Charis Anastopoulos
@upatras.gr
Department of Physics
University of Patras
EDUCATION
1993-1996 Department of Physics, Imperial College, London, UK
Ph.D. Thesis title: Emergence of Classical Behaviour in Quantum Systems
1992 -1993 Department of Physics, Imperial College, London, UK
M.Sc. on Quantum Fields and Fundamental Forces
1988 - 1992 Department of Physics, University of Patras, Greece
B.Sc. in Physics
Scopus Publications
Scholar Citations
Scholar h-index
Scholar i10-index
Scopus Publications
Maria Papageorgiou, Jose de Ramón, and Charis Anastopoulos
American Physical Society (APS)
Pointlike systems coupled to quantum fields are often employed as toy models for measurements in quantum field theory. In this paper, we identify the field observables recorded by such models. We show that in models that work in the strong coupling regime, the apparatus is correlated with smeared field amplitudes, while in models that work in weak coupling the apparatus records particle aspects of the field, such as the existence of a particle-like time of arrival and resonant absorption. Then, we develop an improved field-detector interaction model, adapting the formalism of Quantum Brownian motion, that is exactly solvable. This model confirms the association of field and particle properties in the strong and weak coupling regimes, respectively. Further, it can also describe the intermediate regime, in which the field-particle characteristics `merge'. In contrast to standard perturbation techniques, this model also recovers the relativistic Breit-Wigner resonant behavior in the weak coupling regime. The modulation of field-particle-duality by a single tunable parameter is a novel feature that is, in principle, experimentally accessible.
Charis Anastopoulos and Maria-Electra Plakitsi
MDPI AG
We develop a new formalism for constructing probabilities associated with the causal ordering of events in quantum theory, where an event is defined as the emergence of a measurement record on a detector. We start with constructing probabilities for the causal ordering events in classical physics, where events are defined in terms of worldline coincidences. Then, we show how these notions generalize to quantum systems, where there exists no fundamental notion of trajectory. The probabilities constructed here are experimentally accessible, at least in principle. Our analysis here clarifies that the existence of quantum orderings of events do not require quantum gravity effects: it is a consequence of the quantum dynamics of matter, and it appears in the presence of a fixed background spacetime.
Demetrios Kotopoulis and Charis Anastopoulos
IOP Publishing
Abstract We analyze the thermodynamics of spherically symmetric thin-shell solutions to Einstein’s equations, including solutions with negative interior mass. We show the inclusion of such solutions is essential for the thermodynamic consistency of the system: the maximum energy principle applies when we include an entropy term from the singularity of the negative-mass solutions, in addition to the Bekenstein–Hawking term for the entropy of solutions with positive interior mass. Then, the thermodynamic analysis leads to four distinct thermodynamic phases. We also show that all types of solutions can be either thermodynamically stable or dynamically stable, but only solutions with zero interior mass can be both. Since most of our results are analytic, thin shell models emerge as a useful theoretical paradigm for exploring gravitational thermodynamics. Our results provide an additional argument in support of the assignment of entropy to the singularity of negative-mass Schwarzschild spacetimes, and, consequently, to Penrose’s conjecture about the assignment of entropy to singularities.
Eirini Sourtzinou and Charis Anastopoulos
American Physical Society (APS)
We undertake a first-principles analysis of the thermodynamics of a small body near a black hole horizon. In particular, we study the paradigmatic system of a quantum ideal gas in a small box hovering over the Schwarzschild horizon. We describe the gas in terms of free quantum fields, bosonic and fermionic, massive and massless. We identify thermodynamic properties through the microcanonical distribution. We first analyse the more general case of a box in Rindler spacetime, and then specialize to the black hole case. The physics depends strongly on the distance of the box from the horizon, which we treat as a macroscopic thermodynamic variable. We find that the effective dimension of the system transitions from three-dimensional to two-dimensional as we approach the horizon, that Bekenstein's bound fails when the box is adiabatically lowered towards the black hole, and that the pressure is highly anisotropic. The pressure difference between the upper and lower wall leads to an effective force that must be added to the gravitational acceleration. We also show that the approximation of quantum fields propagating on a fixed background for matter breaks down when the system is brought to microscopic distances from the horizon, in which case backreaction effects must be included.
Charis Anastopoulos, Bei-Lok Hu, and Konstantina Savvidou
Elsevier BV
Charis Anastopoulos, Bei-Lok Hu, and Konstantina Savvidou
IOP Publishing
Abstract We present our program for the development of quantum informational concepts in relativistic systems in terms of the unequal-time correlation functions of quantum fields. We employ two formalisms that provide the basis for further developments. (i) The Quantum Temporal Probabilities (QTP) Method for quantum field measurements and (ii) the Closed-Time-Path (CTP) formalism for causal time evolutions. We present the main ideas of QTP and we show how it relates to the CTP formalism, allowing us to express concepts of measurement theory in terms of path-integrals. We also present many links of our program to non-equilibrium quantum field theories. Details can be found in a recent paper by the authors [1].
Makan Mohageg, Luca Mazzarella, Charis Anastopoulos, Jason Gallicchio, Bei-Lok Hu, Thomas Jennewein, Spencer Johnson, Shih-Yuin Lin, Alexander Ling, Christoph Marquardt,et al.
Springer Science and Business Media LLC
AbstractThe National Aeronautics and Space Administration’s Deep Space Quantum Link mission concept enables a unique set of science experiments by establishing robust quantum optical links across extremely long baselines. Potential mission configurations include establishing a quantum link between the Lunar Gateway moon-orbiting space station and nodes on or near the Earth. This publication summarizes the principal experimental goals of the Deep Space Quantum Link. These goals, identified through a multi-year design study conducted by the authors, include long-range teleportation, tests of gravitational coupling to quantum states, and advanced tests of quantum nonlocality.
Charis Anastopoulos and Bei-Lok Hu
MDPI AG
In recent years an increasing number of papers have attempted to mimic or supplant quantum field theory in discussions of issues related to gravity by the tools and through the perspective of quantum information theory, often in the context of alternative quantum theories. In this article, we point out three common problems in such treatments. First, we show that the notion of interactions mediated by an information channel is not, in general, equivalent to the treatment of interactions by quantum field theory. When used to describe gravity, this notion may lead to inconsistencies with general relativity. Second, we point out that in general one cannot replace a quantum field by a classical stochastic field, or mock up the effects of quantum fluctuations by that of classical stochastic sources (noises), because in so doing important quantum features such as coherence and entanglement will be left out. Third, we explain how under specific conditions semi-classical and stochastic theories indeed can be formulated from their quantum origins and play a role at certain regimes of interest.
Charis Anastopoulos and Bei-Lok Hu
American Vacuum Society
Gravitational decoherence (GD) refers to the effects of gravity in actuating the classical appearance of a quantum system. Because the underlying processes involve issues in general relativity (GR), quantum field theory (QFT) and quantum information, GD has fundamental theoretical significance. There is a great variety of GD models, many of them involving physics that diverge from GR and/or QFT. This overview has two specific goals along one central theme: (i) present theories of GD based on GR and QFT and explore their experimental predictions; (ii) place other theories of GD under the scrutiny of GR and QFT, and point out their theoretical differences. We also describe how GD experiments in space in the coming decades can provide evidences at two levels: a) discriminate alternative quantum theories and non-GR theories; b) discern whether gravity is a fundamental or an effective theory.
Charis Anastopoulos and Ntina Savvidou
MDPI AG
Proposed quantum experiments in deep space will be able to explore quantum information issues in regimes where relativistic effects are important. In this essay, we argue that a proper extension of quantum information theory into the relativistic domain requires the expression of all informational notions in terms of quantum field theoretic (QFT) concepts. This task requires a working and practicable theory of QFT measurements. We present the foundational problems in constructing such a theory, especially in relation to longstanding causality and locality issues in the foundations of QFT. Finally, we present the ongoing Quantum Temporal Probabilities program for constructing a measurement theory that (i) works, in principle, for any QFT, (ii) allows for a first- principles investigation of all relevant issues of causality and locality, and (iii) it can be directly applied to experiments of current interest.
Demetrios Kotopoulis and Charis Anastopoulos
IOP Publishing
We study the thermodynamics of a shell of self-gravitating radiation, bounded by two spherical surfaces. This system provides a consistent model for a gravitating thermal reservoir for different solutions to vacuum Einstein equations in the shell’s interior. The latter include black holes and flat space, hence, this model allows for the study of black hole phase transitions. Following the analysis of Anastopoulos C and Savvidou N (2012 Class. Quantum Grav. 29 025004), we show that the inclusion of appropriate entropy terms to the spacetime boundaries (including the Bekenstein–Hawking entropy for black hole horizons) leads to a consistent thermodynamic description. The system is characterized by four phases, two black hole phases distinguished by the size of the horizon, a flat space phase and one phase that describes naked singularities. We undertake a detailed analysis of black-hole phase transitions, the non-concave entropy function, the properties of temperature at infinity, and system’s heat capacity.
Charis Anastopoulos, Michalis Lagouvardos, and Konstantina Savvidou
IOP Publishing
We analyze the weak-field limit of general relativity with matter and its possible quantisations. This analysis aims toward a predictive quantum theory to provide a first-principles description of gravitational effects in macroscopic quantum systems. This includes recently proposed experiments on the generation of (Newtonian) gravitational forces from quantum distributions of matter, and phenomena like gravity-induced entanglement, gravitational cat states, gravity-induced Rabi oscillations, and quantum causal orderings of events. Our main results include: (i) the demonstration that these phenomena do not involve true gravitational degrees of freedom. (ii) We show that, unlike full general relativity, weak gravity with matter is a parameterised field theory, i.e., a theory obtained by promoting spacetime coordinates to ‘dynamical’ variables. (iii) Quantisation via gauge-fixing leads to an effective field theory that account for some phenomena, but at the price of gauge dependence that manifests more strongly on spacetime observables. This ambiguity is a manifestation of the problem of time that persists even in weak gravity. (iv) A consistent quantisation of parameterised field theories is essential for a predictive and spacetime covariant theory of weak gravity that describes gravitational effects in macroscopic quantum systems. We also discuss the implication of our results to gravitational decoherence theories, the notion of locality in gravity vis-a-vis quantum information theory, and the intriguing possibility that proposed solutions to the problem of time can be tested in weak-gravity quantum experiments.
Charis Anastopoulos
Springer Science and Business Media LLC
Michalis Lagouvardos and Charis Anastopoulos
IOP Publishing
Abstract Models of gravitational decoherence are not commonly applied to ultra-relativistic systems, including photons. As a result, few quantum optical tests of gravitational decoherence have been developed. In this paper, we generalize the gravitational decoherence model of Anastopoulos and Hu (2013 Class. Quantum Grav. 30 165007) to photons. In this model, decoherence originates from a bath of stochastic gravitational perturbations, possibly of fundamental origin. We derive a master equation for general states of the electromagnetic field; the only free parameter is a noise temperature Θ of the gravitational fluctuations. We find that interference experiments with long baselines, accessible in near-future experiments, can, in principle, lead to strong constraints in Θ.
Charis Anastopoulos and Ntina Savvidou
IOP Publishing
Abstract The Tolman–Oppenheimer–Volkoff (TOV) equation admits singular solutions in addition to regular ones. Here, we prove the following theorem. For any equation of state that (i) is obtained from an entropy function, (ii) has positive pressure and (iii) satisfies the dominant energy condition, the TOV equation can be integrated from a boundary inwards to the center. Hence, the thermodynamic consistency of the EoS precludes pathological solutions in which the integration terminates at finite radius (because of horizons, or divergences / zeroes of energy density). At the center, the mass function either vanishes (regular solutions) or it is negative (singular solutions). For singular solutions, the metric at the center is locally isomorphic to negative-mass Schwarzschild spacetime. This means that matter is stabilized because the singularity is strongly repulsive. We show that singular solutions are causally well behaved: they are bounded-acceleration complete, and they are conformal to a globally hyperbolic spacetime with boundary. Finally, we show how to modify unphysical equations of state in order to obtain non-pathological solutions, and we undertake a preliminary investigation of dynamical stability for singular solutions.
Luca Mazzarella, Makan Mohageg, Dmitry V. Strekalov, Aileen J. Zhai, Ulf E. Israelsson, Andrey Matsko, Nan Yu, Charis Anastopoulos, Bradley Carpenter, Jason R. Gallicchio,et al.
SPIE
In this article, we review the proposed experiments for the Deep Space Quantum Link (DSQL) mission concept aiming to probe gravitational effects on quantum optical systems. Quantum theory and general relativity are the two most successful frameworks we have to describe the universe. These theories have been validated through experimental confirmations in their domains of application— the macroscopic domain for relativity, and the microscopic domain for quantum theory. To date, laboratory experiments conducted in a regime where both theories manifest measurable effects on photons are limited. Satellite platforms enable the transmission of quantum states of light between different inertial frames and over distances impossible to emulate in the laboratory. The DSQL concept proposes simultaneous tests of quantum mechanics and general relativity enabled by quantum optical links to one or more spacecrafts.
Theodora Kolioni and Charis Anastopoulos
American Physical Society (APS)
We study the system of two localized detectors (oscillators) interacting through a massless quantum field in a vacuum state via an Unruh-DeWitt coupling. This system admits an exact solution providing a good model for addressing fundamental issues in particle-field interactions, causality and locality in quantum field measurements that are relevant to proposed quantum experiments in space. Our analysis of the exact solution leads to the following results. (i) Common approximations used in the study of analogous open quantum systems fail when the distance between the detectors becomes of the order of the relaxation time. In particular, the creation of correlations between remote detectors is not well described by ordinary perturbation theory and the Markov approximation. (ii) There is a unique asymptotic state that is correlated; it is not entangled unless the detector separation is of the order of magnitude of the wavelength of the exchanged quanta. (iii) The evolution of seemingly localized observables is non-causal. The latter is a manifestation of Fermi's two-atom problem, albeit in an exactly solvable system. We argue that the problem of causality requires a reexamination of the notion of entanglement in relativistic systems, in particular, the physical relevance of its extraction from the quantum vacuum.
C Anastopoulos and B L Hu
IOP Publishing
We extend our earlier work [] on probing a gravitational cat state (gravcat) to the quantum superposition of two gravcats in an exemplary model and in Bose–Einstein condensates (BEC). In addition to its basic theoretical values in gravitational quantum physics and macroscopic quantum phenomena, this investigation can provide some theoretical support to experimental proposals for measuring gravity-induced entanglement and the quantum nature of perturbative gravity. In the first part we consider cat states generated by double-well potentials. A pair of gravcats, each approximated as a two-level system, is characterized by gravity-induced Rabi oscillations, and by gravity-induced entanglement of its energy eigenstates. In the second part we turn to a (non-relativistic) quantum field theory description and derive a gravitational Gross–Pitaevsky equation for gravcats formed in BEC. Using a mathematical analogy to quantum rotors, we explore the properties of the two-gravcat system for BECs, its physical consequences and observational possibilities. Finally we discuss our results in comparison to predictions of alternative quantum theories, and we explain their implications.
Nikolaos Papadatos and Charis Anastopoulos
American Physical Society (APS)
We analyse the thermodynamics of a quantum system in a trajectory of constant velocity that interacts with a static thermal bath. The latter is modeled by a massless scalar field in a thermal state. We consider two different couplings of the moving system to the heat bath, a coupling of the Unruh-DeWitt type and a coupling that involves the time derivative of the field. We derive the master equation for the reduced dynamics of the moving quantum system. It has the same form with the quantum optical master equation, but with different coefficients that depend on velocity. This master equation has a unique asymptotic state for each type of coupling, and it is characterized by a well-defined notion of heat-flow. Our analysis of the second law of thermodynamics leads to a surprising equivalence: a moving heat bath is physically equivalent to a mixture of heat baths at rest, each with a different temperature. There is no unique rule for the Lorentz transformation of temperature. We propose that Lorentz transformations of thermodynamic states are well defined in an extended thermodynamic space that is obtained as a convex hull of the standard thermodynamic space.
Charis Anastopoulos and Konstantina Savvidou
World Scientific Pub Co Pte Lt
We explain how Hawking radiation stores significant amount of information in high-order correlations of quantum fields. This information can be retrieved by multi-time measurements on the quantum fields close to the black hole horizon. This result requires no assumptions about quantum gravity, it takes into account the differences between Gibbs’s and Boltzmann’s accounts of thermodynamics, and it clarifies misconceptions about key aspects of Hawking radiation and about informational notions in QFT.
Charis Anastopoulos and Ntina Savvidou
IOP Publishing
It is believed that no information can be stored in Hawking radiation, because correlations between quanta of different field modes vanish. However, such correlations have been defined only with reference to a single moment of time. In this article, we develop a method for the evaluation of multi-time correlations. We find that these correlations are highly non-trivial: for a scalar field in the Schwarzschild black hole, multi-time correlations have an explicit dependence on angular variables and on the scattering history of Hawking quanta. This result leads us to the conjecture that some pre-collapse information can be stored in multi-time correlations after backreaction effects have been incorporated in the physical description.
Charis Anastopoulos
Springer Science and Business Media LLC
Charis Anastopoulos and Ntina Savvidou
AIP Publishing
Constructing observables that describe the localization of relativistic particles is an important foundational problem in relativistic quantum field theory (QFT). The description of localization in terms of single-time observables leads to conflict with the requirement of causality. In this paper, we describe particle localization in terms of time-of-arrival observables, defined in terms of the interaction between a quantum field and a measuring apparatus. The resulting probabilities are linear functionals of QFT correlation functions. Specializing to the case of a scalar field, we identify several time-of-arrival observables differing on the way that the apparatus localizes particle-detection records. Maximum localization is obtained for a unique observable that is related to the Newton-Wigner position operator. Finally, we define a measure of localizability for relativistic particles in terms of a novel time-energy uncertainty relation for the variance of the time of arrival.
C Anastopoulos and B L Hu
IOP Publishing
We ask the question of how the (weak) equivalence principle established in classical gravitational physics should be reformulated and interpreted for massive quantum objects that may also have internal degrees of freedom (dof). This inquiry is necessary because even elementary concepts like a classical trajectory are not well defined in quantum physics—trajectories originating from quantum histories become viable entities only under stringent decoherence conditions. From this investigation we posit two logically and operationally distinct statements of the equivalence principle for quantum systems. Version A: the probability distribution of position for a free-falling particle is the same as the probability distribution of a free particle, modulo a mass-independent shift of its mean. Version B: any two particles with the same velocity wave-function behave identically in free fall, irrespective of their masses. Both statements apply to all quantum states, including those without a classical correspondence, and also for composite particles with quantum internal dof. We also investigate the consequences of the interaction between internal and external dof induced by free fall. For a class of initial states, we find dephasing occurs for the translational dof, namely, the suppression of the off-diagonal terms of the density matrix, in the position basis. We also find a gravitational phase shift in the reduced density matrix of the internal dof that does not depend on the particle’s mass. For classical states, the phase shift has a natural classical interpretation in terms of gravitational red-shift and special relativistic time-dilation.
Charis Anastopoulos and Ntina Savvidou
American Physical Society (APS)
Attempts to find a quantum-to-classical correspondence in a classically forbidden region leads to non-physical paths, involving, for example, complex time or spatial coordinates. Here, we identify genuine quasi-classical paths for tunneling in terms of probabilistic correlations in sequential time-of-arrival measurements. In particular, we construct the post-selected probability density $P_{p.s.}(x, \\tau)$ for a particle to be found at time $\\tau$ in position $x$ inside the forbidden region, provided that it later crossed the barrier. The classical paths follow from the maximization of the probability density with respect to $\\tau$. For almost monochromatic initial states, the paths correspond to the maxima of the modulus square of the wave-function $|\\psi(x,\\tau)|^2$ with respect to $\\tau$ and for constant $x$ inside the barrier region. The derived paths are expressed in terms of classical equations, but they have no analogues in classical mechanics. Finally, we evaluate the paths explicitly for the case of a square potential barrier.