Relaxation times measured for singlet and triplet electron spin states between two donor qubits

Single electron spins bound to donors can form quantum bits (qubits). They have the ability to encode information in their spin states and their superposition states, which allows complex computational problems to be solved exponentially faster than with classical computers. Taking full advantage of this phenomenon requires an in-depth understanding about the stability over time of a given state to ensure accurate and robust computation.

New research in Applied Physics Letters describes the relaxation time measurements of the transition between the singlet (spin state of zero) and triplet (spin state of one) states of two donor qubits. Researchers fabricated a system of two phosphorus donor quantum dots that contained two electron qubits. The interaction between the qubits was modified so that the two-electron singlet and triplet states became mixed. The mixed state was measured by examining the spin correlations between the individual electrons.

The authors found relaxation times of 10 to 20 seconds between the singlet and triplet states in the new double-donor system, which are much longer than those compared to the readout and gate operation times (nanoseconds to milliseconds). The long relaxation times measured are important for the future demonstration of high-fidelity two-qubit gates.

The results also highlight that single electron spins bound to donors in silicon are well suited for multiqubit devices. The coupling of multiple electron spins is a prerequisite for scalable solid-state quantum computation. While promising, the utilization of the singlet-triplet mixed spin state as a qubit is still dependent on its coherent characteristics, which will be the subject of future work.

Source: “Singlet-triplet minus mixing and relaxation lifetimes in a double donor dot,” by S. K. Gorman, M. A. Broome, M. G. House, S. J. Hile, J. G. Keizer, D. Keith, T. F. Watson, W. J. Baker, and M. Y. Simmons, Applied Physics Letters (2018). The article can be accessed at https://doi.org/10.1063/1.5021500 .