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See detailExact thermal properties of free-fermionic spin chains
Białończyk, Michał; Gómez-Ruiz, Fernando Javier; Del Campo Echevarria, Adolfo UL

in SciPost Physics (2021)

An exact description of integrable spin chains at finite temperature is provided using an elementary algebraic approach in the complete Hilbert space of the system. We focus on spin chain models that ... [more ▼]

An exact description of integrable spin chains at finite temperature is provided using an elementary algebraic approach in the complete Hilbert space of the system. We focus on spin chain models that admit a description in terms of free fermions, including paradigmatic examples such as the one-dimensional transverse-field quantum Ising and XY models. The exact partition function is derived and compared with the ubiquitous approximation in which only the positive parity sector of the energy spectrum is considered. Errors stemming from this approximation are identified in the neighborhood of the critical point at low temperatures. We further provide the full counting statistics of a wide class of observables at thermal equilibrium and characterize in detail the thermal distribution of the kink number and transverse magnetization in the transverse-field quantum Ising chain. [less ▲]

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See detailDigitized-counterdiabatic quantum approximate optimization algorithm
Chandarana, P.; Hegade, N. N.; Paul, K. et al

in Physical Review Research (2021)

The quantum approximate optimization algorithm (QAOA) has proved to be an e ective classical-quantum algorithm serving multiple purposes, from solving combinatorial optimization problems to finding the ... [more ▼]

The quantum approximate optimization algorithm (QAOA) has proved to be an e ective classical-quantum algorithm serving multiple purposes, from solving combinatorial optimization problems to finding the ground state of many-body quantum systems. Since QAOA is an ansatz-dependent algorithm, there is always a need to design ansatz for better optimization. To this end, we propose a digitized version of QAOA enhanced via the use of shortcuts to adiabaticity. Specifically, we use a counterdiabatic (CD) driving term to design a better ansatz, along with the Hamiltonian and mixing terms, enhancing the global performance. We apply our digitizedcounterdiabatic QAOA to Ising models, classical optimization problems, and the P-spin model, demonstrating that it outperforms standard QAOA in all cases we study. [less ▲]

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See detailUniversal statistics of vortices in a newborn holographic superconductor: beyond the Kibble-Zurek mechanism
Del Campo Echevarria, Adolfo UL; Gómez-Ruiz, Fernando Javier; Li, Zhi-Hong et al

in JHEP (2021)

Traversing a continuous phase transition at a finite rate leads to the breakdown of adiabatic dynamics and the formation of topological defects, as predicted by the celebrated Kibble-Zurek mechanism (KZM ... [more ▼]

Traversing a continuous phase transition at a finite rate leads to the breakdown of adiabatic dynamics and the formation of topological defects, as predicted by the celebrated Kibble-Zurek mechanism (KZM). We investigate universal signatures beyond the KZM, by characterizing the distribution of vortices generated in a thermal quench leading to the formation of a holographic superconductor. The full counting statistics of vortices is described by a binomial distribution, in which the mean value is dictated by the KZM and higher-order cumulants share the universal power-law scaling with the quench time. Extreme events associated with large fluctuations no longer exhibit a power-law behavior with the quench time and are characterized by a universal form of the Weibull distribution for different quench rates. [less ▲]

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See detailDark solitons in a trapped gas of long-range interacting bosons
Beau, M.; Del Campo Echevarria, Adolfo UL; Frantzeskakis, D. J. et al

in Physical Review X (2021)

We consider the interplay of repulsive short-range and long-range interactions in the dynamics of dark solitons, as prototypical coherent nonlinear excitations in a trapped quasi-1D Bose gas. Upon ... [more ▼]

We consider the interplay of repulsive short-range and long-range interactions in the dynamics of dark solitons, as prototypical coherent nonlinear excitations in a trapped quasi-1D Bose gas. Upon examining the form of the ground state, both the existence of the solitary waves and their stability properties are explored and corroborated by direct numerical simulations. We find that single- and multiple-dark-soliton states can exist and are generically robust in the presence of long-range interactions. We analyze the modes of vibration of such excitations and find that their respective frequencies are significantly upshifted as the strength of the long-range interactions is increased. Indeed, we find that a prefactor of the long-range interactions considered comparable to the trap strength may upshift the dark soliton oscillation frequency by an order of magnitude, in comparison to the well established one of /√2 in a trap of frequency . [less ▲]

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See detailProbing Quantum Speed Limits with Ultracold Gases
Del Campo Echevarria, Adolfo UL

in Physical Review Letters (2021)

Quantum speed limits (QSLs) rule the minimum time for a quantum state to evolve into a distinguishable state in an arbitrary physical process. These fundamental results constrain a notion of distance ... [more ▼]

Quantum speed limits (QSLs) rule the minimum time for a quantum state to evolve into a distinguishable state in an arbitrary physical process. These fundamental results constrain a notion of distance traveled by the quantum state, known as the Bures angle, in terms of the speed of evolution set by nonadiabatic energy fluctuations. I theoretically propose how to measure QSLs in an ultracold quantum gas confined in a timedependent harmonic trap. In this highly-dimensional system of continuous variables, quantum tomography is prohibited. Yet, QSLs can be probed whenever the dynamics is self-similar by measuring as a function of time the cloud size of the ultracold gas. This makes it possible to determine the Bures angle and energy fluctuations, as I discuss for various ultracold atomic systems. [less ▲]

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See detailThermofield dynamics: Quantum chaos versus decoherence
Xu, Zhenyu; Chenu, Aurélia UL; Prosen, Tomaž et al

in Physical Review. B, Condensed Matter and Materials Physics (2021)

Quantum chaos imposes universal spectral signatures that govern the thermofield dynamics of a many-body system in isolation. The fidelity between the initial and time-evolving thermofield double states ... [more ▼]

Quantum chaos imposes universal spectral signatures that govern the thermofield dynamics of a many-body system in isolation. The fidelity between the initial and time-evolving thermofield double states exhibits as a function of time a decay, dip, ramp, and plateau. Sources of decoherence give rise to a nonunitary evolution and result in information loss. Energy dephasing gradually suppresses quantum noise fluctuations and the dip associated with spectral correlations. Decoherence further delays the appearance of the dip and shortens the span of the linear ramp associated with chaotic behavior. The interplay between signatures of quantum chaos and information loss is determined by the competition among the decoherence, dip, and plateau characteristic times, as demonstrated in the stochastic Sachdev-Ye-Kitaev model. [less ▲]

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See detailSuperadiabatic thermalization of a quantum oscillator by engineered dephasing
Dupays, Léonce UL; Egusquiza, Inigo; Del Campo Echevarria, Adolfo UL et al

in Physical Review Research (2020), 2

Fast nonadiabatic control protocols known as shortcuts to adiabaticity have found a plethora of applications, but their use has been severely limited to speeding up the dynamics of isolated quantum ... [more ▼]

Fast nonadiabatic control protocols known as shortcuts to adiabaticity have found a plethora of applications, but their use has been severely limited to speeding up the dynamics of isolated quantum systems. We introduce shortcuts for open quantum processes that make possible the fast control of Gaussian states in nonunitary processes. Specifically, we provide the time modulation of the trap frequency and dephasing strength that allow preparing an arbitrary thermal state in a finite time. Experimental implementation can be done via stochastic parametric driving or continuous measurements, readily accessible in a variety of platforms. [less ▲]

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See detailShortcuts to Adiabaticity in Driven Open Quantum Systems: Balanced Gain and Loss and Non-Markovian Evolution
Alipour, Sahar; Chenu, Aurélia UL; Rezahkani, Ali et al

in Quantum (2020), 4

A universal scheme is introduced to speed up the dynamics of a driven open quantum system along a prescribed trajectory of interest. This framework generalizes counterdiabatic driving to open quantum ... [more ▼]

A universal scheme is introduced to speed up the dynamics of a driven open quantum system along a prescribed trajectory of interest. This framework generalizes counterdiabatic driving to open quantum processes. Shortcuts to adiabaticity designed in this fashion can be implemented in two alternative physical scenarios: one characterized by the pres- ence of balanced gain and loss, the other involves non-Markovian dynamics with time-dependent Lindblad operators. As an illustration, we engineer superadiabatic cooling, heating, and isothermal strokes for a two-level system, and provide a pro- tocol for the fast thermalization of a quantum oscillator. [less ▲]

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See detailWork Statistics, Loschmidt Echo and Information Scrambling in Chaotic Quantum Systems
Chenu, Aurélia UL; Molina-Vilaplana, J.; Del Campo Echevarria, Adolfo UL

in Quantum (2019), 3

Characterizing the work statistics of driven complex quantum systems is generally challenging because of the exponential growth with the system size of the number of transitions involved between different ... [more ▼]

Characterizing the work statistics of driven complex quantum systems is generally challenging because of the exponential growth with the system size of the number of transitions involved between different energy levels. We consider the quantum work distribution associated with the driving of chaotic quantum systems described by random matrix Hamiltonians and characterize exactly the work statistics associated with a sudden quench for arbitrary temperature and system size. Knowledge of the work statistics yields the Loschmidt echo dynamics of an entangled state between two copies of the system of interest, the thermofield double state. This echo dynamics is dictated by the spectral form factor. We discuss its relation to frame potentials and its use to assess information scrambling. [less ▲]

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See detailExtreme Decoherence and Quantum Chaos
Xu, Z.; Garcis-Pintos, L.P.; Chenu, Aurélia UL et al

in Physical Review Letters (2019), 122

We study the ultimate limits to the decoherence rate associated with dephasing processes. Fluctuating chaotic quantum systems are shown to exhibit extreme decoherence, with a rate that scales ... [more ▼]

We study the ultimate limits to the decoherence rate associated with dephasing processes. Fluctuating chaotic quantum systems are shown to exhibit extreme decoherence, with a rate that scales exponentially with the particle number, thus exceeding the polynomial dependence of systems with fluctuating k-body interactions. Our findings suggest the use of quantum chaotic systems as a natural test bed for spontaneous wave function collapse models. We further discuss the implications on the decoherence of AdS/CFT black holes resulting from the unitarity loss associated with energy dephasing. [less ▲]

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See detailShortcuts to adiabaticity in Fermi gases
Diao, Pengpeng; Deng, Shujin; Li, Fang et al

in New Journal of Physics (2018)

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See detailQuantum work statistics, Loschmidt echo and information scrambling
Chenu, Aurélia UL; Egusquiza, I. L.; Molina-Vilaplana, J. et al

in Scientific Reports (2018)

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See detailSuperadiabatic quantum friction suppression in finite-time thermodynamics
Shujin Deng; Chenu, Aurélia UL; Diao, Pengpeng et al

in Science Advances (2018)

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