Fermi problem with artificial atoms in circuit QED

Causality in a Qubit-Based Implementation of a Quantum Switch

We introduce a qubit-based version of the quantum switch, consisting of a variation of the Fermi problem. Two qubits start in a superposition state in which one qubit is excited and the other is in the ground state. However, it is not defined which is the excited qubit. Then, after some time, if a photon is detected, we know that it must have experienced an emission by one atom and then an absorption and re-emission by the other one, but the ordering of the emission events by both qubits is undefined. While it is tempting to refer to this scenario as one with indefinite causality or a superposition of causal orders, we show that there is still a precise notion of causality: the probability of excitation of each atom is totally independent of the other one when the times are short enough to prevent photon exchange.

Interaction-free bidirectional multi-channel all-optical switching in a multi-level coupling atom-cavity system

We propose a scheme of interaction-free bidirectional multi-channel all-optical switching in a multi-level coupling system consisting of four-level atoms confined in a cavity and coupled by a free-space control laser. A signal laser field is coupled into the cavity and excites two separate transitions of atoms simultaneously under the collective strong coupling condition. The transmission and reflection of signal fields form bidirectional output channels. A free-space control laser induces destructive quantum interference in the multi-level excitation of an atom-cavity system, which can be used to switch on/off the output signal lights of transmission/reflection channels. There is no direct coupling of control light and signal light through the cavity-confined atoms because the output of signal light is nearly totally suppressed to the opposite direction when control light is present. The proposed all-optical switching scheme can be realized with high switching efficiency, broad bandwidth, and weak light intensity. It may be useful for future devices of optical routing, optical communications, and various quantum logic elements.

Quantum simulations of lattice gauge theories using ultracold atoms in optical lattices

Can high-energy physics be simulated by low-energy, non-relativistic, many-body systems such as ultracold atoms? Such ultracold atomic systems lack the type of symmetries and dynamical properties of high energy physics models: in particular, they manifest neither local gauge invariance nor Lorentz invariance, which are crucial properties of the quantum field theories which are the building blocks of the standard model of elementary particles. However, it turns out, surprisingly, that there are ways to configure an atomic system to manifest both local gauge invariance and Lorentz invariance. In particular, local gauge invariance can arise either as an effective low-energy symmetry, or as an exact symmetry, following from the conservation laws in atomic interactions. Hence, one could hope that such quantum simulators may lead to a new type of (table-top) experiments which will be used to study various QCD (quantum chromodynamics) phenomena, such as the confinement of dynamical quarks, phase transitions and other effects, which are inaccessible using the currently known computational methods. In this report, we review the Hamiltonian formulation of lattice gauge theories, and then describe our recent progress in constructing the quantum simulation of Abelian and non-Abelian lattice gauge theories in 1  +  1 and 2  +  1 dimensions using ultracold atoms in optical lattices.