We show that it is possible to use a massless field in the vacuum to communicate in such a way that the signal travels arbitrarily slower than the speed of light and such that no energy is transmitted from the sender to the receiver. Instead, the receiver has to supply a signal-dependent amount of work to switch his detector on and off. Because of that, this kind of communication without energy exchange may be called "Quantum Collect Calling". This type of communication is related to Casimir-like interactions and it is made possible by dimension -and curvature-dependent subtleties of Huygens' principle.Communication by means of massless quantum fields is normally discussed in terms of the exchange of field quanta. Typically, a sender emits photons -which carry energy along null-geodesics-that are then absorbed by a receiver. The receiver has to intercept the beam of light, or he misses the signal. In this letter we introduce a method of communication via massless quantum fields which breaks with each point of the just mentioned intuitive understanding: 1) No emission or absorption of real quanta is necessary; 2) the information flow need not be carried by an energy flow; and 3) the information flow can be arbitrarily slower than light, in spite of the field being massless.The fact that waves of massless fields propagate at exactly the speed of light is merely a peculiar feature of n + 1D Minkowski spacetime, for odd n ≥ 3. When there is generic curvature, or when the spatial dimension is even, then waves of massless fields propagate both on as well as inside the lightcone [1,2,8]. Here, we translate this classical wave phenomenon into quantum field theory (QFT) and analyze the implications for quantum communication.Surprisingly, when applied to QFT, we find that the resulting signal transmission effects appear already at the leading order in perturbation theory. These are not emission and absorption processes of real photons, which only appear at subleading order. Indeed, we find that slowerthan-light quantum signals do not require the transmission of energy from the sender to the receiver. Instead -much as for a collect call-the receiver has to provide energy for the detection of the signal.The strong Huygens' principle in QFT.-The Green's functions of the massless Klein-Gordon field in 3+1D Minkowski space have support only on the lightcone. Hence, any disturbances of the field propagate strictly along null geodesics. This classical phenomenon is called the strong Huygens' principle [1,2].In QFT, communication via the field is possible if and only if the commutator between the field at two events does not vanish [3][4][5][6][7]. The commutator is given by the Green's function [φ(x, t 1 ), φ(y, t 2 )] = iG(x, t 1 , y, t 2 )1 1, where G = G adv − G ret ; thus the commutator always vanishes outside the lightcone. The strong Huygens' principle therefore manifests itself in QFT through the commutator of the field: In 3+1D Minkowski space, the commutator is only supported on the lightcone. In 1+1D and 2+1D, the comm...

We consider quantum signaling between two-level quantum systems in a cavity in the perturbative regime of the earliest possible arrival times of the signal. We present two main results: First, we find that, perhaps surprisingly, the analog of amplitude modulated signaling (Alice using her energy eigenstates |g , |e , as in the Fermi problem) is generally suboptimal for communication, namely, e.g., phase-modulated signaling (Alice using, e.g., {|+ , |− } states) overcomes the quantum noise already at a lower order in perturbation theory. Second, we study the effect of mode truncations that are commonly used in cavity QED on the modeling of the communication between two-level atoms. We show that, on general grounds, namely for causality to be preserved, the UV cutoff must scale at least polynomially with the desired accuracy of the predictions.

We study under what circumstances a separable bipartite system A-B can or cannot become entangled through local interactions with a bi-local entangled source S1-S2. We obtain constraints on the general forms of the interaction Hamiltonians coupling A with S1 and B with S2 necessary for A and B to become entangled. We are able to generalize and provide non-perturbative insight on several previous no-go theorems of entanglement harvesting from quantum fields using these general results. We also discuss the role of communication in the process of entanglement extraction, establishing a distinction between genuine entanglement extraction and communication-assisted entanglement generation.

The quantum channel between two particle detectors provides a prototype framework for the study of wireless quantum communication via relativistic quantum fields. In this article we calculate the classical channel capacity between two Unruh-DeWitt detectors arising from couplings within the perturbative regime. To this end, we identify the detector states which achieve maximal signal strength. We use these results to investigate the impact of relativistic effects on signaling between detectors in inertial and uniformly accelerated motion which communicate via a massless field in Minkowski spacetime.

We study the quantum channel between two localized first-quantized systems that communicate in 3 þ 1 dimensional Schwarzschild spacetime via a quantum field. We analyze the information carrying capacity of direct and black hole-orbiting null geodesics as well as of the timelike contributions that arise because the strong Huygens principle does not hold on the Schwarzschild background. We find, in particular, that the nondirect-null and timelike contributions, which do not possess an analog on Minkowski spacetime, can dominate over the direct null contributions. We cover the cases of both geodesic and accelerated emitters. Technically, we apply tools previously designed for the study of wave propagation in curved spacetimes to a relativistic quantum information communication setup, first for generic spacetimes, and then for the case of Schwarzschild spacetime in particular.