arXiv subjects

Thomas Busch

Publications and source records attributed to Thomas Busch.

At least 109 records · Page 6Linked to original sources

arXiv2016

Topological defect dynamics of vortex lattices in Bose--Einstein condensates

Vortex lattices in rapidly rotating Bose--Einstein condensates are systems of topological excitations that arrange themselves into periodic patterns. Here we show how phase-imprinting techniques can be used to create a controllable number of defects in these lattices and examine the resulting dynamics. Even though we describe our system using the mean-field Gross--Pitaevskii theory, the full range of many particle effects among the vortices can be studied. In particular we find the existence of localized vacancies that are quasi-stable over long periods of time, and characterize the effects on the background lattice through use of the orientational correlation function, and Delaunay triangulation.

cond-mat.quant-gas↗

arXiv2016

Shaken not stirred: Creating exotic angular momentum states by shaking an optical lattice

We propose a method to create higher orbital states of ultracold atoms in the Mott regime of an optical lattice. This is done by periodically modulating the position of the trap minima (known as shaking) and controlling the interference term of the lasers creating the lattice. These methods are combined with techniques of shortcuts to adiabaticity. As an example of this, we show specifically how to create an anti-ferromagnetic type ordering of angular momentum states of atoms. The specific pulse sequences are designed using Lewis-Riesenfeld invariants and a four-level model for each well. The results are compared with numerical simulations of the full Schroedinger equation.

quant-ph↗

arXiv2016

Spatial adiabatic passage via interaction-induced band separation

The development of advanced quantum technologies and the quest for a deeper understanding of many-particle quantum mechanics requires control over the quantum state of interacting particles to a high degree of fidelity. However, the quickly increasing density of the spectrum, together with the appearance of crossings in time-dependent processes, makes any effort to control the system hard and resource intensive. Here we show that in trapped systems regimes can exist, in which isolated energy bands appear that allow to easily generalize known single-particle techniques. We demonstrate this for the well-known spatial adiabatic passage effect, which can control the center-of-mass state of atoms with high fidelity.

quant-ph↗

arXiv2016

Spatial adiabatic passage: a review of recent progress

Adiabatic techniques are known to allow for engineering quantum states with high fidelity. This requirement is currently of large interest, as applications in quantum information require the preparation and manipulation of quantum states with minimal errors. Here we review recent progress on developing techniques for the preparation of spatial states through adiabatic passage, particularly focusing on three state systems. These techniques can be applied to matter waves in external potentials, such as cold atoms or electrons, and to classical waves in waveguides, such as light or sound.

quant-ph↗

arXiv2016

Transport of ultracold atoms between concentric traps via spatial adiabatic passage

Spatial adiabatic passage processes for ultracold atoms trapped in tunnel-coupled cylindrically symmetric concentric potentials are investigated. Specifically, we discuss the matter-wave analogue of the rapid adiabatic passage (RAP) technique for a high fidelity and robust loading of a single atom into a harmonic ring potential from a harmonic trap, and for its transport between two concentric rings. We also consider a system of three concentric rings and investigate the transport of a single atom between the innermost and the outermost rings making use of the matter-wave analogue of the stimulated Raman adiabatic passage (STIRAP) technique. We describe the RAP-like and STIRAP-like dynamics by means of a two- and a three-state models, respectively, obtaining good agreement with the numerical simulations of the corresponding two-dimensional Schrödinger equation.

quant-ph↗

arXiv2015

Emergence of Classical Rotation in Superfluid Bose-Einstein Condensates

Phase transitions can modify quantum behaviour on mesoscopic scales and give access to new and unusual quantum dynamics. Here we investigate the superfluid properties of a rotating two-component Bose--Einstein condensate as a function of changes in the interaction energy and in particular through the phase transition from miscibility to immiscibility. We show that the breaking of one of the hallmarks of superfluid flow, namely the quantisation condition on circulation, is continuous throughout an azimuthal phase separation process and displays intriguing density dynamics. We find that the resulting currents are stable for long times and possess a phase boundary that exhibits classical solid body rotation, despite the quantum nature of superfluid flow. To support this co-existence of classical and quantum behaviour the system develops a unique velocity flow profile, which includes unusual radial flow in regions near the phase boundary.

cond-mat.quant-gas↗

arXiv2015

Gap Solitons in Spin-Orbit-Coupled Bose-Einstein Condensates in Optical Lattices

While different ways to realize spin-orbit coupling in Bose-Einstein condensates exist, not all are currently experimentally implementable. Here we present a detailed study of gap solitons in a Bose- Einstein condensate with experimentally realizable spin-orbit coupling and discuss two cases relating to a spin-dependent parity symmetry. In the parity symmetric case, two families of fundamental gap solitons in second linear energy gap are demonstrated with opposite sign of the parity, with one family having single humped densities and the other double humped ones. In the case of broken parity symmetry, the fundamental modes manifest spin-polarization. Both families possess an opposite sign of the spin-polarization.

cond-mat.quant-gas↗

arXiv2014

Measurement of collective excitations in a spin-orbit-coupled Bose-Einstein condensate

We measure the collective excitation spectrum of a spin-orbit coupled Bose-Einstein condensate using Bragg spectroscopy. The spin-orbit coupling is generated by Raman dressing of atomic hyperfine states. When the Raman detuning is reduced, mode softening at a finite momentum is revealed, which provides insight towards a supersolid-like phase transition. We find that for the parameters of our system, this softening stops at a finite excitation gap and is symmetric under a sign change of the Raman detuning. Finally, using a moving barrier that is swept through the BEC, we also show the effect of the collective excitation on the fluid dynamics.

cond-mat.quant-gas↗

arXiv2014

Quenching small quantum gases: Genesis of the orthogonality catastrophe

We study the dynamics of two strongly interacting bosons with an additional impurity atom trapped in a harmonic potential. Using exact numerical diagonalization we are able to fully explore the dynamical evolution when the interaction between the two distinct species is suddenly switched on (quenched). We examine the behavior of the densities, the entanglement, the Loschmidt echo and the spectral function for a large range of inter-species interactions and find that even in such small systems evidence of Anderson's orthogonality catastrophe can be witnessed.

quant-ph↗

arXiv2014

Single-atom interferometer based on two-dimensional spatial adiabatic passage

In this work we propose a novel single-atom interferometer based on a fully two-dimensional spatial adiabatic passage process using a system of three identical harmonic traps in a triangular geometry. While the transfer of a single atom from the ground state of one trap to the ground state of the most distant one can successfully be achieved in a robust way for a broad range of parameter values, we point out the existence of a specific geometrical configuration of the traps for which a crossing of two energy eigenvalues occurs and the transfer of the atom fails. Instead the wavefunction is robustly split into a coherent superposition between two of the traps. We show that this process can be used to construct a single-atom interferometer and discuss its performance in terms of the final population distribution among the asymptotic eigenstates of the individual traps. This interferometric scheme could be used to study space dependent fields from ultrashort to relatively large distances, or the decay of the coherence of superposition states as a function of the distance.

quant-ph↗

arXiv2014

New Spin Squeezing and Other Entanglement Tests for Two Mode Systems of Identical Bosons

For any quantum state representing a physical system of identical particles, the density operator must satisfy the symmetrisation principle (SP) and for massive particles also conform to super-selection rules (SSR) that prohibit coherences between differing total particle numbers. Here we consider bi-partitite states for systems of massive bosons, where both the system and subsystems are modes (or sets of modes), particles being associated with differing mode occupancies. To define non-entangled or separable states the subsystem density operators are also required to satisfy the SP and conform to SSR, in contrast to some other approaches. Whilst in the presence of these additional constraints some of the known entanglement criteria are still valid, we present a detailed treatment to establish the validity of known criteria and derive new ones.

quant-ph↗

arXiv2014

Tunneling-induced angular momentum for single cold atoms

We study the generation of angular momentum carrying states for a single cold particle by breaking the symmetry of a spatial adiabatic passage process in a two-dimensional system consisting of three harmonic potential wells. By following a superposition of two eigenstates of the system, a single cold particle is completely transferred to the degenerate first excited states of the final trap, which are resonantly coupled via tunneling to the ground states of the initial and middle traps. Depending on the total time of the process, angular momentum is generated in the final trap, with values that oscillate between $\pm\hbar$. This process is discussed in terms of the asymptotic eigenstates of the individual wells and the results have been checked by simulations of the full two-dimensional Schrödinger equation.

quant-ph↗

arXiv2014

Quantum Entanglement for Systems of Identical Bosons. Spin Squeezing and Other Entanglement Tests in Two Mode Systems

The concept of entangled quantum states is considered in the context of systems of identical particles, based on the requirement that in order to represent physical states both for the overall system and the sub-systems which may be entangled, the density operators must satisfy the symmetrisation principle and global and local super-selection rules that prohibit states in which there are coherences between differing particle numbers. These requirements and their justification are fully discussed. In the second quantisation approach used, both the system and the sub-systems are modes (or sets of modes) rather than particles, particles being associated with different occupancies of the modes. The definition of entangled states is based on first defining the non-entangled states - after specifying which modes constitute the sub-systems. This paper mainly focuses on two mode entanglement for massive bosons. Several inequalities involving variances and mean values of operators involving mode annihilation, creation operators have been proposed as tests for two mode entangled states, including the inequalities that define spin squeezing. Spin squeezing criteria in two mode systems are examined, and spin squeezing is best considered for principle spin operator components where the covariance matrix is diagonal. It is shown that the presence of spin squeezing in at least one of the spin components requires entanglement of the relevant pair of modes. Several of the other proposed tests for entanglement, including ones based on the sum of the variances for two spin components are considered. All of the tests are still valid when the present concept of entanglement based on the symmetrisation and super-selection rule criteria is applied, but further tests have been obtained here. Sometimes the new tests are satisfied whilst than those obtained in other papers are not.

quant-ph↗

arXiv2013

Quantum state transfer in the presence of non homogeneous external potentials

Heisenberg-type spin models in the limit of a low number of excitations are useful tools to study basic mechanisms in strongly correlated and magnetic systems. Many of these mechanisms can be experimentally tested using ultracold atoms. Here, we discuss the implementation of a quantum state transfer protocol in a tight-binding chain in the presence of an inhomogeneous external potential. We show that it can be used to extend the parameter range in which high fidelity state transfer can be achieved beyond the well established weak-coupling regime. Among the class of mirror-reflecting potentials that allow for high-fidelity quantum state transfer, the harmonic case is especially relevant because it allows us to formulate a proposal for the experimental implementation of the protocol in the context of optical lattices.

quant-ph↗

arXiv2013

Criticality, factorization and long-range correlations in the anisotropic XY-model

We study the long-range quantum correlations in the anisotropic XY-model. By first examining the thermodynamic limit we show that employing the quantum discord as a figure of merit allows one to capture the main features of the model at zero temperature. Further, by considering suitably large site separations we find that these correlations obey a simple scaling behavior for finite temperatures, allowing for efficient estimation of the critical point. We also address ground-state factorization of this model by explicitly considering finite size systems, showing its relation to the energy spectrum and explaining the persistence of the phenomenon at finite temperatures. Finally, we compute the fidelity between finite and infinite systems in order to show that remarkably small system sizes can closely approximate the thermodynamic limit.

quant-ph↗

arXiv2013

Effect of inter-particle interaction in a free oscillation atomic interferometer

We investigate the dynamics of two interacting bosons repeatedly scattering off a beam-splitter in a free oscillation atom interferometer. Using the inter-particle scattering length and the beam splitter probabilites as our control parameters, we show that even in a simple setup like this a wide range of strongly correlated quantum states can be created. This in particular includes the NOON state, which maximizes the quantum Fisher information and is a foremost state in quantum metrology.

quant-ph↗

arXiv2013

Self-avoiding quantum walks: realisations in subspaces and limit theorems

While completely self-avoiding quantum walks have the distinct property of leading to a trivial unidirectional transport of a quantum state, an interesting and non-trivial dynamics can be constructed by restricting the self-avoidance to a subspace of the complete Hilbert space. Here, we present a comprehensive study of three two-dimensional quantum walks, which are self-avoiding in coin space, in position space and in both, coin and position space. We discuss the properties of these walks and show that all result in delocalisation of the probability distribution for initial states which are strongly localised for a walk with a standard Grover coin operation. We also present analytical results for the evolution in the form of limit distributions for the self-avoiding walks in coin space and in both, coin and position space.

quant-ph↗

arXiv2012

Entangling two defects via a surrounding crystal

We theoretically show how two impurity defects in a crystalline structure can be entangled through coupling with the crystal. We demonstrate this with a harmonic chain of trapped ions in which two ions of a different species are embedded. Entanglement is found for sufficiently cold chains and for a certain class of initial, separable states of the defects. It results from the interplay between localized modes which involve the defects and the interposed ions, it is independent of the chain size, and decays slowly with the distance between the impurities. These dynamics can be observed in systems exhibiting spatial order, viable realizations are optical lattices, optomechanical systems, or cavity arrays in circuit QED.

cond-mat.other↗