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arXiv:1202.1710 · Physical Review A

Optical qudit-type entanglement creation at long distances by means of small cross-Kerr nonlinearities

S. Ya. Kilin, Alexander B. Mikhalychev

Published May 5, 2011• 11 citations
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Abstract

Entanglement represents an important resource for quantum information processing, but its generation itself requires physical resources that are limited. We propose a scheme for generating a wide class of entangled qudit-type states of optical field modes at sites separated by noisy medium when only weak optical nonlinearities are available at both sites. The protocol is also based on exploiting a weak probe field, transmitted between the sites and used for generation of quantum correlations between two spatially separated field modes. The idea of probabilistic entanglement enhancement by measurement is discussed and a corresponding scheme for measuring the probe field state with linear optics and photodetectors not resolving photon numbers is proposed. It is shown that the protocol is applicable in the case when limited efficiency and dark counts of photodetectors, decoherence, and uncertainty of nonlinear coupling constants are present.

DOI
10.1103/physreva.83.052303

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Entanglement represents an important resource for quantum information processing, but its generation itself requires physical resources that are limited. We propose a scheme for generating a wide class of entangled qudit-type states of optical field modes at sites separated by noisy medium when only weak optical nonlinearities are available at both sites. The protocol is also based on exploiting a weak probe field, transmitted between the sites and used for generation of quantum correlations between two spatially separated field modes. The idea of probabilistic entanglement enhancement by measurement is discussed and a corresponding scheme for measuring the probe field state with linear optics and photodetectors not resolving photon numbers is proposed. It is shown that the protocol is applicable in the case when limited efficiency and dark counts of photodetectors, decoherence, and uncertainty of nonlinear coupling constants are present.
We propose a protocol for creating arbitrary qudit state (including entangled states) with arbitrary dimensionality in continuous variable system using weak cross-Kerr nonlinearity, linear beamsplitters, detectors not resolving photon numbers, and sources of coherent states. The equation for unique determination of the used coherent states amplitudes is found. The protocol is applicable for creating entangled states at distances of 100 km using cross-Kerr nonlinearity χ ≥ χ min ≃ 0.01 and optical fiber quantum channel.
We propose methods for generating Bell-type entangled coherent states for quantum information processing. The protocols are completed using linear optical elements, single-photon detectors, and cross-Kerr nonlinearity. We investigate the decoherence effect on our protocols; photon loss is found to affect only the probability, not the fidelity, of the generation process. The entanglement of the generated entangled states is also analyzed using concurrence. Finally, the feasibility of our scheme is discussed using the continuous-time cross-phase modulation model. In the weak nonlinearity region, on the condition that θ2|αp|2∼1, θ≪1, and σ2≪1, our scheme is applicable theoretically.
Processing information quantum mechanically is known to enable new communication and computational scenarios that cannot be accessed with conventional information technology (IT). It is known that such quantum processing can be performed with linear optical techniques, measurement and feed-forward. However, here the gates are intrinsically probabilistic and so scaling up such an approach requires considerable qubit resources. Here we present an alternative approach to optical QIP, based on the use of weak cross-Kerr non-linearities and highly efficient homodyne measurements. This approach enables a parity gate between optical qubits, mediated by an additional optical probe mode, enabling near-deterministic Bell state measurement and entangling gates. Our approach is therefore the basis for very efficient, scalable optical QIP, and provides a natural method for distributing such processing, combining it with quantum communication.
Abstract Employing high‐dimensional photonic encodings (qudits) instead of the traditional 2D encodings (qubits) can significantly enhance loss tolerance and reduce computational resources in photon‐based quantum information technology (QIT). To tap into this potential, effective schemes for the high‐dimensional generation of entangled states are essential. In this study, two arbitrary 4D entanglement generation protocols based on cross‐Kerr effect are developed, including two‐qudit entangled states with two photon pairs and three‐qudit entangled states with three photon pairs. These 4D entangled states require neither auxiliary photons (or entangled states) nor complicated quantum circuits. The success probabilities of high‐dimensional entangled states are close to 1 and their fidelities are robust against the photon loss with the current technology. The 4D entangled states depend on only simple linear‐optics elements, available four‐dimensional single‐qudit operations, and mature measurement methods, making our proposed protocols feasible and efficient in practical QIT.