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Europhysics Letters (EPL)

Measuring the coherence of charge states in undoped GaAs/AlGaAs double quantum dots with photon-assisted tunneling

Jie You, Hai-Ou Li, Gang Cao, Guangwei Deng, Ming Xiao, Guo‐Ping Guo

Published Jul 15, 2015• 9 citations
Publisher

Abstract

Photon-assisted tunneling is used to study coherent properties of a charge qubit which is formed in an undoped GaAs/AlGaAs heterostructure. We found the charge relaxation time T 1 to be around 15 ns and the inhomogeneous decoherence time to be around 330 ps at an electron temperature of 280 mK. This may be slightly better than that previously reported for doped devices, considering its temperature dependence. We discuss the role of donor fluctuation on the charge state coherence and possible ways for making improvements in the undoped devices.

DOI
10.1209/0295-5075/111/17001

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Photon-assisted tunneling is used to study coherent properties of a charge qubit which is formed in an undoped GaAs/AlGaAs heterostructure. We found the charge relaxation time T 1 to be around 15 ns and the inhomogeneous decoherence time to be around 330 ps at an electron temperature of 280 mK. This may be slightly better than that previously reported for doped devices, considering its temperature dependence. We discuss the role of donor fluctuation on the charge state coherence and possible ways for making improvements in the undoped devices.
We perform photon-assisted-tunneling (PAT) experiments on a GaAs double quantum dot device under high microwave excitation power. Photon-assisted absorption of up to 14 photons is observed, when electron temperature (>1 K) is far above the lattice temperature. Signatures of Landau-Zener-Stückelberg interference are found even in this non-equilibrium PAT spectrum. In addition, the charge state relaxation time T1∼8ns measured in this out of thermal equilibrium double quantum dot is in agreement with other previous reports.
Fast quantum oscillations of a charge qubit in a double quantum dot fabricated in a Si/SiGe heterostructure are demonstrated and characterized experimentally. The measured inhomogeneous dephasing time T2∗{T}_{2}^{*}T2∗​ ranges from 127 ps to 2.1 ns; it depends substantially on how the energy difference of the two qubit states varies with external voltages, consistent with a decoherence process that is dominated by detuning noise (charge noise that changes the asymmetry of the qubit's double-well potential). In the regime with the shortest T2∗{T}_{2}^{*}T2∗​, applying a charge-echo pulse sequence increases the measured inhomogeneous decoherence time from 127 to 760 ps, demonstrating that low-frequency noise processes are an important dephasing mechanism.
We study quantum coherence in a semiconductor charge qubit formed from a GaAs double quantum dot containing a single electron. Voltage pulses are applied to depletion gates to drive qubit rotations and noninvasive state readout is achieved using a quantum point contact charge detector. We measure a maximum coherence time of ∼7 ns at the charge degeneracy point, where the qubit level splitting is first-order insensitive to gate voltage fluctuations. We compare measurements of the coherence time as a function of detuning with numerical simulations and predictions from a 1/f noise model.
We describe time-dependent single-electron transport through quantum dots in the Coulomb blockade regime. Coherent dynamics of a single charge qubit in a double quantum dot is discussed with full one-qubit manipulation. Strength of decoherence is controlled with the applied voltage, but uncontrolled decoherence arises from electron–phonon coupling and background fluctuations. Then energy-relaxation dynamics is discussed for orbital and spin degree of freedom in a quantum dot. The electron–phonon interaction and spin–orbit coupling can be investigated as the dissipation problem. Finally, charge detection measurement is presented for statistical analysis of single-electron tunnelling transitions and for a sensitive qubit read-out device.
We consider charge qubits based on shallow donor electron states in silicon and coupled quantum dots in GaAs. Specifically, we study the feasibility of P2+{P}_{2}^{+}P2+​ charge qubits in Si, focusing on single qubit properties in terms of tunnel coupling between the two phosphorus donors and qubit decoherence caused by electron-phonon interaction. By taking into consideration the multivalley structure of the Si conduction band, we show that intervalley quantum interference has important consequences for single-qubit operations of P2+{P}_{2}^{+}P2+​ charge qubits. In particular, the valley interference leads to a tunnel-coupling strength distribution centered around zero. On the other hand, we find that the Si band structure does not dramatically affect the electron-phonon coupling and consequently, qubit coherence. We also critically compare charge qubit properties for Si:P2+\mathrm{Si}:{P}_{2}^{+}Si:P2+​ and GaAs double quantum dot quantum computer architectures.