arXiv subjects

Ronnie Kosloff

Publications and source records attributed to Ronnie Kosloff.

At least 55 records · Page 3Linked to original sources

arXiv2017

The quantum harmonic Otto cycle

The quantum Otto cycle serves as a bridge between the macroscopic world of heat engines and the quantum regime of thermal devices composed from a single element. We compile recent studies of the quantum Otto cycle with a harmonic oscillator as a working medium. This model has the advantage that it is analytically trackable. In addition, an experimental realization has been achieved employing a single ion in a harmonic trap. The review is embedded in the field of quantum thermodynamics and quantum open systems. The basic principles of the theory are explained by a specific example illuminating the basic definitions of work and heat. The relation between quantum observables and the state of the system is emphasized. The dynamical description of the cycle is based on a completely positive map formulated as a propagator for each stroke of the engine. Explicit solutions for these propagators are described on a vector space of quantum thermodynamical observables. These solutions which employ different assumptions and techniques are compared. The tradeoff between power and efficiency is the focal point of finite-time-thermodynamics. The dynamical model enables to study finite time cycles limiting time on the adiabtic and the thermalization times. Explicit finite time solutions are found which are frictionless, meaning that no coherence is generated also known as shortcuts to adiabaticity. The transition from frictionless to sudden adiabats is characterized by a non-hermitian degeneracy in the propagator. In addition the influence of noise on the control is illustrated. These results are used to close the cycles either as engines or as refrigerators.

quant-ph↗

arXiv2016

Speed limits in Liouville space for open quantum systems

One of the defining properties of an open quantum system is the variation of its purity in time. We derive speed limits on the rate of purity change for systems coupled to a Markovian environment. Our speed limits are based on Liouville space where density matrices are represented as vectors. This approach leads to speed limits that are always tighter compared to their parallel speed limits in Hilbert space. These bounds depend solely on the generators of the nonunitary dynamics and are independent of the particular states of the systems. Thus, they are perfectly suited to investigate dephasing, thermalization, and decorrelation processes of arbitrary states. We show that our speed limits can be attained and are therefore tight. As an application of our results we study correlation loss, and the speed of classical and quantum correlation erasure in multi-particle system.

quant-ph↗

arXiv2016

Quantum thermodynamics in strong coupling: heat transport and refrigeration

The performance characteristics of a heat rectifier and a heat pump are studied in a non Markovian framework. The device is constructed from a molecule connected to a hot and cold reservoir. The heat baths are modelled using the stochastic surrogate Hamiltonian method. The molecule is modelled by an asymmetric double-well potential. Each well is semi-locally connected to a heat bath composed of spins. The dynamics is driven by a combined system-bath Hamiltonian. The temperature of the baths is regulated by a secondary spin bath composed of identical spins in thermal equilibrium. A random swap operation exchange spins between the primary and secondary baths. The combined system is studied in various system-bath coupling strengths. In all cases the average heat current always flows from the hot towards the cold bath in accordance to the second law of thermodynamics. The asymmetry of the double well generates a rectifying effect meaning that when the left and right baths are exchanged the heat current follows the hot to cold direction. The heat current is larger when the high frequency is coupled to the hot bath. Adding an external driving field can reverse the transport direction. Such a refrigeration effect is modelled by a periodic driving field in resonance with the frequency difference of the two potential wells. A minimal driving amplitude is required to overcome the heat leak effect. In the strong driving regime the cooling power is non-monotonic with the system-bath coupling.

quant-ph↗

arXiv2016

Parameter estimation in atomic spectroscopy using exceptional points

The dynamics of spontaneous emission of an atomic system is studied in the framework of an open quantum system. The resulting quantum master equation for the atomic system is non hermitian. The generator $\mathcal{L}$ can possess non-hermitian degeneracies, i.e. exceptional points ($EP$), for specific values of the external laser driving amplitude and detuning from the atomic lines. We suggest to employ the special properties of these $EP$s for accurate parameter estimation. The method is demonstrated for the atomic spectrum of $S \rightarrow P$ transitions of $~^{85}$Rb and $~^{40}$Ca$~^+$.

quant-ph↗

arXiv2016

Quantum heat machines equivalence and work extraction beyond Markovianity, and strong coupling via heat exchangers

Various engine types are thermodynamically equivalent in the quantum limit of small 'engine action'. Our previous derivation of the equivalence is restricted to Markovian heat baths and to implicit classical work repository (e.g., laser light in the semi-classical approximation). In this paper all the components, baths, batteries, and engine, are explicitly taken into account. To neatly treat non-Markovian dynamics we use mediating particles that function as a heat exchanger. We find that on top of the previously observed equivalence there is a higher degree of equivalence that cannot be achieved in the Markovian regime. Next we focus on the energy transfer to the work repository. A condition for zero entropy increase in the work repository is given. Moreover, it is shown that in the strong coupling regime it is possible to charge a battery with energy while reducing its entropy at the same time.

quant-ph↗

arXiv2016

Quantum Flywheel

A quantum flywheel is studied with the purpose of storing useful work in quantum levels, while additional power is extracted continuously from the device. The flywheel gains its energy form a quantum heat engine. Generally, when a work repository is quantized the work exchange with the engine is accompanied with heat exchange, which may degrade the charging efficiency. In the particular realization of a quantum harmonic oscillator work repository, quantum and thermal fluctuations dominates the dynamics. Quantum monitoring and feedback control are applied to the flywheel, as it is shown to be an essential part of stabilizing and regulating its state of operation, and bringing the system to a steady state. A particular balance between information gained by measuring the system and the information fed back to the system is found to maximize the charging efficiency. The dynamics of the flywheel are described by a stochastic master equation that accounts for the engine, the external driving, the measurement, and the feedback operations.

quant-ph↗

arXiv2015

Three approaches for representing Lindblad dynamics by a matrix-vector notation

Markovian dynamics of open quantum systems are described by the L-GKS equation, known also as the Lindblad equation. The equation is expressed by means of left and right matrix multiplications. This formulation hampers numerical implementations. Representing the dynamics by a matrix-vector notation overcomes this problem. We review three approaches to obtain such a representation. The methods are demonstrated for a driven two-level system subject to spontaneous emission.

quant-ph↗

arXiv2015

Exceptional points for parameter estimation in open quantum systems: Analysis of the Bloch equations

We suggest to employ the dissipative nature of open quantum systems for the purpose of parameter estimation: The dynamics of open quantum systems is typically described by a quantum dynamical semigroup generator ${\cal L}$. The eigenvalues of ${\cal L}$ are complex, reflecting unitary as well as dissipative dynamics. For certain values of parameters defining ${\cal L}$, non-hermitian degeneracies emerge, i.e. exceptional points ($EP$). The dynamical signature of these $EP$s corresponds to a unique time evolution. This unique feature can be employed experimentally to locate the $EP$s and thereby to determine the intrinsic system parameters with a high accuracy. This way we turn the disadvantage of the dissipation into an advantage. We demonstrate this method in the open system dynamics of a two-level system described by the Bloch equation, which has become the paradigm of diverse fields in physics, from NMR to quantum information and elementary particles.

quant-ph↗

arXiv2015

The interplay between cycle geometry and performance of sudden refrigerators

The relation between the geometry of refrigeration cycles and their performance is explored. The model studied is based on a coupled spin system. Small cycle times termed sudden refrigerators, develop coherence and inner friction. We explore the interplay between coherence and energy of the working medium employing a family of sudden cycles with decreasing cycle times. At the point of minimum ratio between energy and coherence the cycle changes geometry. This region of cycle times is characterised by short circuit cycles where heat is dissipated both to the hot and cold baths. We rationalise the cycle change of geometry as a result of a half integer quantisation which maximises coherence. From this point on, increasing or decreasing the cycle time, eventually leads to refrigeration cycles. The transition point between refrigerators and short circuit cycles is characterised by a transition from finite to singular dynamical temperature. Extremely short cycle times reach a universal limit where all cycles types are equivalent.

quant-ph↗

arXiv2015

Training Schrödinger's cat: quantum optimal control

It is control that turns scientific knowledge into useful technology: in physics and engineering it provides a systematic way for driving a system from a given initial state into a desired target state with minimized expenditure of energy and resources -- as famously applied in the Apollo programme. As one of the cornerstones for enabling quantum technologies, optimal quantum control keeps evolving and expanding into areas as diverse as quantum-enhanced sensing, manipulation of single spins, photons, or atoms, optical spectroscopy, photochemistry, magnetic resonance (spectroscopy as well as medical imaging), quantum information processing and quantum simulation. --- Here state-of-the-art quantum control techniques are reviewed and put into perspective by a consortium uniting expertise in optimal control theory and applications to spectroscopy, imaging, quantum dynamics of closed and open systems. We address key challenges and sketch a roadmap to future developments.

quant-ph↗

arXiv2015

Coherent control of bond making: The performance of rationally phase-shaped femtosecond laser pulses

The first step in the coherent control of a photoinduced binary reaction is bond making or photoassociation. We have recently demonstrated coherent control of bond making in multi-photon femtosecond photoassociation of hot magnesium atoms, using linearly chirped pulses [Levin et al., arXiv:1411.1542]. The detected yield of photoassociated magnesium dimers was enhanced by positively chirped pulses which is explained theoretically by a combination of purification and chirp-dependent Raman transitions. The yield could be further enhanced by pulse optimization resulting in pulses with an effective linear chirp and a sub-pulse structure, where the latter allows for exploiting vibrational coherences. Here, we systematically explore the efficiency of phase-shaped pulses for the coherent control of bond making, employing a parametrization of the spectral phases in the form of cosine functions. We find up to an order of magnitude enhancement of the yield compared to the unshaped transform-limited pulse. The highly performing pulses all display an overall temporally increasing instantaneous frequency and are composed of several overlapping sub-pulses. The time delay between the first two sub-pulses almost perfectly fits the vibrational frequency of the generated intermediate wavepacket.These findings are in agreement with chirp-dependent Raman transitions and exploitation of vibrational dynamics as underlying control mechanisms.

physics.atom-ph↗

arXiv2015

On the Local and Global Approaches to Quantum Transport and Violation of the Second-law of Thermodynamics

Clausius' statement of the second law of thermodynamics reads: Heat will flow spontaneously from a hot to cold reservoir. This statement should hold for transport of energy through a quantum network composed of small subsystems each coupled to a heat reservoir. When the coupling between nodes is small, it seems reasonable to construct a local master equation for each node in contact with the local reservoir. The energy transport through the network is evaluated by calculating the energy flux after the individual nodes are coupled. We show by analysing the most simple network composed of two quantum nodes coupled to a hot and cold reservoir, that the local description can result in heat flowing from cold to hot reservoirs, even in the limit of vanishing coupling between the nodes. A global derivation of the master equation which prediagonalizes the total network Hamiltonian, and within this framework derives the master equation, is always consistent with the second-law of thermodynamics.

quant-ph↗

arXiv2015

Quantum Equivalence and Quantum Signatures in Heat Engines

Quantum heat engines (QHE) are thermal machines where the working substance is quantum. In the extreme case the working medium can be a single particle or a few level quantum system. The study of QHE has shown a remarkable similarity with the standard thermodynamical models, thus raising the issue what is quantum in quantum thermodynamics. Our main result is thermodynamical equivalence of all engine type in the quantum regime of small action. They have the same power, the same heat, the same efficiency, and they even have the same relaxation rates and relaxation modes. Furthermore, it is shown that QHE have quantum-thermodynamic signature, i.e thermodynamic measurements can confirm the presence of quantum coherence in the device. The coherent work extraction mechanism enables power outputs that greatly exceed the power of stochastic (dephased) engines.

quant-ph↗

arXiv2014

Coherent Control of Bond Making

We demonstrate for the first time coherent control of bond making, a milestone on the way to coherent control of photo-induced bimolecular chemical reactions. In strong-field multiphoton femtosecond photoassociation experiments, we find the yield of detected magnesium dimer molecules to be enhanced for positively chirped pulses and suppressed for negatively chirped pulses. Our ab initio model shows that control is achieved by purification via Franck-Condon filtering combined with chirp-dependent Raman transitions. Experimental closed-loop phase optimization using a learning algorithm yields an improved pulse that utilizes vibrational coherent dynamics in addition to chirp-dependent Raman transitions. Our results show that coherent control of binary photo-reactions is feasible even under thermal conditions.

physics.atom-ph↗

arXiv2014

Purity and entropy evolution speed limits for open quantum systems

We derive generic upper bounds on the rate of purity change and entropy increase for open quantum systems. These bounds depend solely on the generators of the nonunitary dynamics and are independent of the particular states of the systems. They are thus perfectly suited to investigate dephasing and thermalization processes of arbitrary systems. We apply these results to single and multiple dephasing channels, to a problem of quantum control in the presence of noise, and to cooling.

quant-ph↗

arXiv2014

Universal features in the efficiency of ultra hot quantum Otto engines

We study internal work optimization over the energy levels of a generic hot quantum Otto engine. We find universal features in the efficiency that resembles the classical external power optimization over the coupling times to the thermal baths. It is shown that in the ultra hot regime the efficiency is determined solely by the optimization constraint, and independent of the engine details. We show that for some constraints the radius of convergence of the perturbative approach used in the classical analysis is zero even for very arbitrarily low efficiencies at small temperature difference.

quant-ph↗

arXiv2014

The Scaling of Weak Field Phase-Only Control in Markovian Dynamics

We consider population transfer in open quantum systems, which are described by quantum dynamical semigroups (QDS). Using second order perturbation theory of the Lindblad equation, we show that it depends on a weak external field only through the field's autocorrelation function (ACF), which is phase independent. Therefore, for leading order in perturbation, QDS cannot support dependence of the population transfer on the phase properties of weak fields. We examine an example of weak-field phase-dependent population transfer, and show that the phase-dependence comes from the next order in the perturbation.

quant-ph↗

arXiv2014

The multilevel four-stroke swap engine and its environment

A multilevel four-stroke engine where the thermalization stokes are generated by unitary collisions with bath particles is analyzed. Our model is solvable even when the engine operates far from thermal equilibrium and in the strong system-bath coupling. Necessary operation conditions for the heat machine to perform as an engine or a refrigerator are derived. We relate the work and efficiency of the device to local and non-local statistical properties of the baths (purity, mutual coincidence etc.). In particular, we relate the Clausius inequality to the symmetrized relative entropy of the baths (Jefferys divergence). Other Clausius-like inequalities are derived as well. Finally, in the ultra-hot regime we optimize the work of the multilevel engine and obtain simpler forms for the work and efficiency.

quant-ph↗