B. Julsgaard

Experimental demonstration of quantum memory for light

The information carrier of todays communications, a weak pulse of light, is an intrinsically quantum object. As a consequence, complete information about the pulse cannot be perfectly recorded in a classical memory, even in principle. In the field of quantum information, this has led to the long-standing challenge of how to achieve a high-fidelity transfer of an independently prepared quantum state of light onto an atomic quantum state. Here we propose and experimentally demonstrate a protocol for such a quantum memory based on atomic ensembles. Recording of an externally provided quantum state of light onto the atomic quantum memory is achieved with 70 per cent fidelity, significantly higher than the limit for classical recording. Quantum storage of light is achieved in three steps: first, interaction of the input pulse and an entangling field with spin-polarized caesium atoms; second, subsequent measurement of the transmitted light; and third, feedback onto the atoms using a radio-frequency magnetic pulse conditioned on the measurement result. The density of recorded states is 33 per cent higher than the best classical recording of light onto atoms, with a quantum memory lifetime of up to 4 milliseconds.

Quantum memory for light

The work reports on the experiments performed implementing quantum memory for light utilizing an atomic spin polarized gas of caesium atoms. The fidelity of the mapping up to 70%, significantly higher than the benchmark classical memory fidelity has been demonstrated. Future plans for extending the memory performance towards other quantum states of light and the memory readout protocols is described in the paper.

Recording quantum properties of light in a long-lived atomic spin state: towards quantum memory.

We report an experiment on mapping a quantum state of light onto the ground state spin of an ensemble of Cs atoms with the lifetime of 2 ms. Recording of one of the two quadrature phase operators of light is demonstrated with vacuum and squeezed states of light. The sensitivity of the mapping procedure at the level of approximately 1 photon/sec per Hz is shown. The results pave the road towards complete (storing both quadrature phase observables) quantum memory for Gaussian states of light. The experiment also sheds new light on fundamental limits of sensitivity of the magneto-optical resonance method.

Deterministic Atom–Light Quantum Interface

Abstract The notion of an atom–light quantum interface has been developed in the past decade, to a large extent due to demands within the new field of quantum information processing and communication. A promising type of such interface using large atomic ensembles has emerged in the past several years. In this chapter we review this area of research with a special emphasis on deterministic high fidelity quantum information protocols. Two recent experiments, entanglement of distant atomic objects and quantum memory for light are described in detail.

Multimode entanglement of light and atomic ensembles via off-resonant coherent forward scattering

Quantum theoretical treatment of coherent forward scattering of light in a polarized atomic ensemble with an arbitrary angular momentum is developed. We consider coherent forward scattering of a weak radiation field interacting with a realistic multi-level atomic transition. Based on the concept of an effective Hamiltonian and on the Heisenberg formalism, we discuss the coupled dynamics of the quantum fluctuations of the polarization Stokes components of propagating light and of the collective spin fluctuations of the scattering atoms. We show that in the process of coherent forward scattering this dynamics can be described in terms of a polariton-type spin wave created in the atomic sample. Our work presents a general example of entangling process in the system of collective quantum states of light and atomic angular momenta, previously considered only for the case of spin 1/2 atoms. We use the developed general formalism to test the applicability of spin 1/2 approximation for modelling the quantum non-demolishing measurement of atoms with a higher angular momentum.

Quantum memory and teleportation using macroscopic gas samples

A long-standing goal in the quantum information community has been to realize quantum networks between distant sites. In this tutorial we describe the experimental demonstration of three crucial components in such a network using the off-resonant Faraday interaction between macroscopic atomic ensembles and coherent light. These are the realization of (a) deterministic entanglement between atomic samples in separate environments, (b) quantum mapping of an unknown light state into an atomic memory and (c) disembodied transport of states between quantum nodes via light–atom teleportation.

Entanglement and quantum teleportation with multi-atom ensembles

Atomic ensembles containing a large number of atoms have been proved to be an effective medium for quantum–state (quantum information) engineering and processing via their coupling with multi–photon light pulses. The general mechanism of this coupling, which involves continuous quantum variables for light and atoms, is described. The efficient quantum interface between light and atoms has led to the recent demonstration of an entangled state of two macroscopic atomic objects, more precisely two caesium gas samples. Based on this result, a proposal for teleportation of an entangled state of two atomic samples (entanglement swapping) is presented.

Distant Entanglement of Macroscopic Gas Samples

One of the main ingredients in most quantum information protocols is a reliable source of two entangled systems. Such systems have been generated experimentally several years ago for light [Aspect 1982 (b); Shih 1988; Ou 1992; Kwiat 1995; Schori 2002 (b)] but has only in the past few years been demonstrated for atomic systems [Hagley 1997; Sackett 2000; Julsgaard 2001; Roos 2004]. None of these approaches however involve two atomic systems situated in separate environments. This is necessary for the creation of entanglement over arbitrary distances which is required for many quantum information protocols such as atomic teleportation [Bennett 1993; Kuzmich 2000]. We present an experimental realization of such distant entanglement based on an adaptation of the entanglement of macroscopic gas samples containing about 1011 cesium atoms shown in Ref. [Julsgaard 2001]. The entanglement is generated via the off-resonant Kerr interaction between the atomic samples and a pulse of light. The achieved entanglement distance is 0.35 m but can be scaled arbitrarily. The feasibility of an implementation of various quantum information protocols using macroscopic samples of atoms has therefore been greatly increased. We also present a theoretical modeling in terms of canonical position and momentum operators (hat X) and (hat P) describing the entanglement generation and verification in presence of decoherence mechanisms.

Quantum limits encountered in atomic spin measurements

Abstract The research program investigating the limits imposed by quantum mechanics on the precision with which a collective atomic spin can be measured is reviewed in this paper. We consider both the cases, when the interaction between atoms and light is weak and strong. In the first case, the precision of the measurement is mainly limited by the quantum noise of light, and the atomic spin noise plays a minor role. In the latter case, we must consider the quantum mechanical measurement induced back action feeding quantum noise of light into the atomic spin system. Here, we show that the precision is limited by atomic quantum spin noise as well as the optical quantum noise. Although it is disturbing for polarization spectroscopy, the measurement induced back action may prove to be useful from a quantum informatics viewpoint, since this enables storage of the polarization state of light in the atomic spin.