearby quantum state that is occupied by another atom. As a result, the scattering cannot occur, and the photon will carry on through the gas. The observable effect of this is an increase in the transparency of the gas as it is cooled to the point where it starts to become degenerate. In the three experiments reported today, this was observed when gases were cooled to temperatures in the microkelvin and nanokelvin range.

Forward scattering

At JILA – a joint institute of the US National Institute of Standards and Technology and the University of Colorado – Christian Sanner and colleagues focused on the angles at which photons scattered from their ultracold gas of strontium-87 atoms. They found that forward scattering – whereby the photons experience small deviations in their trajectories and atomic recoils are small – was suppressed.

This is consistent with atoms in the gas being Pauli blocked from making small transitions to nearby quantum states. However, photon scattering involving larger atomic recoils was not as suppressed. This is consistent with atoms being able to make large transitions to unoccupied states above the Fermi level.

Meanwhile in New Zealand, Amita Deb and Niels Kjærgaard at the University of Otago compared the optical properties of a potassium-40 fermionic gas with that of a gas made of rubidium-87. This isotope of rubidium obeys Bose-Einstein statistics and is therefore not subject to Pauli blocking. When cooled to ultracold temperatures, the transparency of the potassium-40 gas increased whereas the transparency of the rubidium-87 gas did not.

The third group is based at the Massachusetts Institute of Technology (MIT) and includes the Nobel laureate Wolfgang Ketterle. This team cooled fermionic lithium-6 atoms to below the Fermi level and also observed an increase in transparency. This team noted that the effect disappeared as they increased the intensity of the light that they shone on the sample. This, they say, is the result of light scattering heating up the sample.

Density fluctuations

The MIT team has also proposed an alternative explanation for the transparency observations of all three teams. Light can scatter from density fluctuations in a fluid, and if these fluctuations are about the same size as the wavelength of light, the fluid will become opaque. This occurs in milk, where suspended protein clusters create the appropriate density fluctuations.

Ketterle and colleagues point out that a lack of density fluctuations has already been observed in ultracold Fermi gases. This occurs because the Pauli exclusion principle prevents atoms from getting very close together, which makes the density of the gas very homogenous. This dearth of density fluctuations, say the researchers, could also explain the increase in transparency at low temperatures.

Ultracold atomic gases have a growing number of applications including atomic clocks and components for quantum networks. Pauli blocking could be used to improve existing technologies and possibly develop new applications.

What next for India and coal?

Last Saturday a tearful Alok Sharma brought down a hammer to declare the COP26 agreement adopted – but the frustration in Glasgow was palpable. Moments earlier, national representatives had queued up to express disappointment at India’s infamous last minute interjection to weaken the deal’s wording from “phase out” to “phase down” of coal. The sentiment was clear: why can’t India just get on with ditching the coal?

Of course, climate politics is never as straightforward as it appears. India’s current reliance on coal is inextricably linked with its development process, and a just transition to renewable energy requires stronger international support. This contested future of India’s energy sector was one of the topics being discussed this week at Environmental Research 2021, a free-to-attend online event hosted by IOP Publishing.

“In India we have a history of oscillating between being a climate hero and a climate villain,” said Navroz Dubash from the Centre for Policy Research in New Delhi, during his talk on Tuesday. Indeed, Prime Minister Narendra Modi had been praised at COP26 for pledging India to be carbon neutral by 2070 with 50% of its energy generation coming from renewable sources by 2030. Right now coal, oil and natural gas account for 75% of energy use.

Political targets

Dubash is concerned by the number of assumptions built into the projections for India’s future emissions. The models underpinning these figures often contain purely political targets for coal use and they assume India will follow the exact same development pathway as China. Dubash’s work bridges the gap between climate modellers and the policy community by unpacking these assumptions. “When we think about future emissions scenarios we need to think about future Indias,” he said.

Radhika Khosla, a researcher at the Oxford India Centre for Sustainable Development, said researchers need to better understand the amount of carbon “locked in” to different development trajectories. Historically there has been far more focus on energy supply and little consideration of how changing energy demand will shape future emissions. “It’s not just about infrastructure, it’s also about changing behaviours and institutions,” she said. Khosla and Dubash collaborated on a 2018 paper in Environmental Research Letters, which addressed these uncertainties around India’s future emissions.

India’s changing energy demands will be discussed tomorrow (Friday) by Patil Balachandra from the Indian Institute of Science in Bangalore. Speaking with Physics World beforehand, Balachandra said that renewables rollout is essential but that too much focus is being placed on the physical technology alone. He believes India’s reliance on coal can be greatly reduced through more integrated predictions of future supply and demand.

Solar and wind dominate

“My state of Karnataka is already 70% renewable, dominated by solar and wind and the maximum capacity is 2.5 times the maximum peak demand. But we still have power cuts because of weather variability,” he said. Balachandra would like power authorities to combine meteorological predictions with usage forecasts to be better prepared for when the Sun doesn’t shine or the wind doesn’t blow. He’d also like it to be easier for coal-dominant states to buy renewable energy from renewable-dominant states and vice versa.

India’s energy demand is evolving as the economy emerges. In Bangalore the maximum load on the power grid used to be connected with water-heating demand around 8 a.m. but since 2010 the spike occurs now around midday – linked with the activity peak in the city’s thriving services sector. In New Delhi the biggest summer peak is around midnight due to people putting on the air con as they go to bed. “Historically people used to feel comfortable at 30 degrees centigrade, now they only feel comfortable at 22 degrees,” says Balachandra. Economic incentives for individuals and businesses could help to spread the power load across the day.

Given that India imports its gas and oil, you can see why coal remains important to national security for now. But as India phases down coal over the longer term many are concerned about its socioeconomic impacts. Townships have developed around coal mines and Indian Railways – one of the largest employers in the world – relies on coal transportation for nearly 30% of its revenue. “There may be even more new jobs created in renewables but these jobs will not always be for the same people. The government needs to help to retrain people in these coal townships,” says Balachandra.

This week 13 national physical societies committed to tackling the climate crisis and to help bring about new green economies. The Indian Physics Association and the Institute of Physics for the UK and Ireland were among the signatories of the official statement.

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