Suppression of nuclear spin bath fluctuations in self-assembled quantum dots induced by inhomogeneous strain

Abstract

Interaction with nuclear spins leads to decoherence and information loss in solid-state electron-spin qubits. One particular, ineradicable source of electron decoherence arises from decoherence of the nuclear spin bath, driven by nuclear–nuclear dipolar interactions. Owing to its many-body nature nuclear decoherence is difficult to predict, especially for an important class of strained nanostructures where nuclear quadrupolar effects have a significant but largely unknown impact. Here, we report direct measurement of nuclear spin bath coherence in individual self-assembled InGaAs/GaAs quantum dots: spin-echo coherence times in the range 1.2–4.5 ms are found. Based on these values, we demonstrate that strain-induced quadrupolar interactions make nuclear spin fluctuations much slower compared with lattice-matched GaAs/AlGaAs structures. Our findings demonstrate that quadrupolar effects can potentially be used to engineer optically active III-V semiconductor spin-qubits with a nearly noise-free nuclear spin bath, previously achievable only in nuclear spin-0 semiconductors, where qubit network interconnection and scaling are challenging.

Few-second-long correlation times in a quantum dot nuclear spin bath probed by frequency-comb nuclear magnetic resonance spectroscopy

“…A more accurate derivation of τ c is achieved by fitting the entire τ As data set with a stretched exponential function of β The observed τ c 1 s greatly exceeds typical nuclear dipolar flipflop times in strain-free III-V solids τ c ∼ 100 µs (refs 9,10,17). We attribute the extremely long τ c in self-assembled quantum dots to the effect of inhomogeneous nuclear quadrupolar shifts making nuclear spin flip-flops energetically forbidden 20,24 . This interpretation is corroborated by the observation of τ c,ST > τ c,CT , because quadrupolar broadening of the ST transitions is much greater than that of the CT transitions (ref.…”

“…The value of τ c is determined by the spin exchange (flip-flops) of the interacting nuclear bath spins. By contrast, pulsed NMR reveals the spin bath coherence time T 2 , which characterizes the dynamics of the transverse nuclear magnetization 7,8,20 and is much shorter than τ c . The problem is further exacerbated in self-assembled quantum dots, where quadrupolar effects lead to inhomogeneous NMR broadening exceeding 10 MHz (refs 21,22), so that pulsed NMR requires practically unattainable rf field amplitudes exceeding 1 T.…”

“…We then exploit non-resonant nuclear-nuclear interactions: the homogeneous NMR lineshape of one isotope measured with frequency-comb NMR is used as a sensitive non-invasive probe of the correlation times τ c of the nuclear flip-flops of the other isotope. Although initial studies 9,17,19 suggested τ c ∼ 100 µs for nuclear spins in III-V semiconductors, it was recently recognized 20,24,25 that quadrupolar effects may have a significant impact in self-assembled quantum dots. Here, for the first time, we obtain a quantitative measurement of extremely long τ c 1 s, revealing strong freezing of the nuclear spin bath due to the inhomogeneous strain-a crucial advantage for quantum information applications of self-assembled quantum dots.…”

“…20. However, spin-echo could be measured only on the central transitions, for which ν inh is relatively small.…”

Dynamics of a mesoscopic nuclear spin ensemble interacting with an optically driven electron spin

“…The * ma424@cam.ac.uk inhomogeneous electron spin dephasing occurring over a few nanoseconds as a consequence of precession in the slowly changing nuclear Overhauser field is well understood and measured [19][20][21]. Surprisingly, first experimental data on the time scales of the nuclear spin bath dynamics in QDs have only recently emerged, reporting in one case correlation times of 100 μs (5.5 μs) for a resonantly driven negatively charged (neutral) QD [22] and in the other case nuclear coherence times of a few milliseconds for a neutral QD [23]. The dynamics, assigned to nuclear dipolar coupling in both reports, were obtained in the absence of an external magnetic field in the former and at fields of a few Tesla in the latter case.…”