We show that the zero-point fluctuations of the intrinsic electromagnetic environment limit the phase-coherence time in all mesoscopic systems at low temperatures. We derive this quantum-noise-limited dephasing time and its temperature dependence in the crossover to the thermal regime. Our results agree well with most experiments in one-dimensional systems.
We show that the zero-point fluctuations of the intrinsic electromagnetic environment limit the phase-coherence time in all mesoscopic systems at low temperatures. We derive this quantum-noise-limited dephasing time and its temperature dependence in the crossover to the thermal regime. Our results agree well with most experiments in one-dimensional systems.
We point out that even at the absolute zero of temperature environmental decoherence limits the destructive interference between time-reversed paths for an electron in a disordered metal, and thus causes the leading (`weak localization') quantum correction to the conductivity to saturate at T=0. Our calculation, which is intended to be illustrative rather than complete, uses a model in which an electron interacts with the fluctuations of the mean voltage in the sample. The average of the fluctuations produces the steady damping well known in Brownian motion, introduces a direction of time, ensures that arbitrarily long time-reversed electron paths lose phase coherence, and is consistent with the experimental observation of a finite low temperature phase-breaking lifetime.
We point out that the low temperature saturation of the electron phase decoherence time in a disordered conductor can be explained within the existing theory of weak localization provided the effect of quantum (high frequency) fluctuations is taken into account. Making use of the fluctuation-dissipation theorem we evaluate the quantum decoherence time, the crossover temperature below which thermal effects become unimportant, and the weak localization correction $\ensuremath{\delta}\ensuremath{\sigma}$ at T=0. For 1D systems the latter is found to be $\ensuremath{\delta}\ensuremath{\sigma}/\ensuremath{\sigma}\ensuremath{\propto}1/\sqrt{N}$, where N is the number of conducting channels.
Abstract Recent experimental results point to the existence of coherent quantum phenomena in systems made of a large number of particles, despite the fact that for many-body systems the presence of decoherence is hardly negligible and emerging classicality is expected. This behaviour hinges on collective observables, named quantum fluctuations , that retain a quantum character even in the thermodynamic limit: they provide useful tools for studying properties of many-body systems at the mesoscopic level, in-between the quantum microscopic scale and the classical macroscopic one. We herein present the general theory of quantum fluctuations in mesoscopic systems, and study their dynamics in a quantum open system setting, taking into account the unavoidable effects of dissipation and noise induced by the external environment. As in the case of microscopic systems, decoherence is not always the only dominating effect at the mesoscopic scale: certain types of environment can provide means for entangling collective fluctuations through a purely noisy mechanism.
We have derived an explicit nonperturbative expression for decoherence of quantum oscillations in a qubit by Gaussian low-frequency noise. Decoherence strength is controlled by the noise spectral density at zero frequency, while the noise correlation time τ determines the time t of crossover from the $${1 \mathord{\left/ {\vphantom {1 {\sqrt t }}} \right. \kern-\nulldelimiterspace} {\sqrt t }}$$ to the exponential suppression of coherence. We also performed Monte Carlo simulations of qubit dynamics with noise which agree with the analytical results.
The aim of this review is to show how ``ferromagnetic'' states, that is, states having a fully polarization, can produce intrinsic decoherence by unitary evolution. This effect can give an understanding of recent experiments on mesoscopic devices as quantum point contacts showing the 0.7 conductance anomaly and the wide number of data about saturation of dephasing time observed at very low temperatures, as a fully polarized two dimensional electron gas. But similar effects can be seen in different area of physics as for example the Dicke model describing the interaction of two-level systems with a radiation mode. In this case one can show that decoherence is intrinsic and remove a Schrödinger cat state leaving a single coherent state, collapsing the wave function in the thermodynamic limit. So, saturation of dephasing time at low temperatures in mesoscopic devices can be understood by a fully polarized two dimensional electron gas that, by an exchange model, can be reduced to a generalized form of the Dicke Hamiltonian and where the quasiparticles are spin excitations interacting with magnons. In this way, one can see that several experiments on nanowires and quantum dots can be satisfactorily explained. The existence of intrinsic decoherence in the thermodynamic limit could have deep implications in fundamental problems like quantum measurement and irreversibility. Recent experiments with cavities with a large number of photons and with nuclear magnetic resonance in organic molecular crystals give a first strong support to this view.
Conditions in favour of a realistic multilevelled description of a decohering quantum system are examined. In this regard the first crucial observation is that the thermal effects, contrary to the conventional belief, play a minor role at low temperatures in the decoherence properties. The system-environment coupling and the environmental energy spectrum dominantly affect the decoherence. In particular, zero temperature quantum fluctuations or non-equilibrium sources can be present and influential on the decoherence rates in a wide energy range allowed by the spectrum of the environment. A crucial observation against the validity of the two-level approximation is that the decoherence rates are found to be dominated not by the long time resonant but the short time off-resonant processes. This observation is demonstrated in two stages. Firstly, our zero temperature numerical results reveal that the calculated short time decoherence rates are Gaussian-like (the time dependence of the density matrix is led by the second time derivative at t ≤ 0). Exact analytical results are also permitted in the short time limit, which, consistent with our numerical results, reveal that this specific Gaussian-like behaviour is a property of the non-Markovian correlations in the environment. These Gaussian-like rates have no dependence on any spectral parameter (position and the width of the spectrum) except, in totality, the spectral area itself. The dependence on the spectral area is a power law. Furthermore, the Gaussian-like character at short times is independent of the number of levels (N), but the numerical value of the decoherence rates is a monotonic function of N. In this context, we demonstrate that leakage, as a characteristic multilevel effect, is dominated by the non-resonant processes. The long time behaviour of decoherence is also examined. Since our spectral model allows Markovian environmental correlations at long times, the decoherence rates in this regime become exponential independently from the number of levels. The latter and the coupling strengths play the major role in the quantitative values of the rates and the rates are independent of the other spectral parameters. The validity of the presented results is restricted only by their reliance on the Born-Oppenheimer approximation. This approximation is strongly dependent on the external observational time and its reliability depends on an additional timescale. In the rest of the work, the crossover between the short and the long time behaviour of the density matrix of the multilevelled system is examined using an intuitive argument. It is shown that the Born approximation weakens as the resonant couplings become more effective at long times. This implies that, in calculations made with this approximation in the long time regime, a need for a justification arises for the reliability of the results. This justification is made for the present work.