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WSEAS TRANSACTIONS ON SYSTEMS AND CONTROL

Adiabatic Control of Quantum Dot Spin in the Voigt Geometry with Optical Pulses

Dionisis Stefanatos, Nikolaos Iliopoulos, Vasilios D. Karanikolas, Emmanuel Paspalakis

Published Jan 1, 2019• 1 citation
OpenAlex

Abstract

A basic system with important potential applications in quantum technologies is a quantum dot in the Voigt geometry. The spin states of the quantum dot in the Voigt geometry can act as a prototype qubit which can be manipulated by applied optical fields in order to produce the necessary quantum gates. The basic method for spin initialization in a quantum dot in the Voigt geometry is optical pumping. Here, we propose and analyze a new method for the coherent preparation of the quantum dot spin states based on adiabatic control methods. Specifically, we show that the application of two mutually delayed and partially overlapping optical pulses, similar to those used in stimulated Raman adiabatic passage, can lead to initialization of one of the spin states with high fidelity. We also demonstrate that the fidelity of the method may be increased by integrating the quantum dot with a micropillar cavity. Specifically, we show that a preferential Purcell-enhanced decay rate towards the target spin state, in certain cases, increases the fidelity of spin initialization of the adiabatic method. Our results are based on the numerical solution of the relevant density matrix equations for the quantum dot system, either in an isotropic photonic environment or in a micropillar cavity. The calculations presented in this paper are not limited to quantum dots in micropillar cavities. Similar effects can be obtained by other photonic structures as well, as for example, for quantum dots in photonic crystal cavities.

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Adiabatically preparing quantum dot spin states in the Voigt geometry

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We use mutually delayed and partially overlapping optical pulses, similar to those used in stimulated Raman adiabatic passage and its variations, for the coherent control of quantum dot spin states in the Voigt geometry. We consider the quantum dot system initially in an incoherent mixture of the two electron-spin states. We show that the application of regular delayed and partially overlapping pulses can lead to initialization. In addition, if initially delayed, partially overlapping, and simultaneously switched off pulses are applied, the initially incoherent mixture can be changed to a specifically designed coherent superposition state. We also find that due to the initial conditions of the studied quantum system, the proposed methods work for different pulse orderings.

Journal of Applied Physics•Jan 10, 2019•10 citations

Rapid Optical Spin Initialization of a Quantum Dot in the Voigt Geometry Coupled to a Two-Dimensional Semiconductor

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We study the interaction of a quantum dot in the Voigt configuration with a laser pulse and particularly analyze the potential for rapid spin initialization by putting the quantum dot near a molybdenum disulfide (MoS 2 ) monolayer. The MoS 2 monolayer influences the spontaneous decay rates of the quantum dot, leading to anisotropically enhanced decay rates, for the quantum dot’s electric dipole moments parallel and perpendicular to the layer. By solving the relevant density matrix equations, we find that high spin initialization fidelity is obtained at short times. The fidelity is significantly higher than when the quantum dot is in free-space vacuum. We examine two different cases of the interaction of the quantum dot with the applied optical field. First, we use a continuous wave laser field and determine for various quantum dot—MoS 2 layer distances the field strength that leads to acceptable fidelity levels. The effect of the quality of the MoS 2 material on the fidelity of spin initialization is also examined. We also study the interaction of the quantum dot with a laser pulse and apply numerical optimal control to obtain the time-dependent field strength, which leads to maximum final fidelity for short time intervals. The latter approach gives beneficial results in comparison to the continuous wave field excitation.

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Teck Seng Koh, S. N. Coppersmith, Mark G. Friesen

Several logical qubits and quantum gates have been proposed for semiconductor quantum dots controlled by voltages applied to top gates. The different schemes can be difficult to compare meaningfully. Here we develop a theoretical framework to evaluate disparate qubit-gating schemes on an equal footing. We apply the procedure to two types of double-dot qubits: the singlet-triplet and the semiconducting quantum dot hybrid qubit. We investigate three quantum gates that flip the qubit state: a DC pulsed gate, an AC gate based on logical qubit resonance, and a gate-like process known as stimulated Raman adiabatic passage. These gates are all mediated by an exchange interaction that is controlled experimentally using the interdot tunnel coupling g and the detuning [Symbol: see text], which sets the energy difference between the dots. Our procedure has two steps. First, we optimize the gate fidelity (f) for fixed g as a function of the other control parameters; this yields an f(opt)(g) that is universal for different types of gates. Next, we identify physical constraints on the control parameters; this yields an upper bound f(max) that is specific to the qubit-gate combination. We show that similar gate fidelities (~99:5%) should be attainable for singlet-triplet qubits in isotopically purified Si, and for hybrid qubits in natural Si. Considerably lower fidelities are obtained for GaAs devices, due to the fluctuating magnetic fields ΔB produced by nuclear spins.

arXiv:1307.8406•Proceedings of the National Academy of Sciences•Nov 19, 2013•41 citations
A basic system with important potential applications in quantum technologies is a quantum dot in the
Voigt geometry. The spin states of the quantum dot in the Voigt geometry can act as a prototype qubit which
can be manipulated by applied optical fields in order to produce the necessary quantum gates. The basic method
for spin initialization in a quantum dot in the Voigt geometry is optical pumping. Here, we propose and analyze
a new method for the coherent preparation of the quantum dot spin states based on adiabatic control methods.
Specifically, we show that the application of two mutually delayed and partially overlapping optical pulses, similar
to those used in stimulated Raman adiabatic passage, can lead to initialization of one of the spin states with high
fidelity. We also demonstrate that the fidelity of the method may be increased by integrating the quantum dot with
a micropillar cavity. Specifically, we show that a preferential Purcell-enhanced decay rate towards the target spin
state, in certain cases, increases the fidelity of spin initialization of the adiabatic method. Our results are based on
the numerical solution of the relevant density matrix equations for the quantum dot system, either in an isotropic
photonic environment or in a micropillar cavity. The calculations presented in this paper are not limited to quantum
dots in micropillar cavities. Similar effects can be obtained by other photonic structures as well, as for example,
for quantum dots in photonic crystal cavities.
We use mutually delayed and partially overlapping optical pulses, similar to those used in stimulated Raman adiabatic passage and its variations, for the coherent control of quantum dot spin states in the Voigt geometry. We consider the quantum dot system initially in an incoherent mixture of the two electron-spin states. We show that the application of regular delayed and partially overlapping pulses can lead to initialization. In addition, if initially delayed, partially overlapping, and simultaneously switched off pulses are applied, the initially incoherent mixture can be changed to a specifically designed coherent superposition state. We also find that due to the initial conditions of the studied quantum system, the proposed methods work for different pulse orderings.
We study the interaction of a quantum dot in the Voigt configuration with a laser pulse and particularly analyze the potential for rapid spin initialization by putting the quantum dot near a molybdenum disulfide (MoS 2 ) monolayer. The MoS 2 monolayer influences the spontaneous decay rates of the quantum dot, leading to anisotropically enhanced decay rates, for the quantum dot’s electric dipole moments parallel and perpendicular to the layer. By solving the relevant density matrix equations, we find that high spin initialization fidelity is obtained at short times. The fidelity is significantly higher than when the quantum dot is in free-space vacuum. We examine two different cases of the interaction of the quantum dot with the applied optical field. First, we use a continuous wave laser field and determine for various quantum dot—MoS 2 layer distances the field strength that leads to acceptable fidelity levels. The effect of the quality of the MoS 2 material on the fidelity of spin initialization is also examined. We also study the interaction of the quantum dot with a laser pulse and apply numerical optimal control to obtain the time-dependent field strength, which leads to maximum final fidelity for short time intervals. The latter approach gives beneficial results in comparison to the continuous wave field excitation.
The spin of a charged semiconductor quantum dot (QD) coupled to an optical cavity is a promising candidate for high-fidelity spin-photon interfaces; the cavity selectively modifies the decay rates of optical transitions such that spin initialization, manipulation, and readout are all possible in a single magnetic field geometry. By performing cavity QED calculations, we show that a cavity with a single, linearly polarized mode can simultaneously support both high-fidelity optical spin initialization and readout in a single, in-plane (Voigt geometry) magnetic field. Furthermore, we demonstrate that single-mode cavities always outperform bimodal cavities in experimentally favorable driving regimes. Our analysis, when combined with established methods of control in a Voigt geometry field, provides optimal parameter regimes for high-fidelity initialization and readout, and coherent control in both cavity configurations, providing insights for the design and development of QD spin-photon interfaces as the basis of quantum network nodes and for the generation of photonic graph states. Published by the American Physical Society 2024
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.
We investigate cavity-assisted Stimulated Raman Adiabatic passage (STIRAP) schemes in semiconductor quantum dots (QDs) embedded in an optical cavity as a route for generation of high-quality single photons with programmable waveform. This work addresses the need for high-purity, indistinguishable photons in linear quantum computing, boson sampling, and quantum communications. We develop a time-dependent Maxwell-pseudospin model of single-photon generation through cavity-assisted adiabatic passage in a Λ-system isolated in a neutral InAs QD in a realistic GaAs/AlGaAs micropillar cavity. As a model Λ-system, we consider QD biexciton triplet states coupled to dark-exciton states by a circularly polarised pulse and a cavity field. Our simulations demonstrate control of the emitted single-photon pulse waveform by the driving pulse characteristics: shape, duration, intensity and detuning.
Several logical qubits and quantum gates have been proposed for semiconductor quantum dots controlled by voltages applied to top gates. The different schemes can be difficult to compare meaningfully. Here we develop a theoretical framework to evaluate disparate qubit-gating schemes on an equal footing. We apply the procedure to two types of double-dot qubits: the singlet-triplet and the semiconducting quantum dot hybrid qubit. We investigate three quantum gates that flip the qubit state: a DC pulsed gate, an AC gate based on logical qubit resonance, and a gate-like process known as stimulated Raman adiabatic passage. These gates are all mediated by an exchange interaction that is controlled experimentally using the interdot tunnel coupling g and the detuning [Symbol: see text], which sets the energy difference between the dots. Our procedure has two steps. First, we optimize the gate fidelity (f) for fixed g as a function of the other control parameters; this yields an f(opt)(g) that is universal for different types of gates. Next, we identify physical constraints on the control parameters; this yields an upper bound f(max) that is specific to the qubit-gate combination. We show that similar gate fidelities (~99:5%) should be attainable for singlet-triplet qubits in isotopically purified Si, and for hybrid qubits in natural Si. Considerably lower fidelities are obtained for GaAs devices, due to the fluctuating magnetic fields ΔB produced by nuclear spins.