In quantum computing, a qubit or quantum bit is the basic unit of quantum information—the quantum version of the classical binary bit physically realized with a two-state device. A qubit is a two-state (or two-level) quantum-mechanical system, one of the simplest quantum systems displaying the peculiarity of quantum mechanics. Examples include: the spin of the electron in which the two levels can be taken as spin up and spin down; or the polarization of a single photon in which the two states can be taken to be the vertical polarization and the horizontal polarization. In a classical system, a bit would have to be in one state or the other. However, quantum mechanics allows the qubit to be in a coherent superposition of both states simultaneously, a property which is fundamental to quantum mechanics and quantum computing.
Some quantum computers and networks store their information in an electron’s spin, which can be up or down – like the zeros or ones in a conventional computer. They can also be a combination of both up and down. One way to manipulate the electron spins may be through using sound waves. The first challenge in this study was to prime the spins so they were ready. Scientists built a system with curved electrodes to concentrate the sound waves. This is like using a magnifying lens to focus a point of light. Unique x-ray microscopy instrumentation allowed researchers to directly watch atoms move in a quantum material as sound waves passed through them.
Scientist are exploring the use and appearance of phonons in nanomechanical structures in the context of the emerging field of quantum computing and quantum-enabled device applications. Light is made from photons, the quantum of light. In the same way, mechanical vibrations or sound waves can also be described in a quantum-mechanical manner. They are composed of phonons—the smallest possible units of mechanical vibration. In an ideal crystal environment, phonons may play a role analogous to photons, though they propagate with the much slower speed of sound.
Quantum Computing is implemented with various physical approaches that are predicted to complement each other rather than to compete: the photonic (in the optical frequency range, hundred THz) flying qubits will be suitable for long distance quantum state transfer, (due to very low-loss optical channels, even at room temperatures), superconducting Josephson junction (JJ) qubits and cavities (in the microwave range, MHz to GHz) will be suitable for quantum operations (due to potentially high qubit coherence times, very high qubit-cavity coupling), finally, semiconductor spinqubits will be most suitable for quantum information storage (due to very high coherence times, of order of miliseconds to seconds). Sceintists have dicovered one method to inter-couple these various physical systems, eventually using tunable and strong coupling using sound with use of phonons as mediators.
Another challenge is Communicating quantum information. It is difficult to move information stored in electron spin within a device. However, since different quantum systems represent quantum information in different ways, combining more than one type into a hybrid system could take advantage of the strengths of each one. For instance, optical photons can send quantum states across long distances. An electron’s spin state can store information, which allows scientists to expand the binary information storage system used in traditional computing.
In one study, researchers created a hybrid quantum system that acoustically drives transitions in electron spins. The experiment showed a basis for mechanical (strain) control. Scientists then developed a theoretical model from a combination of direct experimental observation and density functional theory calculations. From all of this information, they illustrated different types of mechanical strain that drive longer-lasting spins. The material studied was silicon carbide, which has been shown recently to support long-lived spin states that can be accessed optically. The results offer theoretical understanding and experimental demonstrations of controlling the spin states in silicon carbide and provide a basis for applications in long-distance quantum communication, new approaches to quantum computation, as well as potential quantum sensing with MEMS – microelectromechanical systems.
Scientists with the Institute for Molecular Engineering at the University of Chicago have made breakthroughs in the quest to develop quantum technology. In their study, they entangled two quantum bits using sound for the first time. The work brings us closer to harnessing quantum technology to make more powerful computers, ultra-sensitive sensors and secure transmissions.
The study, published April 26 in Science, shows a way to entangle two superconducting qubits using sound. A challenge for scientists and engineers as they advance quantum technology is to be able to translate quantum signals from one medium to the other. For example, microwave light is perfect for carrying quantum signals around inside chips. “But you can’t send quantum information through the air in microwaves; the signal just gets swamped,” Cleland said.
The team built a system that could translate the qubits’ microwave language into acoustic sound and have it travel across the chip—using a receiver at the other end that could do the reverse translation. It required some creative engineering: “Microwaves and acoustics are not friends, so we had to separate them onto two different materials and stack those on top of each other,” said Audrey Bienfait, a postdoctoral researcher and first author on the study. “But now that we’ve shown it is possible, it opens some interesting new possibilities for quantum sensors.”
Co-author Andrew Cleland, the John A. MacLean Sr. Professor of Molecular Engineering at the IME and UChicago-affiliated Argonne National Laboratory called it a transformative step forward to quantum communications. A leader in the development of superconducting quantum technology, he led the team that built the first “quantum machine,” demonstrating quantum performance in a mechanical resonator.
Sound waves to transmit and control Quantum information
For the last decade, scientists have been making giant leaps in their ability to build and control systems based on the bizarre rules of quantum mechanics, which describe the behavior of particles at the subatomic scale. But a challenge is getting delicate quantum systems to play well with mechanical ones—anything with moving parts—which underlie a great deal of existing technology.
In a first, scientists with the Institute for Molecular Engineering at the University of Chicago and Argonne National Laboratory have built a mechanical system—a tiny “echo chamber” for sound waves—that can be controlled at the quantum level, by connecting it to quantum circuits. Published Nov. 2018 in Nature, the breakthrough could extend the reach of quantum technology to new quantum sensors, communication and memory.
“Getting these two technologies to talk to one another is a key first step for all kinds of quantum applications,” said lead study author Andrew Cleland, the John A. MacLean Sr. Professor for Molecular Engineering Innovation and Enterprise and a senior scientist at Argonne National Laboratory. “With this approach, we’ve achieved quantum control over a mechanical system at a level well beyond what’s been done before.” In particular, Cleland said, there’s been much interest in integrating quantum and mechanical systems in order to make incredibly precise quantum sensors that could detect the tiniest of vibrations or interact with individual atoms.
Cleland’s research focuses in part on quantum electrical circuits, and he wanted to hook up one of these circuits to a device that generates surface acoustic waves—tiny sound waves that run along the surface of a block of solid material, like ripples moving across the surface of a pond. This phenomenon plays a key role in everyday devices like cell phones, garage door openers and radio receivers.
A key breakthrough was building the two systems separately, on different kinds of material, and then connecting them together. This allowed the team to optimize each component and yet still communicate with one another. Both have to be kept very, very cold—just ten thousandths of a degree above absolute zero. Scientists are excited because this gives them a platform to experiment with sound at the quantum level.
“This particular result opens the door to be able to do a lot of things with sound that you can already do with light,” Cleland said. “Sound moves 100,000 times slower than light, which gives you more time to do things. For instance, if you’re storing quantum information in a memory, it can last a lot longer stored in sound than in light.” There are a number of fundamental unanswered questions about how sound waves behave in the quantum realm, he said, and this system could give scientists a platform to address them.
The technique also could point the way toward a quantum “translator” that would allow quantum communication across any distance. The electronic atoms Cleland’s group works with can only operate and communicate at very low temperatures; quantum acoustics could allow these circuits to convert quantum information to optical signals that could then be communicated over large distances at room temperature. It’s possible an acoustic-wave setup could form the basis for such a system, known as a quantum repeater, Cleland said.
The first author was Kevin Satzinger, Ph.D.’, now with Google. Coauthors on the paper included Assoc. Prof. David Schuster and Prof. David Awschalom, as well as postdoctoral researchers Audrey Bienfait and Etienne Dumur; graduate students Youpeng Zhong, Hung-Shen Chang, Greg Peairs, Ming-Han Chou, Joel Grebel, Rhys Povey and Sam Whiteley; and undergraduates Ben November and Ivan Gutierrez (both AB’18).
A separate study in the same edition of Nature, led by Robert Schoelkopf at Yale University, also reports the creation of single-phonon excitations. Taken together, the two studies open a new avenue for storing quantum information, the authors said. The devices were fabricated in the Pritzker Nanofabrication Facility at the IME.
Researchers have invented a way for different types of quantum technology to “talk” to each other using sound.
Researchers are eyeing quantum systems, which tap the quirky behavior of the smallest particles as the key to a fundamentally new generation of atomic-scale electronics for computation and communication. But transferring information between different types of technology, such as quantum memories and quantum processors is a persistent challenge.
The new study is an important step in bringing quantum technology closer to reality. “We approached this question by asking: Can we manipulate and connect quantum states of matter with sound waves?” says senior author David Awschalom, professor with the Institute for Molecular Engineering at the University of Chicago and senior scientist at Argonne National Laboratory.
One way to run a quantum computing operation is to use “spins”—a property of an electron that can be up, down, or both. Scientists can use these like zeroes and ones in today’s binary computer programming language. But getting this information elsewhere requires a translator, and scientists thought sound waves could help. “The object is to couple the sound waves with the spins of electrons in the material,” says graduate student Samuel Whiteley, the co-first author on the paper. “But the first challenge is to get the spins to pay attention.”
So researchers built a system with curved electrodes to concentrate the sound waves, like using a magnifying lens to focus a point of light. Essentially, they used extremely bright, powerful X-rays from the lab’s giant synchrotron, the Advanced Photon Source, as a microscope to peer at the atoms inside the material as the sound waves moved through it at nearly 7,000 kilometers per second (around 4350 miles per second).
“This new method allows us to observe the atomic dynamics and structure in quantum materials at extremely small length scales,” says Awschalom. “This is one of only a few locations worldwide with the instrumentation to directly watch atoms move in a lattice as sound waves passes through them.”
It’s normally difficult to send quantum information for more than a few microns, says Whiteley—that’s the width of a single strand of spider silk. This technique could extend control across an entire chip or wafer. “The results gave us new ways to control our systems, and opens venues of research and technological applications such as quantum sensing,” says postdoctoral researcher Gary Wolfowicz, the other co-first author of the study.
Transferring quantum information using sound
A team of researchers from TU Wien and Harvard University has found a new way to transfer quantum information. They propose using tiny mechanical vibrations. The atoms are coupled via phonons—the smallest quantum mechanical units of vibrations or sound waves.
“We are testing tiny diamonds with built-in silicon atoms—these quantum systems are particularly promising,” says Professor Peter Rabl from TU Wien. “Normally, diamonds are made exclusively of carbon, but adding silicon atoms in certain places creates defects in the crystal lattice where quantum information can be stored.” These microscopic flaws in the crystal lattice can be used like tiny switches that can be toggled between a state of higher energy and a state of lower energy using microwaves.
Together with a team from Harvard University, Peter Rabl’s research group has developed a new idea to achieve the targeted coupling of these quanta within the diamond. One by one, they can be built into a tiny diamond rod measuring only a few micrometres in length, like individual pearls on a necklace. Just like a tuning fork, this rod can then be made to vibrate—however, these vibrations are so small that they can only be described using quantum theory. It is through these vibrations that the silicon atoms can form a quantum-mechanical link to each other.
“Light is made from photons, the quantum of light. In the same way, mechanical vibrations or sound waves can also be described in a quantum-mechanical manner. They are composed of phonons—the smallest possible units of mechanical vibration,” explains Peter Rabl. As the research team has now been able to show using simulation calculations, any number of these quanta can be linked together in the diamond rod via phonons. The individual silicon atoms are switched on and off using microwaves. During this process, they emit or absorb phonons. This creates a quantum entanglement of the silicon defects, thus allowing quantum information to be transferred.
Until now, it was not clear whether something like this was even possible. “Usually you would expect the phonons to be absorbed somewhere, or to come into contact with the environment and thus lose their quantum mechanical properties,” says Peter Rabl. “Phonons are the enemy of quantum information, so to speak. But with our calculations, we were able to show that, when controlled appropriately using microwaves, the phonons are, in fact, useable for technical applications.”
The main advantage of this new technology lies in its scalability. “There are many ideas for quantum systems that, in principle, can be used for technological applications. The biggest problem is that it is very difficult to connect enough of them to be able to carry out complicated computing operations,” says Peter Rabl. The new strategy of using phonons for this purpose could pave the way to a scalable quantum technology.
Quantum microphone detects the presence of phonons
A superconducting qubit can be used to reliably detect the presence of multiple phonons at the same time, US physicists have demonstrated. Patricio Arrangoiz-Arriola and colleagues at Stanford University built their “quantum microphone” using materials that minimized phonon losses, while narrowing the spectra of their qubit’s emissions to reduce uncertainties. The technology could allow for new capabilities in quantum computing, including modems that link together many quantum computers at different locations.
While the quantum properties of photons have been explored and exploited extensively, those of quantized mechanical vibrations, known as phonons, have remained much more difficult to study. Although phonons are important for explaining many properties in solid materials, the technologies required to measure and control them have faced significant challenges because – in contrast to photons – the quantized states of phonons do not have well-defined energies. Instead, they exist as collective excitations at equally spaced energies.
The most successful attempts to detect phonons so far have involved a technique named quantum acoustics, in which an artificial atom is coupled to a vibrating nanostructure. This atom can be in one of two quantum states, depending on whether or not it has absorbed a phonon. In their study, Arrangoiz-Arriola’s team devised a more sophisticated version of this setup – replacing the atom with a superconducting qubit to allow for stronger coupling with the nanostructure. Where the artificial atom would need to entirely absorb a phonon, this coupling allows the qubit to change states simply in the presence of one or more phonons.
To further improve their quantum microphone, Arrangoiz-Arriola and colleagues combined the qubit with a piezoelectric resonator, which produces a large voltage in response to mechanical deformation. This heightens the peaks of the energy spectra emitted by the qubit as it changed states. Shielding the hybrid qubit-resonator platform with a periodic crystal ensures that only the phonons produced by the nanostructure can interact with the qubit, while also minimizing losses of phonons to the surrounding environment.
The physicists then excited phonons through resonant vibrations of the nanostructures, and probed the peak positions of the qubit’s resulting transition spectra – which shifted to different degrees depending on the number of phonons present. They observed energy shifts around five times larger than the linewidths of each peak, revealing the presence of up to three phonons with a high degree of certainty.
In future studies, Arrangoiz-Arriola’s team hope to improve their setup to reveal phonon numbers without changing them, allowing for repeated measurements. Further developments could allow the quantum microphone to provide a basis for quantum modems, potentially creating networks of quantum computers in a variety of locations, and could also inform designs for novel architectures for quantum computers themselves. The full results are reported in Nature