We describe a general technique that allows for an ideal transfer of quantum correlations between light fields and metastable states of matter. The technique is based on trapping quantum states of photons in coherently driven atomic media, in which the group velocity is adiabatically reduced to zero.We discuss possible applications such as quantum state memories, generation of squeezed atomic states, preparation of entangled atomic ensembles and quantum information processing.

We investigate dissipative phase transitions in an open central spin system. In our model the central spin interacts coherently with the surrounding many-particle spin environment and is subject to coherent driving and dissipation. We develop analytical tools based on a self-consistent Holstein-Primakoff approximation that enable us to determine the complete phase diagram associated with the steady states of this system. It includes first and second-order phase transitions, as well as regions of bistability, spin squeezing and altered spin pumping dynamics. Prospects of observing these phenomena in systems such as electron spins in quantum dots or NV centers coupled to lattice nuclear spins are briefly discussed.

We propose to use a new platform -ultracold polar molecules -for quantum computing with switchable interactions. The on/off switch is accomplished by selective excitation of one of the |0 or |1 qubits -long-lived molecular states -to an "excited" molecular state with a considerably different dipole moment. We describe various schemes based on this switching of dipolar interactions where the selective excitation between ground and excited states is accomplished via optical, microwave, or electric fields. We also generalize the schemes to take advantage of the dipole blockade mechanism when dipolar interactions are very strong. These schemes can be realized in several recently proposed architectures.Quantum computing is one of the most rapidly developing areas in physics today. For certain tasks, quantum computers have significant potential to outperform classical computers [1]. Several platforms are being investigated to implement these ideas, e.g., using atomic, molecular and optical, condensed matter, and other systems. A key challenge in all of these approaches is to identify strong and controllable interactions that would allow for the creation of fast quantum operations with minimal decoherence.Quantum information processing makes use of quantum superposition in which the fundamental piece of information, called a qubit, consists of a superposition of quantum states, denoted |0 and |1 . The building blocks of a quantum computer consist of "gate" operations, in which a coherent change in the state of one qubit can be brought about through a carefully controlled interaction with another qubit, and the result is dependent on the state of the second qubit. In order to implement reversible quantum logic operations it is essential to address these quantum states coherently.Of the various platforms proposed to implement quantum computers, trapped ions and neutral atoms are especially attractive [1]. Trapped ions [2] exhibit strong interactions and are relatively easy to control, while neutral atoms [3] have long coherence times and techniques to cool and trap them are well developed. Polar molecules represent a new platform that might incorporate the biggest advantages of both atoms and ions and even bridge the gap with condensed matter physics approaches (e.g. molecule-chips [4] or microtraps connected to superconducting wires [5]). They have long coherence times like neutral atoms, and strong interactions like trapped ions. However, contrary to ions, the interactions can be made "switchable," a feature which would help to simplify phase gates and minimize decoherence. Advances in cooling [6] and storing [7] techniques for molecules are beginning to make possible the required accurate manipulation of single molecules.In this Letter, we investigate the implementation of universal two-qubit logic gates in realistic systems, using ultracold polar molecules. As opposed to other schemes using polar molecules [8], such as vibrational eigenstates [9] and optimal control [10], our approach is based on the ability to "swit...

We consider light scattering off a two-dimensional (2D) dipolar array and show how it can be tailored by properly choosing the lattice constant of the order of the incident wavelength. In particular, we demonstrate that such arrays can operate as nearly perfect mirrors for a wide range of incident angles and frequencies close to the individual atomic resonance. These results can be understood in terms of the cooperative resonances of the surface modes supported by the 2D array. Experimental realizations are discussed, using ultracold arrays of trapped atoms and excitons in 2D semiconductor materials, as well as potential applications ranging from atomically thin metasurfaces to single photon nonlinear optics and nanomechanics.