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We are almost ready to roll out the application of qubits, which are two-state quantum-mechanical systems. Quantum computers are likely to be at the forefront, but sensors using quantum technology represent other platforms that are worthy of consideration. In this case, the best bet for qubits is to use point defects in diamonds, which are called negatively charged nitrogen-vacancy (NV-minus) centers. The qubit situated in the center is ideal in that it operates stably even at room temperature. This property is in stark contrast to that observed in superconductors.

Any quantum state is susceptible to a variety of noise sources, which ultimately result in the decay of the quantum state over time. A comprehensive study has demonstrated that the use of qubits with slow decay rates facilitates the attainment of a more refined sensing resolution. Therefore, one of the most important things to consider in a sensor is how long the quantum states can be maintained. Hence, a slow decay rate is a big plus.

As is easily expected, a short distance between a quantum sensor and a target to be measured is good for sensing. Therefore, having the sensor right next to the diamond surface would be advantageous. However, it's not that simple because surfaces have many of surface-specific sources that can accelerate the decay.

To address this trade-off, this article demonstrates that coupling with another qubit that has a slow decay rate improves the decay rate of a qubit near the diamond surface. For the case that anisotropic Heisenberg coupling between a qubit pair is assumed, the analytical result for the decay rate as a function of the coupling strength has been derived. The formula demonstrates which component of the interaction is the most dominant, and reveals that there is a limit to the potential for improvement in the decay rate.

This finding could be useful in quantum sensing technology, especially cases involving NV-minus centers in diamonds.

(Written by J.-i. Inoue.)