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Quantum Electronics

Measurements of quantum dot level populations using an optical waveguide

Alexander V. Tsukanov

Published Jan 1, 2021• 5 citations
Publisher

Abstract

Abstract A scheme is proposed for measuring electron level populations in a semiconductor quantum dot embedded in a multimode waveguide. The photon transmittance of the waveguide under steady-state pumping is shown to depend on the presence of an electron in the ground state of the quantum dot. The influence of waveguide, quantum dot, and photon source parameters on the electron detection process is examined. It is shown that, even at a moderate mode Q-factor (104 to 105), the measurement contrast and signal-to-noise ratio are rather high (above 104). The feasibility of using the proposed method for probing the state of a charge qubit is discussed.

DOI
10.1070/qel17441

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Abstract A spectroscopic approach to measuring a charge qubit placed in a waveguide consisting of three microcavities (a photonic molecule) is discussed. The logical states of a qubit are represented by the single-electron orbitals of a semiconductor double quantum dot. The impact of different factors that cause deviations of the system’s parameters from specified values is investigated. The possibility of controlling the spectrum of a photonic molecule using a local change in its dielectric properties in order to optimize the qubit measurement procedure is analyzed. The functions of the measuring contrast and signal-to-noise ratio are calculated depending on the main parameters of the system.
A scheme for determining an arbitrary pure state of a charge qubit by measuring a steady-state current through a single-electron transistor is proposed. In order to increase the sensitivity of the transistor its operating part consists of three quantum dots, the energy levels of which form a symmetrical configuration. Such a choice makes it possible to realize high-precision measurements in a nonresonant steady-state regime when a maximum distance of the qubit from this structure is several hundreds of nanometers. Time-dependent populations of the structure states are obtained; a current and a measurement contrast are calculated as functions of system geometric parameters and the optimal values of these parameters are found.
Chip-based cavity quantum electrodynamics (QED) devices consisting of a self-assembled InAs quantum dot (QD) coupled to a high quality factor GaAs microdisk cavity are coherently probed through their optical channel using a fiber taper waveguide. We highlight one particularly important aspect of this all-fiber measurement setup, which is the accuracy to which the optical coupling level and optical losses are known relative to typical free-space excitation techniques. This allows for precise knowledge of the intracavity photon number and measurement of absolute transmitted and reflected signals. Resonant optical spectroscopy of the system under both weak and strong driving conditions are presented, which when compared with a quantum master equation model of the system allows for determination of the coherent coupling rate between QD exciton and optical cavity mode, the different levels of elastic and inelastic dephasing of the exciton state, and the position and orientation of the QD within the cavity. Pump-probe measurements are also performed in which a far off-resonant red-detuned control laser beam is introduced into the cavity. Rather than producing a measurable ac Stark shift in the exciton line of the QD, we find that this control beam induces a saturation of the resonant system response. The broad photoluminescence spectrum resulting from the presence of the control beam in the cavity points to sub-band-gap absorption in the semiconductor, and the resulting free-carrier generation, as the likely source of system saturation.
Abstract A circuit of a measuring chip for determining an arbitrary pure state of a charge qubit is proposed. The strength of the current flowing through a single-electron transistor in the steady-state mode depends on the state of the qubit. To enhance the sensitivity of the transistor, its working part is built from three quantum dots (QDs) with the energy levels forming a symmetric configuration. The parameters of the system are calculated using a microscopic model of two-dimensional QDs. The time dependences of the population of the structure’s states are obtained and the current strength, sensitivity, and measuring contrast as functions of the geometric parameters of the system are determined. The effect of dissipative processes related to the acoustic phonons on the measurements is investigated and the rates of electron relaxation and dephasing in a two-level system are calculated.