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Nature Communications

Ultrafast universal quantum control of a quantum-dot charge qubit using Landau–Zener–Stückelberg interference

Gang Cao, Hai-Ou Li, Tao Tu, Li Wang, Cheng Jun Zhou, Ming Xiao, Guang‐Can Guo, H. W. Jiang, Guo‐Ping Guo

Published Jan 29, 2013• 185 citations
Publisher

Abstract

A basic requirement for quantum information processing is the ability to universally control the state of a single qubit on timescales much shorter than the coherence time. Although ultrafast optical control of a single spin has been achieved in quantum dots, scaling up such methods remains a challenge. Here we demonstrate complete control of the quantum-dot charge qubit on the picosecond scale [corrected], orders of magnitude faster than the previously measured electrically controlled charge- or spin-based qubits. We observe tunable qubit dynamics in a charge-stability diagram, in a time domain, and in a pulse amplitude space of the driven pulse. The observations are well described by Landau-Zener-Stückelberg interference. These results establish the feasibility of a full set of all-electrical single-qubit operations. Although our experiment is carried out in a solid-state architecture, the technique is independent of the physical encoding of the quantum information and has the potential for wider applications.

DOI
10.1038/ncomms2412

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A basic requirement for quantum information processing is the ability to universally control the state of a single qubit on timescales much shorter than the coherence time. Although ultrafast optical control of a single spin has been achieved in quantum dots, scaling up such methods remains a challenge. Here we demonstrate complete control of the quantum-dot charge qubit on the picosecond scale [corrected], orders of magnitude faster than the previously measured electrically controlled charge- or spin-based qubits. We observe tunable qubit dynamics in a charge-stability diagram, in a time domain, and in a pulse amplitude space of the driven pulse. The observations are well described by Landau-Zener-Stückelberg interference. These results establish the feasibility of a full set of all-electrical single-qubit operations. Although our experiment is carried out in a solid-state architecture, the technique is independent of the physical encoding of the quantum information and has the potential for wider applications.
The gate-defined semiconductor quantum dot system, which can be manipulated electrically and fabricated using modern microelectronic technology, is considered as an ideal platform for quantum computation. The efficiency of quantum computation depends on the speed of gate operation. And charge qubit has quick operational speed and strong inter-qubit strength but with technical challenges. In this talk, I will introduce our experiments on ultrafast quantum control in semiconductor charge qubits including ultrafast universal quantum control of single charge qubit using LZS(Landau-Zener-Stückelberg) interference, conditional rotation of two strongly coupled qubits and static Toffoli gate of three qubits. Furthermore, to find a balance between coherence and operation speed we experimentally demonstrated tunable hybrid qubit in GaAs quantum dots system. Finally, we also focuse on cryogenic electronics for quantum computing, we have measured and modeled standard process CMOS at 77K, 4.2K and 300mK. CryoCMOS can be applied to the readout and control system of quantum chips in the future.
The phase coherence of a physical qubit is essential for quantum information processing, motivating fast control methods to preserve that phase. Ultrafast optical techniques allow complete spin control to be performed on a much faster timescale than microwave or electrical control (ps vs. ns at best). Using our ultrafast control techniques, we demonstrate our experimental approach towards a spin echo sequence on the spin of a single electron confined in a semiconductor quantum dot (QD), increasing the observed decoherence time of a single QD electron spin from nanoseconds to several microseconds. The ratio of the observed decoherence time to the demonstrated single-qubit gate time exceeds 105, suggesting strong promise for future quantum information processors.
We demonstrate that a matter physical two-level system, a qubit, can be fully controlled using one ultrafast step. We show that the spin state of an optically excited electron, an exciton, confined in a quantum dot, can be rotated by any desired angle, about any desired axis, during such a step. For this we use a single, resonantly tuned, picosecond long, polarized optical pulse. The polarization of the pulse defines the rotation axis, while the pulse detuning from a nondegenerate absorption resonance defines the magnitude of the rotation angle. We thereby achieve a high fidelity, universal gate operation, applicable to other spin systems, using only one short optical pulse. The operation duration equals the pulse temporal width, orders of magnitude shorter than the qubit evolution life and coherence times.
Landau-Zener (LZ) transition has received renewed interest as an alternative approach to control single-qubit states. An LZ transition occurs when a system passes through an avoided crossing that arises from quantum mechanical coupling of two levels, taking the system to a coherent superposition of the two states. Then, multiple LZ transitions induce interference known as Landau-Zener-Stückelberg (LZS) interference whose amplitude strongly depends on the velocity or adiabaticity of the passage. Here, we study the roles of LZ transitions and LZS interference in coherent charge oscillations of a one-electron semiconductor double quantum dot by time-domain experiments using standard rectangular voltage pulses. By employing density matrix simulations, we show that, in the standard setup using rectangular pulses, even a small distortion of the pulse can give rise to LZ transitions and hence LZS interference, which significantly enhances the measured oscillation amplitude. We further show experimentally that the nature of the coherent charge oscillations changes from Rabi-type to LZS oscillations with increasing pulse distortion. Our results thus demonstrate that it is essential to take into account LZS interference for both precise control of charge qubits and correct interpretation of measurement results.
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.
We perform Landau-Zener-St\"uckelberg interferometry on a single electron GaAs charge qubit by repeatedly driving the system through an avoided crossing. We observe coherent destruction of tunneling, where periodic driving with specific amplitudes inhibits current flow. We probe the quantum dot occupation using a charge detector, observing oscillations in the qubit population resulting from the microwave driving. At a frequency of 9 GHz we observe excitation processes driven by the absorption of up to 17 photons. Simulations of the qubit occupancy are in good agreement with the experimental data.