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European Quantum Electronics Conference (EQEC)

Towards probabilistic entanglement of distant atoms

Markus Hennrich, Felix Rohde, Carsten Schuck, Jürgen Eschner

Published Jan 18, 2006• 0 citations
Publisher

Abstract

The paper discussed on the generation of non-local entanglement. This relies on the state reduction of a projective measurement of the internal atomic states of both atoms. In this scheme both atoms are coherently excited by a weak excitation pulse by which a single photon is scattered with low probability. The authors are currently setting up an experimental system to implement this proposal with two trapped ions in two macroscopically separated traps.

DOI
10.1109/eqec.2005.1567444

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The paper discussed on the generation of non-local entanglement. This relies on the state reduction of a projective measurement of the internal atomic states of both atoms. In this scheme both atoms are coherently excited by a weak excitation pulse by which a single photon is scattered with low probability. The authors are currently setting up an experimental system to implement this proposal with two trapped ions in two macroscopically separated traps.
A scheme for entangling distant atoms is realized, as proposed in the seminal paper by [C. Cabrillo et al., Phys. Rev. A 59, 1025 (1999)]. The protocol is based on quantum interference and detection of a single photon scattered from two effectively one meter distant laser cooled and trapped atomic ions. The detection of a single photon heralds entanglement of two internal states of the trapped ions with high rate and with a fidelity limited mostly by atomic motion. Control of the entangled state phase is demonstrated by changing the path length of the single-photon interferometer.
We assess proposals for entangling two distant atoms by measurement of emitted photons, analyzing how their performance depends on the photon detection efficiency. We consider schemes based on measurement of one or two photons and compare them in terms of the probability to obtain the detection event and of the conditional fidelity with which the desired entangled state is created. Based on an unraveling of the master equation, we quantify the parameter regimes in which one or the other scheme is more efficient, including the possible combination of the one-photon scheme with state purification. In general, protocols based on one-photon detection are more efficient in setups characterized by low photon detection efficiency, while at larger values two-photon protocols are preferable. We give numerical examples based on current experiments.
The generation of atomic entanglement is discussed in a system that atoms are trapped in separate cavities which are connected via optical fibres. Two distant atoms can be projected to Bell-state by synchronized turning off of the local laser fields and then performing a single quantum measurement by a distant controller. The distinct advantage of this scheme is that it works in a regime where Δ ≈ κ ≫ g, which makes the scheme insensitive to cavity strong leakage. Moreover, the fidelity is not affected by atomic spontaneous emission.
Entanglement distribution between distant nodes is a critical requirement for the realization of quantum networks. Recently, entanglement between distant atomic qubits has been demonstrated experimentally [1]. This scheme uses the projective measurement of the photons emitted by the two distant atoms to entangle them. Another strategy is to generate entangled photon pairs by a parametric down-conversion process, and make them interact with the distant atoms, transferring the entanglement from photon to atomic qubits [2, 3]. Here we describe our activities towards studying this second kind of entanglement distribution scheme.
In 1935 Einstein, Podolsky and Rosen used the assumption of local realism to conclude in a Gedankenexperiment with two entangled particles that quantum mechanics is not complete. For this reason EPR motivated an extension of quantum mechanics by so-called local hidden variables. Based on this idea in 1964 Bell constructed a mathematical inequality whereby experimental tests could distinguish between quantum mechanics and local-realistic theories. Many experiments have since been done that are consistent with quantum mechanics, disproving the concept of local realism. But all these tests suffered from loopholes allowing a local-realistic explanation of the experimental observations by exploiting either the low detector efficiency or the fact that the detected particles were not observed space-like separated. In this context, of special interest is entanglement between different quantum objects like atoms and photons, because it allows one to entangle distant atoms by the interference of photons. The resulting space-like separation together with the almost perfect detection efficiency of the atoms allows a first loophole-free test of Bell's inequality. The primary goal of the present thesis is the experimental realization of entanglement between a single localized atom and a single spontaneously emitted photon at a wavelength suitable for the transport over long distances. In the experiment a single optically trapped Rb87 atom is excited to a state which has two selected decay channels. In the following spontaneous decay a photon is emitted coherently with equal probability into both decay channels. This accounts for perfect correlations between the polarization state of the emitted photon and the Zeeman state of the atom after spontaneous decay. Because these decay channels are spectrally and in all other degrees of freedom indistinguishable, the spin state of the atom is entangled with the polarization state of the photon. To verify entanglement, appropriate correlation measurements in complementary bases of the photon polarization and the internal quantum state of the atom are performed. It is shown, that the generated atom-photon state yields an entanglement fidelity of 0.82. The experimental results of this work mark an important step towards the generation of entanglement between space-like separated atoms for a first loophole-free test of Bell's inequality. Furthermore entanglement between a single atom and a single photon is an important tool for new quantum communication and information applications, e.g. the remote state preparation of a single atom over large distances.