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arXiv:1002.4424 · Physical Review Letters

Cavity-Based Single Atom Preparation and High-Fidelity Hyperfine State Readout

Roger Gehr, Jürgen Volz, Guilhem Dubois, T. Steinmetz, Yves Colombe, Benjamin Lev, Romain Long, Jérôme Estève, Jakob Reichel

Published May 20, 2010• 137 citations
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Abstract

We prepare and detect the hyperfine state of a single 87Rb atom coupled to a fiber-based high-finesse cavity on an atom chip. The atom is extracted from a Bose-Einstein condensate and trapped at the maximum of the cavity field, resulting in a reproducibly strong atom-cavity coupling. We use the cavity reflection and transmission signal to infer the atomic hyperfine state with a fidelity exceeding 99.92% in a readout time of 100 μs. The atom is still trapped after detection.

DOI
10.1103/physrevlett.104.203602

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We prepare and detect the hyperfine state of a single 87Rb atom coupled to a fiber-based high-finesse cavity on an atom chip. The atom is extracted from a Bose-Einstein condensate and trapped at the maximum of the cavity field, resulting in a reproducibly strong atom-cavity coupling. We use the cavity reflection and transmission signal to infer the atomic hyperfine state with a fidelity exceeding 99.92% in a readout time of 100 μs. The atom is still trapped after detection.
We introduce lossless state detection of trapped neutral atoms based on cavity-enhanced fluorescence. In an experiment with a single 87Rb atom, a hyperfine-state-detection fidelity of 99.4% is achieved in 85 μs. The quantum bit is interrogated many hundreds of times without loss of the atom while a result is obtained in every readout attempt. The fidelity proves robust against atomic frequency shifts induced by the trapping potential. Our scheme does not require strong coupling between the atom and cavity and can be generalized to other systems with an optically accessible quantum bit.
The capability to measure the state of a neutral atom is vital to an atom-based quantum network, for applications including distributed quantum computing and long-distance quantum communication. However, single neutral atoms suffer from low achievable photon scattering rate and shallow trapping potential, which limits the fidelity and speed of state readout process. Here, by coupling a single neutral atom with a high-finesse fiber-based Fabry-Pérot microcavity in the Purcell regime, we realize strong enhancement of the atomic photoemission rate and high overall system efficiency, which enables ultrafast and high-fidelity discrimination of bright and dark hyperfine states of the atom. The readout fidelity can reach 99.1(2)% within 200 ns and 99.985(8)% within 9 μs. Furthermore, we demonstrate that state preparation via optical pumping can be efficiently accelerated by a real-time decision protocol based on ultrafast state readout. This work demonstrates the potential to serve as a high-performance atom-photon interface and paves the way to the implementation of a practical atom-based quantum network.
We have used a fiber Fabry-Perot cavity on an atom chip to obtain strong, extremely stable atom-cavity coupling for single atoms and Bose-Einstein condensates trapped inside the cavity. The coherent atom-cavity coupling parameter, g0/2pi ~ 209 MHz, is larger than both cavity decay, kappa/2pi ~ 53 MHz and spontaneous emission gamma/2pi ~ 3 MHz. Due to the small mode volume, these values, well inside the strong coupling regime of cavity QED, are realized with a cavity finesse F ~ 36000, which is a moderate value compared to usual strong-coupling cavity QED experiments. The cavity also achieves a very high cooperativity parameter, C=g02/2kappagamma ~ 130.The use of trapped Bose-Einstein condensates (BECs) is a fundamentally new situation for cavity QED, which should enable a new class of experiments, where atoms that share a well-defined internal and external quantum state collectively interact with a quantized mode of the cavity. Besides, as the spatial extension of the BEC assumes the fundamental limit that can be attained for an atom cloud - much smaller than an optical wavelength in our case - an extremely strong, stable and uniform coupling to the cavity mode is achieved. This makes the system interesting for applications such as quantum memories. For single atoms, our combination of high resonator cooperativity with extreme positioning precision enables qubit detection with near-unit efficiency. This realizes a long-awaited milestone for quantum information processing on atom chips.
In this thesis, a model system of quantum optics is experimentally studied - a single atom coupled to a high-finesse optical cavity. Upon excitation with laser pulses, the atom-cavity system undergoes vacuum Rabi oscillations between the atom and the cavity field. This is accompanied by the controlled emission of single photons. The optical resonator also enables efficient state detection of single neutral atoms. We introduce a method based on cavity-enhanced fluorescence and achieve a fidelity of 99.4 % for the detection of the atomic hyperfine state within 85 µs. Further, this thesis presents experiments on electromagnetically induced transparency with a single atom and on phase shaping of single photon wave packets. The atom-cavity system has promising applications in quantum information science.
Subsystem readout during a quantum process, or mid-circuit measurement, is crucial for error correction in quantum computation, simulation, and metrology. Ideal mid-circuit measurement should be faster than the decoherence of the system, high-fidelity, and nondestructive to the unmeasured qubits. Here, we use a strongly coupled optical cavity to read out the state of a single tweezer-trapped ^{87}Rb atom within a small tweezer array. Measuring either atomic fluorescence or the transmission of light through the cavity, we detect both the presence and the state of an atom in the tweezer, within only tens of microseconds, with state preparation and measurement infidelities of roughly 0.5% and atom loss probabilities of around 1%. Using a two-tweezer system, we find measurement on one atom within the cavity causes no observable hyperfine-state decoherence on a second atom located tens of microns from the cavity volume. This high-fidelity mid-circuit readout method is a substantial step toward quantum error correction in neutral atom arrays.
Summary form only given. The strong atom-photon coupling generated inside high-finesse optical micro cavity lets us detect a single atom trajectory in real time. Such a strong coupling is a candidate for trapping a single atom with a single photon. Dynamics of trapped atoms can be observed in situ and in real time by detecting intensity change on the transmission of a blue-shifted laser beam. We cooled down and trapped rubidium atoms by using conventional cell magneto optical trap (MOT) and launched them into the micro optical cavity which locates 35 mm above the MOT. By using this signal as trigger and changing the laser frequency, it is, in principle, possible to trap the atom in the cavity.