A scheme is proposed for converting a transport photonic qubit into a stationary qubit represented by the electronic states of quantum dots (QDs). The choice of the basis states of a qubit in the form of anti-symmetric combinations of the excited states of an array of QDs ensures their stability with respect to pho-ton/phonon relaxation processes. The formation of these states is due to the Stark and Förster interactions between electrons localized in the QDs. An algorithm for the controlled transformation (recording) of a pho-ton state into the electronic states of QDs using optical and electrostatic fields is considered. The possibility of tuning the frequency of the electronic transitions in the QDs in a gallium arsenide nanostructure using metal gates and a charged cantilever needle is studied.
A scheme is proposed for converting a transport photonic qubit into a stationary qubit represented by the electronic states of quantum dots (QDs). The choice of the basis states of a qubit in the form of anti-symmetric combinations of the excited states of an array of QDs ensures their stability with respect to pho-ton/phonon relaxation processes. The formation of these states is due to the Stark and Förster interactions between electrons localized in the QDs. An algorithm for the controlled transformation (recording) of a pho-ton state into the electronic states of QDs using optical and electrostatic fields is considered. The possibility of tuning the frequency of the electronic transitions in the QDs in a gallium arsenide nanostructure using metal gates and a charged cantilever needle is studied.
Abstract A scheme is proposed for converting a transport photonic qubit into a stationary qubit represented by the electronic states of quantum dots (QDs). The choice of the basis states of a qubit in the form of antisymmetric combinations of the excited states of an array of QDs ensures their stability with respect to photon/phonon relaxation processes. The formation of these states is due to the Stark and Förster interactions between electrons localized in the QDs. An algorithm for the controlled transformation (recording) of a photon state into the electronic states of QDs using optical and electrostatic fields is considered. The possibility of tuning the frequency of the electronic transitions in the QDs in a gallium arsenide nanostructure using metal gates and a charged cantilever needle is studied.
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.
We present models of quantum gates based on coupled quantum dots in which a qubit is regarded as the superposition of ground states in each dot. Coherent control on the qubit is performed by a monochromatic light pulse illuminated on the quantum dots. Considering particular types of artificial molecules, we show that the qubit is easily manipulated with the polarization and frequency of a light pulse and, especially, the controlled-NOT operation can be possible exploiting the electron-electron interaction. To examine the decoherence of quantum states, we discuss electronic relaxation contributed mainly by LA phonon processes.
We consider the possibility of using optical vibrational degrees of freedom as an additional resource for controlling the evolution of a charge qubit based on a semiconductor single-electron double quantum dot. The effect of electron – photon – phonon processes on the accuracy of single-qubit operations is investigated. It is shown that, depending on the phonon decay rate and detuning of the electronic transition frequency from the optical phonon frequency, these processes can both facilitate and prevent coherent electron transfer between quantum dots.
Self-assembled quantum dots (QDs) containing a single electron are one of the leading candidate systems for the realization of a quantum computer. The spin states of charges confined in these dots have been shown to possess a number of qualities that are attractive for use as the quantum bits (qubits) in quantum computing implementations, particularly their long lifetimes and coherence times. In addition, these spin qubits may be controlled optically, offering the prospect of ultrafast qubit gate operations, a crucial necessity for the execution of quantum algorithms. Due to the weak interaction of self-assembled QDs with light, however, the coherent optical control and read-out of the spin states of charged self-assembled QDs has proven to be a considerable challenge. This thesis presents two sets of experiments demonstrating the coherent optical control and read-out of the quantum states of a self-assembled InAs QD containing a single electron. The first utilizes mode-locked picosecond optical pulses to control the optical transitions in the dot and to detect QD level occupations. These capabilities are used to observe transient phenomena in a singly charged InAs QD such as the generation and decay of excited state population and spin precession. Further, Rabi oscillations between the electron and the lowest-lying excited state are observed, demonstrating the ability to coherently control the optical transitions of the QD, a prerequisite for spin control. These results are built upon in the second set of experiments, in which a combination of optical pulses, an externally applied DC magnetic field and a continuous-wave (CW) optical field are employed to initialize and completely control the states of a QD confined electron spin. Spin control by the use of pulse-driven two-photon Raman processes, spin precession about the external magnetic field and geometric phases generated by CW-driven cyclic evolutions in the dot is demonstrated. A number of spin qubit gate operations are shown for these spin control mechanisms, forming a foundational set of single qubit gates required for the implementation of quantum computing with singly charged self-assembled QDs.
Single spins in semiconductor quantum dots form a promising platform for solid-state quantum information processing. The spin-up and spin-down states of a single electron or hole, trapped inside a quantum dot, can represent a single qubit with a reasonably long decoherence time. The spin qubit can be optically coupled to excited (charged exciton) states that are also trapped in the quantum dot, which provides a mechanism to quickly initialize, manipulate and measure the spin state with optical pulses, and to interface between a stationary matter qubit and a 'flying' photonic qubit for quantum communication and distributed quantum information processing. The interaction of the spin qubit with light may be enhanced by placing the quantum dot inside a monolithic microcavity. An entire system, consisting of a two-dimensional array of quantum dots and a planar microcavity, may plausibly be constructed by modern semiconductor nano-fabrication technology and could offer a path toward chip-sized scalable quantum repeaters and quantum computers. This article reviews the recent experimental developments in optical control of single quantum dot spins for quantum information processing. We highlight demonstrations of a complete set of all-optical single-qubit operations on a single quantum dot spin: initialization, an arbitrary SU(2) gate, and measurement. We review the decoherence and dephasing mechanisms due to hyperfine interaction with the nuclear-spin bath, and show how the single-qubit operations can be combined to perform spin echo sequences that extend the qubit decoherence from a few nanoseconds to several microseconds, more than 5 orders of magnitude longer than the single-qubit gate time. Two-qubit coupling is discussed, both within a single chip by means of exchange coupling of nearby spins and optically induced geometric phases, as well as over longer-distances. Long-distance spin-spin entanglement can be generated if each spin can emit a photon that is entangled with the spin, and these photons are then interfered. We review recent work demonstrating entanglement between a stationary spin qubit and a flying photonic qubit. These experiments utilize the polarization- and frequency-dependent spontaneous emission from the lowest charged exciton state to single spin Zeeman sublevels.