arXiv subjects

Subhajit Barman

Publications and source records attributed to Subhajit Barman.

At least 19 recordsLinked to original sources

arXiv2026

Mach's principle in atomic transitions

We investigate the atomic transition probabilities in atom-mirror set-ups that are in circular motion. In one scenario, the atom is in circular motion inside a static cylindrical mirror. In the other scenario, the cylindrical mirror rotates around its central axis while the atom remains static. We report structural similarity in the atomic transition probabilities between these two cases -- these probabilities are equivalent upon interchanging the field frequencies between the two scenarios. We interpret such an observation as a semi-classical phenomenon analogous to the classical Mach's principle.

quant-ph↗

arXiv2025

Twin-paradox and Entanglement

We study the quantum version of the classical twin paradox in special relativity by replacing the twins with quantum detectors, and studying the transitions and entanglement induced by coupling them to a quantum field. We show that the \textit{changes} in direction of acceleration leave imprints on detector responses and entanglement, inducing novel features which might have relevance in black hole spacetimes.

gr-qc↗

arXiv2025

Radiative process of tripartite entangled probes in inertial motion

We study the radiative process of three entangled quantum probes initially prepared in a tripartite W state. As a basic set-up, we consider the probes to be inertial in flat spacetime and investigate how the radiative process is affected by different probe configurations. We take the quantum probes as either static or moving with uniform velocities and consider different switching scenarios. Our main observation confirms that the radiative process depends distinctively on the initial configuration in which the probes are arranged, as well as on the direction of the probe velocity. We also extend our analysis to a thermal environment, thereby simulating a more realistic background. We thoroughly discuss the effects due to different switchings, the thermal background, and probe motion on the radiative process of these tripartite entangled probes. We also comment on how the observations from this work can help prepare a set-up least affected by quantum decoherence.

gr-qc↗

arXiv2025

Entanglement between accelerated probes in a de Sitter spacetime

We initiate an investigation into features of vacuum entanglement as probed by accelerated quantum probes in curved spacetime. Focussing specifically on de Sitter (dS) spacetime with curvature $\Lambda$, we obtain several exact results corresponding to different kinematical set-up of the probes. The interaction with the quantum field creates a non-local correlation between initially uncorrelated probes accelerating in different directions. It is well known that a single quantum probe in dS spacetime with uniform acceleration $a$ responds exactly as a quantum probe in Minkowski spacetime with "effective" acceleration $q \equiv\sqrt{a^2+\Lambda}$. However, no such mapping generically exists for the entanglement between probes. Our results suggest that entanglement exhibits independent variations with changes in acceleration and curvature depending on different configurations of detector motion.

gr-qc↗

arXiv2024

Equivalence in virtual transitions between uniformly accelerated and static atoms: from a bird's eye

We study the prospect of the equivalence principle at the quantum regime by investigating the transition probabilities of a two-level atomic detector in different scenarios. In particular, two specific set-ups are considered. ($i$) $Without~a~boundary$: In one scenario the atom is in uniform acceleration and interacting with Minkowski field modes. While in the other the atom is static and in interaction with Rindler field modes. ($ii$) $With~a~reflecting~boundary$: In one scenario, the atom is uniformly accelerated, and the mirror is static, and in the other scenario, the atom is static, and the mirror is in uniform acceleration. In these cases, the atom interacts with the field modes, defined in the mirror's frame. For both the set-ups, the focus is on the excitation and de-excitation probabilities in $(1+1)$ and $(3+1)$ spacetime dimensions. Our observations affirm that in $(1+1)$ dimensions, for both set-ups the transition probabilities from different scenarios become the same when the atomic and the field frequencies are equal. In contrast, in $(3+1)$ dimensions this equivalence is not observed in general, inspiring us to look for a deeper physical interpretation. Our findings suggest that when the equivalence between different scenarios is concerned, the excitation to de-excitation ratio provides a more consistent measure even in $(3+1)$ dimensions. We discuss the physical interpretation and implications of our findings.

gr-qc↗

arXiv2024

Pure Gauss-Bonnet NUT Black Hole Solution: II

In the present article, we have obtained an exact analytical solution of six-dimensional pure Gauss-Bonnet gravity in the presence of both NUT and Maxwell charges. The topology of the horizon is chosen to be the product of two 2-spheres. Upon evaluating the solution, we study the spacetime properties, such as event horizon and singularity, and obtain the ranges of parameter space where the solution is valid. We discuss how the presence of Maxwell charges may impact the solution's asymptotic expansion and what distinctive effects it will bring to the geometry. The thermodynamic properties of the solution are also discussed, emphasizing the interplay between NUT and Maxwell charges.

gr-qc↗

arXiv2024

Timelike transitions in an atom by a mirror in light cone and Kruskal-Szekeres regions: a status of quantum equivalence

We investigate the timelike transitions in a two-level atom in the presence of an infinite reflecting mirror in the future-past light cone regions of a Minkowski spacetime, as well as in the region interior of a $(1+1)$ dimensional Schwarzschild black hole. In particular, when considering the light cone regions, two specific scenarios are dealt with -- $(i)$ a mirror is static in Minkowski spacetime while the atom is attached to a frame confined inside the future light cone region, $(ii)$ an atom is static in Minkowski spacetime, and the mirror is confined inside the future light cone region. For both situations, the atom is interacting with field modes defined in the mirror's frame. Analogous configurations are considered in the black hole spacetime: in one case, the mirror carries field modes represented by the Kruskal time, while the atom follows the Schwarzschild time defined inside the black hole; in the other case, the situations are reversed. The analyses, depending on the frame of the atom, are respectively done within the light cone, Minkowski, Schwarzschild, and Kruskal time-interaction pictures. In all of these scenarios, we observe that the excitation probabilities contain a thermal factor and depend periodically on the separation between the atom and the mirror. At the level of transition probabilities, the aforesaid two scenarios in $(1+1)$ dimensional Minkowski-light cone regions appear to be the same for the equal field and atomic frequencies. However, the same is not true when we consider the $(3+1)$ dimensional Minkowski-light cone or the Schwarzschild interior regions. We also estimate the de-excitation probabilities and encounter similar situations. However, we observe that the excitation to de-excitation ratios (EDRs) corresponding to analogous scenarios are equal for equal atomic and field frequencies.

gr-qc↗

arXiv2024

Optimization of entanglement depends on whether a black hole is extremal

We consider two Unruh-DeWitt detectors interacting with a massless, minimally coupled scalar field in a $(1+1)$ dimensional Reissner-Nordström black hole spacetime. In particular, one of the detectors, corresponding to $Alice$, is moving along an outgoing null trajectory. While the other detector carried by $Bob$ is static. With this set-up, we investigate the entangling condition and the measure of the entanglement, concurrence, in the nonextremal and extremal scenarios. Our observations suggest, as expected, a qualitative similarity in characteristics of the entanglement between these two scenarios. However, we find quantitative differences between the nonextremal and extremal concurrences for a broad range of black hole charges. With moderately large detector transition energy, the extremal background always accounts for the larger entanglement than the nonextremal one. In contrast, with low detector transition energy, entanglement on the nonextremal background can be greater. Therefore, by adjusting the detector transition energy, one can perceive optimum entanglement from either the extremal or the nonextremal background.

gr-qc↗

arXiv2024

Signatures of gravitational wave memory in the radiative process of entangled quantum probes

In this article, we examine entangled quantum probes in geodesic trajectories in a flat background with a gravitational wave (GW) burst. In particular, these quantum probes are prepared initially either in the symmetric or anti-symmetric Bell's states, and we study the radiative process as the GW burst passes. We split a generic GW burst into two profiles with and without memory. GW burst with (without) memory profiles have different (similar) asymptotic strains between early and late times. We observe that for eternal switching, there is a finite change in the collective atomic transition rate due to the memory part of the GW burst, while the contribution from the without memory counterpart vanishes. We also consider finite Gaussian switching and observe characteristic differences in the radiative process between the GW backgrounds with and without memory. Notably, if the Gaussian switching is peaked much later compared to the passing of GW, only the memory part contributes to the radiative process. Thus, although examined in a simplified set-up, our findings suggest the potential to distinguish bursts with and without GW memory based on the radiative process of entangled detectors.

gr-qc↗

arXiv2024

Radiative processes of single and entangled detectors on circular trajectories in (2+1) dimensional Minkowski spacetime

We investigate the radiative processes involving two entangled Unruh-DeWitt detectors that are moving on circular trajectories in $(2+1)$-dimensional Minkowski spacetime. We assume that the detectors are coupled to a massless, quantum scalar field, and calculate the transition probability rates of the detectors in the Minkowski vacuum as well as in a thermal bath. We also evaluate the transition probability rates of the detectors when they are switched on for a finite time interval with the aid of a Gaussian switching function. We begin by examining the response of a single detector before we go on to consider the case of two entangled detectors. As we shall see, working in $(2+1)$ spacetime dimensions makes the computations of the transition probability rates of the detectors relatively simpler. We find that the cross transition probability rates of the two entangled detectors can be comparable to the auto transition probability rates of the individual detectors. We discuss specific characteristics of the response of the entangled detectors for different values of the parameters involved and highlight the effects of the thermal bath as well as switching on the detector for a finite time interval.

gr-qc↗

arXiv2024

Universal role of curvature in vacuum entanglement

We highlight some universal features concerning the role of spacetime curvature in the entanglement induced between quantum probes coupled to a quantum field in a suitable vacuum state. The probes are initially causally disconnected and non-entangled. We explore the parameter space $\{ω, d_0, \boldsymbol{v}_0\}$ spanned by the energy gap $ω$ of the detectors, and the initial values of separation distance $d_0$ and relative velocity $\boldsymbol{v}_0$, both covariantly defined in arbitrary curved spacetime. We also obtain numerical results in de Sitter spacetimes and use these to explore strong curvature regime, while also corroborating our perturbative results in arbitrary curved spacetime. Our analysis shows that curvature can induce entanglement features in certain regions of the above parameter space, in a manner which facilitates using entanglement as a probe of spacetime curvature.

gr-qc↗

arXiv2023

Entanglement harvesting for different gravitational wave burst profiles with and without memory

In the present article, we study how different gravitational wave (GW) burst profiles in linearized gravity, with and without the asymptotic memory, may influence the harvesting between two static Unruh-DeWitt detectors. To this end, we investigate the following burst profiles -- Gaussian, sech-squared, Heaviside step function, and tanh. Out of these, the first two bursts contain no memory, while the latter two consist of a non-vanishing memory effect. We find that in all of these cases, entanglement harvesting is possible, and it decreases with the increasing distance between detectors and the detector transition energy. We observe that the harvesting differs qualitatively based on the presence or absence of the memory, which is prominent in a low transition energy regime. With memory, the harvesting keeps increasing with decreasing transition energy, while without memory, it tends to reach finite values. Furthermore, for the two burst profiles without memory, longer bursts correspond to greater harvesting in the low detector transition energy regime, and this characteristic is reversed for larger transition energy. Meanwhile, for the tanh-type profile with memory, harvesting is always greater for shorter bursts. We discuss various implications of our findings.

gr-qc↗

arXiv2022

Entanglement harvesting from conformal vacuums between two Unruh-DeWitt detectors moving along null paths

It is well-known that the $(1+1)$ dimensional Schwarzschild and spatially flat FLRW spacetimes are conformally flat. This work examines entanglement harvesting from the conformal field vacuums in these spacetimes between two Unruh-DeWitt detectors, moving along outgoing null trajectories. In $(1+1)$ dimensional Schwarzschild spacetime, we considered the Boulware and Unruh vacuums for our investigations. In this analysis, one observes that while entanglement harvesting is possible in $(1+1)$ dimensional Schwarzschild and $(1+3)$ dimensional de Sitter spacetimes, it is not possible in the $(1+1)$ dimensional de Sitter background for the same set of parameters when the detectors move along the same outgoing null trajectory. The qualitative results from the Boulware and the Unruh vacuums are alike. Furthermore, we observed that the concurrence depends on the distance $d$ between the two null paths of the detectors periodically, and depending on the parameter values, there could be entanglement harvesting shadow points or regions. We also observe that the mutual information does not depend on $d$ in $(1+1)$ dimensional Schwarzschild and de Sitter spacetimes but periodically depends on it in $(1+3)$ dimensional de Sitter background. We also provide elucidation on the origin of the harvested entanglement.

gr-qc↗

arXiv2022

Entanglement harvesting between two inertial Unruh-DeWitt detectors from non-vacuum quantum fluctuations

Entanglement harvesting from the quantum field is a well-known fact that, in recent times, is being rigorously investigated further in flat and different curved backgrounds. The usually understood formulation studies the possibility of two uncorrelated Unruh-DeWitt detectors getting entangled over time due to the effects of quantum vacuum fluctuations. Our current work presents a thorough formulation to realize the entanglement harvesting from non-vacuum background fluctuations. In particular, we further consider single excitation field states and a pair of inertial detectors, respectively, in $(1+1)$ and $(1+3)$ dimensions for this investigation. Our main observation asserts that entanglement harvesting is suppressed compared to the vacuum fluctuations in this situation. Our other observations confirm a non-zero individual detector transition probability in this background and vanishing entanglement harvesting for parallel co-moving detectors. We look into the characteristics of the harvested entanglement and discuss its dependence on different system parameters.

gr-qc↗

arXiv2022

Hawking effect in an extremal Kerr black hole spacetime

It is well known that extremal black holes do not Hawking radiate, which is usually realized by taking an extremal limit from the nonextremal case. However, one cannot perceive the same phenomenon using the Bogoliubov transformation method starting from an extremal black hole itself, i.e., without the limiting case consideration. In that case, the Bogoliubov coefficients do not satisfy the required normalization condition. In canonical formulation, which closely mimics the Bogoliubov transformation method, one can consistently reproduce the vanishing number density of Hawking quanta for an extremal Kerr black hole. In this method, the relation between the spatial near-null coordinates, imperative in understanding the Hawking effect, was approximated into a sum of linear and inverse terms only. In the present work, we first show that one can reach the same conclusion in canonical formulation even with the complete relationship between the near-null coordinates, which contains an additional logarithmic term. It is worth mentioning that in the nonextremal case, a similar logarithmic term alone leads to the thermal Hawking radiation. Secondly, we study the case with only the inverse term in the relation (i.e., when the spatial near-null coordinates associated to the past and future observers are inversely related to each other) to understand whether it is the main contributing term in vanishing number density. Third, for a qualitative realization, we consider a simple thought experiment to understand the corresponding Hawking temperature and conclude that the inverse term indeed plays a crucial role in the vanishing number density.

gr-qc↗

arXiv2021

Role of thermal field in entanglement harvesting between two accelerated Unruh-DeWitt detectors

We investigate the effects of field temperature $T^{(f)}$ on the entanglement harvesting between two uniformly accelerated detectors. For their parallel motion, the thermal nature of fields does not produce any entanglement, and therefore, the outcome is the same as the non-thermal situation. On the contrary, $T^{(f)}$ affects entanglement harvesting when the detectors are in anti-parallel motion, i.e., when detectors $A$ and $B$ are in the right and left Rindler wedges, respectively. While for $T^{(f)}=0$ entanglement harvesting is possible for all values of $A$'s acceleration $a_A$, in the presence of temperature, it is possible only within a narrow range of $a_A$. In $(1+1)$ dimensions, the range starts from specific values and extends to infinity, and as we increase $T^{(f)}$, the minimum required value of $a_A$ for entanglement harvesting increases. Moreover, above a critical value $a_A=a_c$ harvesting increases as we increase $T^{(f)}$, which is just opposite to the accelerations below it. There are several critical values in $(1+3)$ dimensions when they are in different accelerations. Contrary to the single range in $(1+1)$ dimensions, here harvesting is possible within several discrete ranges of $a_A$. Interestingly, for equal accelerations, one has a single critical point, with nature quite similar to $(1+1)$ dimensional results. We also discuss the dependence of mutual information among these detectors on $a_A$ and $T^{(f)}$.

gr-qc↗

arXiv2021

Thermal behavior of a radially deformed black hole spacetime

In the present article, we study the Hawking effect and the bounds on greybody factor in a spacetime with radial deformation. This deformation is expected to carry the imprint of a non-Einsteinian theory of gravity, but shares some of the important characteristics of general relativity (GR). In particular, this radial deformation will restore the asymptotic behavior, and also allows for the separation of the scalar field equation in terms of the angular and radial coordinates -- making it suitable to study the Hawking effect and greybody factors. However, the radial deformation would introduce a change in the locations of the horizon, and therefore, the temperature of the Hawking effect naturally alters. In fact, we observe that the deformation parameter has an enhancing effect on both temperature and bounds on the greybody factor, which introduces a useful distinction with the Kerr spacetime. We discuss these effects elaborately, and broadly study the thermal behavior of a radially deformed spacetime.

gr-qc↗

arXiv2021

Radiative process of two entangled uniformly accelerated atoms in a thermal bath: a possible case of anti-Unruh event

We study the radiative process of two entangled two-level atoms uniformly accelerated in a thermal bath, coupled to a massless scalar field. First, using the positive frequency Wightman function from the Minkowski modes with a Rindler transformation we provide the transition probabilities for the transitions from maximally entangled symmetric and anti-symmetric Bell states to the collective excited state in $(1+1)$ and $(1+3)$ dimensions. We observe a possible case of \emph{anti-Unruh-like} event in these transition probabilities, though the $(1+1)$ and $(1+3)$ dimensional results are not completely equivalent. We infer that thermal bath plays a major role in the occurrence of the anti-Unruh-like effect, as it is also present in the transition probabilities corresponding to a single detector in this case. Second, we have considered the Green's functions in terms of the Rindler modes with the vacuum of Unruh modes for estimating the same. Here the anti-Unruh effect appears only for the transition from the anti-symmetric state to the collective excited state. It is noticed that here the $(1+1)$ and $(1+3)$ dimensional results are equivalent, and for a single detector, we do not observe any anti-Unruh effect. This suggests that the entanglement between the states of the atoms is the main cause for the observed anti-Unruh effect in this case. In going through the investigation, we find that the transition probability for a single detector case is symmetric under the interchange between the thermal bath's temperature and the Unruh temperature for Rindler mode analysis; whereas this is not the case for Minkowski mode. We further comment on whether this observation may shed light on the analogy between an accelerated observer and a real thermal bath. An elaborate investigation for the classifications of our observed anti-Unruh effects is also thoroughly demonstrated.

gr-qc↗