Major breakthrough in silicon quantum computing, while three papers achieve 2Q gate fidelity of over 99%
The latest issue of Nature features three papers on quantum computing in silicon at the same time, and all three achieve more than 99% fidelity of the double-qubit gate. One of these papers, Precision tomography of a three-qubit donor quantum processor in silicon by Andrea Morello's team at the University of New South Wales, features on the cover of the journal. These three studies pave the way for the mass production and application of large silicon-based quantum processors.
A double quantum bit gate fidelity of 99% is the fundamental threshold for quantum computing to be useful. "Today's publication of the Nature paper shows that our calculations are 99 percent error-free." Professor Andrea Morello, who led the work with partners in the US, Japan, Egypt, and the Universities of Technology Sydney and Melbourne, said.
He said, "This shows that it is possible to build quantum computers of sufficient scale and sufficient power to handle meaningful computations. This research is an important milestone in achieving our goal of a universal quantum computer."
Serwan Asaad, Andrea Morello, and Mateusz Mądzik are the lead authors of the paper at the University of New South Wales (UNSW).
Silicon quantum computing reaches the 99% threshold
Three papers published today in Nature independently confirm that powerful, reliable silicon quantum computing is now a reality.
● A University of New South Wales (UNSW) team led by Andrea Morello has created two quantum bits of universal quantum logic operation between two nuclear spins formed by a phosphorus donor, introduced into silicon by industry-standard ion implantation methods. The quantum operation involves a single electron with its probability wave distributed over two atomic nuclei. The fidelity is up to 99.95% for a single nucleus and 99.37% for two quantum bits, verified by gate-set laminar imaging (GST). The electron spin, which is itself a quantum bit, can be entangled with two atomic nuclei to form a three-qubit quantum entangled state with 92.5% fidelity.
Thesis: https://www.nature.com/articles/s41586-021-04292-7
Over 99% fidelity of quantum operation was obtained in a three-quantum-bit silicon quantum processor. The first two quantum bits (Q1, Q2) are the nuclear spins of individually injected phosphorus atoms (red spheres). The third quantum bit (Q3) is the spin (shiny ellipse) of an electron around two atomic nuclei.
● A team at the Delft University of Technology in the Netherlands, led by Lieven Vandersypen, has created a two-quantum bit system in a material carefully grown from an alloy of silicon and silicon-germanium (Si/SiGe). The quantum information is encoded in the electron spins in the quantum dots. The application of gate-set laminar imaging not only quantizes but also improves the accuracy of quantum operations, achieving 99.87% single-quantum-bit fidelity and 99.65% double-quantum-bit fidelity. Xiao Xue, the first author of the paper, said, "Increasing the gate fidelity of two quantum bits to much higher than 99% requires improved materials and specially designed quantum bit control and calibration methods." Xiao Xue, born in Shandong in 1992, received his BSc in applied physics from the University of Science and Technology of China in 2014, majoring in condensed matter physics, and undertook postgraduate studies at the Quantum Information Center, Institute of Cross-Information, Tsinghua University, from 2014-2016.
Thesis: https://www.nature.com/articles/s41586-021-04273-w
● The Japanese RIKEN team, led by Tarucha Seigo, one of the founders of the quantum dot field, followed a similar path by creating two electronic quantum bits in Si/SiGe using the same material stack produced by the Delft team. By very fast manipulation, a single quantum bit fidelity of 99.84% and a double quantum bit fidelity of 99.51% were achieved. They used random benchmark tests to measure fidelity.
Thesis: https://www.nature.com/articles/s41586-021-04182-y
Hopping spins learn exact interactions
The UNSW and Delft teams have certified the performance of their quantum processors using a complex method known as Gate-Set Stratification Imaging (GST), first developed by Sandia National Laboratories in the US and made available to the research community. Professor Morello had already demonstrated in 2014 that he could preserve quantum information in silicon for 35 seconds due to the extreme isolation of nuclear spins from their environment.
Thesis: https://www.nature.com/articles/nnano.2014.211
Professor Morello says: "In the quantum world, 35 seconds is eternal. As a comparison, the famous Google and IBM superconducting quantum computers have a lifetime of about a hundred microseconds - nearly a million times shorter."
The cost, however, is that isolating the quantum bits makes it seem impossible for them to interact with each other, whereas the interaction of quantum bits is necessary to perform actual computations.
Today's paper describes how his team overcame this problem by using electrons containing the nuclei of two phosphorus atoms. Dr. Mateusz Madzik, one of the lead authors of the experiment, said: "If there are two nuclei attached to the same electron, it is possible to have them quantum manipulated. When not manipulating electrons, these nuclei can safely store their quantum information. Then, having them interact via electrons allows for universal quantum manipulation that can be adapted to any computational problem."
These three quantum bits can be prepared in quantum entangled states, unleashing the exponential power of quantum computers.
"It really is an effective technique," said Dr. Serwan Asaad, the other lead author. "Nuclear spins are the core quantum processor. If you entangle them with electrons, then the electrons can be moved to another location and entangled with other quantum bit nuclei further away, opening the way to making large arrays of quantum bits capable of powerful and useful computations."
Professor David Jamieson, co-author of the paper and head of research at the University of Melbourne, said, "The phosphorus atoms were introduced into the silicon chip by ion injection, the same method used in all existing silicon computer chips. This ensures that our quantum breakthrough is compatible with the broader semiconductor industry."
Universal quantum logic operations are demonstrated using a pair of ion-injected 31P nuclei in a silicon nanoelectronic device. The fabrication method is compatible with industry-standard processes for existing computer chips.
All computers today deploy some form of error correction and data redundancy, but the laws of quantum physics place strict limits on how error correction can be performed in a quantum computer, explains Professor Morello: "In order to apply quantum error correction protocols, an error rate of less than 1% is typically required. And now that this has been achieved, we can start to design silicon quantum processors - that can scale and operate reliably to perform useful computations."
Semiconductor spin quantum bits in silicon are ideally suited to be the platform of choice for reliable quantum computers. They are stable enough to hold quantum information for long periods of time and can be scaled up using techniques familiar from existing advanced semiconductor fabrication techniques.
Professor Morello said: "Until now, the challenge with silicon quantum bits has been to perform quantum logic operations with sufficiently high precision. All three papers published today show how this challenge can be overcome so that errors can be corrected faster than they appear."
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