Hole spin mode locking and coherent dynamics in a largely inhomogeneous ensemble ofp-doped InAs quantum dots

Abstract

In this study, we evidence hole spin mode locking in a largely inhomogeneous p-doped InAs quantum dot (QD) ensemble, g h x /g h x ≈ 34%, which allows us to reveal a long spin coherence time of T h 2 ≈ 0.8 μs. In addition, with a two-pump experiment, we demonstrate, in a low magnetic range 60-120 mT, the possibility to synchronize and tune a single subset of QDs through the distribution. Experiments are supported by an analysis within the density matrix approach.The resident spin of a carrier confined in a quantum dot (QD) is a promising candidate for quantum information systems. Until recently, most theoretical and experimental studies have been centered on the electron spin, and have demonstrated that the ultimate limitation, at low temperature and zero magnetic field, is the hyperfine interaction of an electron with a nuclear-spin ensemble. Due to the limited number of QD nuclei interacting with the electron spin, on the order of 10 5 , random fluctuations of the nuclearspin bath lead to a dephasing time on the order of ns for InAs QDs. Recently, the initialization of hole spin has been demonstrated, 1-7 and it has emerged as a more attractive candidate, with a one-order-of-magnitude weaker hyperfine coupling. 4,[8][9][10][11][12] Several studies have reported a long hole spin relaxation time T h 1 , from μs to ms, under different conditions of temperature and magnetic field. 1,13-16 In early single-hole experiments of coherent population trapping, the hole spin coherence time was estimated to be larger than 100 ns. 17 In more recent experiments in time domain, the hole-spin coherence time has been found to be 20 ns, but these single hole-spin measurements were affected by electrical-noise 18,19 or nuclear-spin 20 fluctuations. Recent spin-echo experiments allow to circumvent the inhomogeneous decoherence and measure a much longer hole-spin coherence time T h 2 , in the microsecond range between 6 and 10 T. 18 This value is of the same order of magnitude as for electrons 21 and represents an upper bound on the free-induction decay time of hole spin coherence. 22 For electron spins, the possibility to go beyond the ensemble inhomogeneities and to measure a long coherence time by using a periodic train of circularly polarized pulses has been demonstrated. A single pulse creates an ensemble of electron spins along its propagation direction and perpendicular to an external magnetic field (Voigt geometry). Each electron spin precesses about the field and, due to the inhomogeneity of the precession frequencies, the phase coherence is quickly lost. The train of pulses allows the phase synchronization of subsets of the whole spin ensemble, for which the precession frequency is a multiple of the laser repetition rate. This effect was called "electron spin mode locking." 23,24 It has allowed the measurement of an electron spin coherence time of 600 ns at 2 T, 25 and 3 μs at 6 T, 23 in an ensemble of InGaAs QDs with a relative dispersion of the in-plane electron Landé factor g e x /g e x ≈ 1%.Holes are more sensitive than...

Nanosecond spin coherence of excitons bound to acceptors in a CdTe quantum well

“…j values obtained in n-doped 12,14 or p-doped QDs. 15 At B ¼ 0 T, at very small concentration, and because of the localization of the donor-bound electron spins, their dephasing time is likely to be determined by hyperfine interaction. 16,17 The studied A 0 X shows a binding energy of % 12 meV.…”

Spin mode locking in quantum dots revisited

“…For the holes, the hyperfine interaction was found to be about an order of magnitude weaker, as evidenced also from the relative strength of the corresponding signals at negative delays. Therefore, longer coherence times were expected for the hole spins, which could not be confirmed .…”

Model for the light-induced magnetization in singly charged quantum dots

“…2 that the phase φ is not constant, as would be expected from the simplified picture, but changes from zero in the high field limit to φ = π/2 when the magnetic field intensity B approaches zero, which is precisely when the fast dephasing approximation fails. Notice also that recently it was demonstrated that the hole spin coherence lifetime is typically 800 ns [14], which exceeds by three orders of magnitude the characteristic trion recombination time of τ , which is in the nanosecond range [12]. Therefore, an accurate description of the phase of the long-term magnetization oscillation requires modeling the precession of both electrons and holes, which can be obtained by resolving the time-dependent Schrödinger equation.…”