arXiv subjects

Joachim Ankerhold

Publications and source records attributed to Joachim Ankerhold.

At least 91 records · Page 5Linked to original sources

arXiv2014

Incomplete measurement of work in a dissipative two level system

We discuss work performed on a quantum two-level system coupled to multiple thermal baths. To evaluate the work, a measurement of photon exchange between the system and the baths is envisioned. In a realistic scenario, some photons remain unrecorded as they are exchanged with baths that are not accessible to the measurement, and thus only partial information on work and heat is available. The incompleteness of the measurement leads to substantial deviations from standard fluctuation relations. We propose a recovery of these relations, based on including the mutual information given by the counting efficiency of the partial measurement. We further present the experimental status of a possible implementation of the proposed scheme, i.e. a calorimetric measurement of work, currently with nearly single-photon sensitivity.

cond-mat.stat-mech↗

arXiv2014

Entanglement generation through local field and quantum dissipation

Entanglement in a Gaussian two-mode system can be generated by local driving if additional non-local features are introduced to the dynamics. We demonstrate that weak to moderate ohmic friction arising from a dissipative environment can enable entanglement generation in a driven system. This synergy of driving and dissipation is highly sensitive to the pulse shape; several simple pulse shapes fail to produce this effect at all or deposit large amounts of energy in the system as a side effect. Complex pulse shapes, determined by optimal control techniques, however, are effective without detrimental side effects.

quant-ph↗

arXiv2014

Optimal control theory with arbitrary superpositions of waveforms

Standard optimal control methods perform optimization in the time domain. However, many experimental settings demand the expression of the control signal as a superposition of given waveforms, a case that cannot easily be accommodated using time-local constraints. Previous approaches [1,2] have circumvented this difficulty by performing optimization in a parameter space, using the chain rule to make a connection to the time domain. In this paper, we present an extension to Optimal Control Theory which allows gradient-based optimization for superpositions of arbitrary waveforms directly in a time-domain subspace. Its key is the use of the Moore-Penrose pseudoinverse as an efficient means of transforming between a time-local and waveform-based descriptions. To illustrate this optimization technique, we study the parametrically driven harmonic oscillator as model system and reduce its energy, considering both Hamiltonian dynamics and stochastic dynamics under the influence of a thermal reservoir. We demonstrate the viability and efficiency of the method for these test cases and find significant advantages in the case of waveforms which do not form an orthogonal basis.

math.OC↗

arXiv2014

Bipartite entanglement dynamics of two-level systems in sub-Ohmic reservoirs

The quantum dynamics of pairs of two level systems immersed in dissipative reservoirs with sub-Ohmic spectral distributions is studied by means of numerically exact path integral Monte Carlo methods. It is shown that this class of reservoirs, relevant for generic properties of strongly non-Markovian environments with possible realizations in solid state structures at cryogenic temperatures, supports bipartite entanglement in broad ranges of parameter space and even for broken symmetries and in presence of thermal noise. The sensitivity of creation of entanglement in non-equilibrium on initial preparations of the reservoir is investigated in detail.

cond-mat.mes-hall↗

arXiv2014

Lamb shift enhancement and detection in strongly driven superconducting circuits

It is shown that strong driving of a quantum system substantially enhances the Lamb shift induced by broadband reservoirs which are typical for solid-state devices. By varying drive parameters the impact of environmental vacuum fluctuations with continuous spectral distribution onto system observables can be tuned in a distinctive way. This provides experimentally feasible measurement schemes for the Lamb shift in superconducting circuits based on Cooper pair boxes, where it can be detected either in shifted dressed transition frequencies or in pumped charge currents.

cond-mat.mes-hall↗

arXiv2014

Non-classical light from superconducting resonators coupled to voltage-biased Josephson junctions

The interplay of the tunneling transfer of charges and the emission and absorption of light can be investigated in a set-up, where a voltage-biased Josephson junction is placed in series to a microwave cavity. Here, we concentrate on the emission process and show that due to the inherent nonlinearity of the Josephson junction tunneling Cooper-pairs can create a variety of non-classical states of light. Depending on experimental parameters and tuning the device can be described by effective Hamiltonians, indicating specific photon creation mechanisms which lead to strongly bunched or anti-bunched light emission. We derive explicit analytical expressions for the photon correlation function $g^{(2)}(τ=0)$ for these different processes and investigate their full crossover numerically. Photon distribution functions show squeezing and other interesting non-Poissonian behavior.

cond-mat.mes-hall↗

arXiv2013

From Coulomb blockade to nonlinear quantum dynamics in a superconducting circuit with a resonator

Motivated by recent experiments on superconducting circuits consisting of a dc-voltage biased Josephson junction in series with a resonator, quantum properties of these devices far from equilibrium are studied. This includes a crossover from a domain of incoherent to a domain of coherent Cooper pair tunneling, where the circuit realizes a driven nonlinear oscillator. Equivalently, weak photon-charge coupling turns into strong correlations captured by a single degree of freedom. Radiated photons offer a new tool to monitor charge flow and current noise gives access to nonlinear dynamics, which allows to analyze quantum-classical boundaries.

cond-mat.mes-hall↗

arXiv2013

Almost local generation of EPR entanglement in non-equilibrium

The generation of entanglement is studied in a minimal model consisting of two independent Gaussian parties embedded in a common heat bath and subject to local control signals. Neither the reservoir alone nor the external driving can induce quantum non-locality but only the cooperative impact of both in non-equilibrium. It is shown that this corresponds to a dynamical symmetry breaking between the modes. By applying optimal control protocols substantial entanglement is generated even for single-site control and at higher temperatures and is preserved after the control is switched off.

quant-ph↗

arXiv2013

Linear dynamics subject to thermal fluctuations and non-Gaussian noise: From classical to quantum

The dynamics of a linear system embedded in a heat bath environment and subject to non- Gaussian noise is studied. Higher order cumulants in coordinate space are derived and their impact on the dynamics and on asymptotic steady state distributions is analyzed. In the quantum regime non-Gaussian properties are present in the reduced density in coordinate representation which in energy representation exist on a transient time scale only due to symmetry. Within an exactly solvable model our results provide insight into mechanisms of linear detectors as sensors for non- Gaussian noise at high and low temperatures.

cond-mat.mes-hall↗

arXiv2013

Dynamics of quantum coherences at strong coupling to a heat bath

The standard approach for path integral Monte Carlo simulations of open quantum systems is extended as an efficient tool to monitor the time evolution of coherences (off-diagonal elements of the reduced density matrix) also for strong coupling to environments. Specific simulations are performed for two level systems embedded in Ohmic and sub-Ohmic reservoirs in the domains of coherent and incoherent dynamics of the polarization. In the latter regime, the notorious difficulty to access the long time regime is overcome by combining simulations on moderate time scales with iteratively calculated initial densities. This allows to extract relaxation rates for sub-Ohmic environments at finite temperatures and over a broad range of couplings and to compare them to analytical predictions. The time evolution of the von Neumann entropy provides insight into the quantum phase transition at thermal equilibrium from a delocalized to a localized state at zero temperature.

cond-mat.stat-mech↗

arXiv2013

Improvements to Kramers Turnover Theory

The Kramers turnover problem, that is obtaining a uniform expression for the rate of escape of a particle over a barrier for any value of the external friction was solved in the eighties. Two formulations were given, one by Melnikov and Meshkov (MM) (J. Chem. Phys. 85, 1018 (1986)), which was based on a perturbation expansion for the motion of the particle in the presence of friction. The other, by Pollak, Grabert and Haenggi (PGH) (J. Chem. Phys. 91, 4073 (1989)), valid also for memory friction, was based on a perturbation expansion for the motion along the collective unstable normal mode of the particle. Both theories did not take into account the temperature dependence of the average energy loss to the bath. Increasing the bath temperature will reduce the average energy loss. In this paper, we analyse this effect, using a novel perturbation theory. We find that within the MM approach, the thermal energy gained from the bath diverges, the average energy gain becomes infinite, implying an essential failure of the theory. Within the PGH approach increasing the bath temperature reduces the average energy loss but only by a finite small amount, of the order of the inverse of the reduced barrier height. This then does not seriously affect the theory. Analysis and application for a cubic potential and Ohmic friction are presented.

physics.chem-ph↗

arXiv2013

Charge transfer through molecular junctions within Redfield theory: subtleties and pitfalls

Charge transfer through nanoscale junctions connecting metallic leads with quantum dots or single molecules is often described within an open system formulation in terms of Redfield theory. Under non-equilibrium conditions, the usually invoked rotating wave approximation is not justified which may lead to unphysical steady state solutions with e.g. negative populations. In this work we explore subtleties and constraints of the approach and thus clarify its applicability in numerical calculations. General findings are illustrated for an analytically solvable case of a molecule with two electronic states.

cond-mat.mes-hall↗

arXiv2013

Persistence of coherent quantum dynamics at strong dissipation

The quantum dynamics of a two state system coupled to a bosonic reservoir with sub-Ohmic spectral density is investigated for strong friction. Numerically exact path integral Monte Carlo methods reveal that in contrast to conventional expectations, coherent dynamics never turns into incoherent decay for a broad class of spectral distributions. Coherences associated with substantial system-reservoir entanglement exist in non-equilibrium even when strong dissipation makes the thermodynamic state of the system to behave essentially classical. This may be of relevance for current experiments with nanoscale devices and bio-molecular aggregates.

cond-mat.stat-mech↗

arXiv2012

Voltage induced conformational changes and current control in charge transfer through molecules

Transport through molecular contacts with a sluggish intramolecular vibrational mode strongly coupled to excess charges is studied far from equilibrium. A Born-Oppenheimer approximation in steady state reveals voltage dependent energy surfaces which cause abrupt conformational changes of the molecular backbone. These are directly related to transitions between current plateaus which are relatively robust against thermal fluctuations and appear in patterns reminiscent of Coulomb diamonds. In a regime accessible in experiments bias and gate voltages allow for an efficient control of charge transfer or may trigger a molecular rotor.

cond-mat.mes-hall↗

arXiv2012

Quantum phase diffusion in ac-driven superconducting atomic point contacts

The impact of quantum fluctuations on the phase diffusion in resistively shunted superconducting quantum points subject to an external ac-voltage is studied. Based on an extension of the classical Smoluchowski equation to the quantum regime, numerical results for fractional and integer Shapiro resonances are investigated to reveal characteristic features of quantum effects. It is shown that typically in the quantum regime the broadening of resonances cannot be described simply by an effective temperature.

cond-mat.mes-hall↗

arXiv2011

Optimal control of open quantum systems: cooperative effects of driving and dissipation

We investigate the optimal control of open quantum systems, in particular, the mutual influence of driving and dissipation. A stochastic approach to open-system control is developed, using a generalized version of Krotov's iterative algorithm, with no need for Markovian or rotating-wave approximations. The application to a harmonic degree of freedom reveals cooperative effects of driving and dissipation that a standard Markovian treatment cannot capture. Remarkably, control can modify the open-system dynamics to the point where the entropy change turns negative, thus achieving cooling of translational motion without any reliance on internal degrees of freedom.

cond-mat.stat-mech↗

arXiv2011

Charge transfer through single molecule contacts: How reliable are rate descriptions?

The trend to fabricate electrical circuits on nanoscale dimensions has led to impressive progress in the field of molecular electronics in the last decade. A theoretical description of molecular contacts as the building blocks of future devices is challenging though as it has to combine properties of Fermi liquids in the leads with charge and phonon degrees of freedom on the molecule. Apart from ab initio schemes for specific set-ups, generic models reveal characteristics of transport processes. Particularly appealing are descriptions based on transfer rates successfully used in other contexts such as mesoscopic physics and intramolecular electron transfer. However, a detailed analysis of this scheme in comparison with numerically exact data is elusive yet. It turns out that a formulation in terms of transfer rates provides a quantitatively accurate description even in domains of parameter space where in a strict sense it is expected to fail, e.g. for lower temperatures. Typically, intramolecular phonons are distributed according to a voltage driven steady state that can only roughly be captured by a thermal distribution with an effective elevated temperature (heating). An extension of a master equation for the charge-phonon complex to include effectively the impact of off-diagonal elements of the reduced density matrix provides very accurate data even for stronger electron-phonon coupling.

cond-mat.mes-hall↗

arXiv2010

Semiclassical theory of energy diffusive escape in a Duffing oscillator

Motivated by recent experimental progress to read out quantum bits implemented in superconducting circuits via the phenomenon of dynamical bifurcation, transitions between steady orbits in a driven anharmonic oscillator, the Duffing oscillator, are analyzed. In the regime of weak dissipation a consistent master equation in the semiclassical limit is derived to capture the intimate relation between finite tunneling and reflection and bath induced quantum fluctuations. From the corresponding steady state distributions analytical expressions for the switching probabilities are obtained. It is shown that a reduction of the transition rate due to finite reflection at the phase-space barrier is overcompensated by an increase due to environmental quantum fluctuations that are specific for diffusion processes over dynamical barriers. Moreover, it is revealed that close to the bifurcation threshold the escape dynamics enters an overdamped domain such that the quantum mechanical energy scale associated with friction even exceeds the thermal energy scale.

cond-mat.other↗