Physicists have created and optimized a set of high-precision quantum gates for qubits based on a point defect in diamond. The accuracy of single-qubit operations was 99.999(1) percent, and that of two-qubit operations was 99.93(5) percent. The SWAP gate, composed of 17 simpler ones, showed a match rate of about 98.7 percent. The results of the study were published in Physical Review Applied.
Two problems that are slowing down the development of quantum computers today are the short coherence time of qubits (after a very short time, the information in them begins to distort) and the low accuracy of quantum gates, the building blocks of quantum algorithms. For example, if the accuracy of the gates is 99 percent, then an algorithm consisting of ten such gates will yield a fidelity of approximately 90 percent. For comparison, to implement the inverse quantum Fourier transform (a component of Shor's algorithm) for seven qubits, 31 quantum operations must be performed.
The number of errors can be reduced by increasing the number of qubits and correction algorithms (more about them in our article "Quantum Corrector") or by increasing the accuracy of the gates at the physical level. For example, in one of their recent works, physicists demonstrated two-qubit gates for NV centers with a coincidence rate of 99.92 percent, but the method proposed by the scientists gave only the upper, not the best, estimate of accuracy. At the same time, the researchers did not consider single-qubit gates, the implementation of which is complicated by the electron-nuclear interaction in the nitrogen vacancy.
Physicists from the UK and the Netherlands, led by Tim Taminiau from Delft University of Technology, have developed a set of high-precision quantum gates for qubits based on the NV center in diamond and optimized it, achieving a coincidence rate of about 99.999(1) percent for selected single-qubit operations and 99.93(5) percent for two-qubit operations.
To do this, the scientists considered a two-qubit system formed by the electron spin of the NV center and the nuclear spin of nitrogen at a temperature of four kelvins, taking into account the surrounding nuclear spins of carbon as sources of additional noise. The experimenters initialized and read the electron spin using resonant optical excitation, and the preparation and reading of the nitrogen spin was implemented through the electron spin, mapping its states after initialization onto the nitrogen spin. The researchers then determined the characteristics of the gates using gate set tomography (GST). The essence of this method was that the physicists performed a set of quantum circuits consisting of preparing qubits, measuring them, and also controlled introduction of individual errors into the gates.
The scientists optimized the created set of gates — the key step in optimization was the interpulse delay, which eliminated the interaction of the electron-spin qubit with other surrounding spins. The average accuracy of two-qubit gates in experiments was 99.923 ± 0.026 percent. After that, the physicists checked the operation of the system on the SWAP gate, which was constructed from 17 simpler gates — the accuracy of execution was 98.7 percent.
The authors of the work noted that the SWAP gate can be used to create quantum memory based on nitrogen spin in diamond. The experiment showed that such a qubit retained the quantum information recorded in it for one hundred seconds.
We wrote earlier about how physicists changed the coherence time of a diamond qubit using acoustic waves.