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.

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A two-level quantum system coherently driven by a resonant electromagnetic field oscillates sinusoidally between the two levels at frequency $\Omega$ which is proportional to the field amplitude [1]. This phenomenon, known as the Rabi oscillation, has been at the heart of atomic, molecular and optical physics since the seminal work of its namesake and coauthors [2]. Notably, Rabi oscillations in isolated single atoms or dilute gases form the basis for metrological applications such as atomic clocks and precision measurements of physical constants [3]. Both inhomogeneous distribution of coupling strength to the field and interactions between individual atoms reduce the visibility of the oscillation and may even suppress it completely. A remarkable transformation takes place in the limit where only a single excitation can be present in the sample due to either initial conditions or atomic interactions: there arises a collective, many-body Rabi oscillation at a frequency $N^0.5\Omega$ involving all N >> 1 atoms in the sample [4]. This is true even for inhomogeneous atom-field coupling distributions, where single-atom Rabi oscillations may be invisible. When one of the two levels is a strongly interacting Rydberg level, many-body Rabi oscillations emerge as a consequence of the Rydberg excitation blockade. Lukin and coauthors outlined an approach to quantum information processing based on this effect [5]. Here we report initial observations of coherent many-body Rabi oscillations between the ground level and a Rydberg level using several hundred cold rubidium atoms. The strongly pronounced oscillations indicate a nearly complete excitation blockade of the entire mesoscopic ensemble by a single excited atom. The results pave the way towards quantum computation and simulation using ensembles of atoms

Quantum memories for the storage and retrieval of quantum information are extremely sensitive to environmental influences, which limits their storage times. The ground states of atoms and ions are potential candidates for quantum memories, but although coherence times of the order of a few seconds for atoms 1,2 and hundreds of seconds for ions [3][4][5] have been demonstrated, long-lived storage and retrieval of single quantum excitations remains an outstanding challenge. Here, we report a quantum memory using the magnetically insensitive clock transition in atomic rubidium confined in a one-dimensional optical lattice. We observe quantum memory lifetimes exceeding 6 ms, more than two orders of magnitude longer than previously reported 6 . This advance is an important step towards the realization of long-distance quantum networks and the controlled production of complex entangled states of matter and light.Protocols for quantum communication are typically based on remote parties sharing and storing an entangled quantum state. The generation of such remote entanglement must necessarily be done locally and distributed by light transmission over optical fibre links or through free space 7 . For the distribution of entanglement over a length L, the characteristic timescale for storage is the light travel time L/c, where c is the speed of light in the medium. For L = 1,000 km, L/c ≈ 5 ms for an optical fibre.In practice, direct entanglement distribution over optical fibres is limited by absorption to distances l ∼ 100 km. To distribute entanglement over longer distances, the channel should be divided into links of length ≤ l. The division circumvents attenuation in the fibre provided the intermediate memory nodes, which terminate the links, have a non-zero quantum memory time. Entanglement distributed over these shorter links is then connected over length L according to a family of protocols generically known as the quantum repeater 8 . The entanglement distribution rate of a network depends critically on the memory time of these storage elements. For L ∼ 1,000 km, required memory times vary from many seconds for a simple network topology 8,9 to milliseconds for more complex (for example, multiplexed) topologies and architectures 10-12 . Such long-lived quantum memories could revolutionize deterministic singlephoton sources 6 and lead to the generation of entangled states over extended systems 13 .Enhancing the matter coupling to a single spatial light mode is an advantage shared by cold optically thick atomic ensembles 14 and single atoms in high-finesse cavities 15 . The longest quantum memory time previously reported, 32 µs in a cold rubidium ensemble 6 , is insufficient to carry out quantum repeater protocols over the distances where direct transmission fails. The rubidium sample, prepared in a state of zero average magnetization, was allowed to freely fall during the protocol and the quantum memory time was limited by the effects of small uncompensated magnetic fields. In short, equally populated atomic states of...

Light storage on the minute scale is an important capability for future scalable quantum information networks spanning intercontinental distances. We employ an ultracold atomic gas confined in a one-dimensional optical lattice for long-term light storage. The differential ac Stark shift of the ground-level microwave transition used for storage is reduced to a sub-Hz level by the application of a magic-valued magnetic field. The 1/e lifetime for storage of coherent states of light is prolonged up to 16 s by a microwave dynamic decoupling protocol.