Andre G. S. Landulfo

Sudden change in quantum and classical correlations and the Unruh effect

We use the Unruh effect to analyze the dynamics of classical and quantum correlations for a two-qubit system when one of them is uniformly accelerated for a finite amount of proper time. We show that the quantum correlation is completely destroyed in the limit of infinite acceleration, while the classical one remains nonzero. In particular, we show that such correlations exhibit the so-called sudden-change behavior as a function of acceleration. Eventually, we discuss how our results can be interpreted when the system lies in the vicinity of the event horizon of a Schwarzschild black hole.

Influence of detector motion in Bell inequalities with entangled fermions

The discovery of the Bell inequalities can be considered one of the most important physics landmarks of the 20th century [1]. It allows us to probe the essence of quantum theory by distinguishing it from local hidden variable theories. The genesis of this achievement can be traced back to the Einstein-Podolsky-Rosen discussion about the completeness of quantum mechanics [2]. Presently the Clauser-Horne-Shimony-Holt (CHSH) Bell inequality [3] has been shown to be violated 30 standard deviations [4], which strongly supports quantum mechanics. In order to contribute to the intense present debate on the interplay between relativity and quantum mechanics (see e.g. Refs. [5, 6, 7, 8, 9, 10]), we investigate here how the former influences the spin correlation of entangled fermions measured by moving detectors. In particular we show that the CHSH Bell inequality can be satisfied rather than violated by quantum mechanics if the left and right spin detectors are set in fast enough relativistic motion. We adopt natural units ~ = c = 1. Let us assume a system composed of two spin-1/2 particles A and B with mass m and zero total spin angular momentum. Each particle spin is measured along some arbitrary direction defined on the y⊥z plane. The distance between the planes along the x axis is large enough to make both measurements causally disconnected. This is well known that local hidden variable theories satisfy the CHSH Bell inequality |E(a2, b1)+E(a2, b2)+E(a1, b1)−E(a1, b2)| ≤ 2, (1) where ai (i = 1,2) are two arbitrary unit vectors contained in the y⊥z plane along which the spin sA of particle A is measured, and analogously for the two arbitrary unit vectors bj (j = 1,2) and spin sB of particle B. Here

Nonperturbative approach to relativistic quantum communication channels

We investigate the transmission of both classical and quantum information between two arbitrary observers in globally hyperbolic spacetimes using a quantum field as a communication channel. The field is supposed to be in some arbitrary quasifree state and no choice of representation of its canonical commutation relations is made. Both sender and receiver possess some localized two-level quantum system with which they can interact with the quantum field to prepare the input and receive the output of the channel, respectively. The interaction between the two-level systems and the quantum field is such that one can trace out the field degrees of freedom exactly and thus obtain the quantum channel in a nonperturbative way. We end the paper determining the unassisted as well as the entanglement-assisted classical and quantum channel capacities.

Particle creation due to tachyonic instability in relativistic stars

Dense enough compact objects were recently shown to lead to an exponentially fast increase of the vacuum energy density for some free scalar fields properly coupled to the spacetime curvature as a consequence of a tachyonic-like instability. Once the effect is triggered, the star energy density would be overwhelmed by the vacuum energy density in a few milliseconds. This demands that eventually geometry and field evolve to a new configuration to bring the vacuum back to a stationary regime. Here, we show that the vacuum fluctuations built up during the unstable epoch lead to particle creation in the final stationary state when the tachyonic instability ceases. The amount of created particles depends mostly on the duration of the unstable epoch and final stationary configuration, which are open issues at this point. We emphasize that the particle creation coming from the tachyonic instability will occur even in the adiabatic limit, where the spacetime geometry changes arbitrarily slowly, and therefore is quite distinct from the usual particle creation due to the change in the background geometry.

Sending classical information through relativistic quantum channels

We investigate how special relativity influences the transmission of classical information through quantum channels by evaluating the Holevo bound when the sender and the receiver are in (relativistic) relative motion. By using the spin degrees of freedom of spin-1/2 fermions to encode the classical information, we show that, for some configurations, the accessible information in the receiver can be increased when the spin detector moves fast enough. This is possible by allowing the momentum wave packet of one of the particles to be sufficiently wide while the momentum wave packets of other particles are kept relatively narrow. In this way, one can take advantage of the fact that boosts entangle the spin and momentum degrees of freedom of spin-1/2 fermions to increase the accessible information in the former. We close the paper with a discussion of how this relativistic quantum channel cannot in general be described by completely positive quantum maps.

Instability of nonminimally coupled scalar fields in the spacetime of thin charged shells

We investigate the stability of a free scalar field nonminimally coupled to gravity under linear perturbations in the spacetime of a charged spherical shell. Our analysis is performed in the context of quantum field theory in curved spacetimes. This paper completes previous analyses which considered the exponential enhancement of vacuum fluctuations in the spacetime of massive shells.