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arXiv:1107.4927 · Physical Review B

Nonadiabatic quantum control of a semiconductor charge qubit

Yuliya Dovzhenko, J. Stehlik, Karl D. Petersson, J. R. Petta, Hong Lü, Arthur C. Gossard

Published Oct 7, 2011• 61 citations
arXivHTMLPDF

Abstract

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.

DOI
10.1103/physrevb.84.161302

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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 \ensuremathπ/2\ensuremath{\pi}/2\ensuremathπ/2-\ensuremathπ/2\ensuremath{\pi}/2\ensuremathπ/2 pulse sequence and extract T2∗\ensuremath∼60{T}_{2}^{*}\ensuremath{\sim}60T2∗​\ensuremath∼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.
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.
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 present a numerically-optimized multipulse framework for the quantum control of a single-electron double quantum dot qubit. Our framework defines a set of pulse sequences, necessary for the manipulation of the ideal qubit basis, that avoids errors associated with excitations outside the computational subspace. A novel control scheme manipulates the qubit adiabatically, while also retaining high speed and ability to perform a general single-qubit rotation. This basis generates spatially localized logical qubit states, making readout straightforward. We consider experimentally realistic semiconductor qubits with finite pulse rise and fall times and determine the fastest pulse sequence yielding the highest fidelity. We show that our protocol leads to improved control of a qubit. We present simulations of a double quantum dot in a semiconductor device to visualize and verify our protocol. These results can be generalized to other physical systems since they depend only on pulse rise and fall times and the energy gap between the two lowest eigenstates.
For a two-level double quantum-dot (DQD) system, we proposed an alternative ultrafast manipulation approach, Lyapunov-based control method, to transfer the charge qubit from the initial charge state |R) to the target charge state |L) on the picosecond scale. Better control performance is obtained compared with that via Landau-Zener-Stu̅ckberge-Majorana interference, and the designed control fields can be implemented in the actual experiment facility. The control process is composed of three parts: first, a slope pulse takes the system from a positive detuning adiabatically to the anticrossing point, which corresponds to the resonance state of the system; then, a Lyapunov-based control pulse drives the charge qubit transfer nonadiabatically; finally, another slope pulse takes the system away from the anticrossing point to keep the system stable. The charge qubit transfer performance and the function of Lyapunov-based control pulse were studied under different control parameters. Simulation results showed that the designed Lyapunov-based control pulse has a rise time ~20 ps, which is in the scope of the Tektronix AWG70000A series arbitrary waveform generator and results in a significant probability ~96% for the transition from the initial charge state |R) to the target charge state |L). The fidelity of the control process designed can achieve ~94%. This is the first result directly applicable for a DQD charge qubit transfer using the Lyapunov-based control method.
Maturing hybrid circuit quantum electrodynamics architectures composed of semiconductor quantum dots and on-chip microwave resonators promise an effective approach for quantum nondemolition readouts in semiconductor systems. Here, we study quantum coherence in a semiconductor charge qubit coupled with a high-impedance resonator. The qubit is controlled by a periodic pulse sequence, and a dispersive measurement is performed by measuring the time-averaged resonator response. From the Rabi oscillation and Ramsey fringe measurements, the coherence time of the charge qubit is determined as T2, Rabi ≈2.7 ns and T2, Ramsey ≈3.5 ns. Compared with the time-resolved (pulsed) measurement, this time-averaged dispersive measurement process eliminates the limitation caused by the resonator response time, enabling a high repetition rate of the pulse sequence in the experiments.
Developing fast and accurate control and readout techniques is an important challenge in quantum information processing with semiconductor qubits. Here, we study the dynamics and the coherence properties of a GaAs/AlGaAs double quantum dot charge qubit strongly coupled to a frequency-tunable high-impedance resonator. We drive qubit transitions with synthesized microwave pulses and perform qubit readout through the state-dependent frequency shift imparted by the qubit on the dispersively coupled resonator. We perform Rabi oscillation, Ramsey fringe, energy relaxation, and Hahn-echo measurements and find significantly reduced decoherence rates down to γ_{2}/2π∼3 MHz corresponding to coherence times of up to T_{2}∼50 ns for charge states in gate-defined quantum dot qubits. We realize Rabi π pulses of width down to σ∼0.25 ns.