Suchergebnisse

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 dipole interaction in the weak coupling regime the separation of time scales between system and bath can not produce pure dephasing, the process being accompanied by dissipation. Activated and non-activated dephasing processes are analyzed in a diamond nitrogen-vacancy center. ; We thank D Gelman, A Levy, and C P Koch for fruitful discussions. We acknowledge funding by the Israel Science Foundation, the Basque Government (Grant No. IT472-10), Ministerio de Economía y Competitividad (FIS2015-67161-P), the program UFI 11/55 of UPV/EHU, and COST Action MP1209 'Thermodynamics in the quantum regime'. ET is supported by the Basque Government postdoctoral program.

We study the effect of action noise on state-to-state control protocols. Action noise creates dephasing in the instantaneous eigenbasis of the Hamiltonian and hampers the fidelity of the final state with respect to the target state. We find that for shorter protocols the noise more strongly influences the dynamics and degrades fidelity. We suggest improving the fidelity by inducing stronger dephasing rates along the process. The effects of action noise on the dynamics and its manipulation is described for a general Hamiltonian and is then studied by examples. ; We acknowledge J. G. Muga for fruitful discussions and for drawing our attention to his related work. We are grateful to L. McCaslin for useful comments. This work was funded by the US Army Research Office under Contract W911NF15-1-0250 and the Basque Government (Grant No. IT986-16), MINECO/FEDER,UE (Grants No. FIS2015-67161-P and No. FIS2015-70856-P), and QUITEMAD+CM S2013-ICE2801.

Different methods have been recently put forward and implemented experimentally to inverse engineer the time-dependent Hamiltonian of a quantum system and accelerate slow adiabatic processes via nonadiabatic shortcuts. In the "transitionless quantum driving" proposed by Berry, shortcut Hamiltonians are designed so that the system follows exactly, in an arbitrarily short time, the approximate adiabatic path defined by a reference Hamiltonian. A different approach is based on first designing a Lewis-Riesenfeld invariant to carry the eigenstates of a Hamiltonian from specified initial to final configurations, again in an arbitrary time, and then constructing from the invariant the transient Hamiltonian that connects these boundary configurations. We show that the two approaches, apparently quite different in form and so far in results, are, in fact, strongly related and potentially equivalent, so that the inverse-engineering operations in one of them can be reinterpreted and understood in terms of the concepts and operations of the other one. We study, as explicit examples, expansions of time-dependent harmonic traps and the state preparation of two-level systems. ; We thank M. Berry for commenting on the manuscript. This work was supported by the Basque Government (Grant No. IT 472-10), Ministerio de Ciencia e Innovación(Grant No. FIS2009-12773-C02-01), Juan de la Cierva Programme, the National Natural Science Foundation of China (Grant No. 60806041), and the Shanghai Leading Academic Discipline Program (Grant No. S30105). E.T. acknowledges support from the Basque Government (Grant No. BFI08.151).

We use the dynamical algebra of a quantum system and its dynamical invariants to inverse engineer feasible Hamiltonians for implementing shortcuts to adiabaticity. These are speeded up processes that end up with the same populations as slow, adiabatic ones. As application examples, we design families of shortcut Hamiltonians that drive two- and three-level systems between initial and final configurations, imposing physically motivated constraints on the terms (generators) allowed in the Hamiltonian. ; We are grateful to K. Takahashi and R. Kosloff for stimulating discussions. We acknowledge funding by Grants No. IT472-10 and No. FIS2009-12773-C02-01, and theUPV/EHU Program No. UFI 11/55. E.T. is supported by the Basque Government postdoctoral program. S.M.-G. acknowledges support from a UPV/EHU fellowship. ; Publicado

We propose speeding up a single ion heat pump based on a tapered ion trap. If a trapped ion is excited in an oscillatory motion axially the radial degrees of freedom are cyclically expanded and compressed such that heat can be pumped between two reservoirs coupled to the ion at the turning points of oscillation. Through the use of invariant-based inverse engineering we can speed up the process without sacrificing the efficiency of each heat pump cycle. This additional control can be supplied with additional control electrodes or it can be encoded into the geometry of the radial trapping electrodes. We present a novel insight into how speed up can be achieved through the use of inverted harmonic potentials and verify the stability of such trapping conditions. ; We acknowledge funding from MINECO/FEDER (Grants No. FIS2015-70856-P and No. FIS2015-67161-P), CAM PRICYT project QUITEMAD+CM S2013-ICE2801, and Basque Government (Grant No. IT986-16). KS acknowledges support by the German Science Foundation through the Single Ion Heat Engine project.

Diffraction in time (DIT) is a fundamental phenomenon in quantum dynamics due to time-dependent obstacles and slits. It is formally analogous to diffraction of light, and is expected to play an increasing role in the design of coherent matter wave sources, as in the atom laser, to analyze time-of-flight information and emission from ultrafast pulsed excitations, and in applications of coherent matter waves in integrated atom-optical circuits. We demonstrate that DIT emerges robustly in quantum waves emitted by an exponentially decaying source and provide a simple explanation of the phenomenon, as an interference of two characteristic velocities. This allows for its controllability and optimization. ; We acknowledge funding from the Basque Government (Grant No. IT472-10) and Ministerio de Ciencia e Innovación (Grant No. FIS2009-12773-C02-01). Y.B. and E.T. acknowledge financial support from the Basque Government (Grants No. BFI-2010-255 and No. BFI-08-151).

The quantum perceptron is a fundamental building block for quantum machine learning. This is a multidisciplinary field that incorporates abilities of quantum computing, such as state superposition and entanglement, to classical machine learning schemes. Motivated by the techniques of shortcuts to adiabaticity, we propose a speed-up quantum perceptron where a control field on the perceptron is inversely engineered leading to a rapid nonlinear response with a sigmoid activation function. This results in faster overall perceptron performance compared to quasi-adiabatic protocols, as well as in enhanced robustness against imperfections in the controls. ; We acknowledge financial support from Spanish Government via PGC2018-095113-B-I00 (MCIU/AEI/FEDER, UE), Basque Government via IT986-16, as well as from QMiCS (820505) and OpenSuperQ (820363) of the EU Flagship on Quantum Technologies, and the EU FET Open Grant Quromorphic. J. C. acknowledges support from the UPV/EHU grant EHUrOPE. X. C. acknowledges NSFC (11474193), SMSTC (18010500400 and 18ZR1415500), the Program for Eastern Scholar and the Ramón y Cajal program of the Spanish MINECO (RYC-2017-22482). E.T. acknowledges support from Project PGC2018-094792-B-I00 (MCIU/AEI/FEDER,UE), CSIC Research Platform PTI-001, and CAM/FEDER Project No. S2018/TCS-4342 (QUITEMAD-CM).

Sped-up protocols that drive a system quickly to the same populations that can be reached by a slow adiabatic process may involve Hamiltonian terms which are difficult to realize. We use the dynamical symmetry of the Hamiltonian to find, by means of Lie transforms, alternative Hamiltonians that achieve the same goals without the problematic terms. We apply this technique to three-level systems (two interacting bosons in a double well, and beam splitters with two and three output channels) driven by Hamiltonians that belong to the four-dimensional algebra U3S3. ; This work was supported by the National Natural Science Foundation of China (Grant No. 61176118), Grants No. 12QH1400800, No. IT472-10, No. BFI-2010-255, No. 13PJ1403000, and No. FIS2012-36673-C03-01, and the program UFI 11/55. S.M.-G. acknowledges support by a fellowship from UPV/EHU. E.T. is supported by the Basque Government postdoctoral program.

The "fast-forward"approach by Masuda and Nakamura generates driving potentials to accelerate slow quantum adiabatic dynamics. First we present a streamlined version of the formalism that produces the main results in a few steps. Then we show the connection between this approach and inverse engineering based on Lewis-Riesenfeld invariants. We identify in this manner applications in which the engineered potential does not depend on the initial state. Finally we discuss more general applications exemplified by wave splitting processes. ; We are grateful to S. Masuda and K. Nakamura for discussing their method; also to G. Labeyrie for comments on experimental techniques. We acknowledge funding by Projects No. GIU07/40 and No. FIS2009-12773-C02-01, and the UPV/EHU under program UFI 11/55. E.T. acknowledges financial support from the Basque Government (Grant No.BFI08.151).

We study a scaling and coordinate transformation to physically simulate quantum three-body collinear chemical reactions of the type A+BC → AB+C by the motion of a single ultracold atom or a weakly interacting Bose-Einstein condensate on an L-shaped waveguide. We determine its feasibility with current technology and its limitations. As an example we work out the parameters to model the reaction F+H2 → H+HF by the propagation of ultracold lithium atoms. ; We acknowledge the kind hospitality of the Max Planck Institute for the Physics of Complex Systems in Dresden, funding by the Basque Government (Project No. IT 472-10), Ministerio de Ciencia e Innovación (Project No. FIS2009-12773-C02-01), R´egion Midi-Pyrénées, Institut Universitaire de France and Agence Nationale de la Recherche (Project No. ANR-09- BLAN-0134-01). E.T. acknowledges support from the Basque Government (Grant No. BFI08.151).

Two-dimensional (2D) systems with time-dependent controls admit a quadratic Hamiltonian modeling near potential minima. Independent, dynamical normal modes facilitate inverse Hamiltonian engineering to control the system dynamics, but some systems are not separable into independent modes by a point transformation. For these "coupled systems" 2D invariants may still guide the Hamiltonian design. The theory to perform the inversion and two application examples are provided: (i) We control the deflection of wave packets in transversally harmonic wave guides and (ii) we design the state transfer from one coupled oscillator to another. ; This work was supported by the Basque Country Government (Grant No. IT986-16), and by PGC2018-101355-B-I00 (MCIU/AEI/FEDER,UE). E.T. acknowledges support from PGC2018-094792-B-I00 (MCIU/AEI/FEDER,UE), CSIC Research Platform PTI-001, and CAM/FEDER No. S2018/TCS-4342 (QUITEMAD-CM).

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 nonadiabatic 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. ; We acknowledge L McCaslin for fruitful discussions, funding by the Israeli Science Foundation, the US Army Research Office under Contract W911NF- 15-1-0250, the Basque Government (Grant No. IT986-16), MINECO/FEDER,UE (Grants No. FIS2015-70856-P and No. FIS2015-67161-P), and QUITEMAD+CM S2013-ICE2801.

A Schrödinger equation may be unitarily transformed into dynamical equations in different interaction pictures which describe a common physical process, i.e., the same underlying interactions and dynamics. In contrast to this standard scenario, other relations are also possible, such as a common interaction-picture dynamical equation corresponding to several Schrödinger equations that represent different physical processes. This may enable us to design alternative and feasible experimental routes for operations that are a priori difficult or impossible to perform. The power of this concept is exemplified by engineering Hamiltonians that improve the performance or make realizable several shortcuts to adiabaticity. ; We are grateful to M. V. Berry, M. Demirplak, D. Guéry-Odelin, and O. Morsch for discussions. We acknowledge funding by Projects No. GIU07/40, No. FIS2009-12773-C02-01, No. 61176118, and No. 12QH1400800, Juan de la Cierva Program, and the UPV/EHU under program UFI11/55. S. I. and E. T. acknowledge financial support from the Basque Government (Grants No. BFI09.39 and No. BFI08.151).

Different ways to accelerate adiabatic processes in cold atom physics and atomic state preparation are reviewed. The invariant-based inverse engineering approach is applied to trap expansions and contractions, and to atomic transport. Berry's Hamiltonian is applied to produce fast versions of adiabatic passage methods. ; Se resumen distintas maneras de acelerar procesos adiabáticos en la física de átomos fríos y en la preparación de estados atómicos. Se aplica un método inverso basado en invariantes a expansiones y contracciones de trampas y al transporte atómico. También se describen versiones rápidas de los métodos de paso adiabático basadas en un Hamiltoniano propuesto por Berry. ; We are grateful to our collaborators A. Ruschhaupt, D. Guery-Odelin, S. Schmidt and A. del Campo. They have produced with us the original material reviewed here. E. T. and S. I. acknowledge financial support from the Basque Government (Grants No. BFI08.151 and BFI09.39). Funding by the Basque Government (Grant IT472-10), the Ministerio de Ciencia e Innovación (FIS2009-12773-C02-01), the National Natural Science Foundation of China (No. 60806041), the Shanghai Rising-Star Program (No. 08QA14030), the Shanghai Leading Academic Discipline (No. S30105), and Juan de la Cierva Programme is acknowledged. ; Publicado

Postexponential decay of the probability density of a quantum particle leaving a trap can be reproduced accurately, except for interference oscillations at the transition to the postexponential regime, by means of an ensemble of classical particles emitted with constant probability per unit time and the same half-life as the quantum system. The energy distribution of the ensemble is chosen to be identical to the quantum distribution, and the classical point source is located at the scattering length of the corresponding quantum system. A one-dimensional example is provided to illustrate the general argument. ; J.G.M. acknowledges the kind hospitality of the Max Planck Institute for Complex Systems in Dresden. We acknowledge funding by the Basque Country University UPVEHU (GIU07/40), the Ministerio de Educación y Ciencia Spain (FIS2006-10268-C03-01/02 and FIS2009-12773-C02-01), and NSERC Canada (RGPIN-3198). E.T. acknowledges financial support by the Basque Government (BFI08.151).