Y. O. Dudin

Strongly Interacting Rydberg Excitations of a Cold Atomic Gas

Ultimate Blockade A Rydberg atom has an electron in a highly excited energy state, close to being set free, but not quite. Ensembles of such atoms interact strongly, sometimes leading to blockade effects where the excitation of one atom prevents the excitation of another. Dudin and Kuzmich (p. 887, published online 19 April; see the Perspective by Grangier) demonstrate the generation of a many-body excitation with no more than one Rydberg atom in a mesoscopic ensemble of ultracold atoms. When the principal quantum number was increased beyond 70, the excitation was converted into a photon. The ability to control the creation of excitations provides a promising system for quantum information storage, as well as a source of correlated photons. Illumination of an ensemble of cold rubidium atoms ultimately leads to high-level excitation of just a single atom. Highly excited Rydberg atoms have many exaggerated properties. In particular, the interaction strength between such atoms can be varied over an enormous range. In a mesoscopic ensemble, such strong, long-range interactions can be used for fast preparation of desired many-particle states. We generated Rydberg excitations in an ultra-cold atomic gas and subsequently converted them into light. As the principal quantum number n was increased beyond ∼70, no more than a single excitation was retrieved from the entire mesoscopic ensemble of atoms. These results hold promise for studies of dynamics and disorder in many-body systems with tunable interactions and for scalable quantum information networks.

Observation of coherent many-body Rabi oscillations

A two-level quantum system driven by an electromagnetic field can oscillate between its two states. The effects of these so-called Rabi oscillations are usually obscured in many-body systems by the variation in properties of the particles involved. Now, however, coherent many-body Rabi oscillations are observed in a vapour made up of several hundred cold rubidium atoms.

Entanglement of Light-Shift Compensated Atomic Spin Waves with Telecom Light

Entanglement of a 795 nm light polarization qubit and an atomic Rb spin-wave qubit for a storage time of 0.1 s is observed by measuring the violation of Bells inequality (S=2.65±0.12). Long qubit storage times are achieved by pinning the spin wave in a 1064 nm wavelength optical lattice, with a magic-valued magnetic field superposed to eliminate lattice-induced dephasing. Four-wave mixing in a cold Rb gas is employed to perform light qubit conversion between near infrared (795 nm) and telecom (1367 nm) wavelengths, and after propagation in a telecom fiber, to invert the conversion process. Observed Bell inequality violation (S=2.66±0.09), at 10 ms storage, confirms preservation of memory-light entanglement through the two stages of light qubit frequency conversion.

Entanglement between light and an optical atomic excitation

The generation, distribution and control of entanglement across quantum networks is one of the main goals of quantum information science. In previous studies, hyperfine ground states of single atoms or atomic ensembles have been entangled with spontaneously emitted light. The probabilistic character of the spontaneous emission process leads to long entanglement generation times, limiting realized network implementations to just two nodes. The success probability for atom–photon entanglement protocols can be increased by confining a single atom in a high-finesse optical cavity. Alternatively, quantum networks with superior scaling properties could be achieved using entanglement between light fields and atoms in quantum superpositions of the ground and highly excited (Rydberg) electronic states. Here we report the generation of such entanglement. The dephasing of the optical atomic coherence is inhibited by state-insensitive confinement of both the ground and Rydberg states of an ultracold atomic gas in an optical lattice. Our results pave the way for functional, many-node quantum networks capable of deterministic quantum logic operations between long-lived atomic memories.

Dephasing of Multiparticle Rydberg Excitations for Fast Entanglement Generation

An approach to fast entanglement generation based on Rydberg dephasing of collective excitations (spin waves) in large, optically thick atomic ensembles is proposed. Long-range 1/r(3) atomic interactions are induced by microwave mixing of opposite-parity Rydberg states. The required long coherence times are achieved via four-photon excitation and readout of long wavelength spin waves. The dephasing mechanism is shown to have favorable, approximately exponential, scaling for entanglement generation.

Emergence of spatial spin-wave correlations in a cold atomic gas.

Rydberg spin-waves are optically excited in a quasi-one-dimensional atomic sample of Rb atoms. Pairwise spin-wave correlations are observed by a spatially selective transfer of the quantum state onto a light field and photoelectric correlation measurements of the light. The correlations are interpreted in terms of the dephasing of multiply excited spin-waves by long-range Rydberg interactions.

In situ determination of Zeeman content of collective atomic memories

Knowledge and control of atomic Zeeman populations is necessary for the realization of useful, long-lived quantum memories. We propose and implement a method to determine atomic state population distributions for atomic spin waves. Zeeman composition of single atomic spin waves of a cold atomic gas, confined in a one-dimensional optical lattice, is inferred with high precision by measurements of signal–idler polarization correlations as a function of spin-wave storage time.

Cold atom quantum memories and the telecom interface

Long-distance quantum information networks require information storage, retrieval and transmission at telecommunication wavelengths. A quantum memory and its interface to the telecom window based on cold rubidium in an optical lattice is described. With this system it has been demonstrated (i) interconversion of 795 and 1367 nm light with efficiency in excess of 50% and (ii) measurement of non-classical correlations between telecom light and atomic spin-wave excitations.