Category: Physics

A new technology, inspired in part by the design of the James Webb Space Telescope (JWST), uses mirror segments to sort and collect light on the microscopic scale, and capture images of molecules with a new level of resolution: position and orientation, each in three dimensions.

Details of this new system, developed by Oumeng Zhang, a recent Ph.D. graduate from the lab of Matthew Lew, an associate professor of electrical and systems engineering at the McKelvey School of Engineering at Washington University in St. Louis, were published Dec. 5 in the journal Nature Photonics.

Like the space telescope, the radially and azimuthally polarized multi-view reflector (raMVR) microscope depends on gathering as much light as possible. But instead of using that light to see things far away, it uses it to discern different features of tiny, fluorescent molecules attached to proteins and cell membranes.

“The setup is partially inspired by telescopes,” Zhang said. “It’s a very similar setup. Instead of the familiar honeycomb shape of the JWST, we use pyramid-shaped mirrors.”

Currently, microscopes in this domain face challenges creating biological images. For one thing, such small amounts of light given off by the fluorescent molecules are sensitive to the slightest aberrations—including the murky environment inside a cell. Because of this, precise imaging relies more heavily on computer processing to sort out orientation after an image has been captured.

“Think of creating a color picture when all you have are gray-scale camera sensors,” Lew said. “You could try to recreate the color using a computational tool, or you can directly measure it using a color sensor, which uses various absorbing color filters on top of different pixels to detect colors.”

In a similar way, standard microscopes simply do not detect how molecules are oriented. The raMVR microscope uses polarization optics called waveplates along with its pyramid-shaped mirrors to separate light into eight channels, each of which represents a different piece of the molecule’s position and orientation.

Notably, the raMVR microscope is not a small technology. But smaller isn’t always better.

“At the cutting edge of engineering physics, we often have to make tradeoffs to make our instruments compact,” Lew said. “Here, we decided to take a different tack: How could we use every precious bit of light to make the most precise measurement possible? It’s absolutely fun to think differently about the architecture of a microscope, and here, we think the newfound 6D imaging performance will enable new scientific discoveries in the near future.”

More information: Oumeng Zhang et al, Six-dimensional single-molecule imaging with isotropic resolution using a multi-view reflector microscope, Nature Photonics (2022). DOI: 10.1038/s41566-022-01116-6

Journal information: Nature Photonics 

Provided by Washington University in St. Louis 

A research team co-led by a scientist at City University of Hong Kong (CityU) has developed a novel antenna that allows manipulation of the direction, frequency and amplitude of the radiated beam, and is expected to play an important role in the integration of sensing and communications (ISAC) for 6th-generation (6G) wireless communications.

The structure and characteristics of traditional antennas cannot be changed once fabricated. However, the direction, frequency, and amplitude of the electromagnetic waves from this new-generation antenna, which is called a “sideband-free space-time-coding (STC) metasurface antenna,” can be changed through space-time coding (i.e., software control), enabling great user flexibility.

The key to this innovative feature is that the response of the metasurface (artificial, thin-sheet material with sub-wavelength thickness and made of several sub-wavelength meta-atoms) can be changed by switching the meta-atoms on its surface between radiating and non-radiating states, like turning on and off switches, by controlling the electric current.

This allows the STC metasurface antenna to realize complicated wave manipulation in the space and frequency domains through software control, and to create a desired radiation pattern and a highly directed beam.

Professor Chan Chi-hou, Acting Provost and Chair Professor of Electronic Engineering in the Department of Electrical Engineering at CityU, who led the research, highlighted that the antenna relies on the successful combination of two research advances, namely amplitude-modulated (AM) leaky-wave antennas and space-time coding techniques.

Dr. Wu Gengbo, postdoctoral fellow in the State Key Laboratory of Terahertz and Millimeter Waves (SKLTMW) at CityU, first proposed the new concept of AM leaky-wave antennas in 2020 in his Ph.D. studies at CityU. “The concept provides an analytical approach to synthesize antennas with the desired radiation patterns for different specific uses by simply changing the antennas’ shape and structure,” explained Dr. Wu.

But as with other antennas, once the AM leaky-wave antenna is fabricated, its radiation characteristics are fixed. At about that time, Dr. Dai Junyan, from a research group led by Academician Cui Tiejun and Professor Cheng Qiang, from Southeast University at Nanjing, China, who pioneered STC technologies, joined Professor Chan’s group at CityU.

“Dr. Dai’s expertise in space-time coding and digital metasurfaces to dynamically reconfigure antenna performance added a new, important dimension to the antenna research at the SKLTMW,” said Professor Chan, who is also Director of the SKLTMW at CityU.

Moreover, the time modulation of electromagnetic waves on metasurfaces usually generates unwanted harmonic frequencies, called sidebands. These sidebands carry part of the radiated electromagnetic wave energy and interfere with the useful communication channels of the antenna, leading to “spectrum pollution.”

But Professor Chan and his team proposed a novel design, which makes use of a waveguide (a line for transmitting electromagnetic waves by successive reflection from the inner wall) and successfully suppressed the undesired harmonics, achieving a high-directivity beam and enabling secure communication.

“With the AM leaky-wave antenna and space-time coding technologies, we achieve the designated radiation characteristics by controlling the on-off sequences and duration of the ‘switches’ on the antenna through software,” said Professor Chan.

“A high-directivity beam can be generated with the new antenna, allowing a wide range of radiation performance without having to redesign the antenna, except for using different STC inputs,” added Dr. Wu.

The energy from the radiated beam of the STC metasurface antenna can be focused to a focal point with fixed or varying focal lengths, which can be used for real-time imaging and treated as a type of radar to scan the environment and feedback data. “The invention plays an important role in the ISAC for 6G wireless communications,” Professor Chan explained.

“For example, the radiated beam can scan a person and create an image of the person, allowing mobile phone users to talk to each other with 3D hologram imaging. It also performs better against eavesdropping than the conventional transmitter architecture.”

The findings were published in the journal Nature Electronics. Dr. Wu and Dr. Dai are the co-first authors of the paper, and Dr. Dai, Professor Cheng, Academician Cui, and Professor Chan are the corresponding authors.

“Without the collaboration and complementary expertise of the two research teams at CityU and Southeast University, we could not have achieved these research results,” Professor Chan continued. “We hope that the new-generation antenna technology will become more mature in the future and that it can be applied to smaller integrated circuits at lower cost and in a wider range of applications.”

More information: Geng-Bo Wu et al, Sideband-free space–time-coding metasurface antennas, Nature Electronics (2022). DOI: 10.1038/s41928-022-00857-0

Journal information: Nature Electronics 

Provided by City University of Hong Kong 

Researchers have used a quantum processor to make microwave photons uncharacteristically sticky. They coaxed them to clump together into bound states, then found that these photon clusters survived in a regime where they were expected to dissolve into their usual, solitary states. The discovery was first made on a quantum processor, marking the growing role that these platforms are playing in studying quantum dynamics.

Photons—quantum packets of electromagnetic radiation like light or microwaves—typically don’t interact with one another. Two crossed flashlight beams, for example, pass through one another undisturbed. But in an array of superconducting qubits, microwave photons can be made to interact.

In “Formation of robust bound states of interacting photons,” published today in Nature, researchers at Google Quantum AI describe how they engineered this unusual situation. They studied a ring of 24 superconducting qubits that could host microwave photons. By applying quantum gates to pairs of neighboring qubits, photons could travel around by hopping between neighboring sites and interacting with nearby photons.

The interactions between the photons affected their so-called “phase.” The phase keeps track of the oscillation of the photon’s wavefunction. When the photons are non-interacting, their phase accumulation is rather uninteresting. Like a well-rehearsed choir, they’re all in sync with one another. In this case, a photon that was initially next to another photon can hop away from its neighbor without getting out of sync.

Just as every person in the choir contributes to the song, every possible path the photon can take contributes to the photon’s overall wavefunction. A group of photons initially clustered on neighboring sites will evolve into a superposition of all possible paths each photon might have taken.

When photons interact with their neighbors, this is no longer the case. If one photon hops away from its neighbor, its rate of phase accumulation changes, becoming out of sync with its neighbors. All paths in which the photons split apart overlap, leading to destructive interference. It would be like each choir member singing at their own pace—the song itself gets washed out, becoming impossible to discern through the din of the individual singers.

Among all the possible configuration paths, the only possible scenario that survives is the configuration in which all photons remain clustered together in a bound state. This is why interaction can enhance and lead to the formation of a bound state: by suppressing all other possibilities in which photons are not bound together.

To rigorously show that the bound states indeed behaved just as particles did, with well-defined quantities such as energy and momentum, researchers developed new techniques to measure how the energy of the particles changed with momentum. By analyzing how the correlations between photons varied with time and space, they were able to reconstruct the so-called “energy-momentum dispersion relation,” confirming the particle-like nature of the bound states.

The existence of the bound states in itself was not new—in a regime called the “integrable regime,” where the dynamics is much less complicated, the bound states were already predicted and observed ten years ago.

But beyond integrability, chaos reigns. Before this experiment, it was reasonably assumed that the bound states would fall apart in the midst of chaos. To test this, the researchers pushed beyond integrability by adjusting the simple ring geometry to a more complex, gear-shaped network of connected qubits. They were surprised to find that bound states persisted well into the chaotic regime.

The team at Google Quantum AI is still unsure where these bound states derive their unexpected resilience, but it could have something to do with a phenomenon called “prethermalization”, where incompatible energy scales in the system can prevent a system from reaching thermal equilibrium as quickly as it otherwise would.

Researchers hope investigating this system will lead to new insights into many-body quantum dynamics and inspire more fundamental physics discoveries using quantum processors.

More information: Alexis Morvan et al, Formation of robust bound states of interacting microwave photons, Nature (2022). DOI: 10.1038/s41586-022-05348-y

Journal information: Nature 

Provided by Google Quantum AI

Gas clouds across the universe are known to absorb the light produced by distant massive celestial objects, known as quasars. This light manifests as the so-called Lyman alpha forest, a dense structure composed of absorption lines that can be observed using spectroscopy tools.

Over the past decades, astrophysicists have been assessing the value of these absorption lines as a tool to better understand the universe and the relationships between cosmological objects. The Lyman alpha forest could also potentially aid the ongoing search for dark matter, offering an additional tool to test theoretical predictions and models.

Researchers at University of Nottingham, Tel-Aviv University, New York University, and the Institute for Fundamental Physics of the Universe in Trieste have recently compared low-redshift Lyman alpha forest observations to hydrodynamical simulations of the intergalactic medium and dark matter made up of dark photons, a renowned dark matter candidate.

Their paper, published in Physical Review Letters (PRL), builds on an earlier work by some members of their team, which compared simulations of the intergalactic medium (IGM) with Lyman-alpha forest measurements collected by the Cosmic Origins Spectrograph (COS) aboard the Hubble Space Telescope.

“In our analyses, we found that the simulation predicted line widths that were too narrow compared to the COS results, suggesting that there could be additional, noncanonical sources of heating occurring at low redshifts,” Hongwan Liu, Matteo Viel, Andrea Caputo and James Bolton, the researchers who carried out the study, told Phys.org via email.

“We explored several dark matter models that could act as this source of heating. Building on two of the authors’ experience with dark photons in a previous paper published in PRL, we eventually realized that heating from dark photon dark matter could work.”

Based on their previous observations, Liu, Viel, Caputo and Bolton decided to alter a hydrodynamical simulation of the IGM (i.e., a sparse cloud of hydrogen that exists in the spaces between galaxies). In their new simulation, they included the effects of the heat that models predict would be produced by dark photon dark matter.

“In regions of space where the mass of the dark photon matches the effective plasma mass of the photon, conversions from dark photons to photons can occur,” Liu, Viel, Caputo and Bolton explained. “The converted photons are then rapidly absorbed by the IGM in those regions, heating the gas up. The amount of energy transferred from dark matter to the gas can be calculated theoretically.”

The researchers added this estimated energy transfer between dark photons and intergalactic clouds to their simulations. This ultimately allowed them to attain a series of simulated absorption line widths, which they could compare to actual Lyman-alpha forest observations collected by the COS.

“Broadly speaking, we have shown that the Lyman-alpha forest is extremely useful for understanding dark matter models where energy can be converted from dark matter into heating,” Liu, Viel, Caputo and Bolton said. “I think our study will encourage physicists interested in dark matter to pay more attention to the Lyman-alpha forest.”

Overall, the comparison between COS measurements and hydrodynamical simulations performed by this team of researchers suggests that dark photons could in fact be a source of heat in intergalactic gas clouds. Their findings could thus be the first hint of the existence of dark matter that is not observed through its gravitational effects.

While this is a fascinating possibility, Liu, Viel, Caputo and Bolton have not yet ruled out other possible theoretical explanations. They thus hope that their study will inspire other teams to similarly probe the properties of the IGM in the early universe.

“One particularly interesting consequence of dark photon heating is that underdense regions in the IGM are heated up at earlier times compared to overdense regions,” Liu, Viel, Caputo and Bolton said. “This can lead to underdense regions being hotter than overdense regions, which is contrary to standard expectations. There are some indications that the IGM does exhibit this behavior at high redshifts. If so, it could be another important piece of evidence in favor of dark photon dark matter heating.”

More information: James S. Bolton et al, Comparison of Low-Redshift Lyman- α Forest Observations to Hydrodynamical Simulations with Dark Photon Dark Matter, Physical Review Letters (2022). DOI: 10.1103/PhysRevLett.129.211102

James S Bolton et al, Limits on non-canonical heating and turbulence in the intergalactic medium from the low redshift Lyman α forest, Monthly Notices of the Royal Astronomical Society (2022). DOI: 10.1093/mnras/stac862

Andrea Caputo et al, Dark Photon Oscillations in Our Inhomogeneous Universe, Physical Review Letters (2020). DOI: 10.1103/PhysRevLett.125.221303

Journal information: Monthly Notices of the Royal Astronomical Society  , Physical Review Letters 

Our society faces the grand challenge of providing sustainable, secure and affordable means of generating energy, while trying to reduce carbon dioxide emissions to net zero around 2050.

To date, developments in fusion power, which potentially ticks all these boxes, have been funded almost exclusively by the public sector. However, something is changing.

Private equity investment in the global fusion industry has more than doubled in just one year—from US$2.1 billion in 2021 to US$4.7 billion in 2022, according to a survey from the Fusion Industry Association.

So, what is driving this recent change? There’s lots to be excited about.

Before we explore that, let’s take a quick detour to recap what fusion power is.

Merging atoms together

Fusion works the same way our Sun does, by merging two heavy hydrogen atoms under extreme heat and pressure to release vast amounts of energy.

It’s the opposite of the fission process used by nuclear power plants, in which atoms are split to release large amounts of energy.

Sustaining nuclear fusion at scale has the potential to produce a safe, clean, almost inexhaustible power source.

Our Sun sustains fusion at its core with a plasma of charged particles at around 15 million degrees Celsius. Down on Earth, we are aiming for hundreds of millions of degrees Celsius, because we don’t have the enormous mass of the Sun compressing the fuel down for us.

Scientists and engineers have worked out several designs for how we might achieve this, but most fusion reactors use strong magnetic fields to “bottle” and confine the hot plasma.

Generally, the main challenge to overcome on our road to commercial fusion power is to provide environments that can contain the intense burning plasma needed to produce a fusion reaction that is self-sustaining, producing more energy than was needed to get it started.

Joining the public and private

Fusion development has been progressing since the 1950s. Most of it was driven by government funding for fundamental science.

Now, a growing number of private fusion companies around the world are forging ahead towards commercial fusion energy. A change in government attitudes has been crucial to this.

The US and UK governments are fostering public-private partnerships to complement their strategic research programs.

For example, the White House recently announced it would develop a “bold decadal vision for commercial fusion energy“.

In the United Kingdom, the government has invested in a program aimed at connecting a fusion generator to the national electricity grid.

The technology has actually advanced, too

In addition to public-private resourcing, the technologies we need for fusion plants have come along in leaps and bounds.

In 2021, MIT scientists and Commonwealth Fusion Systems developed a record-breaking magnet that will allow them to build a compact fusion device called SPARC “that is substantially smaller, lower cost, and on a faster timeline”.

In recent years, several fusion experiments have also reached the all-important milestone of sustaining plasma temperatures of 100 million degrees Celsius or above. These include the EAST experiment in China, Korea’s flagship experiment KSTAR, and UK-based company Tokamak Energy.

These incredible feats demonstrate an unprecedented ability to replicate conditions found inside our Sun and keep extremely hot plasma trapped long enough to encourage fusion to occur.

In February, the Joint European Torus—the world’s most powerful operational tokamak—announced world-record energy confinement.

And the next-step fusion energy experiment to demonstrate net power gain, ITER, is under construction in France and now about 80% complete.

Magnets aren’t the only path to fusion either. In November 2021, the National Ignition Facility at Lawrence Livermore National Laboratory in California achieved a historic step forward for inertial confinement fusion.

By focusing nearly 200 powerful lasers to confine and compress a target the size of a pencil’s eraser, they produced a small fusion “hot spot” generating fusion energy over a short time period.

In Australia, a company called HB11 is developing proton-boron fusion technology through a combination of high-powered lasers and magnetic fields.

Fusion and renewables can go hand in hand

It is crucial that investment in fusion is not at the cost of other forms of renewable energy and the transition away from fossil fuels.

We can afford to expand adoption of current renewable energy technology like solar, wind, and pumped hydro while also developing next-generation solutions for electricity production.

This exact strategy was outlined recently by the United States in its Net-Zero Game Changers Initiative. In this plan, resource investment will be targeted to developing a path to rapid decarbonisation in parallel with the commercial development of fusion.

History shows us that incredible scientific and engineering progress is possible when we work together with the right resources—the rapid development of COVID-19 vaccines is just one recent example.

It is clear many scientists, engineers, and now governments and private investors (and even fashion designers) have decided fusion energy is a solution worth pursuing, not a pipe dream. Right now, it’s the best shot we’ve yet had to make fusion power a viable reality.

Provided by The Conversation 

Squeezed states of the electromagnetic field find many important applications in quantum information science and quantum metrology. Dr. Jie Li et al. at Zhejiang University put forward a new mechanism for preparing microwave squeezed vacuum states using a cavity magnomechanical system.

Specifically, the spin wave (magnon mode) formed by a large number of spins in a ferrimagnet couples to the phonon mode of the deformation vibration of the ferrimagnet via the magnetostrictive force. The magnetostrictive interaction is a nonlinear effect, which can establish a unique correlation between the amplitude and phase of the magnon mode. This correlation can reduce the quantum noise of the magnon mode, yielding squeezed vacuum of the magnon mode.

Due to the state-swap interaction between magnons and cavity microwave photons, the cavity mode also gets squeezed, leading to squeezed vacuum of the microwave cavity output field. The work shows that the cavity magnomechanical system exhibits some advantages over the most-widely-used method using Josephson parametric amplifiers (JPA) in preparing microwave squeezed states. The working temperature of JPA is typically at 10–20 millikelvin.

This work shows that at temperature of 200 millikelvin, the cavity magnomechanical system can produce microwave squeezed states with the same degree of squeezing as that produced by JPA. This greatly reduces the stringent requirement for ambient temperature. In addition, the operation of JPA requires a large auxiliary circuit, while the cavity magnomechanical system is much simpler, which greatly reduces the cost of the experiment.

The work provides a new mechanism and approach for preparing microwave squeezed vacuum states, which will find many important applications in microwave quantum information processing and quantum metrology.

The paper is published in the journal National Science Review.

More information: Jie Li et al, Squeezing Microwaves by Magnetostriction, National Science Review (2022). DOI: 10.1093/nsr/nwac247

Provided by Science China Press 

Many photochemical processes rely on UV light from inefficient or toxic light sources that the LED technology cannot replace for technical reasons. An international team of scientists led by Professor Christoph Kerzig of Johannes Gutenberg University Mainz (JGU) in Germany and Professor Nobuhiro Yanai of Kyushu University in Japan has now developed the first molecular system for the conversion of blue light into high-energy UV photons with wavelengths below 315 nanometers.

These photons in the so-called UVB range are essential for numerous photochemical processes in the context of light-to-energy conversion, disinfection, or even wastewater treatment applications. However, sunlight cannot provide UVB photons, and their artificial generation typically relies on mercury lamps or other highly inefficient alternatives.

The new findings show that a metal-free photon upconversion (UC) system can transform readily available visible light into UVB photons. Hence, this breakthrough can be regarded as a more environmentally friendly approach. Initial mercury-free applications have already been demonstrated in the lab.

Collaborative research with a long tradition

Both research groups started working on upconversion several years ago. UC is a process in which the absorption of two photons of lower energy leads to the emission of one photon of higher energy. This technique has been developed to increase the efficiency of solar cells, mainly by converting low-energy photons in the infrared region.

“In contrast, highly energetic UV photons are within reach when blue light is used as the energy source,” explained Professor Kerzig of the Department of Chemistry at Mainz University.

Tailor-made molecules have been prepared in Mainz and characterized with a new large-scale laser device recently installed in the Kerzig group. Furthermore, special spectroscopic techniques in the lab of Professor Nobuhiro Yanai have been applied to the UC system to understand its performance in detail.

While the current paper represents the first collaboration between the Kerzig and Yanai groups, the chemistry departments of both universities have a well-established student exchange program. This novel collaboration will further strengthen the network between Mainz and Kyushu.

Development of reusable upconversion materials

The scientists used a commercial blue LED as light source and exploited the generated UV light for the cleavage of strong chemical bonds that would otherwise require very harsh reaction conditions. Moreover, using the laser setup in Mainz, Ph.D. student Till Zähringer managed to observe all intermediates in the complex energy conversion mechanism.

“Our next goal is to develop reusable materials for versatile applications,” said Professor Nobuhiro Yanai.

His group in Kyushu is well known for the development of photoactive materials. The combination of materials science, photochemistry, and photocatalysis in the framework of the Kyushu-Mainz collaboration will pave the way for this ambitious goal.

The research is published in the journal Angewandte Chemie International Edition.

More information: Till J. B. Zähringer et al, Blue‐to‐UVB Upconversion, Solvent Sensitization and Challenging Bond Activation Enabled by a Benzene‐Based Annihilator, Angewandte Chemie International Edition (2022). DOI: 10.1002/anie.202215340

Journal information: Angewandte Chemie International Edition 

Provided by Universitaet Mainz 

Optical photons are ideal carriers of quantum information. But to work together in a quantum computer or network, they need to have the same color—or frequency—and bandwidth. Changing a photon’s frequency requires altering its energy, which is particularly challenging on integrated photonic chips.

Recently, researchers from the Harvard John A. Paulson School of Engineering and Applied Sciences (SEAS) developed an integrated electro-optic modulator that can efficiently change the frequency and bandwidth of single photons. The device could be used for more advanced quantum computing and quantum networks.

The research is published in Light: Science & Applications.

Converting a photon from one color to another is usually done by sending the photon into a crystal with a strong laser shining through it, a process that tends to be inefficient and noisy. Phase modulation, in which photon wave’s oscillation is accelerated or slowed down to change the photon’s frequency, offers a more efficient method, but the device required for such a process, an electro-optic phase modulator, has proven difficult to integrate on a chip.

One material may be uniquely suited for such an application: thin-film lithium niobate.

“In our work, we adopted a new modulator design on thin-film lithium niobate that significantly improved the device performance,” said Marko Lončar, the Tiantsai Lin Professor of Electrical Engineering at SEAS and senior author of the study. “With this integrated modulator, we achieved record-high terahertz frequency shifts of single photons.”

The team also used the same modulator as a “time lens”— a magnifying glass that bends light in time instead of space—to change the spectral shape of a photon from fat to skinny.

“Our device is much more compact and energy-efficient than traditional bulk devices,” said Di Zhu, the first author of the paper. “It can be integrated with a wide range of classical and quantum devices on the same chip to realize more sophisticated quantum light control.”

Di is a former postdoctoral fellow at SEAS and is currently a research scientist at the Agency for Science, Research and Technology (A*STAR) in Singapore.

Next, the team aims to use the device to control the frequency and bandwidth of quantum emitters for applications in quantum networks.

The research was a collaboration between Harvard, MIT, HyperLight, and A*STAR.

The paper was co-authored by Changchen Chen, Mengjie Yu, Linbo Shao, Yaowen Hu, C. J. Xin, Matthew Yeh, Soumya Ghosh, Lingyan He, Christian Reimer, Neil Sinclair, Franco N. C. Wong, and Mian Zhang.

More information: Di Zhu et al, Spectral control of nonclassical light pulses using an integrated thin-film lithium niobate modulator, Light: Science & Applications (2022). DOI: 10.1038/s41377-022-01029-7

Journal information: Light: Science & Applications 

Provided by Harvard John A. Paulson School of Engineering and Applied Sciences 

Physicists at Google Quantum AI have used their quantum computer to study a type of effective particle that is more resilient to environmental disturbances that can degrade quantum calculations. These effective particles, known as Majorana edge modes, form as a result of a collective excitation of multiple individual particles, like ocean waves form from the collective motions of water molecules. Majorana edge modes are of particular interest in quantum computing applications because they exhibit special symmetries that can protect the otherwise fragile quantum states from noise in the environment.

The condensed matter physicist Philip Anderson once wrote, “It is only slightly overstating the case to say that physics is the study of symmetry.” Indeed, studying physical phenomena and their relationship to underlying symmetries has been the main thrust of physics for centuries. Symmetries are simply statements about what transformations a system can undergo—such as a translation, rotation, or inversion through a mirror—and remain unchanged. They can simplify problems and elucidate underlying physical laws. And, as shown in the new research, symmetries can even prevent the seemingly inexorable quantum process of decoherence.

When running a calculation on a quantum computer, we typically want the quantum bits, or “qubits,” in the computer to be in a single, pure quantum state. But decoherence occurs when external electric fields or other environmental noise disturb these states by jumbling them up with other states to create undesirable states. If a state has a certain symmetry, then it could be possible to isolate it, effectively creating an island of stability that is impossible to mix with the other states that don’t also have the special symmetry. In this way, since the noise can no longer connect the symmetric state to the others, it could preserve the coherence of the state.

In 2000, the physicist Alexei Kitaev devised a simple model to generate symmetry-protected quantum states. The model consisted of a chain of interconnected particles called fermions. They could be connected in such a way that two effective particles would appear at the ends of the chain. But these were no ordinary particles—they were delocalized in space, with each appearing at both ends of the chain simultaneously.

These were the Majorana edge modes (MEMs). The two modes had distinctly different behaviors under so-called parity transformation. One mode looked identical under this transformation, so it was a symmetry of the state. The other picked up a minus sign. The difference in parity between these two states meant that they could not be mixed by many external noise sources (i.e. those that also had parity symmetry).

In their new paper published in Science and titled “Noise-resilient Majorana edge modes on a chain of superconducting qubits,” Xiao Mi, Pedram Roushan, Dima Abanin and their colleagues at Google realized these MEMs with superconducting qubits for the first time. They used a mathematical transformation called the Jordan-Wigner transformation to map the model Kitaev had considered to one that they could realize on their quantum computer: the 1D kicked-Ising model. This model connects each qubit in a 1D chain to each of its two nearest neighbors, such that neighboring qubits interact with one another. Then, a “kick” periodically disturbs the chain.

Mi and his colleagues looked for signatures of the MEMs by comparing the behavior of the edge qubits with those in the middle of the chain. While the state of the qubits in the middle decohered rapidly, the states of those on the edge lasted much longer. Mi says this was “preliminary indication for the resilience of the MEMs toward external decoherence.”

The team then conducted a series of systematic studies on the noise resilience of the MEMs. As a first step, they measured the energies corresponding to the various quantum states of the system and observed that they exactly matched the textbook example of the Kitaev model. In particular, they found that the two MEMs at the opposite ends of the chain are exponentially more difficult to mix as the system size grew—a hallmark feature of the Kitaev model.

Next, the team perturbed the system by adding low-frequency noise to the control operations in the quantum circuits. They found that the MEMs were immune to such perturbations, contrasting sharply with other generic edge modes without symmetries. Surprisingly, the team also found that the MEMs are resilient even to some noise that breaks the symmetries of the Ising model. This is due to a mechanism called “prethermalization,” which arises from the large energy cost required to change the MEMs into other possible excitations in the system.

Lastly, the team measured the full wavefunctions of the MEMs. Doing so required simultaneously measuring the states of varying numbers of qubits close to either end of the chain. Here they made another surprising discovery: No matter how many qubits a measurement included, its decay time was identical. In other words, measurements involving even up to 12 qubits decayed over the same time scale as those of just one qubit. This was contrary to the intuitive expectation that larger quantum observables decay faster in the presence of noise, and further highlighted the collective nature and noise resilience of the MEMs.

Mi and Roushan believe that in the future, they might be able to use MEMs to enable symmetry-protected quantum gates. Their work demonstrates that the MEMs are insensitive to both low-frequency noise and small errors, so this is a promising route to making more robust gates in a quantum processor.

The researchers plan to continue to improve the level of protection these MEMs experience, hopefully to rival some of the leading techniques used to fight against decoherence in quantum computers. Abanin says, “A key question for future works is whether these techniques can be extended to achieve the levels of protection comparable to active error-correction codes.”

More information: X. Mi et al, Noise-resilient edge modes on a chain of superconducting qubits, Science (2022). DOI: 10.1126/science.abq5769

Journal information: Science 

Provided by Google Quantum AI

Recently, a team led by Prof. Guo Guangcan achieved long-lived storage of high-dimensional orbital angular momentum (OAM) quantum states of photons based on cold atomic ensembles, using a guiding magnetic field combined with clock state preparation. Their work was published in Physical Review Letters.

Previous work has shown that integrating multimode memory into quantum networks can greatly improve channel capacity, which is crucial for long distance quantum communication. The collective enhancement effect of the cold atomic ensemble makes it an efficient medium for storing photonic information. Although important progress has been made, many problems remain to be solved in long-lived spatial multimode memory based on cold atomic ensembles, one of which is how to achieve high fidelity for multimode memory after a long storage time since multiple spatial modes are more easily affected by the surrounding environment.

Based on the degrees of freedom of OAM, the team carried out research on the long-lived storage of high-dimensional multimode quantum states using the cold 85Rb system. In this work, to overcome the effect of inhomogeneous evolution due to the spatial complexity of stored OAM, the team used a guiding magnetic field to dominate atomic evolution and then employed a pair of magnetically insensitive states to suppress the decoherence in the transverse direction. After the clock states were employed, the destructive interference between different Zeeman sublevels was eliminated, which consequently extended the lifetime of faithful storage.

The team extended the dimension of stored OAM superposition states to three in the experiment, and achieved fidelity that exceeds the quantum-classical criteria after a storage time of 400μs, which is two orders of magnitude longer than previous works. When the storage time was extended from 10μs to 400μs, the retrieval efficiency dropped from 10.7% to 4.7%, showing a clear decreasing trend while the fidelity barely decayed.

More information: Ying-Hao Ye et al, Long-Lived Memory for Orbital Angular Momentum Quantum States, Physical Review Letters (2022). DOI: 10.1103/PhysRevLett.129.193601

Journal information: Physical Review Letters 

Provided by University of Science and Technology of China