Modeling of spin decoherence in a Si hole qubit perturbed by a single charge fluctuator

Spin qubits in semiconductor quantum dots are one of the promising devices to realize a quantum processor. A better knowledge of the noise sources affecting the coherence of such a qubit is therefore of prime importance. In this paper, we study the effect of telegraphic noise induced by the fluctuation of a single electric charge. We simulate as realistically as possible a hole spin qubit in a quantum dot defined electrostatically by a set of gates along a silicon nanowire channel. Calculations combining Poisson and time-dependent Schrodinger equations allow us to simulate the relaxation and the dephasing of the hole spin as a function of time for a classical random telegraph signal. We show that dephasing time T2 is well given by a two-level model in a wide range of frequencies. Remarkably, in the most realistic configuration of a low-frequency fluctuator, the system has a non-Gaussian behavior in which the phase coherence is lost as soon as the fluctuator has changed state. The Gaussian description becomes valid only beyond a threshold frequency omega th, when the two-level system reacts to the statistical distribution of the fluctuator states. We show that the dephasing time T2(omega th) at this threshold frequency can be considerably increased by playing on the orientation of the magnetic field and the gate potentials, by running the qubit along sweet lines. However, T2(omega th) remains bounded due to dephasing induced by the nondiagonal terms of the stochastic perturbation Hamiltonian. On the other hand, our simulations reveal that the spin relaxation, usually characterized by the time T1, cannot be described cleanly in the two -level model because the coupling to higher-energy hole levels impacts very strongly the spin decoherence. This result suggests that multilevel simulations including the coupling to phonons should be necessary to describe the relaxation phenomenon in this type of qubit.

Automated summary

In this paper, the effect of telegraphic noise induced by the fluctuation of a single electric charge on a hole spin qubit in a quantum dot is studied. It is shown that in the most realistic configuration of a low-frequency fluctuator, the system exhibits non-Gaussian behavior with loss of phase coherence when the fluctuator changes state. By manipulating the magnetic field and gate potentials, the dephasing time at the threshold frequency can be significantly increased. However, the dephasing time remains bounded due to dephasing induced by nondiagonal terms in the perturbation Hamiltonian. Additionally, it is found that a clean description of spin relaxation in this type of qubit requires multilevel simulations including the coupling to phonons.