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European Quantum Electronics Conference (EQEC)

Observation of atom - photon entanglement

Jürgen Volz, Markus Weber, Wenjamin Rosenfeld, Daniel Schlenk, Johannes Vrana, Karen Saucke, Christian Kurtsiefer, Harald Weinfurter

Published Jan 18, 2006• 1 citation
Publisher

Abstract

A single /sup 87/Rb atom into an optical dipole trap is loaded from a magneto optical trap. To generate entanglement between the trapped atom and an emitted photon, the atom is excited via an optical /spl phi/-pulse to the | F' = 0, m/sub F'/ = 0} state. Because of conservation of angular momentum, the polarization of the spontaneously emitted photon is entangled with the final Zeeman state of the atom.

DOI
10.1109/eqec.2005.1567479

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A single /sup 87/Rb atom into an optical dipole trap is loaded from a magneto optical trap. To generate entanglement between the trapped atom and an emitted photon, the atom is excited via an optical /spl phi/-pulse to the | F' = 0, m/sub F'/ = 0} state. Because of conservation of angular momentum, the polarization of the spontaneously emitted photon is entangled with the final Zeeman state of the atom.
We demonstrate atom--photon entanglement using a single cesium atom trapped in an optical tweezer. Entanglement is generated by resonant excitation and subsequent spontaneous decay, which entangles the atomic Zeeman state with photon polarization. The photon is collected with a high numerical aperture objective (NA = 0.55) and coupled into a single-mode fiber, enabling atom photon measurements and measurement of the Bell-state fidelity. We obtain raw entanglement fidelity of F=0.942(16){\mathcal F} = 0.942(16)F=0.942(16) and inferred fidelity of Finf=0.962(26){\mathcal F}_{\rm inf} = 0.962(26)Finf​=0.962(26) after correcting independently characterized atom measurement errors. Compared with related free-space experiments using 87^{87}87Rb, the multilevel structure of the relevant excited state in 133^{133}133Cs requires the use of a single short excitation pulse in each entanglement attempt in order to suppress unwanted re-excitation. These results establish a free-space Cs atom--photon interface and provide a step toward dual-species Rb--Cs quantum networking.
The coherent control of single-photon emitters as, e.g., single ions or atoms, is a crucial element for mapping quantum information between light and matter. The possibility of generating entanglement between a photon and the emitter system provides an interface between matter-based quantum memories and photonic quantum communication channels, which is the essential resource for quantum repeaters and other future quantum information applications. To generate entangled atom-photon states, in our experiment, we store a single 87 Rb atom in an optical dipole trap. The single-atom/single-photon character is confirmed by the observation of photon antibunching in the detected fluorescence light. The spectral properties of single photons emitted by the atom allowed us to determine the mean kinetic energy of the atom corresponding to 105 μK. We describe a single-atom state analysis method which allowed us to characterize the entanglement between the atom and a single photon emitted in the spontaneous decay. We obtain an entanglement fidelity of 89% that clearly shows the high degree of entanglement in our system and potential for further applications in quantum communication.
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.
In this thesis a single atom is trapped in the mode of a high-finesse cavity and used to generate single photons in a controlled way. Starting from a magneto-optical trap, rubidium atoms are loaded into an optical dipole-force trap and transported to the cavity. On arrival the atoms are trapped in another trap with a standing wave geometry and excited by a pump laser incident perpendicular to the cavity axis. With the help of cavity cooling forces, a single atom could then be selected and trapped for up to a minute. A sequence of laser pulses is applied to induce the emission of a stream of single photons from the atom. Up to 300000 photons could be generated from the same atom in this way. The single-photon character of the source is verified within a few seconds after which the photons can be distributed to an independent user while monitoring the presence of the atom.
In the experiment, single Rubidium-87 atoms are loaded out of a molasses of Doppler-cooled atoms into the focus of a Gaussian laser beam created by a single mode laser diode with a wavelength of 856 nm. A confocal microscope outside our vacuum chamber is used to focus the trapping beam to a minimal waist of 3.5 /spl mu/m (thus creating a trap depth of 1-2 mK) and to collect the fluorescence light from the center of the dipole trap. The collected fluorescence light is coupled into a single-mode optical fiber for spatial filtering, and the photons are detected with an avalanche photodiode. With this setup the overall detection efficiency of photons emitted from the trap region is about 0.2%. First experimental results show sub-Poissonian occupation statistics of the dipole trap: only single atoms are observed inside our trap, never two ore more. The measured 1/e lifetime of atoms inside the dipole trap in the presence of cooling light is about 1-2 seconds. To prove that we really observe single atoms inside the trap, a Hanbury-Brown-Twiss measurement of the fluorescence light from the trap will be shown. The trapping of single atoms inside our dipole trap marks the first step towards the investigation of the properties of an entangled atom photon state. Progress in preparing and analyzing single atoms in well defined hyperfine ground states will be reported.
We describe a simple experimental technique which allows us to store a single 87Rb^{87}\mathrm{Rb}87Rb atom in an optical dipole trap. Due to light-induced two-body collisions during the loading stage of the trap the maximum number of captured atoms is locked to one. This collisional blockade effect is confirmed by the observation of photon antibunching in the detected fluorescence light. The spectral properties of single photons emitted by the atom were studied with a narrow-band scanning cavity. We find that the atomic fluorescence spectrum is dominated by the spectral width of the exciting laser light field. In addition we observe a spectral broadening of the atomic fluorescence light due to the Doppler effect. This allows us to determine the mean kinetic energy of the trapped atom corresponding to a temperature of $105\phantom{\rule{0.3em}{0ex}}\ensuremath{\mu}\mathrm{K}$. This simple single-atom trap is the key element for the generation of atom-photon entanglement required for future applications in quantum communication and a first loophole-free test of Bell's inequality.