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Optical Control of Exciton Qubits in Semiconductor Quantum Dots Using Pulse Shaping Techniques

Reuble Mathew

Published Aug 18, 2015• 0 citations
OpenAlex

Abstract

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.

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A quantum emitter (QE) is a physical system that can be used to encode a quantum state via some internal degree of freedom (e.g. exciton, electron spin, valley) and is coupled to light via a dipolar transition that enables the conversion of that quantum state into the state of a photon and vice versa. Such QEs can be applied to sources of single and entangled photons for applications in quantum cryptography or quantum imaging and a collection of coupled QEs can be used to realize a small quantum simulator or quantum memory node in a distributed quantum network. Among solid state QE systems, semiconductor quantum dots (QD) are particularly attractive due to their high radiative quantum efficiencies, their strong optical coupling, enabling fast and arbitrary qubit rotations, and their tunable emission in the range of standard telecommunication wavelengths. Adiabatic rapid passage (ARP) is a state preparation scheme that enables the excitation and de-excitation of a QE with a high fidelity even in the presence of fluctuations in experimental parameters such as intensity and frequency of the control laser pulse. This excitation scheme may be used for quantum state initialization in quantum information platforms and for triggering quantum light sources. This thesis work concerns an exploration of the efficacy of ARP for the quantum control of excitons in semiconductor QDs. The experiments carried out in this work show that the impact of phonons may be suppressed for ARP, pointing to the possibility of high-fidelity inversion at elevated temperatures. We also investigate the dependence of the threshold for decoherence suppression on the size and shape of the QD. The experiments performed in this thesis work also quantify the detuning tolerance of ARP. ARP was also used to demonstrate parallel quantum state initialization in two different QDs with different transition energies and dipole moments.

•May 21, 2020•0 citations
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.
Pulse shaping techniques are used to demonstrate quantum control of exciton qubits in InAs quantum dots. Linearly chirped laser pulses are used to demonstrate adiabatic rapid passage in a single quantum dot on a subpicosecond timescale. The observed dependence of the exciton inversion efficiency on the sign of the pulse chirp identifies phonons as the dominant source of dephasing, which can be suppressed for positive chirp at low temperatures. The use of optimal quantum control theory to engineer a single optical pulse to implement simultaneous π and 2π single qubit gates in two uncoupled quantum dots is demonstrated. This work will support the use of pulse shaping in solid-state quantum hardware.
Controlling the quantum states of charge (excitons) or spin-polarized carriers in semiconductor quantum dots (QDs) has been the focus of a considerable research effort in recent years due to the strong promise of using this approach to develop solid state quantum computing hardware. The long-term scalability of this type of quantum computing architecture is enhanced by the use of QDs emitting in the telecom band, which would exploit the established photonic infrastructure. This thesis reports the use of all optical infrared experimental techniques to control exciton dynamics in two different QD samples consisting of InAs/GaAs QDs and InAs/InP QDs within a planar microcavity. An infrared quantum control apparatus was developed and used to apply optimized shaping masks to ultrafast pulses from an optical parametric oscillator. Pulse shaping protocols designed to execute a two-qubit controlled-rotation operation on an individual semiconductor QD were demonstrated and characterized. The quantum control apparatus was then implemented in simultaneous single qubit rotations using two uncoupled, distant InAs/GaAs QDs. These optimal control experiments demonstrated high fidelity optical manipulation of exciton states in the two QDs using a single broadband laser pulse, representing a step forward on the path to a scalable QD architecture and showcasing the power of pulse shaping techniques for quantum control on solid state qubits. As an alternative to single QDs, which have very low optical signals, subsets of QDs within an ensemble can be used in quantum computing applications. To investigate the mediation of inhomogeneities in a QD ensemble, pump-probe experiments were performed on InAs/InP QDs within a dielectric Bragg stack microcavity. Two different excitation geometries showed that the angle dependence of the microcavity transmission allowed for the spectral selection of QD subsets with transition energies resonant with the cavity mode. The microcavity mitigated inhomogeneities in the ensemble while providing a basis for addressing QD subsets which could be used as distinguishable quantum bits. This thesis work shows significant advances towards an optical computing architecture using quantum states in semiconductor QDs.
Pulse-shaping protocols for subpicosecond optically controlled quantum gates in semiconductor quantum dots are reported. Our emphasis is the development of shaping schemes for either amplitude or phase control of the pulse that are easily implemented using commercial pulse shapers and femtosecond laser systems. We illustrate the efficacy of our approach through simulations of a controlled-rotation gate in a realistic In(Ga)As quantum dot with electronic structure calculated using eight-band, strain-dependent $\mathbit{k}\ifmmode\cdot\else\textperiodcentered\fi{}\mathbit{p}$ theory. Our results show that amplitude- and phase-shaping protocols both lead to substantial improvements in fidelity when compared with transform-limited pulses with equivalent gate times. Dephasing was found to have a minimal effect on the gate fidelities due to the ultrafast time scale of the quantum operations.
Shaped ultrafast pulses designed for controlled-rotation (C-ROT) operations on exciton qubits in semiconductor quantum dots are demonstrated using a quantum control apparatus operating at ∼1 eV. Optimum pulse shapes employing amplitude and phase shaping protocols are implemented using the output of an optical parametric oscillator and a programmable pulse shaping system, and characterized using autocorrelation and multiphoton intrapulse interference phase scan techniques. We apply our pulse characterization results and density matrix simulations to assess the fundamental limits on the fidelity of the C-ROT operation, providing a benchmark for the evaluation of sources of noise in other quantum control experiments. Our results indicate the effectiveness of pulse shaping techniques for achieving high fidelity quantum operations in quantum dots with a gate time below 1 ps.
Abstract Femtosecond pulse shaping provides a means to tailor the interaction of light with matter, enabling the optimization of optical control processes and the achievement of a target final quantum state of the matter system. While this approach has found widespread application in the control of atomic and molecular systems, its use for solid state quantum systems remains in its infancy. This review covers our application of this approach to the manipulation of the quantum states of excitons in semiconductor quantum dots (QDs). The achievement of simultaneous π, 2π rotations on excitons in two different QDs using a single engineered infrared pulse illustrates the flexibility of the pulse shaping approach for solid state quantum systems. This versatility is further explored through simulations that show the feasibility of arbitrary SU(2) control of several quantum dots. Shaping of femtosecond infrared control pulses enables the demonstration of adiabatic rapid passage in a single QD on a subpicosecond time scale, representing a substantial speedup and an important step towards the realization of dynamical decoupling. The dependence of the exciton inversion efficiency on the sign of pulse chirp confirms the role of (and ability to control) phonon‐related dephasing. (© 2016 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)
A quantum emitter (QE) is a physical system that can be used to encode a quantum state via some internal degree of freedom (e.g. exciton, electron spin, valley) and is coupled to light via a dipolar transition that enables the conversion of that quantum state into the state of a photon and vice versa. Such QEs can be applied to sources of single and entangled photons for applications in quantum cryptography or quantum imaging and a collection of coupled QEs can be used to realize a small quantum simulator or quantum memory node in a distributed quantum network. Among solid state QE systems, semiconductor quantum dots (QD) are particularly attractive due to their high radiative quantum efficiencies, their strong optical coupling, enabling fast and arbitrary qubit rotations, and their tunable emission in the range of standard telecommunication wavelengths. Adiabatic rapid passage (ARP) is a state preparation scheme that enables the excitation and de-excitation of a QE with a high fidelity even in the presence of fluctuations in experimental parameters such as intensity and frequency of the control laser pulse. This excitation scheme may be used for quantum state initialization in quantum information platforms and for triggering quantum light sources. This thesis work concerns an exploration of the efficacy of ARP for the quantum control of excitons in semiconductor QDs. The experiments carried out in this work show that the impact of phonons may be suppressed for ARP, pointing to the possibility of high-fidelity inversion at elevated temperatures. We also investigate the dependence of the threshold for decoherence suppression on the size and shape of the QD. The experiments performed in this thesis work also quantify the detuning tolerance of ARP. ARP was also used to demonstrate parallel quantum state initialization in two different QDs with different transition energies and dipole moments.