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Optics Express

Quantum nondestructive determination of qubit states in low-Q cavities via single-photon input-output process

Hao Yuan, Lian-Fang Han

Published Mar 4, 2016• 1 citation
Publisher

Abstract

We present an efficient proposal to realize quantum nondestructive determination (QNDD) of unknown single-qubit states in two types of low-Q cavities, i.e., single-sided and double-sided cavity QED systems. In the dispersive regime, we demonstrate that the QNDD of single-qubit states can be realized by detecting qubit-state-dependent phase shifts (QSDPSs) of the reflected or transmitted photons from the cavity in the single-photon input-output process. Our proposal could be straightforwardly extended to the case of multiple-qubit states. Furthermore, the experimental feasibility of our proposal is also analyzed in experimentally-demonstrated circuit QED systems. The distinct feature of our proposal is that our proposal works in the dispersive regime of low-Q cavities and it is robust to both cavity decay and atomic spontaneous emission.

DOI
10.1364/oe.24.005487

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We present an efficient proposal to realize quantum nondestructive determination (QNDD) of unknown single-qubit states in two types of low-Q cavities, i.e., single-sided and double-sided cavity QED systems. In the dispersive regime, we demonstrate that the QNDD of single-qubit states can be realized by detecting qubit-state-dependent phase shifts (QSDPSs) of the reflected or transmitted photons from the cavity in the single-photon input-output process. Our proposal could be straightforwardly extended to the case of multiple-qubit states. Furthermore, the experimental feasibility of our proposal is also analyzed in experimentally-demonstrated circuit QED systems. The distinct feature of our proposal is that our proposal works in the dispersive regime of low-Q cavities and it is robust to both cavity decay and atomic spontaneous emission.
We propose an approach to detect an unknown quantum state of the atom(s) by measuring the phase shifts of the transmitted photons through a dispersively-coupled cavity. In the framework of the input-output theory, we derive the relations between the phase shifts of the transmitted photons and the states of the atom(s) in the cavity. It is shown that due to the dispersive interaction between the cavity and the atom(s), information about the atomic state can then be extracted by measuring the phase shifts of the transmitted photons through the cavity. The feasibility of the proposal is also discussed with the experimental parameters by numerical method.
We propose a scheme for realization of three-qubit controlled-phase gate via passing two three-level atoms through a high-Q optical cavity in a cavity QED system. In the presented protocol, the two stable ground states of the atoms act as the two controlling qubits and the zero- and one-photon Fock states of the cavity-field form the target qubit, and no auxiliary state or any measurement is required. The numerical simulation shows that the gate fidelities remain at a high level under the influence of the atomic spontaneous emission, the decay of the cavity mode and deviation of the coupling strength. The experimental feasibility of our proposal is also discussed.
Both cavity quantum electrodynamics and photons are promising candidates for quantum information processing. We consider a combination of both candidates with a single photon going through spatially separate cavities to entangle the atomic qubits, based on the input-output process of the cavities. We present a general expression for the input-output process regarding the low-QQQ cavity confining a single atom, which works in a wide range of parameters. Focusing on low-QQQ cavity case, we propose some schemes for quantum information processing with Faraday rotation using single photons, which is much different from the high-QQQ cavity and strong-coupling cases.
Nondestructive detection of photonic qubits will enable important applications in photonic quantum information processing and quantum communications. Here we present an approach based on a solid-state cavity containing an ensemble of rare-earth ions. First a probe pulse containing many photons is stored in the ensemble. Then a single signal photon, which represents a time-bin qubit, imprints a phase on the ensemble that is due to the ac Stark effect. This phase does not depend on the exact timing of the signal photon, which makes the detection insensitive to the time-bin qubit state. Then the probe pulse is retrieved and its phase is detected via homodyne detection. We show that the cavity leads to a dependence of the imprinted phase on the probe photon number, which leads to a spreading of the probe phase, in contrast to the simple shift that occurs in the absence of a cavity. However, we show that this scenario still allows nondestructive detection of the signal. We discuss potential implementations of the scheme, showing that high success probability and low loss should be simultaneously achievable.
We investigate an atomic entanglement purification protocol based on the coherent state input-output process by working in low-Q cavity in the atom-cavity intermediate coupling region. The information of entangled states are encoded in three-level configured single atoms confined in separated one-side optical micro-cavities. Using the coherent state input-output process, we design a two-qubit parity check module (PCM), which allows the quantum nondemolition measurement for the atomic qubits, and show its use for remote parities to distill a high-fidelity atomic entangled ensemble from an initial mixed state ensemble nonlocally. The proposed scheme can further be used for unknown atomic states entanglement concentration. Also by exploiting the PCM, we describe a modified scheme for atomic entanglement concentration by introducing ancillary single atoms. As the coherent state input-output process is robust and scalable in realistic applications, and the detection in the PCM is based on the intensity of outgoing coherent state, the present protocols may be widely used in large-scaled and solid-based quantum repeater and quantum information processing.