TL;DR
- In the last few decades, physicists have found ways to super-cool objects so that their atoms are at a near standstill, wrestling small objects such as clouds of millions of atoms, or nanogram-scale objects, into such pure quantum states. Now scientists have cooled a large, human-scale object to close to its motional ground state. The object isn’t tangible in the sense of being situated at one location, but is the combined motion of four separate objects. The ‘object’ that the researchers cooled has an estimated mass of about 10 kilograms, and comprises nearly 1 octillion atoms.
- Researchers have developed a new technique that keeps quantum bits of light stable at room temperature instead of only working at -270 degrees. Their discovery saves power and money and is a breakthrough in quantum research.
- Quantum researchers have developed a method to make previously hardly accessible properties in quantum systems measurable. The new method for determining the quantum state in quantum simulators reduces the number of necessary measurements and makes work with quantum simulators much more efficient.
- Researchers demonstrate a new high-flux and compact cold-atom source with low power consumption that can be a key component of many quantum technologies.
- Scientists from the University of Vienna, the Austrian Academy of Sciences and the Perimeter Institute report in the latest issue of Physical Review Letters that nonlocality is a universal property of the world, regardless of how and at what speed quantum particles move.
- Research from the McKelvey School of Engineering at Washington University in St. Louis has found a missing piece in the puzzle of optical quantum computing.
- Quantum computers have been one-of-a-kind devices that fill entire laboratories. Now, physicists have built a prototype of an ion trap quantum computer that can be used in industry. It fits into two 19-inch server racks like those found in data centers throughout the world. The compact, self-sustained device demonstrates how this technology will soon be more accessible.
- An international team of physicists led by the University of Minnesota has discovered that a unique superconducting metal is more resilient when used as a very thin layer. The research is the first step toward a larger goal of understanding unconventional superconducting states in materials, which could possibly be used in quantum computing in the future.
- A secure quantum link has been created over a distance of 511 kilometres between two Chinese cities by using a relay in the middle that doesn’t have to be trusted. This could help extend secure quantum networks.
- 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
Approaching the motional ground state of a 10-kg object
by Whittle et al. in Science
To the human eye, most stationary objects appear to be just that — still, and completely at rest. Yet if we were handed a quantum lens, allowing us to see objects at the scale of individual atoms, what was an apple sitting idly on our desk would appear as a teeming collection of vibrating particles, very much in motion.
In the last few decades, physicists have found ways to super-cool objects so that their atoms are at a near standstill, or in their “motional ground state.” To date, physicists have wrestled small objects such as clouds of millions of atoms, or nanogram-scale objects, into such pure quantum states.
Now for the first time, scientists at MIT and elsewhere have cooled a large, human-scale object to close to its motional ground state. The object isn’t tangible in the sense of being situated at one location, but is the combined motion of four separate objects, each weighing about 40 kilograms. The “object” that the researchers cooled has an estimated mass of about 10 kilograms, and comprises about 1x1026, or nearly 1 octillion, atoms.
The researchers took advantage of the ability of the Laser Interfrometer Gravitational-wave Observatory (LIGO) to measure the motion of the masses with extreme precision and super-cool the collective motion of the masses to 77 nanokelvins, just shy of the object’s predicted ground state of 10 nanokelvins.
Their results, appearing in Science, represent the largest object to be cooled to close to its motional ground state. The scientists say they now have a chance to observe the effect of gravity on a massive quantum object.
“Nobody has ever observed how gravity acts on massive quantum states,” says Vivishek Sudhir, assistant professor of mechanical engineering at MIT, who directed the project. “We’ve demonstrated how to prepare kilogram-scale objects in quantum states. This finally opens the door to an experimental study of how gravity might affect large quantum objects, something hitherto only dreamed of.”
The study’s authors are members of the LIGO Laboratory, and include lead author and graduate student Chris Whittle, postdoc Evan Hall, research scientist Sheila Dwyer, Dean of the School of Science and the Curtis and Kathleen Marble Professor of Astrophysics Nergis Mavalvala, and assistant professor of mechanical engineering Vivishek Sudhir.
All objects embody some sort of motion as a result of the many interactions that atoms have, with each other and from external influences. All this random motion is reflected in an object’s temperature. When an object is cooled down close to zero temperature, it still has a residual quantum motion, a state called the “motional ground state.”
To stop an object in its tracks, one can exert upon it an equal and opposite force. (Think of stopping a baseball in mid-flight with the force of your glove.) If scientists can precisely measure the magnitude and direction of an atom’s movements, they can apply counteracting forces to bring down its temperature — a technique known as feedback cooling.
Physicists have applied feedback cooling through various means, including laser light, to bring individual atoms and ultralight objects to their quantum ground states, and have attempted to super-cool progressively larger objects, to study quantum effects in bigger, traditionally classical systems.
“The fact that something has temperature is a reflection of the idea that it interacts with stuff around it,” Sudhir says. “And it’s harder to isolate larger objects from all the things happening around them.”
To cool the atoms of a large object to near ground state, one would first have to measure their motion with extreme precision, to know the degree of pushback required to stop this motion. Few instruments in the world can reach such precision. LIGO, as it happens, can.
The gravitational-wave-detecting observatory comprises twin interferometers in separate U.S. locations. Each interferometer has two long tunnels connected in an L-shape, and stretching 4 kilometers in either direction. At either end of each tunnel is a 40-kilogram mirror suspended by thin fibers, that swings like a pendulum in response to any disturbance such as an incoming gravitational wave. A laser at the tunnels’ nexus is split and sent down each tunnel, then reflected back to its source. The timing of the return lasers tells scientists precisely how much each mirror moved, to an accuracy of 1/10,000 the width of a proton.
Sudhir and his colleagues wondered whether they could use LIGO’s motion-measuring precision to first measure the motion of large, human-scale objects, then apply a counteracting force, opposite to what they measure, to bring the objects to their ground state.
The object they aimed to cool is not an individual mirror, but rather the combined motion of all four of LIGO’s mirrors.
“LIGO is designed to measure the joint motion of the four 40-kilogram mirrors,” Sudhir explains. “It turns out you can map the joint motion of these masses mathematically, and think of them as the motion of a single 10-kilogram object.”
When measuring the motion of atoms and other quantum effects, Sudhir says, the very act of measuring can randomly kick the mirror and put it in motion — a quantum effect called “measurement back-action.” As individual photons of a laser bounce off a mirror to gather information about its motion, the photon’s momentum pushes back on the mirror. Sudhir and his colleagues realized that if the mirrors are continuously measured, as they are in LIGO, the random recoil from past photons can be observed in the information carried by later photons.
Armed with a complete record of both quantum and classical disturbances on each mirror, the researchers applied an equal and opposite force with electromagnets attached to the back of each mirror. The effect pulled the collective motion to a near standstill, leaving the mirrors with so little energy that they moved no more than 10–20 meters, less than one-thousandth the size of a proton.
The team then equated the object’s remaining energy, or motion, with temperature, and found the object was sitting at 77 nanokelvins, very close to its motional ground state, which they predict to be 10 nanokelvins.
“This is comparable to the temperature atomic physicists cool their atoms to get to their ground state, and that’s with a small cloud of maybe a million atoms, weighing picograms,” Sudhir says. “So, it’s remarkable that you can cool something so much heavier, to the same temperature.”
“Preparing something in the ground state is often the first step to putting it into exciting or exotic quantum states,” Whittle says. “So this work is exciting because it might let us study some of these other states, on a mass scale that’s never been done before.”
High-flux, adjustable, compact cold-atom source
by Sean Ravenhall, Benjamin Yuen, Chris Foot in Optics Express
Although quantum technology has proven valuable for highly precise timekeeping, making these technologies practical for use in a variety of environments is still a key challenge. In an important step toward portable quantum devices, researchers have developed a new high-flux and compact cold-atom source with low power consumption that can be a key component of many quantum technologies.
“The use of quantum technologies based on laser-cooled atoms has already led to the development of atomic clocks that are used for timekeeping on a national level,” said research team leader Christopher Foot from Oxford University in the U.K. “Precise clocks have many applications in the synchronization of electronic communications and navigation systems such as GPS. Compact atomic clocks that can be deployed more widely, including in space, provide resilience in communications networks because local clocks can maintain accurate timekeeping even if there is a network disruption.”
In The Optical Society (OSA) journal Optics Express, S. Ravenhall, B. Yuen and Foot describe work carried out in Oxford, U.K. to demonstrate a completely new design for a cold atom source. The new device is suitable for a wide range of cold-atom technologies.
“In this project we took a design we made for research purposes and developed it into a compact device,” said Foot. “In addition to timekeeping applications, compact cold-atom devices can also be used for instruments for gravity mapping, inertial navigation and communications and to study physical phenomena in research applications such as dark matter and gravitational waves.”
Although it may seem counterintuitive, laser light can be used to cool atoms to extremely low temperatures by exerting a force that slows the atoms down. This process can be used to create a cold-atom source that generates a beam of laser-cooled atoms directed toward a region where precision measurements for timekeeping or detecting gravitational waves, for example, are carried out.
Laser cooling usually requires a complicated arrangement of mirrors to shine light onto atoms in a vacuum from all directions. In the new work, the researchers created a completely different design that uses just four mirrors. These mirrors are arranged like a pyramid and placed in a way that allows them to slide past each other like the petals of a flower to create a hole at the top of the pyramid through which the cold atoms are pushed out. The size of this hole can be adjusted to optimize the flow of cold atoms for various applications. The pyramid arrangement reflects the light from a single incoming laser beam that enters the vacuum chamber through a single viewport, thus greatly simplifying the optics.
The mirrors, which are located inside the vacuum region of the cold-atom source, were created by polishing metal and applying a dielectric coating. “The adjustability of this design is an entirely new feature,” said Foot. “Creating a pyramid from four identical polished metal blocks simplifies the assembly, and it can be used without the adjustment mechanism.”
To test their new cold-atom source design, the researchers constructed laboratory equipment to fully characterize the flux of atoms emitted through a hole at the apex of the pyramid.
“We demonstrated an exceptionally high flux of rubidium atoms,” said Foot. “Most cold-atom devices take measurements that improve with the number of atoms used. Sources with a higher flux can thus be used to improve measurement accuracy, boost the signal-to-noise ratio or help achieve larger measurement bandwidths.”
The researchers say that the new source is suitable for commercial application. Because it features a small number of components and few assembly steps, scaling up production to produce multiple copies would be straightforward.
Room-temperature single-photon source with near-millisecond built-in memory
by Karsten B. Dideriksen, Rebecca Schmieg, Michael Zugenmaier, Eugene S. Polzik in Nature Communications
As almost all our private information is digitalized, it is increasingly important that we find ways to protect our data and ourselves from being hacked.
Quantum Cryptography is the researchers’ answer to this problem, and more specifically a certain kind of qubit — consisting of single photons: particles of light. Single photons or qubits of light, as they are also called, are extremely difficult to hack. However, in order for these qubits of light to be stable and work properly they need to be stored at temperatures close to absolute zero — that is minus 270 C — something that requires huge amounts of power and resources. Yet in a recently published study, researchers from University of Copenhagen, demonstrate a new way to store these qubits at room temperature for a hundred times longer than ever shown before.
“We have developed a special coating for our memory chips that helps the quantum bits of light to be identical and stable while being in room temperature. In addition, our new method enables us to store the qubits for a much longer time, which is milliseconds instead of microseconds — something that has not been possible before. We are really excited about it,” says Eugene Simon Polzik, professor in quantum optics at the Niels Bohr Institute.
The special coating of the memory chips makes it much easier to store the qubits of light without big freezers, which are troublesome to operate and require a lot of power.
Therefore, the new invention will be cheaper and more compatible with the demands of the industry in the future.
“The advantage of storing these qubits at room temperature is that it does not require liquid helium or complex laser-systems for cooling. Also it is a much more simple technology that can be implemented more easily in a future quantum internet,” says Karsten Dideriksen, a UCPH-PhD on the project.
Normally warm temperatures disturb the energy of each quantum bit of light.
“In our memory chips, thousands of atoms are flying around emitting photons also known as qubits of light. When the atoms are exposed to heat, they start moving faster and collide with one another and with the walls of the chip. This leads them to emit photons that are very different from each other. But we need them to be exactly the same in order to use them for safe communication in the future,” explains Eugene Polzik and adds:
“That is why we have developed a method that protects the atomic memory with the special coating for the inside of the memory chips. The coating consists of paraffin that has a wax like structure and it works by softening the collision of the atoms, making the emitted photons or qubits identical and stable. Also we used special filters to make sure that only identical photons were extracted from the memory chips.”
Even though the new discovery is a breakthrough in quantum research, it stills needs more work.
“Right now we produce the qubits of light at a low rate — one photon per second, while cooled systems can produce millions in the same amount of time. But we believe there are important advantages to this new technology and that we can overcome this challenge in time,” Eugene concludes.
Relativistic Bell Test within Quantum Reference Frames
by Lucas F. Streiter, Flaminia Giacomini, Časlav Brukner in Physical Review Letters
The phenomenon of quantum nonlocality defies our everyday intuition. It shows the strong correlations between several quantum particles, some of which change their state instantaneously when the others are measured, regardless of the distance between them. While this phenomenon has been confirmed for slow moving particles, it has been debated whether nonlocality is preserved when particles move very fast at velocities close to the speed of light, and even more so when those velocities are quantum mechanically indefinite. Now, researchers from the University of Vienna, the Austrian Academy of Sciences and the Perimeter Institute report in the latest issue of Physical Review Letters that nonlocality is a universal property of the world, regardless of how and at what speed quantum particles move.
It is easy to illustrate how correlations can arise in everyday life. Imagine that each day of the month you send two of your friends, Alice and Bob, a toy engine of a set of two for their collection. You can choose each of the engines to be either red or blue or either electric or steam. Your friends are separated by a large distance and do not know about your choice. Once their parcels arrive, they can check the colour of their engine with a device that can distinguish between red and blue or check whether the engine is electric or steam using another device. They compare the measurements made over time to look for particular correlations. In our everyday world, such correlations obey two principles — “realism” and “locality.” “Realism” means that Alice and Bob reveal only what colour or the mechanism of the engine you had chosen in the past, and “locality” means that Alice’s measurement cannot change the colour or the mechanism of Bob’s engine (or vice versa). Bell’s theorem, published in 1964 and considered by some to be one of the most profound discoveries in the foundations of physics, showed that correlations in the quantum world are incompatible with the two principles — a phenomenon known as quantum non-locality.
Quantum nonlocality has been confirmed in numerous experiments, the so-called Bell tests, on atoms, ions and electrons. It not only has deep philosophical implications, but also underpins many of the applications such as quantum computation and quantum satellite communications. However, in all of these experiments, the particles were either at rest or moving at low velocities (scientists call this regime “non-relativistic”). One of the unsolved problems in this field, which still puzzles physicists, is whether nonlocality is preserved when particles are moving extremely fast, close to the speed of light (i.e., in the relativistic regime), or when they are not even moving at a well-defined speed.
For two quantum particles in a Bell’s test which move at high speeds researchers predict that the correlations between the particles are, in principle, reduced. However, if Alice and Bob adapt their measurements in a way that depends on the speed of the particles the correlations between the results of their measurements are still nonlocal. Now imagine that not only are the particles moving very fast, but their velocity is also indefinite: each particle moves in a so-called superposition of different velocities simultaneously, just as the infamous Schrödinger’s cat is simultaneously dead and alive. In such a case, is their description of the world still non-local?
Researchers, led by Brukner at the University of Vienna and the Austrian Academy of Sciences, have shown that Alice and Bob can indeed design an experiment which would prove that the world is nonlocal. For this they used one of the most fundamental principles of physics namely that physical phenomena do not depend on the frame of reference from which we observe them. For example, according to this principle, any observer, whether moving or not, will see that an apple falling from a tree will touch the ground. The researchers went a step further and extended this principle to reference frames “attached” to quantum particles. These are called “quantum reference frames.” The key insight is that if Alice and Bob could move with the quantum reference frames along with their respective particles, they could perform the usual Bell test, since for them the particles would be at rest. In this way, they can prove quantum nonlocality for any quantum particle, regardless of whether the velocity is indefinite or close to that of light.
Flaminia Giacomini, one of the study’s authors, says, “Our result proves that it is possible to design a Bell experiment for particles moving in a quantum superposition at very high speeds.” The co-author, Lucas Streiter, concludes, “We have shown that nonlocality is a universal property of our world.” Their discovery is expected to open applications in quantum technologies, such as quantum satellite communications and quantum computation, using relativistic particles.
Two-photon controlled-phase gates enabled by photonic dimers
by Zihao Chen, Yao Zhou, Jung-Tsung Shen, Pei-Cheng Ku, Duncan Steel in Physical Review A
Research from the McKelvey School of Engineering at Washington University in St. Louis has found a missing piece in the puzzle of optical quantum computing.
Jung-Tsung Shen, associate professor in the Preston M. Green Department of Electrical & Systems Engineering, has developed a deterministic, high-fidelity two-bit quantum logic gate that takes advantage of a new form of light. This new logic gate is orders of magnitude more efficient than the current technology.
“In the ideal case, the fidelity can be as high as 97%,” Shen said.
The potential of quantum computers is bound to the unusual properties of superposition — the ability of a quantum system to contain many distinct properties, or states, at the same time — and entanglement — two particles acting as if they are correlated in a non-classical manner, despite being physically removed from each other.
Where voltage determines the value of a bit (a 1 or a 0) in a classical computer, researchers often use individual electrons as “qubits,” the quantum equivalent. Electrons have several traits that suit them well to the task: they are easily manipulated by an electric or magnetic field and they interact with each other. Interaction is a benefit when you need two bits to be entangled — letting the wilderness of quantum mechanics manifest.
But their propensity to interact is also a problem. Everything from stray magnetic fields to power lines can influence electrons, making them hard to truly control.
For the past two decades, however, some scientists have been trying to use photons as qubits instead of electrons. “If computers are going to have a true impact, we need to look into creating the platform using light,” Shen said.
Photons have no charge, which can lead to the opposite problems: they do not interact with the environment like electrons, but they also do not interact with each other. It has also been challenging to engineer and to create ad hoc (effective) inter-photon interactions. Or so traditional thinking went.
Less than a decade ago, scientists working on this problem discovered that, even if they weren’t entangled as they entered a logic gate, the act of measuring the two photons when they exited led them to behave as if they had been. The unique features of measurement are another wild manifestation of quantum mechanics.
Schematic diagram of a chiral two-level system (TLS). The chiral photon-quantum dot interaction can be induced via either the Zeeman splitting by a magnetic field, or by selective placement of the quantum dot at a chiral point in the reciprocal waveguide.
“Quantum mechanics is not difficult, but it’s full of surprises,” Shen said.
The measurement discovery was groundbreaking, but not quite game-changing. That’s because for every 1,000,000 photons, only one pair became entangled. Researchers have since been more successful, but, Shen said, “It’s still not good enough for a computer,” which has to carry out millions to billions of operations per second.
Shen was able to build a two-bit quantum logic gate with such efficiency because of the discovery of a new class of quantum photonic states — photonic dimers, photons entangled in both space and frequency. His prediction of their existence was experimentally validated in 2013, and he has since been finding applications for this new form of light.
When a single photon enters a logic gate, nothing notable happens — it goes in and comes out. But when there are two photons, “That’s when we predicted the two can make a new state, photonic dimers. It turns out this new state is crucial.”
High-fidelity, two-bit logic gate, designed by Jung-Tsung Shen.
Mathematically, there are many ways to design a logic gate for two-bit operations. These different designs are called equivalent. The specific logic gate that Shen and his research group designed is the controlled-phase gate (or controlled-Z gate). The principal function of the controlled-phase gate is that the two photons that come out are in the negative state of the two photons that went in.
“In classical circuits, there is no minus sign,” Shen said. “But in quantum computing, it turns out the minus sign exists and is crucial.”
When two independent photons (representing two optical qubits) enter the logic gate, “The design of the logic gate is such that the two photons can form a photonic dimer,” Shen said. “It turns out the new quantum photonic state is crucial as it enables the output state to have the correct sign that is essential to the optical logic operations.”
Shen has been working with the University of Michigan to test his design, which is a solid-state logic gate — one that can operate under moderate conditions. So far, he says, results seem positive.
Shen says this result, while baffling to most, is clear as day to those in the know.
“It’s like a puzzle,” he said. “It may be complicated to do, but once it’s done, just by glancing at it, you will know it’s correct.”
Entanglement Hamiltonian tomography in quantum simulation
by Christian Kokail, Rick van Bijnen, Andreas Elben, Benoît Vermersch, Peter Zoller in Nature Physics
Researchers have developed a method to make previously hardly accessible properties in quantum systems measurable. The new method for determining the quantum state in quantum simulators reduces the number of necessary measurements and makes work with quantum simulators much more efficient.
In a few years, a new generation of quantum simulators could provide insights that would not be possible using simulations on conventional supercomputers. Quantum simulators are capable of processing a great amount of information since they quantum mechanically superimpose an enormously large number of bit states. For this reason, however, it also proves difficult to read this information out of the quantum simulator. In order to be able to reconstruct the quantum state, a very large number of individual measurements are necessary. The method used to read out the quantum state of a quantum simulator is called quantum state tomography. “Each measurement provides a ‘cross-sectional image’ of the quantum state. You then put these cross-sectional images together to form the complete quantum state,” explains theoretical physicist Christian Kokail from Peter Zoller’s team at the Institute of Quantum Optics and Quantum Information at the Austrian Academy of Sciences and the Department of Experimental Physics at the University of Innsbruck. The number of measurements needed in the lab increases very rapidly with the size of the system. “The number of measurements grows exponentially with the number of qubits,” the physicist says. The Innsbruck researchers have now succeeded in developing a much more efficient method for quantum simulators.
Insights from quantum field theory allow quantum state tomography to be much more efficient, i.e., to be performed with significantly fewer measurements. “The fascinating thing is that it was not at all clear from the outset that the predictions from quantum field theory could be applied to our quantum simulation experiments,” says theoretical physicist Rick van Bijnen. “Studying older scientific papers from this field happened to lead us down this track.” Quantum field theory provides the basic framework of the quantum state in the quantum simulator. Only a few measurements are then needed to fit the details into this basic framework. Based on this, the Innsbruck researchers have developed a measurement protocol by which tomography of the quantum state becomes possible with a drastically reduced number of measurements. At the same time, the new method allows new insights into the structure of the quantum state to be obtained. The physicists tested the new method with experimental data from an ion trap quantum simulator of the Innsbruck research group led by Rainer Blatt and Christian Roos. “In the process, we were now able to measure properties of the quantum state that were previously not observable in this quality,” Kokail recounts.
A verification protocol developed by the group together with Andreas Elben and Benoit Vermersch two years ago can be used to check whether the structure of the quantum state actually matches the expectations from quantum field theory. “We can use further random measurements to check whether the basic framework for tomography that we developed based on the theory actually fits or is completely wrong,” explains Christian Kokail. The protocol raises a red flag if the framework does not fit. Of course, this would also be an interesting finding for the physicists, because it would possibly provide clues for the not yet fully understood relationship with quantum field theory. At the moment, the physicists around Peter Zoller are developing quantum protocols in which the basic framework of the quantum state is not stored on a classical computer, but is realized directly on the quantum simulator.
Compact Ion-Trap Quantum Computing Demonstrator
by I. Pogorelov, T. Feldker, Ch. D. Marciniak, L. Postler, G. Jacob, O. Krieglsteiner, V. Podlesnic, M. Meth, V. Negnevitsky, M. Stadler, B. Höfer, C. Wächter, K. Lakhmanskiy, R. Blatt, P. Schindler, T. Monz in PRX Quantum
So far, quantum computers have been one-of-a-kind devices that fill entire laboratories. Now, physicists at the University of Innsbruck have built a prototype of an ion trap quantum computer that can be used in industry. It fits into two 19-inch server racks like those found in data centers throughout the world.
Over the past three decades, fundamental groundwork for building quantum computers has been pioneered at the University of Innsbruck, Austria. As part of the EU Flagship Quantum Technologies, researchers at the Department of Experimental Physics in Innsbruck have now built a demonstrator for a compact ion trap quantum computer. “Our quantum computing experiments usually fill 30- to 50-square-meter laboratories,” says Thomas Monz of the University of Innsbruck. “We were now looking to fit the technologies developed here in Innsbruck into the smallest possible space while meeting standards commonly used in industry.” The new device aims to show that quantum computers will soon be ready for use in data centers. “We were able to show that compactness does not have to come at the expense of functionality,” adds Christian Marciniak from the Innsbruck team.
The individual building blocks of the world’s first compact quantum computer had to be significantly reduced in size. For example, the centerpiece of the quantum computer, the ion trap installed in a vacuum chamber, takes up only a fraction of the space previously required. It was provided to the researchers by Alpine Quantum Technologies (AQT), a spin-off of the University of Innsbruck and the Austrian Academy of Sciences which aims to build a commercial quantum computer. Other components were contributed by the Fraunhofer Institute for Applied Optics and Precision Engineering in Jena and laser specialist TOPTICA Photonics in Munich, Germany.
The compact quantum computer can be operated autonomously and will soon be programmable online. A particular challenge was to ensure the stability of the quantum computer. Quantum devices are very sensitive and in the laboratory they are protected from external disturbances with the help of elaborate measures. Amazingly, the Innsbruck team succeeded in applying this quality standard to the compact device as well, thus ensuring safe and uninterrupted operation.
In addition to stability, a decisive factor for the industrial use of a quantum computer is the number of available quantum bits. Thus, in its recent funding campaign, the German government has set the goal of initially building demonstration quantum computers that have 24 fully functional qubits. The Innsbruck quantum physicists have already achieved this goal. They were able to individually control and successfully entangle up to 24 ions with the new device. “By next year, we want to be able to provide a device with up to 50 individually controllable quantum bits,” says Thomas Monz, already looking to the future.
Two-fold symmetric superconductivity in few-layer NbSe2
by Alex Hamill, Brett Heischmidt, Egon Sohn, Daniel Shaffer, Kan-Ting Tsai, Xi Zhang, Xiaoxiang Xi, Alexey Suslov, Helmuth Berger, László Forró, Fiona J. Burnell, Jie Shan, Kin Fai Mak, Rafael M. Fernandes, Ke Wang, Vlad S. Pribiag in Nature Physics
An international team of physicists led by the University of Minnesota has discovered that a unique superconducting metal is more resilient when used as a very thin layer. The research is the first step toward a larger goal of understanding unconventional superconducting states in materials, which could possibly be used in quantum computing in the future.
The collaboration includes four faculty members in the University of Minnesota’s School of Physics and Astronomy — Associate Professor Vlad Pribiag, Professor Rafael Fernandes, and Assistant Professors Fiona Burnell and Ke Wang — along with physicists at Cornell University and several other institutions.
Niobium diselenide (NbSe2) is a superconducting metal, meaning that it can conduct electricity, or transport electrons from one atom to another, with no resistance. It is not uncommon for materials to behave differently when they are at a very small size, but NbSe2 has potentially beneficial properties. The researchers found that the material in 2D form (a very thin substrate only a few atomic layers thick) is a more resilient superconductor because it has a two-fold symmetry, which is very different from thicker samples of the same material.
Motivated by Fernandes and Burnell’s theoretical prediction of exotic superconductivity in this 2D material, Pribiag and Wang started to investigate atomically-thin 2D superconducting devices.
“We expected it to have a six-fold rotational pattern, like a snowflake.” Wang said. “Despite the six-fold structure, it only showed two-fold behavior in the experiment.”
“This was one of the first times [this phenomenon] was seen in a real material,” Pribiag said.
The researchers attributed the newly-discovered two-fold rotational symmetry of the superconducting state in NbSe2 to the mixing between two closely competing types of superconductivity, namely the conventional s-wave type — typical of bulk NbSe2 — and an unconventional d- or p-type mechanism that emerges in few-layer NbSe2. The two types of superconductivity have very similar energies in this system. Because of this, they interact and compete with each other.
Pribiag and Wang said they later became aware that physicists at Cornell University were reviewing the same physics using a different experimental technique, namely quantum tunneling measurements. They decided to combine their results with the Cornell research and publish a comprehensive study.
Burnell, Pribiag, and Wang plan to build on these initial results to further investigate the properties of atomically thin NbSe2 in combination with other exotic 2D materials, which could ultimately lead to the use of unconventional superconducting states, such as topological superconductivity, to build quantum computers.
“What we want is a completely flat interface on the atomic scale,” Pribiag said. “We believe this system will be able to give us a better platform to study materials to use them for quantum computing applications.”
MISC
- Quantum computers have reached the mainstream:
A secure quantum link has been created over a distance of 511 kilometres between two Chinese cities by using a relay in the middle that doesn’t have to be trusted. This could help extend secure quantum networks.
When a pair of photons are quantum entangled, you can instantly deduce the state of one by measuring the other, regardless of the distance separating them. This is the basis of quantum encryption — using entangled particles to create secure keys and ensure that messages are secret.
Previous research has created entangled pairs of photons and transmitted one to a receiver, creating a link that can establish a quantum key. But Qiang Zhang at the University of Science and Technology of China and his colleagues have extended the maximum distance of a quantum key distribution link through a cable by using an intermediate step that doesn’t read the data, but only checks if it matches what was sent by the other end.
Lasers at both ends of a fibre-optic cable send photons towards each other. These particles of light are in random phases, the pattern of peaks and troughs in their movement. When a pair of photons with matching phase meet in the middle hub, the system alerts both the sender and the receiver via a traditional data link.
Because each end knows what it transmitted and whether it matched the phase of the other, they can exchange a quantum key that can be used to encrypt data sent over traditional networks. Crucially, the central hub doesn’t know what was sent, only whether the two signals matched.
A recent experiment by Toshiba Europe in Cambridge, UK, demonstrated a link of 600 kilometres using the same technology, but the apparatus was all housed in a single lab. The Chinese team used a fibre-optic connection 511 kilometres long strung between the cities of Jinan and Qingdao, with a central receiver based in Mazhan.
Zhang says there is a healthy competition between the two labs to extend each other’s distance records. “In the lab, you have an air conditioner, but in the field when the temperature changes you will observe the photon phase drift off,” he says.
“To turn something that works in a lab into something that works in the field, I think they do a good job,” says Peter Kruger at the University of Sussex, UK. “In the lab, nobody’s allowed to talk because it ruins the experiment and clearly in the field you can’t control that. Single photons over hundreds of kilometres is quite remarkable.”
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