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Quantum Interface between an Atom and a Photon

Tatjana Wilk

Published Feb 1, 2008• 11 citations
OpenAlex

Abstract

A single atom strongly coupled to a high-finesse optical-cavity is a versatile tool for quantum information processing. Utilized as a single-photon source, it allows one to generate single photons very efficiently in a well defined spatio-temporal mode. In a first experiment, polarization-control over the photons is shown. A time-resolved two-photon interference experiment proves the indistinguishability of these photons - required in various quantum information processing schemes. Moreover, in a second experiment, entanglement between the polarization of the emitted photon and the population of the atomic Zeeman levels is created. Subsequent state mapping of the atomic state onto another photon results in a pair of polarization-entangled photons emitted one after the other from the cavity. Although these schemes are in principle possible in free space, the cavity boosts the efficiency by several orders of magnitude.

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A single atom strongly coupled to a high-finesse optical-cavity is a versatile tool for quantum information processing. Utilized as a single-photon source, it allows one to generate single photons very efficiently in a well defined spatio-temporal mode. In a first experiment, polarization-control over the photons is shown. A time-resolved two-photon interference experiment proves the indistinguishability of these photons - required in various quantum information processing schemes. Moreover, in a second experiment, entanglement between the polarization of the emitted photon and the population of the atomic Zeeman levels is created. Subsequent state mapping of the atomic state onto another photon results in a pair of polarization-entangled photons emitted one after the other from the cavity. Although these schemes are in principle possible in free space, the cavity boosts the efficiency by several orders of magnitude.
The possibility of creating a scaleable quantum network by interconverting photonic and atomic qubits shows great promise. The fundamental requirement for such a network is deterministic control over the emission and absorption of photons from single atoms. This thesis reports on the experi-mental construction of a photon source that can emit single-photons with arbitrary spatio-temporal shape, phase, and frequency. The photon source itself is a strongly-coupled atom cavity system based on a single 87 Rb atom within a macroscopic high-finesse Fabry-Perot cavity. It operates intermittently for periods of up to 100µs, with single-photon repetition rates of 1.0 MHz and an efficiency of almost 80%. Atoms are loaded into the cavity using an atomic fountain, with the upper turning point near the centre of the cavity mode. This ensures long interaction times without any disturbances introduced by trapping potentials. The photons’ indistinguishability was tested, with a two-photon Hong-Ou-Mandel visibility of 87%. This ability to both generate, and control, the photons’ properties, for example producing photons with symmetric or multi-peaked spatio-temporal shapes, allows for the production of photons in an n-time-bin superposition state where each time-bin has an arbitrarily defined amplitude and phase. These photons can be used as photonic qubits, qutrits and qquads, and their properties have been tested using a small linear-optics network.
This thesis documents the distribution of an entangled state between a single atom trapped in the mode of a high-finesse optical cavity and two subsequently emitted photons. The long atom trapping times achieved in the system together with the high photon collection efficiency of the cavity make the applied protocol in principle deterministic. To verify and characterize the entanglement of the two emitted photons, the violation of a Bell inequality was observed and a full quantum state tomography was performed. The ability to efficiently transfer quantum information between the atom and the two photons reflects the advantages of high-finesse cavities as quantum interfaces in future quantum networks.
Summary form only given. In quantum information processing, a qubit is realised as a superposition of atomic or ionic eigenstates, or as a superposition of the states of a photon. A coupling between these different types of qubits is necessary to perform quantum operations or quantum transmissions, e.g. in a quantum network where single atoms or ions are entangled with individual photons. To generate such an entanglement in a controlled way, a triggered source for single photons, a photon pistol, will be needed. Our scheme will lead to the emission of one photon into a single, predetermined mode of the radiation field in response to a trigger event. This photon is emitted from an atom strongly coupled to a high-finesse optical cavity, and the trigger is a classical light pulse. The device combines cavity-QED with an adiabatic transfer technique. We consider a single lambda-type three-level atom being placed inside an empty cavity.
For quantum computation the efficient preparation and connection of multiple qubits must be addressed. This includes linear optical quantum computation and quantum networks of stationary nodes interlinked by flying qubits. A quantum emitter coupled to a single mode of the electric field is a versatile tool, with the potential to provide the on-demand generation of single photons or a matter-light interface in a quantum network. This thesis presents the construction and characterisation of an \emph{a priori} deterministic single-photon source using single 87Rb atoms coupled to a high finesse optical cavity. These photons show a high degree of indistinguishability and long coherence times with a two-photon Hong-Ou-Mandel visibility of (70.8±4.6)% over the entire interaction time of 300ns long photons, further increasing to ≥97.8% by post-selecting only detection events within 23ns of each other. No degradation of performance is observed when interfering these pairs through a 4x4 multimode interferometer integrated onto a photonic chip with non-classical correlations measured between photon detections orders of magnitude further separated in time than the propagation time through the chip. The production scheme for polarised single photons is considered in detail. Cavity birefringence is found to alter the polarisation state of the emitted photons and a novel model to incorporate this effect is presented. Further deviations from a simple coupling model are observed in the modified hyperfine atomic structure in the intermediate strength magnetic fields required for polarised photon emission. The behaviour in this regime is modelled and shown to explain previously unresolved observations of the asymmetric production efficiencies of orthogonally polarised photons.
Neutral atoms are ideal objects for the deterministic processing of quantum information. Entanglement operations have been performed by photon exchange or controlled collisions. Atom-photon interfaces were realized with single atoms in free space or strongly coupled to an optical cavity. A long standing challenge with neutral atoms, however, is to overcome the limited observation time. Without exception, quantum effects appeared only after ensemble averaging. Here we report on a single-photon source with one-and-only-one atom quasi permanently coupled to a high-finesse cavity. Quasi permanent refers to our ability to keep the atom long enough to, first, quantify the photon-emission statistics and, second, guarantee the subsequent performance as a single-photon server delivering up to 300,000 photons for up to 30 seconds. This is achieved by a unique combination of single-photon generation and atom cooling. Our scheme brings truly deterministic protocols of quantum information science with light and matter within reach.