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Europhysics Letters (EPL)

Coherent control and charge echo in a GaAs charge qubit

Bao‐Chuan Wang, Bao-Bao Chen, Gang Cao, Hai-Ou Li, Ming Xiao, Guo‐Ping Guo

Published Mar 1, 2017• 6 citations
Publisher

Abstract

In fulfilling the non-adiabatic requirement of pulse sequences, it is challenging to perform multi-pulse quantum control of a charge qubit. By optimizing our charge qubit and pulse parameters, we experimentally demonstrate the coherent control and echo process of a GaAs charge qubit. We firstly employed a single non-adiabatic voltage pulse to perform the Larmor oscillation experiment and determine the optimal operation pulses. Then, we produced Ramsey fringes using two pulses and extracted the decoherence time from the Ramsey fringes. More importantly, we successfully applied a charge echo pulse sequence to increase the extracted inhomogeneous dephasing time to ∼1360 ps. Our results show that the low-frequency noise is the important dephasing limiter of the coherence time of the charge qubit in the GaAs system.

DOI
10.1209/0295-5075/117/57006

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We demonstrate multipulse quantum control of a single electron charge qubit. The qubit is manipulated by applying nonadiabatic voltage pulses to a surface depletion gate and readout is achieved using a quantum point-contact charge sensor. We observe Ramsey fringes in the excited-state occupation in response to a $\ensuremath{\pi}/2$-$\ensuremath{\pi}/2$ pulse sequence and extract ${T}_{2}^{*}\ensuremath{\sim}60$ ps away from the charge degeneracy point. Simulations suggest these results may be extended to implement a charge echo by reducing the interdot tunnel coupling and pulse rise time, thereby increasing the nonadiabaticity of the pulses.
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We experimentally demonstrate a tunable hybrid qubit in a five-electron GaAs double quantum dot. The qubit is encoded in the (1,4) charge regime of the double dot and can be manipulated completely electrically. More importantly, dot anharmonicity leads to quasiparallel energy levels and a new anticrossing, which help preserve quantum coherence of the qubit and yield a useful working point. We have performed Larmor precession and Ramsey fringe experiments near the new working point and find that the qubit decoherence time is significantly improved over a charge qubit. This work shows a new way to encode a semiconductor qubit that is controllable and coherent.
Many quantum dot qubits operate in regimes where the energy splittings between qubit states are large and phonons can be the dominant source of decoherence. The recently proposed charge quadrupole qubit, based on one electron in a triple quantum dot, employs a highly symmetric charge distribution to suppress the influence of long-wavelength charge noise. To study the effects of phonons on the charge quadrupole qubit, we consider Larmor and Ramsey pulse sequences to identify favorable operating parameters. We show that phonon-induced decoherence increases with the qubit frequency, in contrast to the effects of charge noise. We also show that there is an optimum value of the tunnel coupling of the qubit at which the decohering effects of phonons and charge noise are small enough to be consistent with single qubit gate fidelities > 99.99%.