arXiv subjects

Doron Cohen

Publications and source records attributed to Doron Cohen.

At least 109 records · Page 6Linked to original sources

arXiv2009

Occupation Statistics of a BEC for a Driven Landau-Zener Crossing

We consider an atomic Bose-Einstein condensate (BEC) loaded in a biased double-well trap with tunneling rate $K$ and inter-atomic interaction $U$. The BEC is prepared such that all $N$ atoms are in the left well. We drive the system by sweeping the potential difference $ε$ between the two wells. Depending on the interaction $u=NU/K$, and the sweep rate $\dotε$, we distinguish three dynamical regimes: adiabatic, diabatic, and sudden, and consider the occupation statistics of the final state. The analysis goes beyond mean-field theory and is complemented by a semiclassical picture.

cond-mat.other↗

arXiv2009

Phase-diffusion dynamics in weakly coupled Bose-Einstein condensates

We study the phase-sensitivity of collisional phase-diffusion between weakly coupled Bose-Einstein condensates, using a semiclassical picture of the two-mode Bose-Hubbard model. When weak-coupling is allowed, zero relative phase locking is attained in the Josephson-Fock transition regime, whereas a $π$ relative phase is only locked in Rabi-Josephson point. Our analytic semiclassical estimates agree well with the numerical results.

cond-mat.other↗

arXiv2009

Semilinear response for the heating rate of cold atoms in vibrating traps

The calculation of the heating rate of cold atoms in vibrating traps requires a theory that goes beyond the Kubo linear response formulation. If a strong "quantum chaos" assumption does not hold, the analysis of transitions shows similarities with a percolation problem in energy space. We show how the texture and the sparsity of the perturbation matrix, as determined by the geometry of the system, dictate the result. An improved sparse random matrix model is introduced: it captures the essential ingredients of the problem, and leads to a generalized variable range hopping picture.

quant-ph↗

arXiv2008

Quantum dynamics and transport in a double well system

The simplest one-dimensional model for the studying of non-trivial geometrical effects is a ring shaped device which is formed by joining two arms. We explore the possibility to model such a system as a two level system (TLS). Of particular interest is the analysis of quantum stirring, where it is not evident that the topology is properly reflected within the framework of the TLS modeling. On the technical side we provide a practical "neighboring level" approximation for the analysis of such quantum devices, which remains valid even if the TLS modeling does not apply.

cond-mat.mes-hall↗

arXiv2008

Controlled quantum stirring of Bose-Einstein condensates

By cyclic adiabatic change of two control parameters of an optical trap one can induce a circulating current of condensed bosons. The amount of particles that are transported per period depends on the "radius" of the cycle, and this dependence can be utilized in order to probe the interatomic interactions. For strong repulsive interaction the current can be regarded as arising from a sequence of Landau-Zener crossings. For weaker interaction one observes either gradual or coherent mega crossings, while for attractive interaction the particles are glued together and behave like a classical ball. For the analysis we use the Kubo approach to quantum pumping with the associated Dirac monopoles picture of parameter space.

cond-mat.other↗

arXiv2008

Quantum Stirring in low dimensional devices

A circulating current can be induced in the Fermi sea by displacing a scatterer, or more generally by integrating a quantum pump into a closed circuit. The induced current may have either the same or the opposite sense with respect to the "pushing" direction of the pump. We work out explicit expressions for the associated geometric conductance using the Kubo-Dirac monopoles picture, and illuminate the connection with the theory of adiabatic passage in multiple path geometry.

cond-mat.mes-hall↗

arXiv2008

Control of atomic currents using a quantum stirring device

We propose a BEC stirring device which can be regarded as the incorporation of a quantum pump into a closed circuit: it produces a DC circulating current in response to a cyclic adiabatic change of two control parameters of an optical trap. We demonstrate the feasibility of this concept and point out that such device can be utilized in order to probe the interatomic interactions.

cond-mat.mes-hall↗

arXiv2008

The mesoscopic conductance of disordered rings, its random matrix theory, and the generalized variable range hopping picture

The calculation of the conductance of disordered rings requires a theory that goes beyond the Kubo-Drude formulation. Assuming "mesoscopic" circumstances the analysis of the electro-driven transitions show similarities with a percolation problem in energy space. We argue that the texture and the sparsity of the perturbation matrix dictate the value of the conductance, and study its dependence on the disorder strength, ranging from the ballistic to the Anderson localization regime. An improved sparse random matrix model is introduced to captures the essential ingredients of the problem, and leads to a generalized variable range hopping picture.

cond-mat.mes-hall↗

arXiv2008

Renormalization of the dephasing by zero point fluctuations

We study the role of zero-point-fluctuations (ZPF) in dephasing at low temperature. Unlike the Caldeira-Leggett model where the interaction is with an homogeneous fluctuating field of force, here we consider the effect of short range scattering by localized bath modes. We find that in presence of ZPF the inelastic cross-section gets renormalized. Thus indirectly ZPF might contribute to the dephasing at low temperature.

cond-mat.mes-hall↗

arXiv2008

The conductance of a multi-mode ballistic ring: beyond Landauer and Kubo

The Landauer conductance of a two terminal device equals to the number of open modes in the weak scattering limit. What is the corresponding result if we close the system into a ring? Is it still bounded by the number of open modes? Or is it unbounded as in the semi-classical (Drude) analysis? It turns out that the calculation of the mesoscopic conductance is similar to solving a percolation problem. The "percolation" is in energy space rather than in real space. The non-universal structures and the sparsity of the perturbation matrix cannot be ignored.

cond-mat.mes-hall↗

arXiv2008

Counting statistics in multiple path geometries and the fluctuations of the integrated current in a quantum stirring device

The amount $Q$ of particles that are transported via a path of motion is characterized by its expectation value $ $ and by its variance $Var(Q)$. We analyze what happens if a particle has two optional paths available to get from one site to another site, and in particular what is $Var(Q)$ for the current which is induced in a quantum stirring device. It turns out that coherent splitting and the stirring effect are intimately related and cannot be understood within the framework of the prevailing probabilistic theory.

quant-ph↗

arXiv2008

Restricted quantum-classical correspondence and counting statistics for a coherent transition

The conventional probabilistic point of view implies that if a particle has a probability $p$ to make a transition from one site to another site, then the average transport should be $ =p}$ with a variance $Var(Q)=(1-p)p$. In the quantum mechanical context this observation becomes a non-trivial manifestation of restricted quantum-classical correspondence. We demonstrate this observation by considering the full counting statistics which is associated with a two level coherent transition in the context of a continuous quantum measurement process. In particular we test the possibility of getting a valid result for $Var(Q)$ within the framework of the adiabatic picture, analyzing the simplest non-trivial example of a Landau-Zener crossing.

cond-mat.mes-hall↗

arXiv2007

Decoherence of a particle in a ring

We consider a particle coupled to a dissipative environment and derive a perturbative formula for the dephasing rate based on the purity of the reduced probability matrix. We apply this formula to the problem of a particle on a ring, that interacts with a dirty metal environment. At low but finite temperatures we find a dephasing rate $\propto T^{3/2}$, and identify dephasing lengths for large and for small rings. These findings shed light on recent Monte Carlo data regarding the effective mass of the particle. At zero temperature we find that spatial fluctuations suppress the possibility of having a power law decay of coherence.

cond-mat.mes-hall↗

arXiv2007

Dephasing of a particle in a dissipative environment

The motion of a particle in a ring of length L is influenced by a dirty metal environment whose fluctuations are characterized by a short correlation distance $\ell << L$. We analyze the induced decoherence process, and compare the results with those obtained in the opposing Caldeira-Leggett limit ($\ell >> L$). A proper definition of the dephasing factor that does not depend on a vague semiclassical picture is employed. Some recent Monte-Carlo results about the effect of finite temperatures on "mass renormalization" in this system are illuminated.

cond-mat.mes-hall↗

arXiv2007

EPR, Bell, Schrodinger's cat, and the Monty Hall Paradox

The purpose of this manuscript is to provide a short pedagogical explanation why "quantum collapse" is not a metaphysical event, by pointing out the analogy with a "classical collapse" which is associated with the Monty Hall Paradox.

quant-ph↗

arXiv2007

From the Kubo formula to variable range hopping

Consider a multichannel closed ring with disorder. In the semiclassical treatment its conductance is given by the Drude formula. Quantum mechanics challenge this result both in the limit of strong disorder (eigenstates are not quantum-ergodic in real space) and in the limit of weak disorder (eigenstates are not quantum-ergodic in momentum space). Consequently the analysis of conductance requires going beyond linear response theory, leading to a resistor network picture of transitions between energy levels. We demonstrate that our semi-linear response theory provides a firm unified framework from which the "hopping" phenomenology of Mott can be derived.

cond-mat.mes-hall↗

arXiv2007

The mesoscopic conductance of ballistic rings

The calculation of the conductance of ballistic rings requires a theory that goes well beyond the Kubo-Drude formula. Assuming "mesoscopic" circumstance of very weak environmental relaxation, the conductance is much smaller compared with the naive expectation. Namely, the electro-motive-force induces an energy absorption with a rate that depends crucially on the possibility to make connected sequences of transitions. Thus the calculation of the mesoscopic conductance is similar to solving a percolation problem. The "percolation" is in energy space rather than in real space. Non-universal structures and sparsity of the perturbation matrix cannot be ignored. The latter are implied by lack of quantum-chaos ergodicity in ring shaped ballistic devices.

cond-mat.mes-hall↗

arXiv2006

Rate of energy absorption by a closed ballistic ring

We make a distinction between the spectroscopic and the mesoscopic conductance of closed systems. We show that the latter is not simply related to the Landauer conductance of the corresponding open system. A new ingredient in the theory is related to the non-universal structure of the perturbation matrix which is generic for quantum chaotic systems. These structures may created bottlenecks that suppress the diffusion in energy space, and hence the rate of energy absorption. The resulting effect is not merely quantitative: For a ring-dot system we find that a smaller Landauer conductance implies a smaller spectroscopic conductance, while the mesoscopic conductance increases. Our considerations open the way towards a realistic theory of dissipation in closed mesoscopic ballistic devices.

cond-mat.mes-hall↗