arXiv subjects

R. Kosloff

Publications and source records attributed to R. Kosloff.

At least 19 recordsLinked to original sources

arXiv2019

Imaging recoil ions from optical collisions between ultracold, metastable neon isotopes

We present an experimental scheme which combines the well established method of velocity-mapimaging, with a cold trapped metastable neon target. The device is used for obtaining the branching ratios and recoil-ion energy distributions for the penning ionization process in optical collisions of ultracold metastable neon. The potential depth of the highly excited dimer potential is extracted and compared with theoretical calculations. The simplicity to construct, characterize and apply such a device, makes it a unique tool for the low-energy nuclear physics community, enabling opportunities for precision measurements in beta- and beta-delayed-neutron decays of cold, trapped, short-lived radioactive isotopes.

physics.atom-ph↗

arXiv2017

Noise Resistant Quantum Control Using Dynamical Invariants

A systematic approach to design robust control protocols against the influence of different types of noise is introduced. We present control schemes which protect the decay of the populations avoiding dissipation in the adiabatic and non-adiabatic regimes and minimize the effect of dephasing. The effectiveness of the protocols is demonstrated in two different systems. Firstly we present the case of population inversion of a two level system in the presence of either one or two simultaneous noise sources. Secondly, we present an example of the expansion of coherent and thermal states in harmonic traps, subject to noise arising from monitoring and modulation of the control respectively.

quant-ph↗

arXiv2017

Energy consumption for Shortcuts To Adiabaticity

Shortcuts to adiabaticity let a system reach the results of a slow adiabatic process in a shorter time. We propose to quantify the "energy cost" of the shortcut by the energy consumption of the system enlarged by including the control device. A mechanical model where the dynamics of the system and control device can be explicitly described illustrates that a broad range of possible values for the consumption are possible, including zero (above the adiabatic energy increment) when friction is negligible and the energy given away as negative power is stored and recovered by perfect regenerative braking.

quant-ph↗

arXiv2017

Directional Quantum-Controlled Chemistry: Generating Aligned Ultracold Molecules via Photoassociation

Photoassociation of ultracold atoms is shown to lead to alignment of the product molecules along the excitation laser polarization axis. We theoretically investigate pulsed photoassociation of $^{87}Rb$ atoms into a specific weakly-bound level of the a $^3Σ_u^+$ metastable electronic state and find both stationary and time-dependent field-free alignment. Although a transform-limited pulse yields significant alignment, a frequency-chirped pulse dramatically enhances the molecular formation rate at the cost of a slight decrease in the alignment. Employing multiple pulses synchronized with the vibrational and rotational periods leads to coherent enhancement of both population and alignment of the target state.

quant-ph↗

arXiv2016

Efficient Formation of Ultracold Molecules with Chirped Nanosecond Pulses

We describe experiments and associated quantum simulations involving the production of ultracold $^{87}$Rb$_{2}$ molecules with nanosecond pulses of frequency-chirped light. With appropriate chirp parameters, the formation is dominated by coherent processes. For a positive chirp, excited molecules are produced by photoassociation early in the chirp, then transferred into high vibrational levels of the lowest triplet state by stimulated emission later in the chirp. Generally good agreement is seen between the data and the simulations. Shaping of the chirp can lead to a significant enhancement of the formation rate. Further improvements using higher intensities and different intermediate states are predicted.

physics.atom-ph↗

arXiv2016

Enhancement of Ultracold Molecule Formation Using Shaped Nanosecond Frequency Chirps

We demonstrate that judicious shaping of a nanosecond-time-scale frequency chirp can dramatically enhance the formation rate of ultracold $^{87}$Rb$_{2}$ molecules. Starting with ultracold $^{87}$Rb atoms, we apply pulses of frequency-chirped light to first photoassociate the atoms into excited molecules and then, later in the chirp, de-excite these molecules into a high vibrational level of the lowest triplet state, $a \, ^{3}Σ_{u}^{+}$. The enhancing chirp shape passes through the absorption and stimulated emission transitions relatively slowly, thus increasing their adiabaticity, but jumps quickly between them to minimize the effects of spontaneous emission. Comparisons with quantum simulations for various chirp shapes support this enhancement mechanism.

physics.atom-ph↗

arXiv2015

Activated and non activated dephasing demonstrated in NV center dynamics

We analyze different decoherence processes in a system coupled to a bath. Apart from the well known standard dephasing mechanism which is temperature dependent an alternative mechanism is presented, the spin-swap dephasing which does not need initial bath activation and is temperature independent. We show that for dipolar interaction the separation of time scales between system and bath can not produce pure dephasing, the process being accompained by dissipation. Activated and non activated dephasing processes are demonstrated in a diamond nitrogen-vacancy (NV) center.

quant-ph↗

arXiv2013

Quantum control with noisy fields: computational complexity vs. sensitivity to noise

A closed quantum system is defined as completely controllable if an arbitrary unitary transformation can be executed using the available controls. In practice, control fields are a source of unavoidable noise, which has to be suppressed to retain controllability. Can one design control fields such that the effect of noise is negligible on the time-scale of the transformation? This question is intimately related to the fundamental problem of a connection between the computational complexity of the control problem and the sensitivity of the controlled system to noise. The present study considers a paradigm of control, where the Lie-algebraic structure of the control Hamiltonian is fixed, while the size of the system increases with the dimension of the Hilbert space representation of the algebra. We find two types of control tasks, easy and hard. Easy tasks are characterized by a small variance of the evolving state with respect to the operators of the control operators. They are relatively immune to noise and the control field is easy to find. Hard tasks have a large variance, are sensitive to noise and the control field is hard to find. The influence of noise increases with the size of the system, which is measured by the scaling factor $N$ of the largest weight of the representation. For fixed time and control field as O(N) for easy tasks and as $O(N^2)$ for hard tasks. As a consequence, even in the most favorable estimate, for large quantum systems, generic noise in the controls dominates for a typical class of target transformations, i.e., complete controllability is destroyed by noise.

quant-ph↗

arXiv2013

The quest for absolute zero in presence of external noise

A reciprocating quantum refrigerator is analyzed with the intention to study the limitations imposed by external noise. In particular we focus on the behavior of the refrigerator when it approaches the absolute zero. The cooling cycle is based on the Otto cycle with a working medium constituted by an ensemble of non interacting harmonic oscillators. The compression and expansion segments are generated by changing an external parameter in the Hamiltonian. In this case the force constant of the harmonic oscillators $m ω^2$ is modified from an initial to a final value. As a result, the kinetic and potential energy of the system do not commute causing frictional losses. By proper choice of scheduling function $ω(t)$ frictionless solutions can be obtained in the noiseless case. We examine the performance of a refrigerator subject to noise. By expanding from the adiabatic limit we find that the external noise, gaussian phase and amplitude noises, reduce the amount of heat that can be extracted but nevertheless the zero temperature can be approached.

quant-ph↗

arXiv2012

Production of Ultracold Molecules with Chirped Nanosecond Pulses: Evidence for Coherent Effects

We use frequency-chirped light on the nanosecond time scale to produce ultracold $^{87}$Rb$_{2}$ molecules in the lowest triplet state via the process of photoassociation. Comparing to quantum simulations of the molecular formation, we conclude that coherent stimulated emission plays an important role and is primarily responsible for the significant difference observed between positive and negative chirps.

physics.atom-ph↗

arXiv2011

Coherent control of ultracold 85Rb trap-loss collisions with nonlinearly frequency-chirped light

We present results on coherent control of ultracold trap-loss collisions using 40 ns pulses of nonlinearly frequency-chirped light. The chirps, either positive or negative, sweep ~1 GHz in 100 ns and are centered at various detunings below the D2 line of 85Rb. At each center detuning, we compare the collisional rate constant beta for chirps that are linear in time, concave-down and concave-up. For positive chirps, we find that beta generally depends very little on the shape of the chirp. For negative chirps, however, we find that beta can be enhanced by up to 50(20)% for the case of the concave-down shape. This occurs at detunings where the evolution of the wavepacket is expected to be coherent. An enhancement at these detunings is also seen in quantum mechanical simulations of the collisional process.

physics.atom-ph↗

arXiv2010

Dynamic Matter-Wave Pulse Shaping

In this paper we discuss possibilities to manipulate a matter-wave with time-dependent potentials. Assuming a specific setup on an atom chip, we explore how one can focus, accelerate, reflect, and stop an atomic wave packet, with, for example, electric fields from an array of electrodes. We also utilize this method to initiate coherent splitting. Special emphasis is put on the robustness of the control schemes. We begin with the wave packet of a single atom, and extend this to a BEC, in the Gross-Pitaevskii picture. In analogy to laser pulse shaping with its wide variety of applications, we expect this work to form the base for additional time-dependent potentials eventually leading to matter-wave pulse shaping with numerous applications

quant-ph↗

arXiv2009

Algorithm for simulation of quantum many-body dynamics using dynamical coarse graining

An algorithm for simulation of quantum many-body dynamics having su(2) spectrum-generating algebra is developed. The algorithm is based on the idea of dynamical coarse-graining. The original unitary dynamics of the target observables, the elements of the spectrum-generating algebra, is simulated by a surrogate open-system dynamics, which can be interpreted as weak measurement of the target observables, performed on the evolving system. The open-system state can be represented by a mixture of pure states, localized in the phase-space. The localization reduces the scaling of the computational resources with the Hilbert space dimension n by factor n^{3/2}/ln n compared to conventional sparse-matrix methods. The guidelines for the choice of parameters for the simulation are presented and the scaling of the computational resources with the Hilbert space dimension of the system is estimated. The algorithm is applied to the simulation of the dynamics of systems of 2*10^4 and 2*10^6 cold atoms in the double-well trap, described by the two-sites Bose-Hubbard model.

quant-ph↗

arXiv2008

Efficient simulation of quantum evolution using dynamical coarse-graining

A novel scheme to simulate the evolution of a restricted set of observables of a quantum system is proposed. The set comprises the spectrum-generating algebra of the Hamiltonian. The idea is to consider a certain open-system evolution, which can be interpreted as a process of weak measurement of the distinguished observables performed on the evolving system of interest. Given that the observables are "classical" and the Hamiltonian is moderately nonlinear, the open system dynamics displays a large time-scales separation between the dephasing of the observables and the decoherence of the evolving state in the basis of the generalized coherent states (GCS), associated with the spectrum-generating algebra. The time scale separation allows the unitary dynamics of the observables to be efficiently simulated by the open-system dynamics on the intermediate time-scale.The simulation employs unraveling of the corresponding master equations into pure state evolutions, governed by the stochastic nonlinear Schroedinger equantion (sNLSE). It is proved that GCS are globally stable solutions of the sNLSE, if the Hamilonian is linear in the algebra elements.

quant-ph↗

arXiv2008

The globally stable solution of a stochastic Nonlinear Schrodinger Equation

Weak measurement of a subset of noncommuting observables of a quantum system can be modeled by the open-system evolution, governed by the master equation in the Lindblad form. The open-system density operator can be represented as statistical mixture over non unitarily evolving pure states, driven by the stochastic Nonlinear Schrodinger equation (sNLSE). The globally stable solution of the sNLSE is obtained in the case where the measured subset of observables comprises the spectrum-generating algebra of the system. This solution is a generalized coherent state (GCS), associated with the algebra. The result is based on proving that GCS minimize the trace-norm of the covariance matrix, associated with the spectrum-generating algebra.

quant-ph↗

arXiv2007

Grid methods for cold molecules : determination of photoassociation lineshapes and rate constants

A description of photoassociation by CW laser is formulated in the framework of grid methods. The Hamiltonian describing one or several bound states coupled to a multiple of continuum manifolds via a radiative field is written in the energy representation and diagonalized. The generality of the treatment allows to compute accurately and efficiently physical properties such as non-linear high-intensity energy shifts, line shapes, and photoassociation rates both for isolated and non-isolated resonances. Application is given to sodium photoassociation in the experimental conditions of Mc Kenzie {\it et al} [Phys. Rev. Lett. {\bf 88}, 090403 (2002)]. Inverted region for the dependency of the rate vs. the intensity and non-symmetric lineshapes were predicted to occur above the saturation limit. Comparison with the model of Bohn and Julienne [Phys. Rev. A {\bf 60}, 414 (1999)] is discussed.

physics.chem-ph↗

arXiv2007

Negativity as a distance from a separable state

The computable measure of the mixed-state entanglement, the negativity, is shown to admit a clear geometrical interpretation, when applied to Schmidt-correlated (SC) states: the negativity of a SC state equals a distance of the state from a pertinent separable state. As a consequence, a SC state is separable if and only if its negativity vanishes. Another remarkable consequence is that the negativity of a SC can be estimated "at a glance" on the density matrix. These results are generalized to mixtures of SC states, which emerge in certain quantum-dynamical settings.

quant-ph↗

arXiv2006

Coherent Control of Ultracold Collisions with Chirped Light: Direction Matters

We demonstrate the ability to coherently control ultracold atomic Rb collisions using frequency-chirped light on the nanosecond time scale. For certain center frequencies of the chirp, the rate of inelastic trap-loss collisions induced by negatively chirped light is dramatically suppressed compared to the case of a positive chirp. We attribute this to a fundamental asymmetry in the system: an excited wavepacket always moves inward on the attractive molecular potential. For a positive chirp, the resonance condition moves outward in time, while for a negative chirp, it moves inward, in the same direction as the excited wavepacket; this allows multiple interactions between the wavepacket and the light, enabling the wavepacket to be returned coherently to the ground state. Classical and quantum calculations support this interpretation.

physics.atom-ph↗