Decoherence induced by coupling a system with an environment may display universal features. Here we demostrate that when the coupling to the system drives a quantum phase transition in the environment, the temporal decay of quantum coherences in the system is Gaussian with a width independent of the system-environment coupling strength. The existence of this effect opens the way for a new type of quantum simulation algorithm, where a single qubit is used to detect a quantum phase transition. We discuss possible implementations of such algorithm and we relate our results to available data on universal decoherence in NMR echo experiments. PACS numbers:The coupling between a quantum system and its environment leads to decoherence, the process by which quantum information is degraded. Decoherence plays a crucial role in the understanding of the quantum to classical transition [1]. It also has practical importance: its understanding is essential in technologies that actively use quantum coherence, such as quantum information processing [2]. In general, the timescale t dec of decoherence depends on the system-environment coupling strength, which we arbitrarily denote λ. For example, in the well studied case of quantum Brownian motion (where the environment consists of a large number of non-interacting harmonic oscillators), quantum coherence generally decays exponentially with a rate 1/t dec proportional to λ 2 [3]. In this letter we describe a class of systems with a drastically different behavior: Gaussian decay of coherence with a rate independent of λ. This independence signals a universal behavior whose study is the aim of this work. In general, one should avoid building physical quantum information processing devices in presence of universal decoherence. However, we show that universality is a powerful property we can use to our advantadge: by detecting decoherence in the universal regime we can extract valuable information about the environment.Environment-independent decoherence rates are also found in other circumstances. For example, systems with a classically chaotic Hamiltonian display a "Lyapunov regime" where the decay is exponential and given by the Lyapunov exponent of the underlying classical dynamics [4,5]. These models are also often used to represent a complex environment. In fact, chaoticity is the widespread explanation [5,6] for the perturbationindependent decay of polarization detected in recent NMR echo experiments [7] (where, however, a nonexponential but Gaussian decay is actually observed). Our findings are different from the usual exponential Lyapunov regime: we discuss systems where the universal (independent of λ) decoherence is Gaussian. In our model, the complexity and sensitivity of the environment arise from the susceptibility of the environmental spectrum to the system's state. The relation between our results and the experiments of Ref.[7] will also be discussed below.Let us consider a spin 1/2 particle (a qubit) coupled to an environment that is "structurally unstable" with respe...

Cold atoms, driven by a laser and simultaneously coupled to the quantum field of an optical resonator, may self-organize in periodic structures. These structures are supported by the optical lattice, which emerges from the laser light they scatter into the cavity mode, and form when the laser intensity exceeds a threshold value. We study theoretically the quantum ground state of these structures above the pump threshold of self-organization, by mapping the atomic dynamics of the self-organized crystal to a Bose-Hubbard model. We find that the quantum ground state of the self-organized structure can be the one of a Mott-insulator or a superfluid, depending on the pump strength of the driving laser. For very large pump strengths, where the intracavity intensity is maximum and one would expect a Mott-insulator state, we find intervals of parameters where the system is superfluid. These states could be realized in existing experimental setups.

We investigate a paradigm example of cavity quantum electrodynamics with many body systems: an ultracold atomic gas inside a pumped optical resonator, confined by the mechanical potential emerging from the cavity-field spatial mode structure. When the optical potential is sufficiently deep, the atomic gas is in the Mott-insulator state as in open space. Inside the cavity, however, the potential depends on the atomic distribution, which determines the refractive index of the medium, thus altering the intracavity-field amplitude. We derive the effective Bose-Hubbard model describing the physics of the system in one dimension and study the crossover between the superfluid -Mottinsulator quantum states. We predict the existence of overlapping stability regions corresponding to competing insulator-like states. Bistable behavior, controlled by the pump intensity, is encountered in the vicinity of the shifted cavity resonance.

We investigate arrays of three traps with two fermionic or bosonic atoms. The tunneling interaction between neighboring sites is used to prepare multisite dark states for the empty site (i.e., the hole) which allows for the coherent manipulation of its external degrees of freedom. By means of an ab initio integration of the Schrödinger equation, we investigate the adiabatic transport of a hole between the two extreme traps of a triple-well potential. Furthermore, a quantum-trajectory approach based on the de Broglie-Bohm formulation of quantum mechanics is used to get physical insight into the transport process. Finally, we discuss the use of the hole for the construction of a coherent single hole diode and a coherent single hole transistor.