arXiv subjects

Jorma Louko

Publications and source records attributed to Jorma Louko.

At least 73 records · Page 4Linked to original sources

arXiv2014

Unruh-DeWitt detector response along static and circular geodesic trajectories for Schwarzschild-AdS black holes

We present novel methods to numerically address the problem of characterizing the response of particle detectors in curved spacetimes. These methods allow for the integration of the Wightman function, at least in principle, in rather general backgrounds. In particular we will use this tool to further understand the nature of conformal massless scalar Hawking radiation from a Schwarzschild black hole in anti-de Sitter space. We do that by studying an Unruh-DeWitt detector at rest above the horizon and in circular geodesic orbit. The method allows us to see that the response rate shows peaks at certain characteristic frequencies, which correspond to the quasinormal modes (QNMs) of the space-time. It is in principle possible to apply these techniques to more complicated and interesting physical scenarios, e.g. geodesic infall or multiple detector entanglement evolution, or the study of the behaviour of quantum correlations in spacetimes with black hole horizons.

quant-ph↗

arXiv2014

Particle detectors and the zero mode of a quantum field

We study the impact of the zero-mode of a quantum field on the evolution of a particle detector. For a massless scalar field in a periodic cavity, we show that the impact of the zero mode on the Unruh-DeWitt detector and its derivative-coupling generalisation is necessarily nonvanishing but can be made negligible in some limits, including those commonly occurring in non-relativistic quantum optics. For the derivative-coupling detector this can be accomplished by just tuning the zero mode's initial state, but the standard Unruh-DeWitt detector requires a more subtle and careful tuning. Applications include an inertial detector with arbitrary velocity, where we demonstrate the regularity of the ultrarelativistic limit, and a detector with uniform acceleration.

quant-ph↗

arXiv2014

Static detectors and circular-geodesic detectors on the Schwarzschild black hole

We examine the response of an Unruh-DeWitt particle detector coupled to a massless scalar field on the (3+1)-dimensional Schwarzschild spacetime, in the Boulware, Hartle-Hawking and Unruh states, for static detectors and detectors on circular geodesics, by primarily numerical methods. For the static detector, the response in the Hartle-Hawking state exhibits the known thermality at the local Hawking temperature, and the response in the Unruh state is thermal at the local Hawking temperature in the limit of a large detector energy gap. For the circular-geodesic detector, we find evidence of thermality in the limit of a large energy gap for the Hartle-Hawking and Unruh states, at a temperature that exceeds the Doppler-shifted local Hawing temperature. Detailed quantitative comparisons between the three states are given. The response in the Hartle-Hawking state is compared with the response in the Minkowski vacuum and in the Minkowski thermal state for the corresponding Rindler, drifted Rindler, and circularly accelerated trajectories. The analysis takes place within first-order perturbation theory and relies in an essential way on stationarity.

gr-qc↗

arXiv2014

Singularity resolution by lattice shifts in discretised quantum mechanics

We investigate the robustness of singularity avoidance mechanisms in nonrelativistic quantum mechanics on the discretised real line when lattice points are allowed to approach a singularity of the classical potential. We consider the attractive Coulomb potential and the attractive scale invariant potential, on an equispaced parity-noninvariant lattice and on a non-equispaced parity-invariant lattice, and we examine the energy eigenvalues by a combination of analytic and numerical techniques. While the lowest one or two eigenvalues descend to negative infinity in the singular limit, we find that the higher eigenvalues remain finite and form degenerate pairs, close to the eigenvalues of a theory in which a lattice point at the singularity is regularised either by Thiemann's loop quantum gravity singularity avoidance prescription or by a restriction to the odd parity sector. The approach to degeneracy can be reproduced from a nonsingular discretised half-line quantum theory by tuning a boundary condition parameter. The results show that Thiemann's singularity avoidance prescription and the discretised half-line boundary condition reproduce quantitatively correct features of the singular limit spectrum apart from the lowest few eigenvalues.

gr-qc↗

arXiv2014

Voyage to Alpha Centauri: Entanglement degradation of cavity modes due to motion

We propose a scheme to investigate whether non-uniform motion degrades entanglement of a relativistic quantum field that is localised both in space and in time. For a Dirichlet scalar field in a cavity in Minkowski space, in small but freely-adjustable acceleration of finite but arbitrarily long duration, degradation of observable magnitude occurs for massless transverse quanta of optical wavelength at Earth gravity acceleration and for kaon mass quanta already at microgravity acceleration. We outline a space-based experiment for observing the effect and its gravitational counterpart.

quant-ph↗

arXiv2013

Scalar, spinor, and photon fields under relativistic cavity motion

We analyse quantised scalar, spinor and photon fields in a mechanically rigid cavity that is accelerated in Minkowski spacetime, in a recently-introduced perturbative small acceleration formalism that allows the velocities to become relativistic, with a view to applications in relativistic quantum information. A scalar field is analysed with both Dirichlet and Neumann boundary conditions, and a photon field under perfect conductor boundary conditions is shown to decompose into Dirichlet-like and Neumann-like polarisation modes. The Dirac spinor is analysed with a nonvanishing mass and with dimensions transverse to the acceleration, and the MIT bag boundary condition is shown to exclude zero modes. Unitarity of time evolution holds for smooth accelerations but fails for discontinuous accelerations in spacetime dimensions (3+1) and higher. As an application, the experimental desktop mode-mixing scenario proposed for a scalar field by Bruschi et al. [New J. Phys. 15, 073052 (2013)] is shown to apply also to the photon field.

quant-ph↗

arXiv2013

Relativistic Motion Generates Quantum Gates and Entanglement Resonances

We show that the relativistic motion of a quantum system can be used to generate quantum gates. The nonuniform acceleration of a cavity is used to generate well-known two-mode quantum gates in continuous variables. Observable amounts of entanglement between the cavity modes are produced through resonances which appear by repeating periodically any trajectory.

quant-ph↗

arXiv2013

Mode-mixing quantum gates and entanglement without particle creation in periodically accelerated cavities

We show that mode-mixing quantum gates can be produced by non-uniform relativistic acceleration. Periodic motion in cavities exhibits a series of resonant conditions producing entangling quantum gates between different frequency modes. The resonant condition associated with particle creation is the main feature of the dynamical Casimir effect which has been recently demonstrated in superconducting circuits. We show that a second resonance, which has attracted less attention since it implies negligible particle production, produces a beam splitting quantum gate leading to a resonant enhancement of entanglement which can be used as the first evidence of acceleration effects in mechanical oscillators. We propose a desktop experiment where the frequencies associated with this second resonance can be produced mechanically.

quant-ph↗

arXiv2013

A topological state sum model for fermions on the circle

A simple state sum model for fermions on a 1-manifold is constructed. The model is independent of the triangulation and gives exactly the same partition function as the Dirac functional integral with zeta-function regularisation. Some implications for more realistic physical models are discussed.

math-ph↗

arXiv2013

Relativistic Quantum Information in Detectors-Field Interactions

We review Unruh-DeWitt detectors and other models of detector-field interaction in a relativistic quantum field theory setting as a tool for extracting detector-detector, field-field and detector-field correlation functions of interest in quantum information science, from entanglement dynamics to quantum teleportation. We in particular highlight the contrast between the results obtained from linear perturbation theory which can be justified provided switching effects are properly accounted for, and the nonperturbative effects from available analytic expressions which incorporate the backreaction effects of the quantum field on the detector behaviour.

quant-ph↗

arXiv2013

Entanglement generation in relativistic cavity motion

We analyse particle creation and mode mixing for a quantum field in an accelerated cavity, assuming small accelerations but allowing arbitrary velocities, travel times and travel distances, and in particular including the regime of relativistic velocities. As an application, we identify a desktop experimental scenario where the mode mixing resonance frequency in linear sinusoidal motion or in uniform circular motion is significantly below the particle creation resonance frequencies of the Dynamical Casimir Effect, and arguably at the threshold of current technology. The mode mixing acts as a beamsplitter quantum gate, experimentally detectable not only via fluxes or particle numbers but also via entanglement generation.

quant-ph↗

arXiv2012

Static, stationary and inertial Unruh-DeWitt detectors on the BTZ black hole

We examine an Unruh-DeWitt particle detector coupled to a scalar field in three-dimensional curved spacetime. We first obtain a regulator-free expression for the transition probability in an arbitrary Hadamard state, working within first-order perturbation theory and assuming smooth switching, and we show that both the transition probability and the instantaneous transition rate remain well defined in the sharp switching limit. We then analyse a detector coupled to a massless conformally coupled field in the Hartle-Hawking vacua on the Banados-Teitelboim-Zanelli black hole, under both transparent and reflective boundary conditions at the infinity. A selection of stationary and freely-falling detector trajectories are examined, including the co-rotating trajectories, for which the response is shown to be thermal. Analytic results in a number of asymptotic regimes, including those of large and small mass, are complemented by numerical results in the interpolating regimes. The boundary condition at infinity is seen to have a significant effect on the transition rate.

gr-qc↗

arXiv2012

Unruh-DeWitt detector on the BTZ black hole

We examine an Unruh-DeWitt particle detector coupled to a scalar field in three-dimensional curved spacetime, within first-order perturbation theory. We first obtain a causal and manifestly regular expression for the instantaneous transition rate in an arbitrary Hadamard state. We then specialise to the Bañados-Teitelboim-Zanelli black hole and to a massless conformally coupled field in the Hartle-Hawking vacuum. A co-rotating detector responds thermally in the expected local Hawking temperature, while a freely-falling detector shows no evidence of thermality in regimes that we are able to probe, not even far from the horizon. The boundary condition at the asymptotically anti-de Sitter infinity has a significant effect on the transition rate.

gr-qc↗

arXiv2012

How often does the Unruh-DeWitt detector click beyond four dimensions?

We analyse the response of an arbitrarily-accelerated Unruh-DeWitt detector coupled to a massless scalar field in Minkowski spacetimes of dimensions up to six, working within first-order perturbation theory and assuming a smooth switch-on and switch-off. We express the total transition probability as a manifestly finite and regulator-free integral formula. In the sharp switching limit, the transition probability diverges in dimensions greater than three but the transition rate remains finite up to dimension five. In dimension six, the transition rate remains finite in the sharp switching limit for trajectories of constant scalar proper acceleration, including all stationary trajectories, but it diverges for generic trajectories. The divergence of the transition rate in six dimensions suggests that global embedding spacetime (GEMS) methods for investigating detector response in curved spacetime may have limited validity for generic trajectories when the embedding spacetime has dimension higher than five.

gr-qc↗

arXiv2012

Boundary conditions in quantum mechanics on the discretized half-line

We investigate nonrelativistic quantum mechanics on the discretized half-line, constructing a one-parameter family of Hamiltonians that are analogous to the Robin family of boundary conditions in continuum half-line quantum mechanics. For classically singular Hamiltonians, the construction provides a singularity avoidance mechanism that has qualitative similarities with singularity avoidance encountered in loop quantum gravity. Applications include the free particle, the attractive Coulomb potential, the scale invariant potential and a black hole described in terms of the Einstein-Rosen wormhole throat. The spectrum is analyzed by analytic and numerical techniques. In the continuum limit, the full Robin family of boundary conditions can be recovered via a suitable fine-tuning but the Dirichlet-type boundary condition emerges as generic.

gr-qc↗

arXiv2012

Particle and anti-particle bosonic entanglement in non-inertial frames

We analyse the entanglement tradeoff between particle and anti-particle modes of a charged bosonic field between inertial and uniformly accelerated observers. In contrast with previous results for fermionic fields, we find that the entanglement redistribution between particle and antiparticle modes does not prevent the entanglement from vanishing in the infinite acceleration limit.

quant-ph↗

arXiv2012

Motion generates entanglement

We demonstrate entanglement generation between mode pairs of a quantum field in a nonuniformly accelerated cavity in Minkowski space-time. The effect is sensitive to the initial state, the choice of the mode pair and bosonic versus fermionic statistics, and it can be stronger by orders of magnitude than the entanglement degradation between a nonuniformly accelerated cavity and an inertial cavity. Detailed results are obtained for massless scalar and spinor fields in (1+1) dimensions. By the equivalence principle, the results provide a model of entanglement generation by gravitational effects.

quant-ph↗

arXiv2012

Kinematic entanglement degradation of fermionic cavity modes

We analyse the entanglement and the non-locality of a (1+1)-dimensional massless Dirac field confined to a cavity on a worldtube that consists of inertial and uniformly accelerated segments, for small accelerations but arbitrarily long travel times. The correlations between the accelerated field modes and the modes in an inertial reference cavity are periodic in the durations of the individual trajectory segments, and degradation of the correlations can be entirely avoided by fine-tuning the individual or relative durations of the segments. Analytic results for selected trajectories are presented. Differences from the corresponding bosonic correlations are identified and extensions to massive fermions are discussed.

quant-ph↗