Relaxation of Electron and Hole Spin Qubits in III–V Quantum Dots

The control of solid-state qubits for quantum information processing requires a detailed understanding of the mechanisms responsible for decoherence. During the past decade, considerable progress has been achieved for describing the qubit dynamics in relatively strong external magnetic fields. However, testing theoretical predictions at very low magnetic fields has proven difficult in optically active dots. Here, we describe our studies of electron and hole spin qubit dephasing in single InGaAs quantum dots using spin memory devices. The results show that without applied magnetic fields, the initially orientated electron spin rapidly loses its polarization due to precession around the fluctuating Overhauser field with an effective magnetic field amplitude of 10.5 mT. The inhomogeneous dephasing time associated with these hyperfine mediated dynamics is T*₂ ~ 2 ns. Over longer timescales, an unexpected stage of central spin relaxation is observed, namely the appearance of a second feature in the relaxation curve around T_Q = 750 ns arising from quadrupolar coupling. In comparison, hole spin qubits are shown couple significantly more weakly to the nuclear spin bath. We measure a ~ 100× times longer dephasing time T*₂ ~ 210 ns for hole spin qubits compared with the electron spin. We also obtain evidence for the impact of anisotropic hyperfine coupling on the spin polarization decay, allowing us to quantify the degree of anisotropy α = 0.19 which is fundamental to the character of the confined hole spin wave function. By modeling this behavior, we derive the degree of light-hole heavy-hole mixing, which is an essential mechanism for enabling hole spin dephasing and thus refining the description of hole hyperfine coupling beyond the initially suggested pure Ising form.