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Physical Review B

Decoherence-protected storage of exciton qubits through ultrafast multipulse control

Thomas E. Hodgson, Lorenza Viola, Irene D’Amico

Published Oct 9, 2008• 29 citations
Publisher

Abstract

We quantitatively investigate the usefulness and viability of the scheme developed by Viola and Lloyd [Phys. Rev. A 58, 2733 (1998)] to control dephasing in the context of exciton-based quantum computation with self-assembled quantum dots. We demonstrate that optical coherence of a confined exciton qubit exposed to phonon-induced dephasing can be substantially enhanced through the application of a simple periodic sequence of control pulses. The shape of the quantum dot has a significant effect on the dephasing properties. Remarkably, we find that quantum dots with parameters optimized for implementing quantum computation are among the most susceptible to dephasing, yet periodic decoupling is most efficient for exactly that type of dot. We also show that the presence of an electric field, which is a necessary ingredient for many exciton-based quantum computing schemes, may further increase the control efficiency. Our results suggest that dynamical decoupling may be a method of choice for robust storage of exciton qubits during idle stages of quantum algorithms.

DOI
10.1103/physrevb.78.165311

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We quantitatively investigate the usefulness and viability of the scheme developed by Viola and Lloyd [Phys. Rev. A 58, 2733 (1998)] to control dephasing in the context of exciton-based quantum computation with self-assembled quantum dots. We demonstrate that optical coherence of a confined exciton qubit exposed to phonon-induced dephasing can be substantially enhanced through the application of a simple periodic sequence of control pulses. The shape of the quantum dot has a significant effect on the dephasing properties. Remarkably, we find that quantum dots with parameters optimized for implementing quantum computation are among the most susceptible to dephasing, yet periodic decoupling is most efficient for exactly that type of dot. We also show that the presence of an electric field, which is a necessary ingredient for many exciton-based quantum computing schemes, may further increase the control efficiency. Our results suggest that dynamical decoupling may be a method of choice for robust storage of exciton qubits during idle stages of quantum algorithms.
We investigate the usefulness and viability of the scheme developed by Viola and Lloyd [Phys. Rev. A 58, 2733 (1998)] to control dephasing in the context of exciton-based quantum computation with self-assembled quantum dots. We demonstrate that optical coherence of a confined exciton qubit exposed to phonon-induced dephasing can be substantially enhanced through the application of a simple periodic sequence of control pulses. The shape of the quantum dot has a significant effect on the dephasing properties. Remarkably, we find that quantum dots with parameters optimized for implementing quantum computation are among the most susceptible to dephasing, yet periodic decoupling is most efficient for exactly that type of dot. We also show that the presence of an electric field, which is a necessary ingredient for many exciton-based quantum computing schemes, may further increase the control efficiency. Our results suggest that dynamical decoupling may be a method of choice for robust storage of exciton qubits during idle stages of quantum algorithms.
We investigate the effectiveness of different dynamical decoupling protocols for storage of a single qubit in the presence of a purely dephasing bosonic bath, with emphasis on comparing quantum coherence preservation under uniform versus nonuniform delay times between pulses. In the limit of instantaneous bit-flip pulses, this is accomplished by establishing a different representation of the controlled qubit evolution, where the decoherence behavior after an arbitrary number of pulses is directly expressed in terms of the uncontrolled decoherence function. In particular, analytical expressions are obtained for approximation of the long- and short-term coherence behavior for both Ohmic and supra-Ohmic environments. By focusing on the realistic case of pure dephasing in an excitonic qubit, we quantitatively assess the impact of physical constraints on achievable pulse separations, and show that little advantage of high-level decoupling schemes based on concatenated or optimal design may be expected if pulses cannot be applied sufficiently fast. In such constrained scenarios, we demonstrate how simple modifications of repeated periodic-echo protocols can offer significantly improved coherence preservation in realistic parameter regimes. We expect similar conclusions to be relevant to other constrained qubit devices exposed to quantum or classical phase noise.
Semiconductor quantum dots have been featured in a number of proposals for quantum computing because of the advantages afforded by confinement on the nanoscale, such as well separated, discrete energy levels and ease of optical manipulation. In addition, they can leverage established semiconductor fabrication techniques and, like the quantum dots used in this work, can be designed to match the telecommunication band, improving the scalability and potential integration of the platform into existing technologies. This thesis work applied optical pulse engineering to manipulate exciton qubits in self-assembled InAs quantum dots. Optimal quantum control theory was used to design pulse shapes that implement high fidelity single-qubit and two-qubit operations, with constraints on the numerical optimization that ensure that the pulses can be accurately implemented using a commercial 4f pulse shaper. In the case of single-qubit operations, the results showed that two uncoupled qubits can be manipulated in parallel using a single phase-shaped laser pulse, provided their optoelectronic properties are sufficiently different. It was demonstrated that targeted differences in inversion and phase between the qubits can be achieved on an ultrafast timescale with high fidelity. In the case of two-qubit operations, the controlled-rotation gate was optimized using amplitude-only and phase-only pulse shaping schemes. The shaped pulses for both schemes were shown to produce higher fidelity operations. The ability to implement robust state inversion on short timescales is particularly useful for technologies such as ultrafast optical switches, single-photon sources, and entangled-photon sources. This work has demonstrated state inversion in a single InAs quantum dot via adiabatic rapid passage using linearly chirped laser pulses. The achieved gate times were an order of magnitude shorter than previous demonstrations. Theoretical predictions of the dependence of the inversion efficiency of the exciton on the sign of the pulse chirp were also verified experimentally, allowing for the identification of phonon-mediated dephasing as the dominant source of decoherence.
Semiconductor quantum dots are excellent candidates for ultrafast coherent manipulation of qubits by laser pulses on picosecond timescales or even faster. In inhomogeneous ensembles a macroscopic optical polarization decays rapidly due to dephasing, which, however, is reversible in photon echoes carrying complete information about the coherent ensemble dynamics. Control of the echo emission time is mandatory for applications. Here, we propose a novel concept to reach this goal. In a two-pulse photon echo sequence, we apply an additional resonant control pulse with multiple of 2pi area. Depending on its arrival time, the control slows down dephasing or rephasing of the exciton ensemble during its action. We demonstrate for self-assembled (In,Ga)As quantum dots that the photon echo emission time can be retarded or advanced by 5 ps relative to its nominal appearance time without control. This versatile protocol may be used to obtain significantly longer temporal shifts for suitably tailored control pulses.