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Physical Review A

Photon-assisted conditionality for double-dot charge qubits in a single-mode cavity

Alexander V. Tsukanov

Published Jan 30, 2012• 10 citations
Publisher

Abstract

The problems of the design, control, and interaction of the single-electron double-dot charge qubits coherently coupled to the optical cavity mode are studied theoretically. A way to overcome the challenges concerned with the use of classical laser pulses for a qubit state engineering is described, replacing the lasers by a quantized photon field in a semiconductor cavity and a gate voltage. Using this strategy, a simple and efficient scheme is proposed for the creation of highly entangled multiqubit states like the nine-qubit Shor states.

DOI
10.1103/physreva.85.012331

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The problems of the design, control, and interaction of the single-electron double-dot charge qubits coherently coupled to the optical cavity mode are studied theoretically. A way to overcome the challenges concerned with the use of classical laser pulses for a qubit state engineering is described, replacing the lasers by a quantized photon field in a semiconductor cavity and a gate voltage. Using this strategy, a simple and efficient scheme is proposed for the creation of highly entangled multiqubit states like the nine-qubit Shor states.
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.
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.
Abstract Semiconductor quantum dots are ideal candidates for quantum information applications in solid-state technology. However, advanced theoretical and experimental tools are required to coherently control, for example, the electronic charge in these systems. Here we demonstrate how quantum optimal control theory provides a powerful way to manipulate the electronic structure of coupled quantum dots with an extremely high fidelity. As alternative control fields we apply both laser pulses as well as electric gates, respectively. We focus on double and triple quantum dots containing a single electron or two electrons interacting via Coulomb repulsion. In the two-electron situation we also briefly demonstrate the challenges of timedependent density-functional theory within the adiabatic local-density approximation to produce comparable results with the numerically exact approach.
Implementations for quantum computing require fast single- and multiqubit quantum gate operations. In the case of optically controlled quantum dot qubits, theoretical designs for long-range two- or multiqubit operations satisfying all the requirements in quantum computing are not yet available. We have developed a design for a fast, long-range two-qubit gate mediated by a photonic microcavity mode using excited states of the quantum-dot-cavity system that addresses these needs. This design does not require identical qubits, it is compatible with available optically induced single-qubit operations, and it advances opportunities for scalable architectures. We show that the gate fidelity can exceed 90% in experimentally accessible systems.
Based on superconducting charge qubits (SCCQs) coupled to a single-mode microwave cavity, we propose a scheme for generating charge cluster states. For all SCCQs, the controlled gate voltages are all in their degeneracy points, the quantum information is encoded in two logic states of charge basis. The generation of the multi-qubit cluster state can be achieved step by step on a pair of nearest-neighbor qubits. Considering effective long-rang coupling, we provide an efficient way to one-step generating of a highly entangled cluster state, in which the qubit-qubit coupling is mediated by the cavity mode. Our quantum operations are insensitive to the initial state of the cavity mode by removing the influence of the cavity mode via the periodical evolution of the system. Thus, our operation may be against the decoherence from the cavity.
The recent realization of a coherent interface between a single electron in a silicon quantum dot and a single photon trapped in a superconducting cavity opens the way for implementing photon-mediated two-qubit entangling gates. In order to couple a spin to the cavity electric field, some type of spin-charge hybridization is needed, which impacts spin control and coherence. In this work we propose a cavity-mediated two-qubit gate and calculate cavity-mediated entangling gate fidelities in the dispersive regime, accounting for errors due to the spin-charge hybridization, as well as photon- and phonon-induced decays. By optimizing the degree of spin-charge hybridization, we show that two-qubit gates mediated by cavity photons are capable of reaching fidelities exceeding 9090%90 in present-day device architectures. High iswap gate fidelities are achievable even in the presence of charge noise at the level of $2\phantom{\rule{0.28em}{0ex}}\ensuremath{\mu}\text{eV}$.