Setups in which a system Alice emits field quanta that a system Bob receives are prototypical for wireless communication and have been extensively studied. In the most basic setup, Alice and Bob are modeled as Unruh-DeWitt detectors for scalar quanta, and the only noise in their communication is due to quantum fluctuations. For this basic setup, we construct the corresponding information-theoretic quantum channel. We calculate the classical channel capacity as a function of the spacetime separation, and we confirm that the classical as well as the quantum channel capacity are strictly zero for spacelike separations. We show that this channel can be used to entangle Alice and Bob instantaneously. Alice and Bob are shown to extract this entanglement from the vacuum through a Casimir-Polder effect.
Setups in which a system Alice emits field quanta that a system Bob receives are prototypical for wireless communication and have been extensively studied. In the most basic setup, Alice and Bob are modeled as Unruh-DeWitt detectors for scalar quanta, and the only noise in their communication is due to quantum fluctuations. For this basic setup, we construct the corresponding information-theoretic quantum channel. We calculate the classical channel capacity as a function of the spacetime separation, and we confirm that the classical as well as the quantum channel capacity are strictly zero for spacelike separations. We show that this channel can be used to entangle Alice and Bob instantaneously. Alice and Bob are shown to extract this entanglement from the vacuum through a Casimir-Polder effect.
The setup in which two quantum systems, Alice and Bob, communicate using \nbosonic field quanta can be viewed as a prototype for wireless quantum communication. \nIn this thesis we focus on the most basic case, where Alice and Bob \nare modeled as Unruh-DeWitt detectors, i.e., as two-level quantum \nsystems that interact locally through a scalar quantum field. Our aim is to study how information propagation and entanglement generation between the two detectors are impacted by both relativity and by the unavoidable noise that is due to the quantum fluctuations of the field. \n \nWe start by studying information propagation between the two detectors. Concretely, we construct and study the information-theoretic quantum channel, ξ, i.e., the completely positive trace preserving map between the input density matrix ϱ, in which Alice prepares her detector for the emission, and the output density matrix ϱ '=ξ(ϱ) of Bob's detector at a later time. We confirm that the classical as well as the quantum channel capacity are strictly zero to all orders in perturbation theory for spacelike separations. \n \nWe then study entanglement generation between the two detectors. Specifically, we discuss how two Unruh-DeWitt detectors can extract entanglement from the vacuum. We show that the detectors can naturally and instantaneously become entangled through a Casimir-Polder effect. We then analyze the impact of various additions to this setup, such as the presence of a weak gravitational field, the presence of boundary conditions in the field, the presence of a weak classical potential, etc.
Abstract 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.
Abstract We construct a relativistic quantum communication channel between two localized qubit systems, mediated by a relativistic quantum field, that can achieve the theoretical maximum for the quantum capacity in arbitrary curved spacetimes using the Unruh–DeWitt detector formalism. Using techniques from algebraic quantum field theory, we express the quantum capacity of the quantum communication channel purely in terms of the correlation functions of the field and the causal propagator for the wave equation. Consequently, the resulting quantum channel, and hence the quantum capacity, are by construction manifestly covariant, respect the causal structure of spacetime, and are independent of the details of the background geometry, topology, and the choice of Hilbert space (quasifree) representations of the field.
We consider the communication of classical and quantum information between two arbitrary observers in asymptotically flat spacetimes (possibly containing black holes) and investigate what the energy cost is for such information transmission. By means of localized two-level quantum systems, sender and receiver can use a quantum scalar field as a communication channel. As we have already shown in a previous paper, such a channel has nonvanishing classical capacity as well as entanglement-assisted classical and quantum capacities. Here we will show that the change in the expectation value of the energy of the system during the communication process can be separated in (i) a contribution coming from the particle creation due to the change of the spacetime, (ii) a contribution associated with the energy needed to switch on or off each qubit, and (iii) a term which comes from the communication process itself. For the quantum channel considered here, we show that the extra energy cost needed for communication vanishes. As a result, if one has already created a system of qubits for some specific task (e.g., quantum computation), one can also reliably convey information between its parts with no extra energy cost. We conclude the paper by illustrating the form of the channel capacities and energy contributions in two paradigmatic cases in Minkowski spacetime: (i) sender and receiver in inertial motion and (ii) sender in inertial motion while the receiver is uniformly accelerated.
We study the ability of qubit detectors to i) extract correlations from, and ii) transmit quantum information through, a quantum field. \n \nWe start by perturbatively studying the harvesting of correlations from thermal and squeezed coherent field states. We find that an increase in field temperature is detrimental to entanglement harvesting, but beneficial to mutual information harvesting. We also show that entanglement harvesting is independent of the field's coherent amplitude - which we relate to fundamental results regarding the entanglement structure of coherent field states - but strongly dependent on the field's squeezing amplitude. We conclude by analyzing the practical feasibility of entangling qubits using squeezed field states. \n \nWe then go on to study, non-perturbatively, the entanglement extraction by targets A and B from a quantum source S. After proving a general no-go theorem which applies for any A, B and S, we apply this theorem to the entanglement harvesting setup to prove that a wide class of i) degenerate, or ii) point-in-time coupled, detectors cannot harvest entanglement from any field state. We also discuss the role of communication in the process of entanglement extraction, and we end the chapter by presenting the simplest successful example of a non-perturbative entanglement harvesting protocol. \n \nWe conclude by studying the ability of flat spacetime observers Alice and Bob to transmit quantum information through a quantum field. We construct a perfect, field-mediated quantum channel, each use of which allows Alice to transmit a full qubit of information to Bob. This construction provides us with an understanding of how quantum information propagates through a relativistic field, which we find to be consistent with our understanding of the strong Huygens principle. Lastly, we analyze the possibility of simultaneously broadcasting a quantum message through a quantum field to multiple receivers, and discover severe fundamental limitations to such a setup.
Given that any communication is communication through quantum fields, we study here the scenario where a sender, Alice, causes information-carrying disturbances in a quantum field. We track the exact spread of these disturbances in space and time by using the technique of quantum information capsules (QIC). We find that the channel capacity between Alice and a receiver, Bob, is enhanced by Bob placing detectors not only inside but in addition also outside the causal future of Alice's encoding operation. Intuitively, this type of superadditivity arises because the field outside the causal future of Alice is entangled with the field inside Alice's causal future. Hence, the quantum noise picked up by Bob's detectors outside Alice's causal future is correlated with the noise of Bob's detectors inside Alice's causal future. In effect, this correlation allows Bob to improve the signal-to-noise ratio of those of his detectors which are in the causal future of Alice. Further, we develop the multimode generalization of the QIC technique. This allows us to extend the analysis to the case where Alice operates multiple localized and optionally entangled emitters. We apply the new techniques to the case where Alice enhances the channel capacity by operating multiple emitters that are suitably lined up and pretimed to generate a quantum shockwave in the field.