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%), and for two-qubit operations, 99.93% (5%). The SWAP gate, composed of 17 simpler gates, demonstrated a match rate of approximately 98.7%. The results of the study were published in Physical Review Applied.
Two problems currently slowing the development of quantum computers are the short coherence time of qubits (after a very short time, information within them begins to distort) and the low precision of quantum gates—the building blocks of quantum algorithms. For example, if the gates have a 99 percent accuracy, then an algorithm consisting of ten such gates will yield a fidelity of approximately 90 percent. By comparison, implementing the inverse quantum Fourier transform (a component of Shor's algorithm) for seven qubits requires 31 quantum operations.
Errors can be reduced by increasing the number of qubits and correction algorithms (more on these in our article "Quantum Corrector") or by improving the precision of the gates at the physical level. For example, in a recent study, physicists demonstrated two-qubit gates for NV centers with a 99.92 percent match rate, but the method proposed by the scientists only yielded an upper bound, not the best, accuracy estimate. Furthermore, the researchers did not consider single-qubit gates, the implementation of which is complicated by electron-nuclear interactions in the nitrogen vacancy.
Physicists from the UK and the Netherlands, led by Tim Taminiau of 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 match 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 examined a two-qubit system formed by the electron spin of the NV center and the nuclear spin of nitrogen at a temperature of four Kelvin, accounting for the surrounding carbon nuclear spins as sources of additional noise. The experimenters initialized and read the electron spin using resonant optical excitation, while the preparation and reading of the nitrogen spin were implemented via 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). This method involved the physicists implementing a set of quantum circuits consisting of preparing qubits, measuring them, and injecting individual errors into the gates.
The scientists optimized the gate set they created. The key optimization step was the interpulse delay, which eliminated interactions between the electron-spin qubit and other surrounding spins. The average accuracy of the two-qubit gates across experiments was 99.923 ± 0.026 percent. The physicists then tested the system using a SWAP gate, constructed from 17 simpler gates, and the accuracy was 98.7 percent.
The authors of the study noted that the SWAP gate could be used to create quantum memory based on nitrogen spin in diamond. An experiment showed that such a qubit retained the quantum information stored in it for 100 seconds.
We previously wrote about how physicists changed the coherence time of a diamond qubit using acoustic waves.
