Abstract:
Derived from semi-classical quantum field theory in curved spacetime, Unruh effect was known as a quantum effect. We find that there does exist a classical correspondence of this effect in electrodynamics. The thermal nature of the vacuum in correlation function for the uniformly accelerated detector is coming from the non-linear relationship between the proper time and the propagating length of the electromagnetic wave. Both the Coulomb field of the detector itself and the radiation supporting the detector's uniformly accelerating motion contribute to the non-vanishing vacuum energy. From this observation we conclude that Unruh temperature experienced by a uniformly accelerated classical electron has no additional effects to Born's solution for laboratory observers far away from the classical electron.

Abstract:
We have shown the classical correspondence of Unruh effect in the classical relativistic electron theory in our previous work (gr-qc/0105051). Here we demonstrate the analogy between the classical relativistic electron theory and the classical Unruh-DeWitt type monopole detector theory. The field configuration generated by a uniformly accelerated detector is worked out. The classical correspondence of Unruh effect for scalar fields is shown by calculating the modified energy density for the scalar field around the detector. We conclude that a classical monopole detector cannot find any evidence about its acceleration unless it has a finite size.

Abstract:
Suppose the postulate of measurement in quantum mechanics can be extended to quantum field theory, then a local projective measurement at some moment on an object locally coupled with a relativistic quantum field will result in a projection or collapse of the wavefunctional of the combined system defined on the whole time-slice associated with the very moment of the measurement, if the relevant degrees of freedom have nonzero correlations. This implies that the wavefunctionals in the same Hamiltonian system but defined in different reference frames would collapse on different time-slices passing through the same local event where the measurement was done. Are these post-measurement states consistent with each other? We illustrate that the quantum states of the Raine-Sciama-Grove detector-field system started with the same initial Gaussian state defined on the same initial time-slice, then collapsed by the measurements on the pointlike detectors on different time-slices in different frames, will evolve to the same state of the combined system up to a coordinate transformation when compared on the same final time-slice. Such consistency is guaranteed by the spatial locality of interactions and the general covariance in a relativistic system, together with the spatial locality of measurements and the linearity of quantum dynamics in its quantum theory.

Abstract:
Using nonperturbative results obtained recently for an uniformly accelerated Unruh-DeWitt detector, we discover new features in the dynamical evolution of the detector's internal degree of freedom, and identified the Unruh effect derived originally from time-dependent perturbation theory as operative in the ultra-weak coupling and ultra-high acceleration limits. The mutual interaction between the detector and the field engenders entanglement between them, and tracing out the field leads to a mixed state of the detector even for a detector at rest in Minkowski vacuum. Our findings based on this exact solution shows clearly the differences from the ordinary result where the quantum field's backreaction is ignored in that the detector no longer behaves like a perfect thermometer. From a calculation of the evolution of the reduced density matrix of the detector, we find that the transition probability from the initial ground state over an infinitely long duration of interaction derived from time-dependent perturbation theory is existent in the exact solution only in transient under special limiting conditions corresponding to the Markovian regime. Furthermore, the detector at late times never sees an exact Boltzmann distribution over the energy eigenstates of the free detector, thus in the non-Markovian regime covering a wider range of parameters the Unruh temperature cannot be identified inside the detector.

Abstract:
This is a synopsis of our recent work on quantum entanglement, recoherence and information flow between an uniformly accelerated detector and a massless quantum scalar field. The availability of exact solutions to this model enables us to explore the black hole information issue with some quantifiable results and new insights. To the extent this model can be used as an analog to the system of a black hole interacting with a quantum field, our result seems to suggest in the prevalent non-Markovian regime, assuming unitarity for the combined system, that black hole information is not lost but transferred to the quantum field degrees of freedom. This combined system will evolve into a highly entangled state between a remnant of large area (in Bekenstein's black hole atom analog) without any information of its initial state, while the quantum field is imbued with complex information content not-so-easily retrievable by a local observer.

Abstract:
We study an exactly solvable model where an uniformly accelerated detector is linearly coupled to a massless scalar field initially in the Minkowski vacuum. Using the exact correlation functions we show that as soon as the coupling is switched on one can see information flowing from the detector to the field and propagating with the radiation into null infinity. By expressing the reduced density matrix of the detector in terms of the two-point functions, we calculate the purity function in the detector and study the evolution of quantum entanglement between the detector and the field. Only in the ultraweak coupling regime could some degree of recoherence in the detector appear at late times, but never in full restoration. We explicitly show that under the most general conditions the detector never recovers its quantum coherence and the entanglement between the detector and the field remains large at late times. To the extent this model can be used as an analog to the system of a black hole interacting with a quantum field, our result seems to suggest in the prevalent non-Markovian regime, assuming unitarity for the combined system, that black hole information is not lost but transferred to the quantum field degrees of freedom. Our combined system will evolve into a highly entangled state between a remnant of large area (in Bekenstein's black hole atom analog) without any information of its initial state, and the quantum field, now imbued with complex information content not-so-easily retrievable by a local observer.

Abstract:
We consider the entanglement dynamics between two Unruh-DeWitt detectors at rest separated at a distance $d$. This simple model when analyzed properly in quantum field theory shows many interesting facets and helps to dispel some misunderstandings of entanglement dynamics. We find that there is spatial dependence of quantum entanglement in the stable regime due to the phase difference of vacuum fluctuations the two detectors experience, together with the interference of the mutual influences from the backreaction of one detector on the other. When two initially entangled detectors are still outside each other's light cone, the entanglement oscillates in time with an amplitude dependent on spatial separation $d$. When the two detectors begin to have causal contact, an interference pattern of the relative degree of entanglement (compared to those at spatial infinity) develops a parametric dependence on $d$. The detectors separated at those $d$ with a stronger relative degree of entanglement enjoy longer disentanglement times. In the cases with weak coupling and large separation, the detectors always disentangle at late times. For sufficiently small $d$, the two detectors can have residual entanglement even if they initially were in a separable state, while for $d$ a little larger, there could be transient entanglement created by mutual influences. However, we see no evidence of entanglement creation outside the light cone for initially separable states.

Abstract:
In this paper we analyze the interaction of a uniformly accelerated detector with a quantum field in (3+1)D spacetime, aiming at the issue of how kinematics can render vacuum fluctuations the appearance of thermal radiance in the detector (Unruh effect) and how they engender flux of radiation for observers afar. Two basic questions are addressed in this study: a) How are vacuum fluctuations related to the emitted radiation? b) Is there emitted radiation with energy flux in the Unruh effect? We adopt a method which places the detector and the field on an equal footing and derive the two-point correlation functions of the detector and of the field separately with full account of their interplay. From the exact solutions, we are able to study the complete process from the initial transient to the final steady state, keeping track of all activities they engage in and the physical effects manifested. We derive a quantum radiation formula for a Minkowski observer. We find that there does exist a positive radiated power of quantum nature emitted by the detector, with a hint of certain features of the Unruh effect. We further verify that the total energy of the dressed detector and a part of the radiated energy from the detector is conserved. However, this part of the radiation ceases in steady state. So the hint of the Unruh effect in radiated power is actually not directly from the energy flux that the detector experiences in Unruh effect. Since all the relevant quantum and statistical information about the detector (atom) and the field can be obtained from the results presented here, they are expected to be useful, when appropriately generalized, for addressing issues of quantum information processing in atomic and optical systems, such as quantum decoherence, entanglement and teleportation.

Abstract:
We study the full entanglement dynamics of two uniformly accelerated Unruh-DeWitt detectors with no direct interaction in between but each coupled to a common quantum field and moving back-to-back in the field vacuum. For two detectors initially prepared in a separable state our exact results show that quantum entanglement between the detectors can be created by the quantum field under some specific circumstances, though each detector never enters the other's light cone in this setup. In the weak coupling limit, this entanglement creation can occur only if the initial moment is placed early enough and the proper acceleration of the detectors is not too large or too small compared to the natural frequency of the detectors. Once entanglement is created it lasts only a finite duration, and always disappears at late times. Prior result by Reznik derived using the time-dependent perturbation theory with extended integration domain is shown to be a limiting case of our exact solutions at some specific moment. In the strong coupling and high acceleration regime, vacuum fluctuations experienced by each detector locally always dominate over the cross correlations between the detectors, so entanglement between the detectors will never be generated.

Abstract:
We obtained an exact solution for a uniformly accelerated Unruh-DeWitt detector interacting with a massless scalar field in (3+1) dimensions which enables us to study the entire evolution of the total system, from the initial transient to late-time steady state. We find that the Unruh effect as derived from time-dependent perturbation theory is valid only in the transient stage and is totally invalid for cases with proper acceleration smaller than the damping constant. We also found that, unlike in (1+1)D results, the (3+1)D uniformly accelerated Unruh-DeWitt detector in a steady state does emit a positive radiated power of quantum nature at late-times, but it is not connected to the thermal radiance experienced by the detector in the Unruh effect proper.