Research promises to accelerate the use of silicon technology in quantum computing
Research by physicists at Princeton University is paving the way for the use of silicon-based technologies in quantum computing, particularly in the form of quantum bits – the basic units of quantum computers. This research promises to accelerate the use of silicon technology as a viable alternative to other quantum computing technologies, such as superconductors or trapped ions.
In research published in the journal Scientists progress, Princeton physicists used a two-qubit silicon quantum device to achieve an unprecedented level of fidelity. At over 99%, this is the highest fidelity achieved to date for a two-qubit gate in a semiconductor and is comparable to the best results achieved by competing technologies. Fidelity, which is a measure of a qubit’s ability to perform error-free operations, is key in the quest to develop practical and efficient quantum computing.
Researchers around the world are trying to determine which technologies -; such as superconducting qubits, trapped ions or silicon spin qubits, for example -; can best be used as basic units of quantum computing. And, just as important, researchers are exploring which technologies will have the ability to scale most effectively for commercial use.
“Silicon spin qubits are gaining momentum [in the field]”, said Adam Mills, a graduate student in Princeton University’s Department of Physics and lead author of the recently published study. “It looks like a big year for silicon as a whole.”
Using a silicon device called a double quantum dot, the Princeton researchers were able to capture two electrons and force them to interact. The spin state of each electron can be used as a qubit and the interaction between electrons can entangle these qubits. This operation is crucial for quantum computing and the research team, led by Jason Petta, Professor of Physics Eugene Higgins at Princeton, was able to perform this entanglement operation at over 99.8% fidelity.
A qubit, in simple terms, is a quantum version of a computer bit, which is the smallest unit of data in a computer. Like its classical counterpart, the qubit is encoded with information that can have the value of one or zero. But unlike the bit, the qubit is able to harness the concepts of quantum mechanics to be able to perform tasks that classical bits cannot.
“In a qubit, you can encode zeros and ones, but you can also have superpositions of those zeros and ones,” said Mills. This means that each qubit can simultaneously be a zero and a one. This concept, called superposition, is a fundamental quality of quantum mechanics and allows qubits to perform operations that seem amazing and otherworldly. Concretely, this gives the quantum computer a greater advantage over conventional computers, for example by factoring very large numbers or isolating the most optimal solution to a problem.
The “spin” in spin qubits is the angular momentum of the electron. It is a quantum property that manifests as a tiny magnetic dipole that can be used to encode information. A classic analogue is a compass needle, which has north and south poles, and rotates to align with the earth’s magnetic field. In quantum mechanics, the spin of the electron can align with the magnetic field generated in the laboratory (spin-up), or be oriented anti-parallel to the field (spin-down), or be in a quantum superposition of spin- up and spin-down. Spin is the property of the electron exploited in silicon-based quantum devices; conventional computers, on the other hand, work by manipulating the negative charge of an electron.
Mills asserted that in general, silicon spin qubits have advantages over other types of qubits. “The idea is that each system will need to scale up to many qubits,” he said. “And right now other qubit systems have real physical limits to scalability. Size could be a real issue with those systems. There’s only so much space you can cram these things into. .”
By comparison, silicon spin qubits are made from single electrons and are extremely small.
“Our devices are about 100 nanometers in diameter, whereas a conventional superconducting qubit is more like 300 microns in diameter, so if you want to fabricate several on one chip, it will be difficult to use a superconducting approach,” Petta said.
The other advantage of silicon spin qubits, Petta added, is that conventional electronics today are based on silicon technology. “Our feeling is that if you really want to create a million or ten million qubits that are going to be needed to do something practical, that’s only going to happen in a solid-state system that can be scaled up using the ‘standard semiconductor manufacturing industry.’
Yet the operating spin qubits -; like other types of qubits – ; with high fidelity has been a challenge for researchers.
“One of the bottlenecks in spin qubit technology is that the fidelity of the two-qubit gate until very recently wasn’t that high,” Petta said. “It’s been well below 90% in most experiments.”
But it was a challenge that Petta and Mills and the research team thought they could overcome.
To perform the experiment, the researchers first had to capture a single electron -; no small task.
“We trap a single electron, a very small particle, and we have to make it enter a specific region of space and then make it dance,” Petta said.
To do this, Mills, Petta and their colleagues had to build a “cage”. This took the form of an ultra-thin semiconductor made mostly of silicon. In addition to this, the team modeled small electrodes, which create the electrostatic potential used to corral the electron. Two of these cages joined together, each separated by a barrier, or door, constituted the double quantum box.
“We have two spinners sitting on adjacent sites next to each other,” Petta said. “By adjusting the voltage on these grids, we can momentarily bring the electrons together and make them interact. This is called a two-qubit gate.”
The interaction causes each spin qubit to evolve according to the state of its neighboring spin qubits, which leads to entanglement in quantum systems. The researchers were able to perform this two-qubit interaction with greater than 99% fidelity. To date, this is the highest fidelity for a two-qubit gate that has so far been achieved in spin qubits.
Petta said the results of this experiment place this technology -; silicon spin qubits -; on a par with the best results achieved by other major competing technologies. “This technology is on a steeply increasing slope,” he said, “and I think it’s only a matter of time before it overtakes superconducting systems.”
“Another important aspect of this article,” Petta added, “is not just a demonstration of a high-fidelity two-qubit gate, but this device does it all. This is the first demonstration of a solid-state spin qubit system where we We’ve integrated system-wide performance – state readiness, readout, single-qubit control, two-qubit control; all with performance metrics that exceed the threshold you need to run a larger scale system.”
In addition to Mills and Petta, the work also included the efforts of Princeton graduate students Charles Guinn and Mayer Feldman, as well as University of Pennsylvania assistant professor of electrical engineering Anthony Sigillito. Michael Gullans, Department of Physics, Princeton University and Center for Quantum Information and Computer Science, NIST/University of Maryland, and Erik Nielsen, Sandia National Laboratories, Albuquerque, New Mexico, also contributed to the article. and looking.
The research was sponsored by Army Research Office Grant W911NF-15-1-0149 (to ARM, AJS, CRG, MJG, and JRP), DARPA Grant D18AC0025 (to MJG, MMF, and JRP), National Science Foundation Grant DMR- 2011750 (to ARM and JRP), and Department of Energy grant DE-NA0003525 (to EN). The Sandia National Laboratories portion of this work was funded, in part, by the US Department of Energy, Office of Science, Office of Advanced Scientific Computing Research’s Quantum Testbed Pathfinder. The devices were fabricated in Princeton University’s Quantum Device Nanofabrication Laboratory, which is operated by the physics department.
The study, “Two-Qubit Silicon Quantum Processor with Greater Than 99% Operation Fidelity”, by Adam Mills, Charles Guinn, Michael Gullans, Anthony Sigillito, Mayer Feldman, Erik Nielsen, and Jason Petta was published online in the journal Science Advances on April 6, 2022. DOI: 10.1126/sciadv.abn5130.