We address the validity of the single-mode approximation that is commonly invoked in the analysis of entanglement in non-inertial frames and in other relativistic quantum information scenarios. We show that the single-mode approximation is not valid for arbitrary states, finding corrections to previous studies beyond such approximation in the bosonic and fermionic cases. We also exhibit a class of wave packets for which the single-mode approximation is justified subject to the peaking constraints set by an appropriate Fourier transform.

We present an experiment demonstrating entanglement-enhanced classical communication capacity of a quantum channel with correlated noise. The channel is modelled by a fiber optic link exhibiting random birefringence that fluctuates on a time scale much longer than the temporal separation between consecutive uses of the channel. In this setting, introducing entanglement between two photons travelling down the fiber allows one to encode reliably up to one bit of information into their joint polarization degree of freedom. When no quantum correlations between two separate uses of the channel are allowed, this capacity is reduced by a factor of more than three. We demonstrated this effect using a fiber-coupled source of entagled photon pairs based on spontaneous parametric down-conversion, and a linear-optics Bell state measurement.PACS numbers: 03.67. Hk, 03.65.Yz, 42.50.Dv, 89.70.+c Quantum mechanics admits existence of correlations that cannot be explained simply as statistical uncertainty in assigning definite realistic properties locally to each of the subsystems. This feature of quantum mechanics was quantified first in the form of Bell's inequalities that discriminate it against theories based on the assumption of local realism [1]. In recent years, this specifically quantum form of correlations, known commonly as entanglement, is being exploited to develop novel modes of information processing that offer advantages not available through the classical approach [2]. The well-known examples include solving certain computation tasks via collective unitary operations on coherent registers of quantum particles [3], or distributing a cryptographic key with the security verified by the detection of entanglement [4]. Entanglement plays also a non-trivial role in a number of scenarios for sending both classical information and quantum states over noisy quantum channels [5].In this Letter, we demonstrate experimentally how entanglement can be used to enhance classical communication over a noisy channel. Our experiment follows recent theoretical studies [6,7,8] that analyzed classical communication over quantum channels in which the noise affecting consecutive uses is correlated. It is then natural to consider two types of input ensembles used for communication. The first one is restricted only to introducing classical correlations between separate uses of the channel, and it can be prepared by adjusting individually the quantum state of each particle sent through the channel in a single use. The second, completely general input ensemble includes entangled states of many particles that are subsequently sent through the channel one-byone. Theoretical analysis of model situations showed that indeed the use of entangled input ensemble can substantially enhance the classical capacity, and our work provides a proofof-principle experimental verification of this prediction.Our experiment implements the idea described in Ref.[8] which we will now briefly review. The model of a noisy quantum channel analyzed there was motivat...

We propose a projective operator formalism that is well-suited to study the correlations of quantum fields in non-inertial frames. We generalise a Glauber model of detection of a single localised field mode that is capable of making measurements in an arbitrary reference frame. We show that the model correctly reproduces the Unruh temperature formula of a single accelerated detector, and use it to extract vacuum entanglement by a pair of counter-accelerating detectors. This latter example is a proof of principle that this approach will be appropriate to further studies on the nature of entanglement in non-inertial frames and, in general, to model experimentally feasible scenarios in quantum field theory in non-inertial frames. Finally, as further confirmation of the validity of our approach, we introduce an explicit perturbative matter-radiation interaction model which reproduces both the generalised Glauber model and the projective measurement results in the weak coupling regime. arXiv:1203.0655v2 [quant-ph] 1 Dec 2012

We propose an implementation of a twin paradox scenario in superconducting circuits, with velocities as large as a few percent of the speed of light. Ultrafast modulation of the boundary conditions for the electromagnetic field in a microwave cavity simulates a clock moving at relativistic speeds. Since our cavity has a finite length, the setup allows us to investigate the role of clock size as well as interesting quantum effects on time dilation. In particular, our theoretical results show that the time dilation increases for larger cavity lengths and is shifted due to quantum particle creation.Comment: 6 pages, 3 figures. I. F. previously published as I. Fuentes-Guridi and I. Fuentes-Schulle