Fundamental Limitations to Information Transfer in Accelerated Frames.

E. Martín-Martínez, D. Hosler, M. Montero.
We study communication between an inertial observer and one of two causally disconnected counteraccelerating observers. We will restrict the quantum channel considering inertial-to-accelerated bipartite classical and quantum communication over different sets of Unruh modes (single-rail or dual-rail encoding). We find that the coherent information (and therefore the amount of entanglement that can be generated via state merging protocol) in this strongly restricted channel presents some interesting monogamy properties between the inertial and only one of the accelerated observers if we take a fixed choice of the Unruh mode used in the channel. The optimization of the controllable parameters is also studied and we find that they deviate from the values usually employed in the literature.

The Fate of Non-Trivial Entanglement Under a Gravitational Collapse.

E. Martín-Martínez, L. J. Garay, J. León.
We analyse the evolution of the entanglement of a non-trivial initial quantum field state (which, for simplicity, has been taken to be a bipartite state made out of vacuum and the first excited state) when it undergoes a gravitational collapse. We carry out this analysis by generalizing the tools developed to study entanglement behaviour in stationary scenarios and making them suitable to deal with dynamical spacetimes. We also discuss what kind of problems can be tackled using the formalism spelled out here, as well as single out future avenues of research.

Dynamics of Atom-Atom Correlations in the Fermi Problem.

M. Borrelli, C. Sabín, G. Adesso, F. Plastina, S. Maniscalco.
We present a detailed perturbative study of the dynamics of several types of atom–atom correlations in the famous Fermi problem. This is an archetypal model to study micro-causality in the quantum domain, where two atoms, one initially excited and the other prepared in its ground state, interact with the vacuum electromagnetic field. The excitation can be transferred to the second atom via a flying photon, and various kinds of quantum correlations between the two are generated during this process. Among these, prominent examples are given by entanglement, quantum discord and non-local correlations. The aim of this paper is to analyze the role of the light cone in the emergence of such correlations.

Quantum Tomography in Position and Momentum Space.

J. Casanova, C. E. López, J. J. García-Ripoll, C. F. Roos, E. Solano.
We introduce a method of quantum tomography for a continuous variable system in position and momentum space. We consider a single two-level probe interacting with a quantum harmonic oscillator by means of a class of Hamiltonians, linear in position and momentum variables, during a tunable time span. We study two cases: the reconstruction of the wavefunctions of pure states and the direct measurement of the density matrix of mixed states. We show that our method can be applied to several physical systems where high quantum control can be experimentally achieved.

Extracting Past-Future Vacuum Correlations Using Circuit QED.

C. Sabín, B. Peropadre, M. del Rey, E. Martín-Martínez.
We propose a realistic circuit QED experiment to test the extraction of past-future vacuum entanglement to a pair of superconducting qubits. The qubit P interacts with the quantum field along an open transmission line for an interval T_on and then, after a time-lapse T_off, the qubit F starts interacting for a time T_on in a symmetric fashion. After that, past-future quantum correlations will have transferred to the qubits, even if the qubits do not coexist at the same time. We show that this experiment can be realized with current technology and discuss its utility as a possible implementation of a quantum memory.

Microwave Photonics with Josephson Junction Arrays: Negative Refraction Index and Entanglement through Disorder.

D. Zueco, J. J. Mazo, E. Solano, J. J. García-Ripoll.
We study different architectures for a photonic crystal in the microwave regime based on superconducting transmission lines interrupted by Josephson junctions, both in one and two dimensions. A study of the scattering properties of a single junction in the line shows that the junction behaves as a perfect mirror when the photon frequency matches the Josephson plasma frequency. We generalize our calculations to periodic arrangements of junctions, demonstrating that they can be used for tunable band engineering, forming what we call a quantum circuit crystal. Two applications are discussed in detail. In a two-dimensional structure we demonstrate the phenomenon of negative refraction. We finish by studying the creation of stationary entanglement between two superconducting qubits interacting through a disordered media.

Nonmonotonic Entanglement of Physical Electromagnetic Field States in Noninertial Frames.

M. Montero, M. del Rey, E. Martín-Martínez.
We develop a general technique to analyze the quantum effects of acceleration on realistic spatially localized electromagnetic field states entangled in the polarization degree of freedom. We show that for this setting, quantum entanglement may build up as the acceleration increases, providing a clear signature of phenomena related to the Unruh and Hawking effects.

Quantum Simulation of the Ultrastrong-Coupling Dynamics in Circuit Quantum Electrodynamics.

D. Ballester, G. Romero, J. J. García-Ripoll, F. Deppe, E. Solano.
We propose a method to get experimental access to the physics of the ultrastrong- and deep-strong-coupling regimes of light-matter interaction through the quantum simulation of their dynamics in standard circuit QED. The method makes use of a two-tone driving scheme, using state-of-the-art circuit-QED technology, and can be easily extended to general cavity-QED setups. We provide examples of ultrastrong- and deep-strong-coupling quantum effects that would be otherwise inaccessible.

Encoding Relativistic Potential Dynamics into Free Evolution.

C. Sabín, J. Casanova, J. J. García-Ripoll, L. Lamata, E. Solano, J. León.
We propose a method to simulate a Dirac or Majorana equation evolving under particular potentials with the use of the corresponding free evolution, while the potential dynamics is encoded in a static transformation upon the initial state. We extend our results to interacting two-body systems.

Solvable Model of Dissipative Dynamics in the Deep Strong Coupling Regime.

M. Bina, G. Romero, J. Casanova, J. J. García-Ripoll, A. Lulli, E. Solano.
We describe the dynamics of a qubit interacting with a bosonic mode coupled to a zero-temperature bath in the deep strong coupling (DSC) regime. We provide an analytical solution for this open system dynamics in the off-resonance case of the qubit-mode interaction. Collapses and revivals of parity chain populations and the oscillatory behavior of the mean photon number are predicted. At the same time, photon number wave packets, propagating back and forth along parity chains, become incoherently mixed. Finally, we investigate numerically the effect of detuning on the validity of the analytical solution.

Convergence of Fermionic-Field Entanglement at Finite Acceleration in Relativistic Quantum Information.

M. Montero, E. Martín-Martínez.
We provide a simple argument showing that in the limit of infinite acceleration, the entanglement in a fermionic-field bipartite system must be independent of the choice of Unruh modes. This implies that most tensor product structures used previously to compute field entanglement in relativistic quantum information cannot give rise to physical results.

Short-Time Quantum Detection: Probing Quantum Fluctuations.

M. del Rey, C. Sabín, J. León.
We study the information provided by a detector click about the state of an initially excited two-level system. By computing the time evolution of the corresponding conditioned probability beyond the rotating-wave approximation, we show that a click in the detector is related to the decay of the source only for long interaction times. For short times, non-rotating-wave approximation effects such as self-excitations of the detector forbid a naive interpretation of the detector readings. These effects might appear in circuit QED experiments.

Simulating Accelerated Atoms Coupled to a Quantum Field.

M. del Rey, D. Porras, E. Martín-Martínez.
We show an analogy between static quantum emitters coupled to a single mode of a quantum field and accelerated Unruh-DeWitt detectors. We envision a way to simulate a variety of relativistic quantum field settings beyond the reach of current computational power, such as a high number of qubits coupled to a quantum field following arbitrary noninertial trajectories. Our scheme may be implemented with trapped ions and circuit QED setups.

Temperature-Independent Quantum Logic for Molecular Spectroscopy.

J. Mur-Petit, J. J. García-Ripoll, J. Pérez-Ríos, J. Campos-Martínez, M. I. Hernández, S. Willitsch.
We propose a fast and nondestructive spectroscopic method for single molecular ions that implements quantum logic schemes between an atomic ion and the molecular ion of interest. Our proposal relies on a hybrid coherent manipulation of the two-ion system, using optical or magnetic forces depending on the types of molecular levels to be addressed (e.g., Zeeman, rotational, vibrational or electronic degrees of freedom). The method is especially suited for the nondestructive precision spectroscopy of single molecular ions and sets a starting point for new hybrid quantum computation schemes that combine molecular and atomic ions, covering the measurement and entangling steps.

Shaping an Itinerant Quantum Field into a Multimode Squeezed Vacuum by Dissipation.

D. Porras, J. J. García-Ripoll.
We show that inducing sidebands in the emission of a single emitter into a one-dimensional waveguide, together with a dissipative repumping process, a photon field is cooled down to a multimode squeezed vacuum. Our method does not require being in the strong coupling regime, works with a continuum of propagating field modes, and leads to sources of tunable multimode squeezed light in circuit-QED systems.

Reply to 'Comment on 'Fermionic Entanglement Ambiguity in Noninertial Frames''.

M. Montero, E. Martín-Martínez.
We show that, contrary to the claims of the preceding Comment [Brádler and Jáuregui, Phys. Rev. A 85, 016301 (2012)], our original work [Phys. Rev. A 83, 062323 (2011)] is devoid of flaws or mistakes. We show that the criticism comes from a misunderstanding of part of our work.