Holger P. Specht

A single-atom quantum memory

The faithful storage of a quantum bit (qubit) of light is essential for long-distance quantum communication, quantum networking and distributed quantum computing. The required optical quantum memory must be able to receive and recreate the photonic qubit; additionally, it must store an unknown quantum state of light better than any classical device. So far, these two requirements have been met only by ensembles of material particles that store the information in collective excitations. Recent developments, however, have paved the way for an approach in which the information exchange occurs between single quanta of light and matter. This single-particle approach allows the material qubit to be addressed, which has fundamental advantages for realistic implementations. First, it enables a heralding mechanism that signals the successful storage of a photon by means of state detection; this can be used to combat inevitable losses and finite efficiencies. Second, it allows for individual qubit manipulations, opening up avenues for in situ processing of the stored quantum information. Here we demonstrate the most fundamental implementation of such a quantum memory, by mapping arbitrary polarization states of light into and out of a single atom trapped inside an optical cavity. The memory performance is tested with weak coherent pulses and analysed using full quantum process tomography. The average fidelity is measured to be 93%, and low decoherence rates result in qubit coherence times exceeding 180 microseconds. This makes our system a versatile quantum node with excellent prospects for applications in optical quantum gates and quantum repeaters.

Photon-photon entanglement with a single trapped atom.

An experiment is performed where a single rubidium atom trapped within a high-finesse optical cavity emits two independently triggered entangled photons. The entanglement is mediated by the atom and is characterized both by a Bell inequality violation of S=2.5, as well as full quantum-state tomography, resulting in a fidelity exceeding F=90%. The combination of cavity-QED and trapped atom techniques makes our protocol inherently deterministic--an essential step for the generation of scalable entanglement between the nodes of a distributed quantum network.

Polarization-Controlled Single Photons

Vacuum-stimulated Raman transitions are driven between two magnetic substates of a 87Rb atom strongly coupled to an optical cavity. A magnetic field lifts the degeneracy of these states, and the atom is alternately exposed to laser pulses of two different frequencies. This produces a stream of single photons with alternating circular polarization in a predetermined spatiotemporal mode. MHz repetition rates are possible as no recycling of the atom between photon generations is required. Photon indistinguishability is tested by time-resolved two-photon interference.

Phase shaping of single-photon wave packets

While the phase of a coherent light field can be precisely known, the phase of the individual photons that create this field, considered individually, cannot [1]. Phase changes within singlephoton wave packets, however, have observable effects. In fact, actively controlling the phase of individual photons has been identified as a powerful resource for quantum communication protocols [2, 3]. Here we demonstrate the arbitrary phase control of a single photon. The phase modulation is applied without affecting the photon’s amplitude profile and is verified via a two-photon quantum interference measurement [4, 5], which can result in the fermionic spatial behaviour of photon pairs. Combined with previously demonstrated control of a single photon’s amplitude [6, 7, 8, 9, 10], frequency [11], and polarisation [12], the fully deterministic phase shaping presented here allows for the complete control of single-photon wave packets. Consider two identical photons mode-matched at the two input ports (A and B) of a 50/50 non-polarising beam splitter (NPBS), represented by the initial state |Ψi〉 = |1A1B〉 (see Fig. 1). Due to the indistinguishability of the photons, the detection of one photon in output port C or D at time t0 projects the initial product state |Ψi〉 into the “which path” superposition state |Ψ±(t0)〉 = (|1A, 0B〉 ± |0A, 1B〉)/ √ 2 of the remaining photon. As first demonstrated by Hong, Ou and Mandel [4], the bosonic nature of photons always results in the detection of the second photon in the same output port as the first. However, we can alter this coalescence behaviour by introducing an arbitrary differential phase ∆φ between the two components of |Ψ±〉. This results in a phase-dependent wave function of the remaining single photon

Fast Excitation and Photon Emission of a Single-Atom-Cavity System

We report on the fast excitation of a single atom coupled to an optical cavity using laser pulses that are much shorter than all other relevant processes. The cavity frequency constitutes a control parameter that allows the creation of single photons in a superposition of two tunable frequencies. Each photon emitted from the cavity thus exhibits a pronounced amplitude modulation determined by the oscillatory energy exchange between the atom and the cavity. Our technique constitutes a versatile tool for future quantum networking experiments.

Scheme for generating a sequence of single photons of alternating polarization

Single-photons of well-defined polarization that are deterministically generated in a single spatio-temporal field mode are the key to the creation of multi-partite entangled states in photonic networks. Here, we present a novel scheme to produce such photons from a single atom in an optical cavity, by means of vacuum-stimulated Raman transitions between the Zeeman substates of a single hyperfine state. Upon each transition, a photon is emitted into the cavity, with a polarization that depends on the direction of the Raman process.

A single-atom optical quantum memory

A prerequisite for the realization of quantum networks [1] is a coherent interface between flying and stationary qubits. So far, most of these interfaces have been based on the exchange of information between photons and collective atomic excitations. A promising alternative is the development of an interface between single photons and a single atom, employing an optical cavity to achieve sufficient coupling strength. This approach has fundamental advantages, as it allows for the individual manipulation of the atomic qubit and the implementation of a heralding scheme based on quantum non-demolition (QND) measurements of the atomic state.

A universal single-atom based quantum node

We report our progress in the development of a universal node of a quantum network, capable of fully controlled photon generation, qubit storage and with intriguing perspectives towards the development of quantum gates.

A Single-Photon Server with Just One Atom

We trap a single atom in a cavity, and use it to produce a stream of up to 300000 single photons. Such a single-photon server is useful for quantum information science.

Generation of single photons of alternating polarization

We present first results of single-photon generation from an atom-cavity system, where Raman transitions are driven between Zeeman sublevels. The process results in photons with alternating polarisations and requires no repumping.