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Optics and Spectroscopy

Quantum memory on a charge qubit in an optical microresonator

Alexander V. Tsukanov

Published Oct 1, 2017• 6 citations
Publisher

Abstract

A quantum-memory unit scheme on the base of a semiconductor structure with quantum dots is proposed. The unit includes a microresonator with single and double quantum dots performing frequencyconverter and charge-qubit functions, respectively. The writing process is carried out in several stages and it is controlled by optical fields of the resonator and laser. It is shown that, to achieve high writing probability, it is necessary to use high-Q resonators and to be able to suppress relaxation processes in quantum dots.

DOI
10.1134/s0030400x17100241

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A quantum-memory unit scheme on the base of a semiconductor structure with quantum dots is proposed. The unit includes a microresonator with single and double quantum dots performing frequencyconverter and charge-qubit functions, respectively. The writing process is carried out in several stages and it is controlled by optical fields of the resonator and laser. It is shown that, to achieve high writing probability, it is necessary to use high-Q resonators and to be able to suppress relaxation processes in quantum dots.
The concept of a quantum node consisting of a memory qubit and a frequency convertor is proposed and analysed. The memory qubit is presented by a semiconductor four-level double quantum dot (DQD) placed in an optical microresonator (MR). The DQD contains an electron in the quantised part of the conduction band and the MR can be populated by a certain number of photons. The DQD and MR states are controlled be applying the laser and electrostatic fields. The difference between the telecommunication frequency of the photon (transport qubit) supplied to the system through a waveguide and the frequency of the electronic transition in the DQD is compensated for using an auxiliary element, i.e. a frequency convertor based on a single quantum dot (QD). This design allows the electron – photon state of the hybrid system to be controlled by an appropriate variation of the field parameters and the switching between resonance and nonresonance DQD and MR interaction regimes. As an example, a GaAs DQD placed in a microdisk MR is studied. A numerical technique for modelling an optical spectrum of a microdisk MR with an additional layer (AL) deposited on its surface is developed. Using this technique, the effect of the AL on the MR eigenmode properties is investigated and the possibility of tuning its frequency to the QD electronic transition frequency by depositing an AL on the disk surface is demonstrated.
We study the possibility of implementing a two-qubit controlled-NOT gate operation in a structure consisting of two semiconductor double quantum dots placed in a high- Q optical microcavity and governed by a resonant laser field. The effect of relaxation processes on the dynamics of the two-electron system is discussed. The dissipation rates allowing quantum error correction algorithms to be used are determined. The existing additional excitation channel (laser pulse) is shown to weaken the effect of the nonideality of the cavity on the evolution of the states. Optimal electron coupling coefficients in the quantum dots with control fields are fitted, in which the controlled qubit is switched with the highest probability and the gate implementation takes several hundreds of picoseconds.
Abstract A scheme for measuring the state of a charge qubit on a semiconductor single-electron double quantum dot (DQD) coupled to a photonic molecule (PM) consisting of three optical microresonators is proposed. The DQD that is the qubit plays the role of a nonlinear element whose electron state affects a PM response to an external laser field. Analysis of the spectroscopic response of the structure in the steady-state regime allows one to determine the state of the qubit. As an example, the spectrum of the PM formed by three GaAs microdisk resonators are calculated. The effect of various system parameters on the measuring contrast and the signal-to-noise ratio is studied. It is shown that this ratio can reach values of 15 000–20 000 for certain sets of parameters.
We consider one- and two-qubit operations on charge qubits that represent double quantum dots with one electron in each of them. The dots are formed inside a high-Q semiconductor cavity (disk, toroidal, or spherical) in the antinodes of one of its optical eigenmodes; the frequencies of the transitions between the ground (logic) and excited (auxiliary) states of the discrete electron spectrum in quantum dots are close to the frequency of this mode. The precise tuning of the transition frequency is performed by applying an electric potential on the qubit control gate. Within the model of qubits coherently interacting with a cavity quantum field, a few methods for controlling their states are developed. In particular, we propose different variants for implementing two-qubit CNOT and CZ gates and generating qubit entangled states. By the example of a micro-disk cavity, we calculate the operating characteristics that ensure a high rate of quantum gate implementation.
The concept of multifunctional device (a quantum node) composed of a semiconductor single-electron four-level doublequantum dot coupled to an optical microcavity resonator is developed. The terahertz laser field and voltage biases provide an external control. The structure enables flexible driving via appropriate variations of field amplitudes and switching between resonant and off-resonant modes. As shown this hybrid electron-photon system can be used as the charge qubit with flying-to-stationary qubit conversion or the single-photon transistor and several others. Each of listed devices works in the specific regime of system evolution. For example, the qubit is robust when the optical resonator and laser Rabi frequencies dominate the dissipation rates – the so-called strong coupling or coherent regime. From another hand, in order to attain the steady-state one has to work in the so-called weak coupling or incoherent regime when the dissipation rates are comparable to or greater than the Rabi frequencies. Further, the single-photon driving is required for spectroscopic applications of this system. We numerically investigate the population dynamics to reveal the parameter choice corresponding to each device. The model is based on Lindblad formalism where all incoherent processes are considered as the markovian ones. The time dependencies of populations and spectrograms for different pairs of parameters are obtained. The specific features concerned with working characteristics of the quantum node in different modes are discussed.
A scheme for controlling a charge qubit on a semiconductor asymmetric double quantum dot with suppressed tunnel coupling between individual quantum dots is proposed. Laser pulses convert the logical single-electron states of the qubit into auxiliary trion states, for which the resonant tunneling condition is restored due to the compensation of structural asymmetry by the Coulomb interaction of the particles. The conditions for performing orthogonal single-qubit rotations are formulated. For a two-dimensional structure, the parameters of the qubit are calculated, and the dependence of the fidelity of the inversion operation on the parameters of the laser field and the rates of dissipative processes is obtained. It is shown that the considered algorithm is characterized by high speed and reliability.