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

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.

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.

Quantum physics allows for entanglement between microscopic and macroscopic objects, described by discrete and continuous variables, respectively. As in Schrödinger's famous cat gedanken experiment, a box enclosing the objects can keep the entanglement alive. For applications in quantum information processing, however, it is essential to access the objects and manipulate them with suitable quantum tools. Here we reach this goal and deterministically generate entangled light-matter states by reflecting a coherent light pulse with up to four photons on average from an optical cavity containing one atom. The quantum light propagates freely and reaches a remote receiver for quantum state tomography. We produce a plethora of quantum states and observe negative-valued Wigner functions, a characteristic sign of non-classicality. As a first application, we demonstrate a quantum-logic gate between an atom and a light pulse, with the photonic qubit encoded in the phase of the light field.As early as 1935, Schrödinger formulated a gedanken experiment 1 with a living cat and a radioactive atom placed inside a box, which is then closed. When the atom decays, it triggers a death mechanism that kills the cat. According to the laws of quantum physics, the decay of the atom occurs at some random time. Consequently, the time of death of the cat is unknown. Mathematically, the situation inside the box is described by an entangled superposition state that is known as a 'Schrödinger-cat state'. Most remarkable, this state offers a unique access to the atom-cat system, at least in principle. For example, a measurement apparatus that is capable of measuring the atom in a superposition of 'not decayed' and 'decayed' immediately projects the cat into a superposition of 'alive' and 'dead'. Such observation thus transfers the superposition state of the microscopic quantum object into the macroscopic classical world, something weird for a cat. In contrast to the entangled Schrödinger-cat state, the coherent superposition state of the cat is here denoted as a 'cat state'.While notoriously difficult to create 2 , several implementations of Schrödinger-cat states or just cat states have emerged during recent decades. In all these experiments, coherent states with distinguishable phases mimic the two cat states 'dead' and 'alive'. Most prominently, Schrödinger-cat states were explored using a trapped ion 3,4 , with the vibrational state in the trap taking the role of the cat, and coherent microwave fields confined to superconducting boxes were used in combination with Rydberg atoms 5,6 and superconducting qubits 7 . In the latter experiment, the cat state was also released from the microwave resonator 8 .Modern applications in an open quantum-communication and distributed quantum-networking architecture could benefit from cat states that propagate over some distances. As long as superconducting transmission lines exist for short distances only, optical fields propagating through low-loss optical fibres in the (near) visible part of the electrom...

High-resolution noncontact atomic force microscopy of SiO2 reveals previously unresolved roughness at the few-nm length scale, and scanning tunneling microscopy of graphene on SiO2 shows graphene to be slightly smoother than the supporting SiO2 substrate. A quantitative energetic analysis explains the observed roughness of graphene on SiO2 as extrinsic, and a natural result of highly conformal adhesion. Graphene conforms to the substrate down to the smallest features with nearly 99% fidelity, indicating conformal adhesion can be highly effective for strain engineering of graphene.