arXiv subjects

Ana Maria Rey

Publications and source records attributed to Ana Maria Rey.

At least 109 records · Page 6Linked to original sources

arXiv2020

Quantum Many-Body Physics with Ultracold Polar Molecules: Nanostructured Potential Barriers and Interactions

We design dipolar quantum many-body Hamiltonians that will facilitate the realization of exotic quantum phases under current experimental conditions achieved for polar molecules. The main idea is to modulate both single-body potential barriers and two-body dipolar interactions on a spatial scale of tens of nanometers to strongly enhance energy scales and, therefore, relax temperature requirements for observing new quantum phases of engineered many-body systems. We consider and compare two approaches. In the first, nanoscale barriers are generated with standing wave optical light fields exploiting optical nonlinearities. In the second, static electric field gradients in combination with microwave dressing are used to write nanostructured spatial patterns on the induced electric dipole moments, and thus dipolar interactions. We study the formation of inter-layer and interface bound states of molecules in these configurations, and provide detailed estimates for binding energies and expected losses for present experimental setups.

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arXiv2020

Relaxation of the collective magnetization of a dense 3D array of interacting dipolar S=3 atoms

We report on measurements of the dynamics of the collective spin length (total magnetization) and spin populations in an almost unit filled lattice system comprising about 10^4 spin S=3 chromium atoms, under the effect of dipolar interactions. The observed spin population dynamics is unaffected by the use of a spin echo, and fully consistent with numerical simulations of the S=3 XXZ spin model. On the contrary, the observed spin length decays slower than in simulations, and surprisingly reaches a small but nonzero asymptotic value within the longest timescale. Our findings show that spin coherences are sensitive probes to systematic effects affecting quantum many-body behavior that cannot be diagnosed by merely measuring spin populations.

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arXiv2020

Thermodynamics of a deeply degenerate SU($N$)-symmetric Fermi gas

Many-body quantum systems can exhibit a striking degree of symmetry unparalleled by their classical counterparts. While in real materials SU($N$) symmetry is an idealization, this symmetry is pristinely realized in fully controllable ultracold alkaline-earth atomic gases. Here, we study an SU($N$)-symmetric Fermi liquid of $^{87}$Sr atoms, where $N$ can be tuned to be as large as 10. In the deeply degenerate regime, we show through precise measurements of density fluctuations and expansion dynamics that the large $N$ of spin states under SU($N$) symmetry leads to pronounced interaction effects in a system with a nominally negligible interaction parameter. Accounting for these effects we demonstrate thermometry accurate to one-hundredth of the Fermi energy. We also demonstrate record speed for preparing degenerate Fermi seas, reaching $T/T_F = 0.12$ in under 3 s, enabled by the SU($N$) symmetric interactions. This, along with the introduction of a new spin polarizing method, enables operation of a 3D optical lattice clock in the band insulating-regime.

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arXiv2020

Short-time expansion of Heisenberg operators in open collective quantum spin systems

We present a new method to compute short-time expectation values in large collective spin systems with generic Markovian decoherence. Our method is based on a Taylor expansion of a formal solution to the equations of motion for Heisenberg operators. This expansion can be truncated at finite order to obtain virtually exact results at short times that are relevant for metrological applications such as spin squeezing. In order to evaluate the expansion for Heisenberg operators, we compute the relevant structure constants of a collective spin operator algebra. We demonstrate the utility of our method by computing spin squeezing, two-time correlation functions, and out-of-time-ordered correlators for $10^4$ spins in strong-decoherence regimes that are otherwise inaccessible via existing numerical methods. Our method can be straightforwardly generalized to the case of a collective spin coupled to bosonic modes, relevant for trapped ion and cavity QED experiments, and may be used to investigate short-time signatures of quantum chaos and information scrambling.

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arXiv2019

Engineering spin squeezing in a 3D optical lattice with interacting spin-orbit-coupled fermions

One of the most important tasks in modern quantum science is to coherently control and entangle many-body systems, and to subsequently use these systems to realize powerful quantum technologies such as quantum-enhanced sensors. However, many-body entangled states are difficult to prepare and preserve since internal dynamics and external noise rapidly degrade any useful entanglement. Here, we introduce a protocol that counterintuitively exploits inhomogeneities, a typical source of dephasing in a many-body system, in combination with interactions to generate metrologically useful and robust many-body entangled states. Motivated by current limitations in state-of-the-art three-dimensional (3D) optical lattice clocks (OLCs) operating at quantum degeneracy, we use local interactions in a Hubbard model with spin-orbit coupling to achieve a spin-locking effect. In addition to prolonging inter-particle spin coherence, spin-locking transforms the dephasing effect of spin-orbit coupling into a collective spin-squeezing process that can be further enhanced by applying a modulated drive. Our protocol is fully compatible with state-of-the-art 3D OLC interrogation schemes and may be used to improve their sensitivity, which is currently limited by the intrinsic quantum noise of independent atoms. We demonstrate that even with realistic experimental imperfections, our protocol may generate $\sim10$--$14$ dB of spin squeezing in $\sim1$ second with $\sim10^2$--$10^4$ atoms. This capability allows OLCs to enter a new era of quantum enhanced sensing using correlated quantum states of driven non-equilibrium systems.

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arXiv2019

Doublon dynamics of Bose-Fermi mixtures in optical lattices

We study the out-of-equilibrium dynamics of a dilute, lattice-confined Bose-Fermi mixture initialized in a highly excited state consisting of boson-fermion pairs (doublons) occupying single lattice sites. This system represents a paradigmatic case for studying relaxation dynamics in strongly correlated systems, and provides a versatile platform for studying thermalization and localization phenomena. We provide analytical expressions for the short-time decay of isolated doublons and small doublon clusters due to the competition between tunneling and interparticle interactions. We also discuss a mechanism for long-time decay that crucially depends on the quantum statistics of the particles constituting the doublon, namely, the conversion of pairs of neighboring doublons into an unpaired fermion and a site with a fermion and two bosons. Building on these insights, we develop a cluster expansion method to describe the dynamics in extended systems and compare it to numerically exact matrix product state simulations in one dimension. Finally, we discuss how our predictions can be observed in experiments with ultracold heteronuclear molecules.

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arXiv2019

Quantum computation toolbox for decoherence-free qubits using multi-band alkali atoms

We introduce protocols for designing and manipulating qubits with ultracold alkali atoms in 3D optical lattices. These qubits are formed from two-atom spin superposition states that create a decoherence-free subspace immune to stray magnetic fields, dramatically improving coherence times while still enjoying the single-site addressability and Feshbach resonance control of state-of-the-art alkali atom systems. Our protocol requires no continuous driving or spin-dependent potentials, and instead relies upon the population of a higher motional band to realize naturally tunable in-site exchange and cross-site superexchange interactions. As a proof-of-principle example of their utility for entanglement generation for quantum computation, we show the cross-site superexchange interactions can be used to engineer 1D cluster states. Explicit protocols for experimental preparation and manipulation of the qubits are also discussed, as well as methods for measuring more complex quantities such as out-of-time-ordered correlation functions (OTOCs).

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arXiv2019

A cavity-QED protocol for precise field sensing in the optical domain

In the context of quantum metrology, optical cavity-QED platforms have primarily been focused on the generation of entangled atomic spin states useful for next-generation frequency and time standards. Here, we report a complementary application: The use of optical cavities to generate non-classical states of light for electric field sensing below the standard quantum limit. We show that cooperative atom-light interactions in the strong collective coupling regime can be used to engineer generalized atom-light cat states which enable quantum enhanced sensing of small displacements of the cavity field even in the presence of photon loss. We demonstrate that metrological gains of 10-20 dB below the standard quantum limit are within reach for current cavity-QED systems operating with long-lived alkaline-earth atoms.

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arXiv2019

Variational spin-squeezing algorithms on programmable quantum sensors

Arrays of atoms trapped in optical tweezers combine features of programmable analog quantum simulators with atomic quantum sensors. Here we propose variational quantum algorithms, tailored for tweezer arrays as programmable quantum sensors, capable of generating entangled states on-demand for precision metrology. The scheme is designed to generate metrological enhancement by optimizing it in a feedback loop on the quantum device itself, thus preparing the best entangled states given the available quantum resources. We apply our ideas to generate spin-squeezed states on Sr atom tweezer arrays, where finite-range interactions are generated through Rydberg dressing. The complexity of experimental variational optimization of our quantum circuits is expected to scale favorably with system size. We numerically show our approach to be robust to noise, and surpassing known protocols.

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arXiv2019

A generalized phase space approach for solving quantum spin dynamics

Numerical techniques to efficiently model out-of-equilibrium dynamics in interacting quantum many-body systems are key for advancing our capability to harness and understand complex quantum matter. Here we propose a new numerical approach which we refer to as GDTWA. It is based on a discrete semi-classical phase-space sampling and allows to investigate quantum dynamics in lattice spin systems with arbitrary $S\geq 1/2$. We show that the GDTWA can accurately simulate dynamics of large ensembles in arbitrary dimensions. We apply it for $S>1/2$ spin-models with dipolar long-range interactions, a scenario arising in recent experiments with magnetic atoms. We show that the method can capture beyond mean-field effects, not only at short times, but it also correctly reproduces long time quantum-thermalization dynamics. We benchmark the method with exact diagonalization in small systems, with perturbation theory for short times, and with analytical predictions made for closed system which feature quantum-thermalization at long times. By computing the Renyi entropy, currently an experimentally accessible quantifier of entanglement, we reveal that large $S$ systems can feature larger entanglement than corresponding $S=1/2$ systems. Our analyses demonstrate that the GDTWA can be a powerful tool for modeling complex spin dynamics in regimes where other state-of-the art numerical methods fail.

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arXiv2019

Quantum entropic self-localization with ultracold fermions

We study a driven, spin-orbit coupled fermionic system in a lattice at the resonant regime where the drive frequency equals the Hubbard repulsion, for which non-trivial constrained dynamics emerge at fast timescales. An effective density-dependent tunneling model is derived, and examined in the sparse filling regime in 1D. The system exhibits entropic self-localization, where while even numbers of atoms propagate ballistically, odd numbers form localized bound states induced by an effective attraction from a higher configurational entropy. These phenomena occur in the strong coupling limit where interactions only impose a constraint with no explicit Hamiltonian term. We show how the constrained dynamics lead to quantum few-body scars and map to an Anderson impurity model with an additional intriguing feature of non-reciprocal scattering. Connections to many-body scars and localization are also discussed.

quant-ph↗

arXiv2019

Dynamics of an itinerant spin-3 atomic dipolar gas in an optical lattice

Arrays of ultra-cold dipolar gases loaded in optical lattices are emerging as powerful quantum simulators of the many-body physics associated with the rich interplay between long-range dipolar interactions, contact interactions, motion, and quantum statistics. In this work we report on our investigation of the quantum many-body dynamics of a large ensemble of bosonic magnetic chromium atoms with spin S = 3 in a three-dimensional lattice as a function of lattice depth. Using extensive theory and experimental comparisons we study the dynamics of the population of the different Zeeman levels and the total magnetization of the gas across the superfluid to the Mott insulator transition. We are able to identify two distinct regimes: At low lattice depths, where atoms are in the superfluid regime, we observe that the spin dynamics is strongly determined by the competition between particle motion, onsite interactions and external magnetic field gradients. Contact spin dependent interactions help to stabilize the collective spin length, which sets the total magnetization of the gas. On the contrary, at high lattice depths, transport is largely frozen out. In this regime, while the spin populations are mainly driven by long range dipolar interactions, magnetic field gradients also play a major role in the total spin demagnetization. We find that dynamics at low lattice depth is qualitatively reproduced by mean-field calculations based on the Gutzwiller ansatz; on the contrary, only a beyond mean-field theory can account for the dynamics at large lattice depths. While the cross-over between these two regimes does not correspond to sharp features in the observed dynamical evolution of the spin components, our simulations indicate that it would be better revealed by measurements of the collective spin length.

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arXiv2019

Effective multi-body SU($N$)-symmetric interactions of ultracold fermionic atoms on a 3-D lattice

Rapid advancements in the experimental capabilities with ultracold alkaline-earth-like atoms (AEAs) bring to a surprisingly near term the prospect of performing quantum simulations of spin models and lattice field theories exhibiting SU($N$) symmetry. Motivated in particular by recent experiments preparing high density samples of strongly interacting ${}^{87}$Sr atoms in a three-dimensional optical lattice, we develop a low-energy effective theory of fermionic AEAs which exhibits emergent multi-body SU($N$)-symmetric interactions, where $N$ is the number of atomic nuclear spin levels. Our theory is limited to the experimental regime of (i) a deep lattice, with (ii) at most one atom occupying each nuclear spin state on any lattice site. The latter restriction is a consequence of initial ground-state preparation. We fully characterize the low-lying excitations in our effective theory, and compare predictions of many-body interaction energies with direct measurements of many-body excitation spectra in an optical lattice clock. Our work makes the first step in enabling a controlled, bottom-up experimental investigation of multi-body SU($N$) physics.

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arXiv2019

Cluster State Generation with Spin-Orbit Coupled Fermionic Atoms in Optical Lattices

Measurement-based quantum computation, an alternative paradigm for quantum information processing, uses simple measurements on qubits prepared in cluster states, a class of multiparty entangled states with useful properties. Here we propose and analyze a scheme that takes advantage of the interplay between spin-orbit coupling and superexchange interactions, in the presence of a coherent drive, to deterministically generate macroscopic arrays of cluster states in fermionic alkaline earth atoms trapped in three dimensional (3D) optical lattices. The scheme dynamically generates cluster states without the need of engineered transport, and is robust in the presence of holes, a typical imperfection in cold atom Mott insulators. The protocol is of particular relevance for the new generation of 3D optical lattice clocks with coherence times $>10$ s, two orders of magnitude larger than the cluster state generation time. We propose the use of collective measurements and time-reversal of the Hamiltonian to benchmark the underlying Ising model dynamics and the generated many-body correlations.

quant-ph↗

arXiv2019

Observation of a Transition Between Dynamical Phases in a Quantum Degenerate Fermi Gas

A proposed paradigm for out-of-equilibrium quantum systems is that an analogue of quantum phase transitions exists between parameter regimes of qualitatively distinct time-dependent behavior. Here, we present evidence of such a transition between dynamical phases in a cold-atom quantum simulator of the collective Heisenberg model. Our simulator encodes spin in the hyperfine states of ultracold fermionic potassium. Atoms are pinned in a network of single-particle modes, whose spatial extent emulates the long-range interactions of traditional quantum magnets. We find that below a critical interaction strength, magnetization of an initially polarized fermionic gas decays quickly, while above the transition point, the magnetization becomes long-lived, due to an energy gap that protects against dephasing by the inhomogeneous axial field. Our quantum simulation reveals a non-equilibrium transition predicted to exist but not yet directly observed in quenched s-wave superconductors.

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arXiv2019

Shattered Time: Can a Dissipative Time Crystal Survive Many-Body Correlations?

We investigate the emergence of a time crystal in a driven-dissipative many-body spin array. In this system the interplay between incoherent spin pumping and collective emission stabilizes a synchronized non-equilibrium steady state which in the thermodynamic limit features a self-generated time-periodic pattern imposed by collective elastic interactions. In contrast to prior realizations where the time symmetry is already broken by an external drive, here it is only spontaneously broken by the elastic exchange interactions and manifest in the two-time correlation spectrum. Employing a combination of exact numerical calculations and a second-order cumulant expansion, we investigate the impact of many-body correlations on the time crystal formation and establish a connection between the regime where it is stable and a slow growth rate of the mutual information, signalling that the time crystal studied here is an emergent semi-classical out-of-equilibrium state of matter. We also confirm the rigidity of the time crystal to single-particle dephasing. Finally, we discuss an experimental implementation using long-lived dipoles in an optical cavity.

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arXiv2018

Driven-Dissipative Quantum Dynamics in Ultra Long-lived Dipoles in an Optical Cavity

We study the quantum dynamics of many-body arrays of two-level atoms in a driven cavity subject to collective decay and interactions mediated by the cavity field. We work in the bad cavity limit accessible, for example, using long-lived electronic clock states of alkaline earth atoms, for which the bare atomic linewidth is much less than the cavity linewidth. In the absence of interactions, our system reduces to previously studied models of collective fluorescence. We show that while interactions do not qualitatively change the steady state properties, they lead to a drastic change in the dynamical properties. We find that, for some interval of driving strengths, the system shows two very distinct types of transient behaviors that depend on the initial state of the system. In particular, there is a parameter regime where the system features oscillatory dynamics with a period of oscillation that becomes much shorter than the duration of the overall transient dynamics as the atom number increases. We use both mean field and exact numerical calculations of the quantum system to investigate the dynamics.

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arXiv2018

Spectrum estimation of density operators with alkaline-earth atoms

We show that Ramsey spectroscopy of fermionic alkaline-earth atoms in a square-well trap provides an efficient and accurate estimate for the eigenspectrum of a density matrix whose $n$ copies are stored in the nuclear spins of $n$ such atoms. This spectrum estimation is enabled by the high symmetry of the interaction Hamiltonian, dictated, in turn, by the decoupling of the nuclear spin from the electrons and by the shape of the square-well trap. Practical performance of this procedure and its potential applications to quantum computing, quantum simulation, and time-keeping with alkaline-earth atoms are discussed.

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