The implementation of solid-state spin qubits for quantum information processing requires a detailed understanding of the decoherence mechanisms. At low magnetic fields, when the magnitude of the Zeeman energy becomes comparable to intrinsic couplings, electron spins confined to quantum dots (QDs) have been shown to feature a characteristic decoherence signature with several distinct stages: The electron spin undergoes fast ensemble dephasing due to the coherent precession of spin qubits around nearly static but randomly distributed hyperfine fields (∼2ns). At intermediate timescales (∼750ns) we identify an additional stage which corresponds to the effect of coherent dephasing processes that occur in the nuclear spin bath itself induced by quadrupolar coupling of nuclear spins to strain-driven electric field gradients. Finally, a slower process (>1μs) of irreversible relaxation of the spin polarization due to nuclear spin co-flips with the central spin causes the complete loss of coherence [1].
For hole spins, the mainly p-type Bloch function reduces the contact term of the hyperfine interaction due to the reduced wave function overlap with the nuclei. Consequently, at low magnetic fields we observe a more than two orders of magnitude slower dephasing compared to electrons. Time domain measurements of T2* show faster dephasing rates with increasing external magnetic field. We attribute this to electrical noise, which broadens the distribution of Zeeman frequencies. Strategies to counteract this noise source and measurements of T2 (via spin-echo) are discussed [2].
[1] A. Bechtold et al., Nature Physics 11, 1005–1008 (2015)
[2] T. Simmet et al., in preparation
KEYWORDS: Quantum communications, Quantum dots, Active optics, Magnetism, Solid state electronics, Quantum information processing, Spin polarization, Time metrology, Physics
Using solid-state spin qubits for quantum information processing requires a detailed understanding of the decoherence mechanisms. For electron spins in quantum dots (QDs), considerable progress has been achieved in strong external magnetic fields; however, decoherence at very low magnetic fields remains puzzling when the magnitude of the Zeeman energy becomes comparable with intrinsic couplings. Phenomenological models of decoherence currently recognize two types of spin relaxation; fast ensemble dephasing due to the coherent precession of spin qubits around nearly static but randomly distributed hyperfine fields (∼2ns) and a much slower process (>1μs) of irreversible relaxation of the spin polarization due to nuclear spin co-flips with the central spin. Here, we demonstrate that not only two but three distinct stages of decoherence can be identified in the relaxation. Measurements and simulations of the spin projection without an external field clearly reveal an additional decoherence stage at intermediate timescales (∼750ns) [1]. The additional stage corresponds to the effect of coherent dephasing processes that occur in the nuclear spin bath itself induced by quadrupolar coupling of nuclear spins to strain driven electric field gradients, leading to a relatively fast but incomplete non-monotonic relaxation of the central spin. For hole spins we observe a more than two-orders of magnitude slower dephasing due to the reduced hyperfine interaction of the p-like wavefunction. In addition, time domain measurements of T2* and T2 (via spin-echo) are discussed.
[1] A. Bechtold et al., Nature Physics 11, 1005-1008 (2015)
[2] T. Simmet et al., in preparation
We demonstrate an entirely new method to probe quantum measurement phenomena in semiconductor quantum dot (QD) spin qubits [1]. In addition to providing direct evidence for the quantum nature of solid state qubits, we show that our method has practical importance since it provides a completely alternative route for measuring ensemble and quantum dephasing times, T2* and T2, using only repeated projective measurements and without the need for coherent spin control. Our approach is based on measuring time-correlators of a spin qubit in an optically active QD beyond the second order. We utilize a quantum dot spin-storage structure to initialize a single electron spin in a quantum dot subject to a magnetic field applied in Voigt geometry through tunnel ionization and perform repeated projective measurements of the spin at times t1 and t2. This measurement is repeated, corresponding to ensemble averaging, and the resulting third-order time correlations reveals rich physics: For times t1 or t2 < T2* Larmor precession is observed which reveals the ensemble dephasing time T2*. Importantly, even though the time-correlators were obtained through averaging many measurements for times t1 and t2 > T2* oscillations are observed that decay with the dephasing time T2 and allow its determination even without the need for coherent spin control. Finally, combining the third-order time correlator with the second-order time correlator allows to demonstrate a violation of Leggett-Garg type inequalities for certain times providing clear evidence for the quantum nature of the quantum dot spin. [1] A. Bechtold et al. Phys. Rev. Lett. 117, 027402 (2016)
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