QT/ Researchers make a significant step towards reliably processing quantum information
Quantum news biweekly vol.59, 5th September — 21st September
TL;DR
- Using laser light, researchers have developed the most robust method currently known to control individual qubits made of the chemical element barium. The ability to reliably control a qubit is an important achievement for realizing future functional quantum computers.
- Researchers have used machine learning to perform error correction for quantum computers — a crucial step for making these devices practical — using an autonomous correction system that despite being approximate, can efficiently determine how best to make the necessary corrections.
- Scientists have mathematically derived the fundamental limit of heat current flowing into a quantum system comprising numerous quantum mechanical particles in relation to the particle count. Further, they established a clearer understanding of how the heat current rises with increasing particle count, shedding light on the performance constraints of potential future quantum thermal devices.
- The Mpemba effect is originally referred to the non-monotonic initial temperature dependence of the freezing start time, but it has been observed in various systems — including colloids — and has also become known as a mysterious relaxation phenomenon that depends on initial conditions. However, very few have previously investigated the effect in quantum systems. Now, the temperature quantum Mpemba effect can be realized over a wide range of initial conditions.
- Quantum physics has allowed for the creation of sensors far surpassing the precision of classical devices. Now, several new studies show that the precision of these quantum sensors can be significantly improved using entanglement produced by finite-range interactions. Researchers were able to demonstrate this enhancement using entangled ion-chains with up to 51 particles.
- A team has created an artificial quantum magnet featuring a quasiparticle made of entangled electrons, the triplon.
- Quantum physicists have successfully simulated super diffusion in a system of interacting quantum particles on a quantum computer. This is the first step in doing highly challenging quantum transport calculations on quantum hardware and, as the hardware improves over time, such work promises to shed new light in condensed matter physics and materials science.
- Researchers have developed a new approach to building quantum repeaters, devices that can link quantum computers over long distances. The new system transmits low-loss signals over optical fiber using light in the telecom band, a longstanding goal in the march toward robust quantum communication networks.
- Scientists demonstrated a conceptual breakthrough by fabricating atomically precise quantum antidots using self-assembled single vacancies in a two-dimensional transition metal dichalcogenide.
- Scientists have developed a new method for detecting mid-infrared (MIR) light at room temperature using quantum systems.
- And more!
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
A guided light system for agile individual addressing of Ba+ qubits with 10−4 level intensity crosstalk
by Ali Binai-Motlagh, Matthew L Day, Nikolay Videnov, Noah Greenberg, Crystal Senko, Rajibul Islam in Quantum Science and Technology
Using laser light, researchers have developed the most robust method currently known to control individual qubits made of the chemical element barium. The ability to reliably control a qubit is an important achievement for realizing future functional quantum computers.
This new method, developed at the University of Waterloo’s Institute for Quantum Computing (IQC), uses a small glass waveguide to separate laser beams and focus them four microns apart, about four-hundredths of the width of a single human hair. The precision and extent to which each focused laser beam on its target qubit can be controlled in parallel is unmatched by previous research.
“Our design limits the amount of crosstalk-the amount of light falling on neighbouring ions-to the very small relative intensity of 0.01 per cent, which is among the best in the quantum community,” said Dr. K. Rajibul Islam, a professor at IQC and Waterloo’s Department of Physics and Astronomy. “Unlike previous methods to create agile controls over individual ions, the fibre-based modulators do not affect each other.
“This means we can talk to any ion without affecting its neighbours while also retaining the capability to control each individual ion to the maximum possible extent. This is the most flexible ion qubit control system with this high precision that we know of anywhere, in both academia and industry.”
The researchers targeted barium ions, which are becoming increasingly popular in the field of trapped ion quantum computation. Barium ions have convenient energy states that can be used as the zero and one levels of a qubit and be manipulated with visible green light, unlike the higher energy ultraviolet light needed for other atom types for the same manipulation. This allows the researchers to use commercially available optical technologies that are not available for ultraviolet wavelengths.
The researchers created a waveguide chip that divides a single laser beam into 16 different channels of light. Each channel is then directed into individual optical fibre-based modulators which independently provide agile control over each laser beam’s intensity, frequency, and phase. The laser beams are then focused down to their small spacing using a series of optical lenses similar to a telescope. The researchers confirmed each laser beam’s focus and control by measuring them with precise camera sensors.
“This work is part of our effort at the University of Waterloo to build barium ion quantum processors using atomic systems,” said Dr. Crystal Senko, Islam’s co-principal investigator and a faculty member at IQC and Waterloo’s Department of Physics and Astronomy. “We use ions because they are identical, nature-made qubits, so we don’t need to fabricate them. Our task is to find ways to control them.”
The new waveguide method demonstrates a simple and precise method of control, showing promise for manipulating ions to encode and process quantum data and for implementation in quantum simulation and computing.
Approximate Autonomous Quantum Error Correction with Reinforcement Learning
by Yexiong Zeng, Zheng-Yang Zhou, Enrico Rinaldi, Clemens Gneiting, Franco Nori in Physical Review Letters
Researchers from the RIKEN Center for Quantum Computing have used machine learning to perform error correction for quantum computers — a crucial step for making these devices practical — using an autonomous correction system that despite being approximate, can efficiently determine how best to make the necessary corrections.
In contrast to classical computers, which operate on bits that can only take the basic values 0 and 1, quantum computers operate on “qubits,” which can assume any superposition of the computational basis states. In combination with quantum entanglement, another quantum characteristic that connects different qubits beyond classical means, this enables quantum computers to perform entirely new operations, giving rise to potential advantages in some computational tasks, such as large-scale searches, optimization problems, and cryptography.
The main challenge towards putting quantum computers into practice stems from the extremely fragile nature of quantum superpositions. Indeed, tiny perturbations induced, for instance, by the ubiquitous presence of an environment give rise to errors that rapidly destroy quantum superpositions and, as a consequence, quantum computers lose their edge.
To overcome this obstacle, sophisticated methods for quantum error correction have been developed. While they can, in theory, successfully neutralize the effect of errors, they often come with a massive overhead in device complexity, which itself is error-prone and thus potentially even increases the exposure to errors. As a consequence, full-fledged error correction has remained elusive.
In this work, the researchers leveraged machine learning in a search for error correction schemes that minimize the device overhead while maintaining good error correcting performance. To this end, they focused on an autonomous approach to quantum error correction, where a cleverly designed, artificial environment replaces the necessity to perform frequent error-detecting measurements. They also looked at “bosonic qubit encodings,” which are, for instance, available and utilized in some of the currently most promising and widespread quantum computing machines based on superconducting circuits.
Finding high-performing candidates in the vast search space of bosonic qubit encodings represents a complex optimization task, which the researchers address with reinforcement learning, an advanced machine learning method, where an agent explores a possibly abstract environment to learn and optimize its action policy. With this, the group found that a surprisingly simple, approximate qubit encoding could not only greatly reduce the device complexity compared to other proposed encodings, but also outperformed its competitors in terms of its capability to correct errors.
Yexiong Zeng, the first author of the paper, says, “Our work not only demonstrates the potential for deploying machine learning towards quantum error correction, but it may also bring us a step closer to the successful implementation of quantum error correction in experiments.”
According to Franco Nori, “Machine learning can play a pivotal role in addressing large-scale quantum computation and optimization challenges. Currently, we are actively involved in a number of projects that integrate machine learning, artificial neural networks, quantum error correction, and quantum fault tolerance.”
Universal Scaling Bounds on a Quantum Heat Current
by Shunsuke Kamimura, Kyo Yoshida, Yasuhiro Tokura, Yuichiro Matsuzaki in Physical Review Letters
Over the past few years, research has been conducted on quantum technologies that exploit the quantum mechanical properties of microscopic entities. Quantum thermodynamics is a notable field in this domain. Within this field, quantum heat engines and quantum batteries, leveraging quantum characteristics, have been theoretically studied and practically tested. A critical indicator of the performance of such devices is the magnitude of heat current (heat transferred per unit time) flowing from the ambient environment to the quantum system as the system’s size increases. However, the fundamental limit of the heat current flowing into such an ensemble of quantum systems remains undefined.
In this study, the researchers mathematically derived a novel inequality that defines the limit of the heat current flowing into a quantum system. Based on this inequality, they demonstrated that as a quantum system incorporates increasing number of particles, the heat current flowing into the system does not rise faster than a cubic function of the particle count.
Furthermore, they derived an inequality applicable under more realistic conditions wherein the heat current does not rise faster than a square function of the particle count. Interestingly, the phenomenon related to energy radiation termed as “super-radiance” was identified as the most efficient mechanism for achieving the fundamental heat current limit derived in this study.
While earlier research has hinted at nonlinear heat current surge with respect to the particle count in various specific scenarios, this study is pioneering in pinpointing a fundamental limit that is universally applicable. Notably, these findings could be instrumental for cooling engines associated with quantum devices and other similar applications.
Quantum Mpemba Effect in a Quantum Dot with Reservoirs
by Amit Kumar Chatterjee, Satoshi Takada, Hisao Hayakawa in Physical Review Letters
Does hot water freeze faster than cold water? Aristotle may have been the first to tackle this question that later became known as the Mpemba effect.
This phenomenon originally referred to the non-monotonic initial temperature dependence of the freezing start time, but it has been observed in various systems — including colloids — and has also become known as a mysterious relaxation phenomenon that depends on initial conditions. However, very few have previously investigated the effect in quantum systems. Now, a team of researchers from Kyoto University and the Tokyo University of Agriculture and Technology has shown that the temperature quantum Mpemba effect can be realized over a wide range of initial conditions.
“The quantum Mpemba effect bears the memory of initial conditions that result in anomalous thermal relaxation at later times,” explains project leader and co-author Hisao Hayakawa at KyotoU’s Yukawa Institute for Theoretical Physics.
Hayakawa’s team prepared two systems with quantum dots connected to a heat bath, one with a current flowing and the other in an equilibrium state. Both were quenched to a low-temperature equilibrium state, allowing the team to follow their time evolution toward a steady state regarding the density matrix, energy, entropy, and — most critically — temperature.
“When the two copies crossed each other before reaching the same equilibrium state — so that the hotter part became colder and vice versa in an identity reversal — we knew we had achieved the thermal quantum Mpemba effect,” says co-author Satoshi Takada of TUAT.
“After analyzing the quantum master equation, we also discovered we had obtained the thermal quantum Mpemba effect in a wide range of parameters, including reservoir temperatures and chemical potentials,” adds first and corresponding author Amit Kumar Chatterjee, also of KyotoU.
“Our results encourage us to explore the potential use of the quantum Mpemba effect in future applications beyond thermal analyses,” reflects Hayakawa.
Quantum-enhanced sensing on optical transitions through finite-range interactions
by Johannes Franke, Sean R. Muleady, Raphael Kaubruegger, Florian Kranzl, Rainer Blatt, Ana Maria Rey, Manoj K. Joshi, Christian F. Roos in Nature
Quantum physics has allowed for the creation of sensors far surpassing the precision of classical devices. Now, several studies show that the precision of these quantum sensors can be significantly improved using entanglement produced by finite-range interactions. Innsbruck researchers led by Christian Roos were able to demonstrate this enhancement using entangled ion-chains with up to 51 particles.
Metrological institutions around the world administer our time, using atomic clocks based on the natural oscillations of atoms. These clocks, pivotal for applications like satellite navigation or data transfer, have recently been improved by using ever higher oscillation frequencies in optical atomic clocks. Now, scientists at the University of Innsbruck and the Institute of Quantum Optics and Quantum Information (IQOQI) of the Austrian Academy of Sciences led by Christian Roos show how a particular way of creating entanglement can be used to further improve the accuracy of measurements integral to an optical atomic clock’s function.
Observations of quantum systems are always subject to a certain statistical uncertainty. “This is due to the nature of the quantum world,” explains Johannes Franke from Christian Roos’ team. “Entanglement can help us reduce these errors.” With the support of theorist Ana Maria Rey from JILA in Boulder, USA, the Innsbruck physicists tested the measurement accuracy on an entangled ensemble of particles in the laboratory.
The researchers used lasers to tune the interaction of ions lined up in a vacuum chamber and entangled them. “The interaction between neighboring particles decreases with the distance between the particles. Therefore, we used spin-exchange interactions to allow the system to behave more collectively,” explains Raphael Kaubrügger from the Department of Theoretical Physics at the University of Innsbruck. Thus, all particles in the chain were entangled with each other and produced a so-called squeezed quantum state. Using this, the physicists were able to show that measurement errors can be roughly halved by entangling 51 ions in relation to individual particles. Previously, entanglement-enhanced sensing mainly relied on infinite interactions, limiting its applicability to only certain quantum platforms.
With their experiments, the Innsbruck quantum physicists were able to show that quantum entanglement makes sensors even more sensitive. “We used an optical transition in our experiments that is also employed in atomic clocks,” says Christian Roos. This technology could improve areas where atomic clocks are currently used, such as satellite-based navigation or data transfer. Moreover, these advanced clocks could open new possibilities in pursuits like the search for dark matter or the determination of time-variations of fundamental constants.
Real-Space Imaging of Triplon Excitations in Engineered Quantum Magnets
by Robert Drost, Shawulienu Kezilebieke, Jose L. Lado, Peter Liljeroth in Physical Review Letters
Triplons are tricky little things. Experimentally, they’re exceedingly difficult to observe. And even then, researchers usually conduct the tests on macroscopic materials, in which measurements are expressed as an average across the whole sample.
That’s where designer quantum materials offer a unique advantage, says Academy Research Fellow Robert Drost, the first author of a paper. These designer quantum materials let researchers create phenomena not found in natural compounds, ultimately enabling the realization of exotic quantum excitations.
‘These materials are very complex. They give you very exciting physics, but the most exotic ones are also challenging to find and study. So, we are trying a different approach here by building an artificial material using individual components,’ says Professor Peter Liljeroth, head of the Atomic Scale physics research group at Aalto University.
Quantum materials are governed by the interactions between electrons at the microscopic level. These electronic correlations lead to unusual phenomena like high-temperature superconductivity or complex magnetic states, and quantum correlations give rise to new electronic states.
In the case of two electrons, there are two entangled states known as singlet and triplet states. Supplying energy to the electron system can excite it from the singlet to the triplet state. In some cases, this excitation can propagate through a material in an entanglement wave known as a triplon. These excitations are not present in conventional magnetic materials, and measuring them has remained an open challenge in quantum materials.
In the new study, the team used small organic molecules to create an artificial quantum material with unusual magnetic properties. Each of the cobalt-phthalocyanine molecules used in the experiment contains two frontier electrons.
‘Using very simple molecular building blocks, we are able to engineer and probe this complex quantum magnet in a way that has never been done before, revealing phenomena not found in its independent parts,’ Drost says. ‘While magnetic excitations in isolated atoms have long been observed using scanning tunnelling spectroscopy, it has never been accomplished with propagating triplons.’
‘We use these molecules to bundle electrons together, we pack them into a tight space and force them to interact,’ continues Drost. ‘Looking into such a molecule from the outside, we will see the joint physics of both electrons. Because our fundamental building block now contains two electrons, rather than one, we see a very different kind of physics.’
The team monitored magnetic excitations first in individual cobalt-phthalocyanine molecules and later in larger structures like molecular chains and islands. By starting with the very simple and working towards increasing complexity, the researchers hope to understand emergent behaviour in quantum materials. In the present study, the team could demonstrate that the singlet-triplet excitations of their building blocks can traverse molecular networks as exotic magnetic quasiparticles known as triplons.
‘We show that we can create an exotic quantum magnetic excitation in an artificial material. This strategy shows that we can rationally design material platforms that open up new possibilities in quantum technologies,’ says Assistant Professor Jose Lado, one of the study’s co-authors, who heads the Correlated Quantum Materials research group at Aalto University.
The team plans to extend their approach towards more complex building blocks to design other exotic magnetic excitations and ordering in quantum materials. Rational design from simple ingredients will not only help understand the complex physics of correlated electron systems but also establish new platforms for designer quantum materials.
Evidence of Kardar-Parisi-Zhang scaling on a digital quantum simulator
by Nathan Keenan, Niall F. Robertson, Tara Murphy, Sergiy Zhuk, John Goold in npj Quantum Information
Trinity’s quantum physicists in collaboration with IBM Dublin have successfully simulated super diffusion in a system of interacting quantum particles on a quantum computer.
This is the first step in doing highly challenging quantum transport calculations on quantum hardware and, as the hardware improves over time, such work promises to shed new light in condensed matter physics and materials science. The work is one of the first outputs of the TCD-IBM predoctoral scholarship programme which was recently established where IBM hires PhD students as employees while being co-supervised at Trinity.
IBM is a global leader in the exciting field of quantum computation. The early stage quantum computer used in this study consists of 27 superconducting qubits (qubits are the building blocks of quantum logic) and is physically located in IBMs lab in Yorktown Heights in New York and programmed remotely from Dublin. Quantum computing is currently one of the most exciting technologies and is expected to be edging closer towards commercial applications in the next decade. Commercial applications aside there are fascinating fundamental questions which quantum computers can help with. The team at Trinity and IBM Dublin tackled one such question concerning quantum simulation.
Explaining the significance of the work and the idea of quantum simulation in general, Trinity’s Professor John Goold, Director of the newly established Trinity Quantum Alliance, who led the research, explains:
“Generally speaking the problem of simulating the dynamics of a complex quantum system with many interacting constituents is a formidable challenge for conventional computers. Consider the 27 qubits on this particular device. In quantum mechanics the state of such a system is described mathematically by an object called a wave function. In order to use a standard computer to describe this object you require a huge number of coefficients to be stored in memory and the demands scale exponentially with the number of qubits; roughly 134 million coefficients, in the case of this simulation.
“As you grow the system to say 300 qubits you would need more coefficients than there are atoms in the observable universe to describe such a system and no classical computer will be able to exactly capture the system’s state. In other words we hit a wall when simulating quantum systems. The idea of using quantum systems to simulate quantum dynamics goes back to the American Nobel prize winning Physicist Richard Feynman who proposed that quantum systems are best simulated using quantum systems. The reason is simple — you naturally exploit the fact that the quantum computer is described by a wave function thus circumventing the need for exponential classical resources for storage of the state.”
So what exactly did the team simulate. Prof. Goold continues: “Some of the simplest non-trivial quantum systems are spin chains. These are systems of little connected magnets called spins, which mimic more complex materials and are used to understand magnetism. We were interested in a model called the Heisenberg chain and we were particularly interested in the long-time behaviour of how spin excitations are transported across the system. In this long-time limit, quantum many-body systems enter a hydrodynamic regime and transport is described by equations that describe classical fluids.
“We were interested in a particular regime where something called super-diffusion occurs due to the underlying physics being governed by something called the Kardar-Parisi-Zhang equation. This is an equation which typically describes the stochastic growth of a surface or interface like how the height of snow grows during a snowstorm, how the stain of a coffee cup on cloth grows with time, or how a fluff fire grows. The propagation is known to give super diffusive transport. This is transport which becomes faster as you increase the system size. It is amazing that the same equations that govern these phenomena crop up in quantum dynamics and we were able to use the quantum computer to verify that. This was the main achievement of the work.”
IBM-Trinity predoctoral scholar Nathan Keenan, who programmed the device as part of the project tells us of some of the challenges to programme quantum computers.
“The biggest problem with programming quantum computers, is performing useful calculations in the presence of noise,” he said. “The operations performed at the chip-level are imperfect, and the computer is very sensitive to disturbances from its laboratory environment. As a result, you want to minimise the runtime of a useful programme, as this will shorten the time in which these errors and disturbances can occur and affect your result.”
As the world moves into a new era of quantum simulation it is reassuring to know that Trinity’s quantum physicists are at the forefront — programming the devices of the future.
Indistinguishable telecom band photons from a single Er ion in the solid state
by Salim Ourari, Łukasz Dusanowski, Sebastian P. Horvath, et al in Nature
Researchers have a new way to connect quantum devices over long distances, a necessary step toward allowing the technology to play a role in future communications systems.
While today’s classical data signals can get amplified across a city or an ocean, quantum signals cannot. They must be repeated in intervals — that is, stopped, copied and passed on by specialized machines called quantum repeaters. Many experts believe these quantum repeaters will play a key role in future communication networks, allowing enhanced security and enabling connections between remote quantum computers.
The Princeton study details the basis for a new approach to building quantum repeaters. It sends telecom-ready light emitted from a single ion implanted in a crystal. The effort was many years in the making, according to Jeff Thompson, the study’s principal author. The work combined advances in photonic design and materials science.
Other leading quantum repeater designs emit light in the visible spectrum, which degrades quickly over optical fiber and must be converted before traveling long distances. The new device is based on a single rare earth ion implanted in a host crystal. And because this ion emits light at an ideal infrared wavelength, it requires no such signal conversion, which can lead to simpler and more robust networks.
The device has two parts: a calcium tungstate crystal doped with just a handful of erbium ions, and a nanoscopic piece of silicon etched into a J-shaped channel. Pulsed with a special laser, the ion emits light up through the crystal. But the silicon piece, a whisp of a semiconductor stuck onto the top of the crystal, catches and guides individual photons out into the fiber optic cable.
Ideally, this photon would be encoded with information from the ion, Thompson said. Or more specifically, from a quantum property of the ion called spin. In a quantum repeater, collecting and interfering the signals from distant nodes would create entanglement between their spins, allowing end-to-end transmission of quantum states despite losses along the way.
Thompson’s team first started working with erbium ions several years before, but first versions used different crystals that harbored too much noise. In particular, this noise caused the frequency of the emitted photons to jump around randomly in a process known as spectral diffusion. This prevented the delicate quantum interference that is necessary to operate quantum networks. To solve this problem, his lab started working with Nathalie de Leon, associate professor of electrical and computer engineering, and Robert Cava, a leading solid-state materials scientist and Princeton’s Russell Wellman Moore Professor of Chemistry, to explore new materials that could host single erbium ions with much less noise.
They winnowed the list of candidate materials from hundreds of thousands down to a few hundred, then a couple dozen, then three. Each of the three finalists took half a year to test. The first material turned out to be not quite clear enough. The second caused the erbium to have poor quantum properties. But the third, the calcium tungstate, was just right.
To demonstrate that the new material is suitable for quantum networks, the researchers built an interferometer where photons randomly pass through one of two paths: a short path that is several feet long, or a long path that is 22 miles long (made of spooled optical fiber). Photons emitted from the ion can go on the long path or the short path, and about half the time, consecutive photons take opposite paths, and arrive at the output at the same time.
When such a collision occurs, quantum interference causes the photons to leave the output in pairs if and only if they are fundamentally indistinguishable — having the same shape and frequency. Otherwise, they leave the interferometer individually. By observing a strong suppression — up to 80 percent — of individual photons at the interferometer output, the team proved conclusively that the erbium ions in the new material emit indistinguishable photons. According to Salim Ourari, a graduate student who co-led the research, that puts the signal well above the hi-fi threshold.
While this work crosses an important threshold, additional work is required to improve the storage time of quantum states in the spin of the erbium ion. The team is currently working on making more highly refined calcium tungstate, with fewer impurities that disturb the quantum spin states.
Atomically precise vacancy-assembled quantum antidots
by Hanyan Fang, Harshitra Mahalingam, Xinzhe Li, Xu Han, et al in Nature Nanotechnology
National University of Singapore (NUS) scientists demonstrated a conceptual breakthrough by fabricating atomically precise quantum antidots (QAD) using self-assembled single vacancies (SVs) in a two-dimensional (2D) transition metal dichalcogenide (TMD).
Quantum dot confines electrons on a nanoscale level. In contrast, an antidot refers to a region characterised by a potential hill that repels electrons. By strategically introducing antidot patterns (“voids”) into carefully designed antidot lattices, intriguing artificial structures emerge. These structures exhibit periodic potential modulation to change 2D electron behaviour, leading to novel transport properties and unique quantum phenomena. As the trend towards miniaturised devices continue, it is important to accurately control the size and spacing of each antidot at the atomic level. This control together with resilience to environmental perturbations is crucial to address technological challenges in nanoelectronics.
A research team led by Associate Professor Jiong LU from the NUS Department of Chemistry and the NUS Institute for Functional Intelligent Materials introduced a method to fabricate a series of atomic-scale QADs with elegantly engineered quantum hole states in a 2D three-atom-layer TMD. QADs can serve as a promising new-generation candidate that can be used for applications such as quantum information technologies. This was achieved through the self-assembly of the SVs into a regular pattern. The atomic and electronic structure of the QADs is analysed using both scanning tunnelling microscopy and non-contact atomic force microscopy . This work is performed in collaboration with Assistant Professor Aleksandr RODIN’s research group from the Yale-NUS College.
A defective platinum ditelluride (PtTe2) sample containing numerous tellurium (Te) SVs was intentionally grown for this study. After thermal annealing, the Te SVs behave as “atomic Lego,” self-assembling into highly ordered vacancy-based QADs. These SVs inside QADs are spaced by a single Te atom, representing the minimal distance possible in conventional antidot lattices. When the number of SVs in QADs increases, it strengthens the cumulative repulsive potential. This leads to enhanced interference of the quasiparticles within the QADs. This, in turn, results in the creation of multi-level quantum hole states, featuring an adjustable energy gap spanning from the telecommunication to far-infrared ranges.
Due to their geometry-protected characteristics, these precisely engineered quantum hole states survived in the structure even when vacancies in QADs are occupied by oxygen after exposure to air. This exceptional robustness against environmental influences is an added advantage of this method.
Assoc Prof Lu said, “The conceptual demonstration of the fabrication of these QADs opens the door for the creation of a new class of artificial nanostructures in 2D materials with discrete quantum hole states. These structures provide an excellent platform to enable the exploration of novel quantum phenomena and the dynamics of hot electron in previously inaccessible regimes.”
Single-molecule mid-infrared spectroscopy and detection through vibrationally assisted luminescence
by Rohit Chikkaraddy, Rakesh Arul, Lukas A. Jakob, Jeremy J. Baumberg in Nature Photonics
Scientists from the University of Birmingham and the University of Cambridge have developed a new method for detecting mid-infrared (MIR) light at room temperature using quantum systems.
The research was conducted at the Cavendish Laboratory at the University of Cambridge and marks a significant breakthrough in the ability for scientists to gain insight into the working of chemical and biological molecules. In the new method using quantum systems, the team converted low-energy MIR photons into high-energy visible photons using molecular emitters. The new innovation has the capability to help scientists detect MIR and perform spectroscopy at a single-molecule level, at room temperature.
Dr Rohit Chikkaraddy, an Assistant Professor at the University of Birmingham, and lead author on the study explained: “The bonds that maintain the distance between atoms in molecules can vibrate like springs, and these vibrations resonate at very high frequencies. These springs can be excited by mid-infrared region light which is invisible to the human eye. At room temperature, these springs are in random motion which means that a major challenge in detecting mid-infrared light is avoiding this thermal noise. Modern detectors rely on cooled semiconductor devices that are energy-intensive and bulky, but our research presents a new and exicting way to detect this light at room temperature.”
The new approach is called MIR Vibrationally-Assisted Luminescence (MIRVAL), and uses molecules that have the capability of being both MIR and visible light. The team were able to assemble the molecular emitters into a very small plasmonic cavity which was resonant in both the MIR and visible ranges. They further engineered it so that the molecular vibrational states and electronic states were able to interact, resulting in an efficient transduction of MIR light into enhanced visible luminescence.
Dr Chikkaraddy continued: “The most challenging aspect was to bring together three widely different length scales — the visible wavelength which are hundreds of nanometres, molecular vibrations which are less than a nanometre, and the mid-infrared wavelengths which are ten thousand nanometres — into a single platform and combine them effectively.”
Through the creation of picocavities, incredibly small cavities that trap light and are formed by single-atom defects on the metallic facets, the researchers were able to achieve extreme light confinement volume below one cubic nanometre. This meant the team could confine MIR light all the way down to the scale of a single molecule.
This breakthrough has the ability to deepen understanding of complex systems, and opens the gateway to infrared-active molecular vibrations, which are typically inaccessible at the single-molecule level. But MIRVAL could prove beneficial in a number of fields, byeond pure scientific research.
Dr Chikkaraddy concluded: “MIRVAL could have a number of uses such as real-time gas sensing, medical diagnostics, astronomical surveys and quantum communication, as we can now see the vibrational fingerprint of individual molecules at MIR frequencies. The ability to detect MIR at room temperature means that it is that much easier to explore these applications and conduct further research in this field. Through further advancements, this novel method could not only find its way into practical devices that will shape the future of MIR technologies, but also unlock the ability to coherently manipulate the intricate interplay of ‘balls with springs’ atoms in molecular quantum systems.”
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