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arXiv:quant-ph/0609214

Long-distance teleportation of atomic qubit via optical interferometry

Y. P. Huang, M. G. Moore

Published Sep 27, 2006• 0 citations
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Abstract

The problem of long-distance teleportation of single-atom qubits via a common photonic channel is examined within the framework of a Mach-Zender optical interferometer. As expected, when a coherent state is used as input, a high-finesse optical cavity is required to overcome sensitivity to spontaneous emission. However, we find that a number-squeezed light field in a twin-Fock state can in principle create useful entanglement without cavity-enhancement. Both approaches require single photon counting detectors, and best results are obtained by combining cavity-feedback with twin-fock inputs. Such an approach may allow a fidelity of $.99$ using a two-photon input and currently available mirror and detector technology. In addition, the present approach can be conveniently extended to generate multi-site entanglement and entanglement swapping, both of which are necessities in quantum networks.

DOI
10.48550/arxiv.quant-ph/0609214

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The problem of long-distance teleportation of single-atom qubits via a common photonic channel is examined within the framework of a Mach-Zender optical interferometer. As expected, when a coherent state is used as input, a high-finesse optical cavity is required to overcome sensitivity to spontaneous emission. However, we find that a number-squeezed light field in a twin-Fock state can in principle create useful entanglement without cavity-enhancement. Both approaches require single photon counting detectors, and best results are obtained by combining cavity-feedback with twin-fock inputs. Such an approach may allow a fidelity of .99.99.99 using a two-photon input and currently available mirror and detector technology. In addition, the present approach can be conveniently extended to generate multi-site entanglement and entanglement swapping, both of which are necessities in quantum networks.
The problem of on-demand generation of entanglement between single-atom qubits via a common photonic channel is examined within the framework of optical interferometry. As expected, for a Mach-Zehnder interferometer with coherent laser beam as input, a high-finesse optical cavity is required to overcome sensitivity to spontaneous emission. We show, however, that with a twin-Fock input, useful entanglement can in principle be created without cavity enhancement. Both approaches require single-photon resolving detectors, and best results would be obtained by combining both cavity feedback and twin-Fock inputs. Such an approach may allow a fidelity of 0.99 using a two-photon input and currently available mirror and detector technology. In addition, we study interferometers based on NOON states, i.e., maximally entangled NNN-particle states, and show that they perform similarly to the twin-Fock states, yet without the need for high-precision photodetectors. The present interferometrical approach can serve as a universal, scalable circuit element for quantum information processing, from which fast quantum gates, deterministic teleportation, entanglement swapping, etc., can be realized with the aid of single-qubit operations.
The schemes proposed by S. Bose et al [Phys. Rev. Lett. 83, 5158 (1999)] and others for long distance atomic teleportation using cavity decay, gives message state dependent fidelity on successful attempt and in case of failure message state destroys. We propose a different scheme for teleportation of an atomic state from one cavity to a distant cavity by reflecting optical-pulse modes of an entangled coherent state (ECS) from single atom-cavity systems and detecting whether light is on or off. Unit success with unit fidelity is obtained for large mean photon number (|α|^2). For small |α|^2, however, there is some probability of failure but message state does not destroy, and this allows us to achieve unit success in a few repeated attempts. Unlike previous schemes, our scheme also enjoys advantages of deterministic generation of ECS, robustness of ECS against de-coherence due to photon absorption, and it does not require many-stage cavity interactions and single photon detection ability.
Summary form only given. Quantum teleportation allows to transfer the quantum state of a particle to a remote location without sending the particle itself. This makes it a versatile tool for the distribution of quantum states over long distances or the remote preparation of quantum memories as they are necessary in various scenarios of quantum information processing and quantum communication. In this context atomic systems are often discussed as promising candidates for the storage and manipulation of qubit states whereas photonic qubit states are easy to prepare and to distribute. Hybrid systems of entangled atom-photon pairs can serve as an interface between those different carriers of quantum information. We utilize this property to perform teleportation of the polarization state of an attenuated laser pulse onto a single atom over a distance of 20m.Our experimental sequence consists of two steps. First the spin state of the single atom is entangled with the polarization state of a single photon in a spontaneous emission process [1]. The single photon is then sent to a second laboratory 20m away where an attenuated laser pulse with a well defined polarization state |Ψ) is prepared. In a second step a Bell-state measurement on the joint polarization state of the single photon from the atom and the attenuated laser pulse is performed (Fig. 1a). This is done via interference of the photon and the pulse at a fiber beam-splitter and subsequent coincidence detection of photons at the outputs of the beam-splitter (Fig. 1b). This teleports the input state |Ψ) onto the atom - up to a unitary tranformation depending on the outcome of the Bell-state measurement.need to be indistinguishable in all degrees of freedom except for their polarization. In particular, the temporal shape of the pulse was matched to that of the single photon using an arbitrary waveform generator and an AOM in a closed-loop feedback control. We performed this experiment with input states |Ψ) = |H), |+45°) and |R) and evaluated the fidelity to find the atom in the correspondig output state. For |+45°) and |R) the fraction of coherent pulses containing two or more photons can cause the same signal as expected from a successful Bell-state projection which reduces the overall fidelity to 0.81 and 0.84, respectively. However, for |H) the fidelty is practically only limited by the precision of the atomic state readout and reaches a value of 0.93.
Institute of Quantum Optics — Teleportation is a prerequisite for the transfer of quantum information over large distances when the losses inherent in any quantum channel preclude a direct transfer. We demonstrate teleportation between two single-atom quantum memories in distant laboratories. By implementing a timeresolved photonic Bell-state measurement (BSM), which is based on two-photon quantum inference, we achieve a teleportation fidelity of 88 %, largely determined by our entanglement fidelity. The problem of limited photon collection efficiency in free space is overcome by trapping each atom in an optical cavity. Compared to previous experiments with remote single material qubits, our approach boosts the overall efficiency by almost five orders of magnitude. This results in success probabilities not predominantly limited by the photon generation and collection efficiency but by the transmission and detection losses inherent in the photonic BSM.
Individual atoms in optical cavities can provide an efficient interface between stationary qubits and flying qubits (photons), which is an essential building block for quantum communication. Furthermore, cavity-assisted controlled-not (cnot) gates can be used for swapping entanglement to long distances in a quantum repeater setup. Nonetheless, dissipation introduced by the cavity during the cnot may increase the experimental difficulty in obtaining long-distance entanglement distribution using these systems. We analyze and compare a number of cavity-based repeater schemes combining various entanglement generation schemes and cavity-assisted cnot gates. We find that a scheme, where high-fidelity entanglement is first generated in a two-photon detection scheme and then swapped to long distances using a recently proposed heralded controlled-ZZZ (cz) gate, exhibits superior performance compared to the other schemes. The heralded gate moves the effect of dissipation from the fidelity to the success probability of the gate thereby enabling high-fidelity entanglement swapping. As a result, high-rate entanglement distribution can be achieved over long distances even for low cooperativities of the atom-cavity systems. This high-fidelity repeater is shown to outperform the other cavity-based schemes by up to two orders of magnitude in the rate for realistic parameters and large distances (1000 km).
Long-distance entanglement distribution is the key task for quantum networks, enabling applications such as secure communication and distributed quantum computing. Here we report on novel developments extending the reach for sharing entanglement between a single 87^{87}87Rb atom and a single photon over long optical fibers. To maintain a high fidelity during the long flight times through such fibers, the coherence time of the single atom is prolonged to 7 ms by applying a long-lived qubit encoding. In addition, the attenuation in the fibers is minimized by converting the photon's wavelength to the telecom S-Band via polarization-preserving quantum frequency conversion. This enables to observe entanglement between the atomic quantum memory and the emitted photon after passing 101 km of optical fiber with a fidelity better than 70.8$\pm$2.4%. The fidelity, however, is no longer reduced due to loss of coherence of the atom or photon but in the current setup rather due to detector dark counts, showing the suitability of our platform to realize city-to-city scale quantum network links.