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Physical Review A

Detection of nonlocal atomic entanglement assisted by single photons

Lan Zhou, Yu‐Bo Sheng

Published Aug 21, 2014• 67 citations
Publisher

Abstract

We present an efficient way for measuring the entanglement of atoms. The concurrence of atomic entanglement can be obtained according to the probability of picking up the singlet states of the atoms. In this protocol, we require the phenomenon of optical Faraday rotation, but the entangled atoms do not need to be in contact with each other and the atomic entanglement can be distributed nonlocally. Moreover, our way of measuring the concurrence for atoms can also be suitable for other solid systems, such as quantum dots inside microcavities and the nitrogen-vacancy (N-V) defect centers in diamonds, which can also induce the optical Faraday rotation. All these advantages provide important applications in future distributed quantum computations.

DOI
10.1103/physreva.90.024301

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We present an efficient way for measuring the entanglement of atoms. The concurrence of atomic entanglement can be obtained according to the probability of picking up the singlet states of the atoms. In this protocol, we require the phenomenon of optical Faraday rotation, but the entangled atoms do not need to be in contact with each other and the atomic entanglement can be distributed nonlocally. Moreover, our way of measuring the concurrence for atoms can also be suitable for other solid systems, such as quantum dots inside microcavities and the nitrogen-vacancy (N-V) defect centers in diamonds, which can also induce the optical Faraday rotation. All these advantages provide important applications in future distributed quantum computations.
We present an efficient way for measuring the entanglement of the atoms. Through the auxiliary single photons input-output process in cavity quantum electrodynamics (QED), the concurrence of the atomic entanglement can be obtained according to the success probability of picking up the singlet states of the atoms. This protocol has three advantages: First, we do not require the sophisticated controlled-not (CNOT) gates. Second, the distributed atoms are not required to intact with each other. Third, the atomic entanglement can be distributed nonlocally, which provides its important applications in distributed quantum computation.
We propose an efficient protocol for measuring the concurrence of arbitrary two-photon pure entangled state with the help of the photonic Faraday rotation. In the protocol, the concurrence of the photonic entangled state can be conversed into the total success probability for picking up the odd-parity photonic state. For completing the measurement task, we require some auxiliary three-level atoms, which are trapped in the low-quality cavities. Our protocol can be well realized under current experimental conditions. Moreover, under practical imperfect atom state detection and photonic Faraday rotation conditions, our protocol can also work well. Based on these features, our protocol may be useful in current quantum information processing.
We propose efficient schemes of direct concurrence measurement for two-qubit spin and photon-polarization entangled states via the interaction between single-photon pulses and nitrogen-vacancy (NV) centers in diamond embedded in optical microcavities. For different entangled-state types, diversified quantum devices and operations are designed accordingly. The initial unknown entangled states are possessed by two spatially separated participants, and nonlocal spin (polarization) entanglement can be measured with the aid of detection probabilities of photon (NV center) states. This non-demolition entanglement measurement manner makes initial entangled particle-pair avoid complete annihilation but evolve into corresponding maximally entangled states. Moreover, joint inter-qubit operation or global qubit readout is not required for the presented schemes and the final analyses inform favorable performance under the current parameters conditions in laboratory. The unique advantages of spin qubits assure our schemes wide potential applications in spin-based solid quantum information and computation.
Remote implementation of nonlocal quantum gates between different nodes in quantum communication network plays an important role in distributed quantum computation. Compared to protocol for nonlocal implementation of quantum operation in one degree of freedom (DoF), the protocol for parallel remote implementation of nonlocal operations simultaneously in two DoFs has the advantage of having high channel capacity. Under the balance condition of diamond nitrogen-vacancy center coupled to a single-side optical microcavity, we present a protocol for remote implementation of robust hyperparallel nonlocal operation in polarization, time-bin DoFs. Alice and Bob let photons pass through the diamond nitrogen-vacancy center system successively, perform single-qubit measurement on the entangled photons and the electrons. The robust nonlocal CNOT operation can be simultaneously implemented by applying unitary operations in accordance with the measurement results. The protocol has the advantage of transmitting less entangled photons for remote implementation of nonlocal CNOT operations in two DoFs. Moreover, the protocol is more convenient in application since the noise in the protocol for hyperparallel nonlocal CNOT operation via diamond nitrogen-vacancy center can be efficiently depressed.
Abstract The generation of entanglement between remote matter qubits has developed into a key capability for fundamental investigations as well as for emerging quantum technologies. In the single-photon, protocol entanglement is heralded by generation of qubit-photon entangled states and subsequent detection of a single photon behind a beam splitter. In this work we perform a detailed theoretical and experimental investigation of this protocol and its various sources of infidelity. We develop an extensive theoretical model and subsequently tailor it to our experimental setting, based on nitrogen-vacancy centers in diamond. Experimentally, we verify the model by generating remote states for varying phase and amplitudes of the initial qubit superposition states and varying optical phase difference of the photons arriving at the beam splitter. We show that a static frequency offset between the optical transitions of the qubits leads to an entangled state phase that depends on the photon detection time. We find that the implementation of a Charge-Resonance check on the nitrogen-vacancy center yields transform-limited linewidths. Moreover, we measure the probability of double optical excitation, a significant source of infidelity, as a function of the power of the excitation pulse. Finally, we find that imperfect optical excitation can lead to a detection-arm-dependent entangled state fidelity and rate. The conclusion presented here are not specific to the nitrogen-vacancy centers used to carry out the experiments, and are therefore readily applicable to other qubit platforms.