TL;DR

• To build highly performing quantum computers, researchers should be able to reliably derive information about the noise inside them, while also identifying effective strategies to suppress this noise. In recent years, significant progress has been made in this direction, enabling operation errors below 1% in various quantum computing platforms. A research team at Tokyo Institute of Technology and RIKEN recently set out to reliably quantify the correlations between the noise produced by pairs of semiconductor-based qubits, which are very appealing for the development of scalable quantum processors. Their paper, published in Nature Physics, unveiled strong interqubit noise correlations between a pair of neighboring silicon spin qubits.
• Researchers have used the nitrogen-vacancy (NV) center inside a single nanodiamond for quantum sensing to overcome the problem of random particle rotation. Their study is published in Nature Communications.
• Scientists from Lancaster University in the UK have discovered how superfluid helium 3He would feel if you could put your hand into it. Dr. Samuli Autti is the lead author of the research published in Nature Communications.
• New research shows that imperfect timekeeping places a fundamental limit to quantum computers and their applications. The team claims that even tiny timing errors add up to have a significant impact on any large-scale algorithm, posing another problem that must eventually be solved if quantum computers are to fulfill the lofty aspirations that society has for them.
• In physics, quasiparticles are used to describe complex processes in solids. In ultracold quantum gases, these quasiparticles can be reproduced and studied. Now scientists have been able to observe in experiments how Fermi polarons — a special type of quasiparticle — can interact with each other.
• Researchers have proposed a new way of using quantum light to ‘see’ quantum sound. A new paper reveals the quantum-mechanical interplay between vibrations and particles of light, known as photons, in molecules. It is hoped that the discovery may help scientists better understand the interactions between light and matter on molecular scales. And it potentially paves the way for addressing fundamental questions about the importance of quantum effects in applications ranging from new quantum technologies to biological systems.
• Researchers report a significant advance in quantum computing. They have prolonged the coherence time of their single-electron qubit to an impressive 0.1 milliseconds, nearly a thousand-fold improvement.
• Quantum physicists show that imperfect timekeeping places a fundamental limit on quantum computers and their applications. The team claims that even tiny timing errors add up to place a significant impact on any large-scale algorithm, posing another problem that must eventually be solved if quantum computers are to fulfill the lofty aspirations that society has for them.
• The search is on for better semiconductors. A team of chemists describes the fastest and most efficient semiconductor yet: a superatomic material called Re6Se8Cl2.

Quantum Computing Market

According to the recent market research report ‘Quantum Computing Market with COVID-19 impact by Offering (Systems and Services), Deployment (On Premises and Cloud Based), Application, Technology, End-use Industry and Region — Global Forecast to 2026’, published by MarketsandMarkets, the Quantum Computing market is expected to grow from USD 472 million in 2021 to USD 1,765 million by 2026, at a CAGR of 30.2%. The early adoption of quantum computing in the banking and finance sector is expected to fuel the growth of the market globally. Other key factors contributing to the growth of the quantum computing market include rising investments by governments of different countries to carry out research and development activities related to quantum computing technology. Several companies are focusing on the adoption of QCaaS post-COVID-19. This, in turn, is expected to contribute to the growth of the quantum computing market. However, stability and error correction issues are expected to restrain the growth of the market.

According to ‘Quantum Computing Market Research Report: By Offering, Deployment Type, Application, Technology, Industry — Industry Share, Growth, Drivers, Trends and Demand Forecast to 2030’ report, the quantum computing market is projected to reach $64,988 million by 2030. Machine learning (ML) is expected to progress at the highest CAGR, during the forecast period, among all application categories, owing to the fact that quantum computing is being integrated in ML for improving the latter’s use case.

Latest Research

Noise-correlation spectrum for a pair of spin qubits in silicon

by J. Yoneda et al in Nature Physics

To build highly performing quantum computers, researchers should be able to reliably derive information about the noise inside them, while also identifying effective strategies to suppress this noise. In recent years, significant progress has been made in this direction, enabling operation errors below 1% in various quantum computing platforms.

A research team at Tokyo Institute of Technology and RIKEN recently set out to reliably quantify the correlations between the noise produced by pairs of semiconductor-based qubits, which are very appealing for the development of scalable quantum processors. Their paper, published in Nature Physics, unveiled strong interqubit noise correlations between a pair of neighboring silicon spin qubits.

Characterization of qubit error correlation in a silicon qubit array. Credit: Yoneda et al

“A useful quantum computer would practically require millions of densely packed, well-controlled qubits with errors not only small but also sufficiently uncorrelated,” Jun Yoneda, one of the researchers who carried out the study, told Phys.org. “We set out to address the potentially serious issue of error correlation in silicon qubits, as they have become a compelling platform for large quantum computations otherwise.”

Fabricating highly performing quantum processors based on many closely positioned silicon qubits has so far proved challenging. These systems would exhibit noise that is correlated between different qubits. This reduces the devices’ fault tolerance, increasing their error rate and thus impairing their performance.

As part of their recent study, Yoneda and his colleagues set out to explore the extent of these interqubit noise correlations, in the hope of informing the future development of semiconductor-based quantum computing systems. To do this, they analyzed and tried to quantify the correlation between the noise seen by two silicon-based qubits that were placed 100 nm away from each other.

Measured noise correlation in a silicon qubit pair. Dots show the correlation strength (the amplitude of the normalized cross power spectral density) of qubit energies with color representing correlation phase. Credit: Yoneda et al

“Errors in silicon spin qubits are dominated by fluctuations of the qubit energy, that is, the energy difference between the spin-up and -down states,” Yoneda explained. “We measured the simultaneous time evolution of qubit energies and assessed the ‘degree of similarity’ between the two time traces via a quantity called the cross power spectral density.”

The researchers subsequently used a Bayesian estimation technique they developed as part of their previous research work, which is designed to give the probability distributions of cross power spectral densities. This technique allowed them to validate the statistical relevance of the correlations they observed, confirming that the two qubits were subject to strongly correlated noise.

“We observed strong noise correlations between silicon qubits — with a correlation strength as large as 0.7 at some frequencies,” Yoneda said. “Such correlations due to electrical noise are unlikely to decay quickly with distance, so we are now keenly aware that error correlation needs to be taken seriously in dense qubit arrays in silicon. We also showed that noise correlation analysis provides novel insights into the source of qubit noise.”

The statistical methods employed by this team of researchers is unique and powerful, as contrarily to conventional approaches, it requires no prior knowledge of the auto-spectrum (e.g., 1/f) to assess and quantify qubit noise. Overall, the findings of this recent work confirm the challenges associated with noise correlation between closely situated silicon qubits, highlighting the need to devise new approaches to suppress or mitigate noise in semiconductor-based quantum computers.

“Our future research will include investigating how far the correlation will extend in a qubit array, leveraging the methods of including cross correlations in noise analysis that we pioneered here experimentally,” Yoneda added. “This is a critical question concerning fault-tolerance, as well as understanding of the noise source.”

In situ electron paramagnetic resonance spectroscopy using single nanodiamond sensors

by Zhuoyang Qin et al in Nature Communications

Researchers have used the nitrogen-vacancy (NV) center inside a single nanodiamond for quantum sensing to overcome the problem of random particle rotation. Their study is published in Nature Communications.

Being able to detect and analyze molecules under physiological in situ conditions is an important goal in the field of life sciences. Only by observing biomolecules under this condition can we reveal conformation changes when they realize physiological functions.

Thanks to its high sensitivity, good biocompatibility, and the characteristics of magnetic resonance detection of single molecules at room temperature atmosphere, the NV center quantum sensor is more suitable for physiological in situ detection than traditional magnetic spectrum resonance instruments.

However, the results of tracking the movement of nanodiamond in living cells show that it rotates randomly both inside the cell and on the cell membrane, making the current common magnetic resonance detection methods ineffective.

Methods for EPR measurements based on tumbling NDs. a Simplified model of the ND sensor and the target spin. A microwave field marked by the yellow arrow is applied to control the spin state of the NV center, where only the component perpendicular marked by the black arrows (Ω1,2,3) to the N-V axis matters. b Generalized Hartmann-Hahn scheme. The driving field can transfer the NV center from lab frame to dressed frame in order to eliminate the huge energy mismatch between the NV center and the target spins, where D is the zero-field splitting of the NV center and ω is the energy splitting of the target spin. c Pulse sequence and corresponding energy match condition for direct drive. The black arrows mark the scanning variables, microwave (MW) amplitude B1. d Simulated EPR spectra for direct drive. Left side is a simulation of the spectral dependence on θ, while right side is the expected spectra after average over random θ. We omit the gyromagnetic ratio γNV for simplicity. PL, photoluminescence. e Pulse sequence and corresponding energy match condition for amplitude-modulated drive. The black arrows mark the scanning variables, amplitude-modulation frequency f. f Simulated EPR spectra for amplitude-modulated drive. Credit: Nature Communications (2023). DOI: 10.1038/s41467–023–41903–5

To solve this problem, the research team designed an amplitude-modulation sequence, which will generate a series of equally spaced energy levels on the NV center.

When the energy level of the NV center matches the energy level of the measured target, resonance will occur and the state of the NV center will change.

By scanning the modulation frequency, the electron paramagnetic resonance (EPR) spectroscopy of the target can be obtained, and the position of the spectral peak is no longer affected by the spatial orientation of the NV center.

In this work, the ions in the solution environment of nanodiamond were measured by EPR spectroscopy under the condition of in situ. The research team simulated the movement of nanodiamonds in the cell to detect the solution of oxygen vanadium ions.

When there is nanodiamond rotation, it is difficult to conduct accurate quantum manipulation of NV centers, but zero-field EPR spectrum of oxo-vanadium ions can still be measured.

This result proves in principle that it is feasible to use the NV center in nanodiamond to realize the detection of intracellular physiological in-situ magnetic resonance.

The oxygen vanadium ions detected in this work itself have biological functions. The ultra-fine constant of oxygen vanadium ions can be analyzed and obtained by the EPR spectrum measured by a single moving nanodiamond.

The research team has previously relaxed the detection conditions of single-molecular magnetic resonance detection from solid conditions to an aqueous solution environment, and this work has further promoted it to the in situ environment.

Transport of bound quasiparticle states in a two-dimensional boundary superfluid

by S. Autti et al in Nature

Researchers from Lancaster University in the UK have discovered how superfluid helium 3He would feel if you could put your hand into it. Dr. Samuli Autti is the lead author of the research published in Nature Communications.

The interface between the exotic world of quantum physics and classical physics of the human experience is one of the major open problems in modern physics.

Dr. Autti said, “In practical terms, we don’t know the answer to the question ‘How does it feel to touch quantum physics?’ These experimental conditions are extreme and the techniques complicated, but I can now tell you how it would feel if you could put your hand into this quantum system.

“Nobody has been able to answer this question during the 100-year history of quantum physics. We now show that — at least in superfluid 3He — this question can be answered.”

The experiments were carried out at about a 10,000th of a degree above absolute zero in a special refrigerator, and made use of mechanical resonator the size of a finger to probe the very cold superfluid.

The experiments were carried out at about a 10000th of a degree above absolute zero in a special refrigerator and made use of mechanical resonator the size of a finger to probe the very cold superfluid; Dr Samuli Autti (right) at Lancaster University. Credit: Mike Thompson

When stirred with a rod, superfluid 3He carries the generated heat away along the surfaces of the container. The bulk of the superfluid behaves like a vacuum and remains entirely passive.

Dr. Autti said, “This liquid would feel two-dimensional if you could stick your finger into it. The bulk of the superfluid feels empty, while heat flows in a two-dimensional subsystem along the edges of the bulk — in other words, along your finger.”

The researchers conclude that the bulk of superfluid 3He is wrapped by an independent two-dimensional superfluid that interacts with mechanical probes instead of the bulk superfluid, only providing access to the bulk superfluid if given a sudden burst of energy.

That is, superfluid 3He at the lowest temperatures and applied energies is thermo-mechanically two-dimensional.

“This also redefines our understanding of superfluid 3He. For the scientist, that may be even more influential than hands-in quantum physics.”

Superfluid 3He is one of the most versatile macroscopic quantum systems in the laboratory. It often influences seemingly distant fields such as particle physics (for example the Higgs mechanism), cosmology (Kibble mechanism), and quantum information processing (time crystals).

Impact of Imperfect Timekeeping on Quantum Control

by Jake Xuereb et al in Physical Review Letters

New research from a consortium of quantum physicists, led by Trinity College Dublin’s Dr. Mark Mitchison, shows that imperfect timekeeping places a fundamental limit to quantum computers and their applications. The team claims that even tiny timing errors add up to place a significant impact on any large-scale algorithm, posing another problem that must eventually be solved if quantum computers are to fulfill the lofty aspirations that society has for them.

It is difficult to imagine modern life without clocks to help organize our daily schedules; with a digital clock in every person’s smartphone or watch, we take precise timekeeping for granted — although that doesn’t stop people from being late.

And for quantum computers, precise timing is even more essential, as they exploit the bizarre behavior of tiny particles — such as atoms, electrons, and photons — to process information.

While this technology is still at an early stage, it promises to dramatically speed up the solution of important problems, like the discovery of new pharmaceuticals or materials. This potential has driven significant investment across the private and public sector, such as the establishment of the Trinity Quantum Alliance academic-industrial partnership launched earlier in 2023.

Credit: Physical Review Letters (2023). DOI: 10.1103/PhysRevLett.131.160204

Currently, however, quantum computers are still too small to be useful. A major challenge to scaling them up is the extreme fragility of the quantum states that are used to encode information.

In the macroscopic world, this is not a problem. For example, you can add numbers perfectly using an abacus, in which wooden beads are pushed back and forth to represent arithmetic operations. The wooden beads have very stable states: each one sits in a specific place and it will stay in place unless intentionally moved. Importantly, whether you move the bead quickly or slowly does not affect the result.

But in quantum physics, it is more complicated.

“Mathematically speaking, changing a quantum state in a quantum computer corresponds to a rotation in an abstract high-dimensional space,” says Jake Xuereb from the Atomic Institute at the Vienna University of Technology, the first author of the paper. “In order to achieve the desired state in the end, the rotation must be applied for a very specific period of time — otherwise you turn the state either too little or too far.”

Given that real clocks are never perfect, the team investigated the impact of imperfect timing on quantum algorithms.

“A quantum algorithm is like an app that runs on a quantum computer,” explains Trinity’s Dr. Mitchison. “It was already known that timing errors could disrupt individual quantum logic gates, which are the building blocks of quantum algorithms. Our work extends this to full quantum algorithms, showing exactly how precise the clock must be to achieve a given computational accuracy.”

Since the error gets worse for more complex algorithms, it will ultimately pose a challenge for quantum computers.

“It’s not a problem at the moment,” said Prof. Marcus Huber who leads the research team in Vienna. “Currently, the accuracy of quantum computers is still limited by other factors, for example the precision of the hardware components or the effect of stray electromagnetic fields. But our calculations also show that today we are not far from the regime in which the fundamental limits of time measurement will play the decisive role.”

The team is quick to emphasize that the message is not entirely pessimistic, because the problem could be mitigated in the future by designing clever error correction protocols.

Mediated interactions between Fermi polarons and the role of impurity quantum statistics

by Cosetta Baroni, Bo Huang, Isabella Fritsche, Erich Dobler, Gregor Anich, Emil Kirilov, Rudolf Grimm, Miguel A. Bastarrachea-Magnani, Pietro Massignan, Georg M. Bruun in Nature Physics

In physics, quasiparticles are used to describe complex processes in solids. In ultracold quantum gases, these quasiparticles can be reproduced and studied. Now, for the first time, Austrian scientists led by Rudolf Grimm have been able to observe in experiments how Fermi polarons — a special type of quasiparticle — can interact with each other.

An electron moving through a solid generates polarization in its environment due to its electric charge. In his theoretical considerations, the Russian physicist Lev Landau extended the description of such particles by their interaction with the environment and spoke of quasiparticles.

More than ten years ago, the team led by Rudolf Grimm at the Institute of Quantum Optics and Quantum Information (IQQOI) of the Austrian Academy of Sciences (ÖAW) and the Department of Experimental Physics of the University of Innsbruck succeeded in generating such quasiparticles for both attractive and repulsive interactions with the environment.

For this purpose, the scientists use an ultracold quantum gas consisting of lithium and potassium atoms in a vacuum chamber. With the help of magnetic fields, they control the interactions between the particles, and by means of radio-frequency pulses push the potassium atoms into a state in which they attract or repel the lithium atoms surrounding them. In this way, the researchers simulate a complex state similar to the one produced in the solid state by a free electron.

The energy of attractive and repulsive polarons is presented as a function of the dimensionless interaction parameter X according to equation (3) using the static limit for the polaron–polaron-mediated interaction defined in equation (2). The results are shown for the limit of a single impurity (that is, �=0; black dashed lines) and for impurity concentration �=0.5 in FB (solid green lines) and FF (solid red lines) K–Li mixtures.

Now, the scientists led by Rudolf Grimm have been able to generate several such quasiparticles simultaneously in the quantum gas and observe their interactions with each other.

“In a naive notion, one would assume that polarons always attract each other, regardless of whether their interaction with the environment is attractive or repulsive,” says the experimental physicist. “However, this is not the case. We see attractive interaction in bosonic polarons, repulsive interaction in fermionic polarons. Here, quantum statistics plays a crucial role.”

The researchers have now been able to demonstrate this behavior, which in principle already follows as a consequence of Landau’s theory, in an experiment for the first time. The theoretical calculations for this were done by colleagues from Mexico, Spain and Denmark.

“High experimental skills were required to implement this in the laboratory,” explains Cosetta Baroni, first author of the study, “because even the smallest deviations could have skewed the measurements.”

“Such investigations provide us with insights into very fundamental mechanisms of nature and offer us excellent opportunities to study them in detail,” says Rudolf Grimm excitedly.

Phonon Signatures in Photon Correlations

by Ben S. Humphries, Dale Green, Magnus O. Borgh, Garth A. Jones in Physical Review Letters

Researchers at the University of East Anglia have proposed a new way of using quantum light to ‘see’ quantum sound.

A new paper published today reveals the quantum-mechanical interplay between vibrations and particles of light, known as photons, in molecules.

It is hoped that the discovery may help scientists better understand the interactions between light and matter on molecular scales.

And it potentially paves the way for addressing fundamental questions about the importance of quantum effects in applications ranging from new quantum technologies to biological systems.

Dr Magnus Borgh from UEA’s School of Physics said: “There is a long-standing controversy in chemical physics about the nature of processes where energy from particles of light is transferred within molecules.

“Are they fundamentally quantum-mechanical or classical? Molecules are complex and messy systems, constantly vibrating. How do these vibrations affect any quantum-mechanical processes in the molecule?

“These processes are typically investigated using techniques that rely on polarisation — the same property of light used in sunglasses to reduce reflections. But this is a classical phenomenon.

“Techniques from quantum optics, the field of physics that studies the quantum nature of light and its interactions with matter on the atomic scale, can offer a way to investigate genuine quantum effects directly in molecular systems.”

Quantum behaviour can be revealed by studying correlations in the emitted light from a molecule placed in a laser field. Correlations answer the question how likely it is that two photons are emitted very close together and can be measured using standard techniques.

Ben Humphries, PhD student in theoretical chemistry, at UEA said: “Our research shows that when a molecule exchanges phonons — quantum-mechanical particles of sound — with its environment, this produces a recognisable signal in the photon correlations.”

While photons are routinely created and measured in laboratories all over the world, individual quanta of vibrations, which are the corresponding particles of sound, phonons, cannot in general be similarly measured.

The new findings provide a toolbox for investigating the world of quantum sound in molecules.

Lead researcher Dr Garth Jones, from UEA’s School of Chemistry, said: “We have also computed correlations between photon and phonons.

“It would be very exciting if our paper could inspire the development of new experimental techniques to detect individual phonons directly,” he added.

Electron charge qubit with 0.1 millisecond coherence time

by Xianjing Zhou, Xinhao Li, Qianfan Chen, Gerwin Koolstra, Ge Yang, Brennan Dizdar, Yizhong Huang, Christopher S. Wang, Xu Han, Xufeng Zhang, David I. Schuster, Dafei Jin in Nature Physics

Coherence stands as a pillar of effective communication, whether it is in writing, speaking or information processing. This principle extends to quantum bits, or qubits, the building blocks of quantum computing. A quantum computer could one day tackle previously insurmountable challenges in climate prediction, material design, drug discovery and more.

A team led by the U.S. Department of Energy’s (DOE) Argonne National Laboratory has achieved a major milestone toward future quantum computing. They have extended the coherence time for their novel type of qubit to an impressive 0.1 milliseconds — nearly a thousand times better than the previous record.

In everyday life, 0.1 milliseconds is as fleeting as a blink of an eye. However, in the quantum world, it represents a long enough window for a qubit to perform many thousands of operations.

Unlike classical bits, qubits seemingly can exist in both states, 0 and 1. For any working qubit, maintaining this mixed state for a sufficiently long coherence time is imperative. The challenge is to safeguard the qubit against the constant barrage of disruptive noise from the surrounding environment.

The team’s qubits encode quantum information in the electron’s motional (charge) states. Because of that, they are called charge qubits.

“Among various existing qubits, electron charge qubits are especially attractive because of their simplicity in fabrication and operation, as well as compatibility with existing infrastructures for classical computers,” said Dafei Jin, a professor at the University of Notre Dame with a joint appointment at Argonne and the lead investigator of the project. “This simplicity should translate into low cost in building and running large-scale quantum computers.”

Jin is a former staff scientist at the Center for Nanoscale Materials (CNM), a DOE Office of Science user facility at Argonne. While there, he led the discovery of their new type of qubit, reported last year.

The team’s qubit is a single electron trapped on an ultraclean solid-neon surface in a vacuum. The neon is important because it resists disturbance from the surrounding environment. Neon is one of a handful of elements that do not react with other elements. The neon platform keeps the electron qubit protected and inherently guarantees a long coherence time.

“Thanks to the small footprint of single electrons on solid neon, qubits made with them are more compact and promising for scaling up to multiple linked qubits,” said Xu Han, an assistant scientist in CNM with a joint appointment at the Pritzker School of Molecular Engineering at the University of Chicago. “These attributes, along with coherence time, make our electron qubit exceptionally compelling.”

Following continued experimental optimization, the team not only improved the quality of the neon surface but also significantly reduced disruptive signals. As reported in Nature Physics, their work paid off with a coherence time of 0.1 milliseconds. That is about a thousand-fold increase from the initial 0.1 microseconds.

“The long lifetime of our electron qubit allows us to control and read out the single qubit states with very high fidelity,” said Xinhao Li, a postdoctoral appointee at Argonne and the co-first author of the paper. This time is well above the requirements for quantum computing.

“Rather than 10 to 100 operations over the coherence times of conventional electron charge qubits, our qubits can perform 10,000 with very high precision and speed,” Jin said.

Yet another important attribute of a qubit is its scalability to link with many other qubits. The team achieved a significant milestone by showing that two-electron qubits can couple to the same superconducting circuit such that information can be transferred between them through the circuit. This marks a pivotal stride toward two-qubit entanglement, a critical aspect of quantum computing.

Impact of Imperfect Timekeeping on Quantum Control

by Jake Xuereb, Paul Erker, Florian Meier, Mark T. Mitchison, Marcus Huber in Physical Review Letters

New research from a consortium of quantum physicists, led by Trinity College Dublin’s Dr Mark Mitchison, shows that imperfect timekeeping places a fundamental limit to quantum computers and their applications. The team claims that even tiny timing errors add up to place a significant impact on any large-scale algorithm, posing another problem that must eventually be solved if quantum computers are to fulfil the lofty aspirations that society has for them.

It is difficult to imagine modern life without clocks to help organise our daily schedules; with a digital clock in every person’s smartphone or watch, we take precise timekeeping for granted — although that doesn’t stop people from being late!

And for quantum computers, precise timing is even more essential, as they exploit the bizarre behaviour of tiny particles — such as atoms, electrons, and photons — to process information. While this technology is still at an early stage, it promises to dramatically speed up the solution of important problems, like the discovery of new pharmaceuticals or materials. This potential has driven significant investment across the private and public sector, such as the establishment of the Trinity Quantum Alliance academic-industrial partnership launched earlier this year.

Currently, however, quantum computers are still too small to be useful. A major challenge to scaling them up is the extreme fragility of the quantum states that are used to encode information. In the macroscopic world, this is not a problem. For example, you can add numbers perfectly using an abacus, in which wooden beads are pushed back and forth to represent arithmetic operations. The wooden beads have very stable states: each one sits in a specific place and it will stay in place unless intentionally moved. Importantly, whether you move the bead quickly or slowly does not affect the result.

But in quantum physics, it is more complicated.

“Mathematically speaking, changing a quantum state in a quantum computer corresponds to a rotation in an abstract high-dimensional space,” says Jake Xuereb from the Atomic Institute at the Vienna University of Technology, the first author of the paper. “In order to achieve the desired state in the end, the rotation must be applied for a very specific period of time — otherwise you turn the state either too little or too far.”

Given that real clocks are never perfect, the team investigated the impact of imperfect timing on quantum algorithms.

“A quantum algorithm is like an app that runs on a quantum computer,” explains Trinity’s Dr Mitchison. “It was already known that timing errors could disrupt individual quantum logic gates, which are the building blocks of quantum algorithms. Our work extends this to full quantum algorithms, showing exactly how precise the clock must be to achieve a given computational accuracy.”

Since the error gets worse for more complex algorithms, it will ultimately pose a challenge for quantum computers.

“It’s not a problem at the moment,” clarifies Prof. Marcus Huber who leads the research team in Vienna. “Currently, the accuracy of quantum computers is still limited by other factors, for example the precision of the hardware components or the effect of stray electromagnetic fields. But our calculations also show that today we are not far from the regime in which the fundamental limits of time measurement will play the decisive role.”

The team is quick to emphasise that the message is not entirely pessimistic, because the problem could be mitigated in the future by designing clever error correction protocols.

Room-temperature wavelike exciton transport in a van der Waals superatomic semiconductor

by Jakhangirkhodja A. Tulyagankhodjaev, Petra Shih, Jessica Yu, Jake C. Russell, Daniel G. Chica, Michelle E. Reynoso, Haowen Su, Athena C. Stenor, Xavier Roy, Timothy C. Berkelbach, Milan Delor in Science

The search is on for better semiconductors. Writing in Science, a team of chemists at Columbia University led by Jack Tulyag, a PhD student working with chemistry professor Milan Delor, describes the fastest and most efficient semiconductor yet: a superatomic material called Re6Se8Cl2.

Semiconductors — most notably, silicon — underpin the computers, cellphones, and other electronic devices that power our daily lives, including the device on which you are reading this article. As ubiquitous as semiconductors have become, they come with limitations. The atomic structure of any material vibrates, which creates quantum particles called phonons. Phonons in turn cause the particles — either electrons or electron-hole pairs called excitons — that carry energy and information around electronic devices to scatter in a matter of nanometers and femtoseconds. This means that energy is lost in the form of heat, and that information transfer has a speed limit.

The search is on for better options. Writing in Science, a team of chemists at Columbia University led by Jack Tulyag, a PhD student working with chemistry professor Milan Delor, describes the fastest and most efficient semiconductor yet: a superatomic material called Re6Se8Cl2.

Rather than scattering when they come into contact with phonons, excitons in Re6Se8Cl2 actually bind with phonons to create new quasiparticles called acoustic exciton-polarons. Although polarons are found in many materials, those in Re6Se8Cl2 have a special property: they are capable of ballistic, or scatter-free, flow. This ballistic behavior could mean faster and more efficient devices one day.

In experiments run by the team, acoustic exciton-polarons in Re6Se8Cl2 moved fast — twice as fast as electrons in silicon — and crossed several microns of the sample in less than a nanosecond. Given that polarons can last for about 11 nanoseconds, the team thinks the exciton-polarons could cover more than 25 micrometers at a time. And because these quasiparticles are controlled by light rather than an electrical current and gating, processing speeds in theoretical devices have the potential to reach femtoseconds — six orders of magnitude faster than the nanoseconds achievable in current Gigahertz electronics. All at room temperature.

“In terms of energy transport, Re6Se8Cl2 is the best semiconductor that we know of, at least so far,” Delor said.

Re6Se8Cl2 is a superatomic semiconductor created in the lab of collaborator Xavier Roy. Superatoms are clusters of atoms bound together that behave like one big atom, but with different properties than the elements used to build them. Synthesizing superatoms is a specialty of the Roy lab, and they are a main focus of Columbia’s NSF-funded Material Research Science and Engineering Center on Precision Assembled Quantum Materials. Delor is interested in controlling and manipulating the transport of energy through superatoms and other unique materials developed at Columbia. To do this, the team builds super-resolution imaging tools that can capture particles moving at ultrasmall, ultrafast scales.

When Tulyag first brought Re6Se8Cl2 into the lab, it wasn’t to search for a new and improved semiconductor — it was to test the resolution of the lab’s microscopes with a material that, in principle, shouldn’t have conducted much of anything.

“It was the opposite of what we expected,” said Delor. “Instead of the slow movement we expected, we saw the fastest thing we’ve ever seen.”

Tulyag and his peers in the Delor group spent the next two years working to pinpoint why Re6Se8Cl2 showed such remarkable behavior, including developing an advanced microscope with extreme spatial and temporal resolution that can directly image polarons as they form and move through the material. Theoretical chemist Petra Shih, a PhD student working in Timothy Berkelbach’s group, also developed a quantum mechanical model that provides an explanation for the observations.

The new quasiparticles are fast, but, counterintuitively, they accomplish that speed by pacing themselves — a bit like the story of the tortoise and the hare, Delor explained. What makes silicon a desirable semiconductor is that electrons can move through it very quickly, but like the proverbial hare, they bounce around too much and don’t actually make it very far, very fast in the end. Excitons in Re6Se8Cl2 are, comparatively, very slow, but it’s precisely because they are so slow that they are able to meet and pair up with equally slow-moving acoustic phonons. The resulting quasiparticles are “heavy” and, like the tortoise, advance slowly but steadily along. Unimpeded by other phonons along the way, acoustic exciton-polarons in Re6Se8Cl2 ultimately move faster than electrons in silicon.

Like many of the emerging quantum materials being explored at Columbia, Re6Se8Cl2 can be peeled into atom-thin sheets, a feature that means they can potentially be combined with other similar materials in the search for additional unique properties. Re6Se8Cl2, however, is unlikely to ever make its way into a commercial product — the first element in the molecule, Rhenium, is one of the rarest on earth and extremely expensive as a result.

But with the new theory from the Berkelbach group in hand along with the advanced imaging technique that Tulyag and the Delor group developed to directly track the formation and movement of polarons in the first place, the team is ready to see if there are other superatomic contenders capable of beating Re6Se8Cl2'’s speed record.

“This is the only material that anyone has seen sustained room-temperature ballistic exciton transport in. But we can now start to predict what other materials might be capable of this behavior that we just haven’t considered before,” said Delor. “There is a whole family of superatomic and other 2D semiconductor materials out there with properties favorable for acoustic polaron formation.”

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