1,721,011 research outputs found
Open Quantum System dynamics: Applications to Decoherence and Collapse models
No full textQuantum mechanics exhibits a broad collection of theoretical results in complete agreement with experimental evidence. Besides showing unquestionable success in the description of isolated systems, it can be also successfully used to characterize non-isolated quantum system. In such a case, phenomena like dissipation, diffusion or decoherence. The theory of open quantum systems provides the framework where such features can be conveniently explained.
This thesis is about decoherence and collapse models. Albeit conceptually they are very far from each other, they both belong the framework of open quantum systems. Indeed, they consider systems interacting with an external entity: an environment for decoherence models, a noise for collapse models. Although the external influence has different origin, they can be described by similar dynamical equations and, to confirm or falsify them, the same experimental tests can be performed.
The quantum Brownian motion model can be considered the paradigm of decoherence models. We provide an exact and analytic equation for the time evolution of the operators and we show that the corresponding equation for the states is equivalent to well-known results in the literature. Our derivation allows to compute the time evolution of physically relevant quantities in a much easier way than previous formulations. Moreover, we are not bound to compute the time evolution of the state of the system, which in general is a complicated task. The explicit dependence on the initial state appears only in the initial expectation values and not in the dynamics. This makes possible the derivation of expectation values also for nontrivial states.
Another decoherence model we considered is a recently proposed model, based on the mass-energy equivalence. The model describes a decoherence source acting universally on every system whose superposition is extended on positions experiencing different gravitational potentials. We studied the conditions under which this mechanism becomes the dominant decoherence effect in typical interferometric experiments. We show that current experiments are off by several orders of magnitude. New ideas are needed to achieve the necessary requirements.
The second part of this thesis concerns collapse models, in particular the Continuous Spontaneous Localization (CSL) model. We focus on experimental tests that can probe it. In this respect, experiments can be grouped in two classes: interferometric tests and non-interferometric ones. The first class includes those experiments, which directly create and detect quantum superpositions of the center of mass of massive systems. The strongest bounds on the CSL parameters come from the second class of non-interferometric experiments, which are sensitive to small position displacements and detect CSL-induced diffusion in position.
We investigate how we can benefit from the non-interferometric perspective given by optomechanical setups, which have reached high sensitivities as force and position sensors. Three examples are considered.
First, we compute the upper bounds on the CSL parameters, which can be inferred by the gravitational wave detectors LIGO, LISA Pathfinder and AURIGA.
Second, we report new results from an experiment based on a high-quality cantilever cooled to millikelvin temperature. High accuracy measurements of the cantilever thermal fluctuations reveal a nonthermal force noise of unknown origin. This excess noise is compatible with the CSL heating predicted by Adler. Several physical mechanisms able to explain the observed noise have been ruled out.
Third, we propose an unattempted non-interferometric test aimed to investigate the still unexplored region of the CSL parameter space. Our proposal relies on torsional degrees of freedom rather than the usual vibrational ones. We believe that the test that has been put forward here, will eventually probe the unexplored CSL parameter space
Dataset for Narrowing the parameter space of collapse models with ultracold layered force sensors
No full textPSD - Original power spectral densities at different temperatures (in mK). For accounting the spectral leakage due to the FFT resolution, one has to remove the central 6 points. The columns are: frequency (Hz), value of PSD (Phi_0^2/Hz), PSD error (same units) Final data - Excel reporting the results of the PSD fits and that of the linear fit of B for the high temperatures. Final upper bound for the value of S_{F0}</span
Decoherence due to gravitational time dilation: Analysis of competing decoherence effects
Full text linkRecently the Earth gravitational field was proposed as a new source of decoherence [1]. We study the conditions under which, at least in principle, it becomes the dominant decoherence effect in a typical matter-wave or optomechanical experiment aiming at testing quantum coherence for massive systems. The following competing sources of decoherence are considered: spontaneous emission of light, absorption, scattering with the thermal photons and collisions with the residual gas. The conclusion is that the gravitational decoherence cannot be observed using the present experimental technology
Multilayer test masses to enhance the collapse noise
No full textRecently, a non-thermal excess noise, compatible with the theoretical prediction provided by collapse models, was measured in a millikelvin nanomechanical cantilever experiment [Vinante et al., Phys. Rev. Lett. 119, 110401 (2017)]. We propose a feasible implementation of the cantilever experiment able to probe such a noise. The proposed modification, completely within the grasp of current technology and readily implementable also in other type of mechanical non-interferometric experiments, consists in substituting the homogeneous test mass with one composed of different layers of different materials. This will enhance the action of a possible collapse noise above that given by standard noise sources
Colored collapse models from the non-interferometric perspective
Full text linkModels of spontaneous wave function collapse describe the quantum-to-classical transition by assuming a progressive breakdown of the superposition principle when the mass of the system increases, providing a well-defined phenomenology in terms of a non-linearly and stochastically modified Schro ̈dinger equation, which can be tested experimentally. The most popular of such models is the continuous spontaneous localization (CSL) model: in its original version, the collapse is driven by a white noise, and more recently, generalizations in terms of colored noises, which are more realistic, have been formulated. We will analyze how current non-interferometric tests bound the model, depending on the spectrum of the noise. We will find that low frequency purely mechanical experiments provide the most stable and strongest bounds
The continuous spontaneous localization layering effect from a lattice perspective
Full text linkFor a solid lattice, we rederive the Continuous Spontaneous Localization (CSL) noise total
energy gain of a test mass starting from a Lindblad formulation, and from a similar starting
point rederive the geometry factor governing center of mass energy gain. We then suggest
that the geometry factor can be used as a way to distinguish between low temperature
cantilever motion saturation arising from CSL noise, and saturation arising from thermal
leakage
Perturbative algorithm for rotational decoherence
No full textRecent advances in levitated optomechanics provide new perspectives for the use of rotational degrees of freedom for the development of quantum technologies as well as for testing fundamental physics. As for the translational case, their use, especially in the quantum regime, is limited by environmental noises, the characterization of which is fundamental in order to assess, control, and minimize their effect, in particular decoherence. Here, we present a general perturbative approach to compute decoherence for a quantum system in a superposition of its rotational degrees of freedom. The specific cases of the dipole-dipole and quadrupole-quadrupole interactions are solved explicitly, and we show that the rotational degrees of freedom decohere on a time scale that can be longer than the translational one
Adjoint master equation for quantum Brownian motion
Full text linkQuantum Brownian motion is a fundamental model for a proper understanding of open quantum systems in different contexts such as chemistry, condensed-matter physics, biophysics, and optomechanics. In this paper we propose a different approach to describe this model. We provide an exact and analytic equation for the time evolution of the operators and we show that the corresponding equation for the states is equivalent to well-known results in the literature. The dynamics is expressed in terms of the spectral density, regardless of the strength of the coupling between the system and the bath. Our derivation allows to compute the time evolution of physically relevant quantities in a much easier way than previous formulations. An example is explicitly studied
Multilayer test masses to enhance the collapse noise
Full text linkRecently, nonthermal excess noise, compatible with the theoretical prediction provided by collapse models, was measured in a millikelvin nanomechanical cantilever experiment [A. Vinante et al., Phys. Rev. Lett. 119, 110401 (2017)]. We propose a feasible implementation of the cantilever experiment able to probe such noise. The proposed modification, completely within the grasp of current technology and readily implementable also in other types of mechanical noninterferometric experiments, consists in replacing the homogeneous test mass with one composed of different layers of different materials. This will enhance the action of a possible collapse noise above that given by standard noise sources
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