M. Sanz

Dynamical Casimir Effect Entangles Artificial Atoms

We show that the physics underlying the dynamical Casimir effect may generate multipartite quantum correlations. To achieve it, we propose a circuit quantum electrodynamics scenario involving superconducting quantum interference devices, cavities, and superconducting qubits, also called artificial atoms. Our results predict the generation of highly entangled states for two and three superconducting qubits in different geometric configurations with realistic parameters. This proposal paves the way for a scalable method of multipartite entanglement generation in cavity networks through dynamical Casimir physics.

Upconversion processes in Nd3+-doped fluorochloride glass

Abstract Infrared (IR) to visible (VIS) and ultraviolet (UV) upconversion in Nd3+-doped fluorochloride glass has been observed between 4.2 and 300 K under continuous wave (cw) and pulsed IR laser excitation in resonance with the 4 F 3/2 and 4 F 5/2 levels. The emission spectra have UV bands at 359 and 383 nm and VIS bands located around 416, 452, 527, 589, and 660 nm. The green, yellow, and red emissions are attributed to transitions from the 4 G 7/2 excited state. The excitation spectra of the upconverted emissions which are similar to the one-photon absorption spectrum, together with the features appearing in the decays of these emissions, indicate that an energy transfer upconversion (ETU) process seems to be responsible for the VIS luminescence observed, though other possible mechanisms cannot be disregarded. Referring to the blue and UV bands, the most probable origin of the upconverted fluorescence seems to be the 4 D 3/2 , 4 D 5/2 , 4 D 1/2 levels. In the last, the participation of three neodymium ions is necessary in the ETU process.

Quantum memristors with superconducting circuits

Memristors are resistive elements retaining information of their past dynamics. They have garnered substantial interest due to their potential for representing a paradigm change in electronics, information processing and unconventional computing. Given the advent of quantum technologies, a design for a quantum memristor with superconducting circuits may be envisaged. Along these lines, we introduce such a quantum device whose memristive behavior arises from quasiparticle-induced tunneling when supercurrents are cancelled. For realistic parameters, we find that the relevant hysteretic behavior may be observed using current state-of-the-art measurements of the phase-driven tunneling current. Finally, we develop suitable methods to quantify memory retention in the system.

The Forbidden Quantum Adder

Quantum information provides fundamentally different computational resources than classical information. We prove that there is no unitary protocol able to add unknown quantum states belonging to different Hilbert spaces. This is an inherent restriction of quantum physics that is related to the impossibility of copying an arbitrary quantum state, i.e., the no-cloning theorem. Moreover, we demonstrate that a quantum adder, in absence of an ancillary system, is also forbidden for a known orthonormal basis. This allows us to propose an approximate quantum adder that could be implemented in the lab. Finally, we discuss the distinct character of the forbidden quantum adder for quantum states and the allowed quantum adder for density matrices.

Upconversion processes in Nd3+-doped fluoroarsenate glasses

Abstract In this work we have studied the infrared to visible upconversion luminescence of Nd3+-doped fluoroarsenate glasses with different neodymium concentrations (0.1, 0.5, 2, 3, 4, and 5 mol%) under continuous wave and pulsed laser excitation. Green, orange, and red emissions are observed with excitation at 802 and 874 nm within the 4F5/2 and 4F3/2 levels. In addition, a weak blue emission appears under continuous wave (cw) laser excitation which under pulsed laser excitation only occurs for the most concentrated sample at 77 K. The green, orange, and red emissions can be attributed to transitions from the 4G7/2 excited state. The pump power dependencies of these upconverted emissions together with the shape of the upconverted excitation spectra which are similar to the one-photon absorption spectrum and the temporal behavior of the decays, indicate that an ETU process seems to be responsible for the observed visible luminescence, though other possible mechanisms cannot be disregarded.

Site-selective spectroscopy and infrared-to-visible upconversion in a Nd3+-doped Pb5Al3F19 crystal

Abstract In this work, we analyzed the spectroscopic features of the absorption and emission of Nd 3+ in a fluoroaluminate Pb 5 Al 3 F 19 crystal. Two main crystal field sites were identified for the rare earth in this crystal which may correspond to the two different crystallographic sites of Pb 2+ . Infrared-to-visible and -ultraviolet upconversion, under continuous wave and pulsed laser excitation, was observed at room and low temperatures. Analysis of the steady-state excitation spectra and of the decays of the upconverted emissions shows that the most likely mechanism for the observed upconversion emissions is an energy-transfer upconversion (ETU) process.

Advanced-Retarded Differential Equations in Quantum Photonic Systems

We propose the realization of photonic circuits whose dynamics is governed by advanced-retarded differential equations. Beyond their mathematical interest, these photonic configurations enable the implementation of quantum feedback and feedforward without requiring any intermediate measurement. We show how this protocol can be applied to implement interesting delay effects in the quantum regime, as well as in the classical limit. Our results elucidate the potential of the protocol as a promising route towards integrated quantum control systems on a chip.

Quantum chemistry and charge transport in biomolecules with superconducting circuits

We propose an efficient protocol for digital quantum simulation of quantum chemistry problems and enhanced digital-analog quantum simulation of transport phenomena in biomolecules with superconducting circuits. Along these lines, we optimally digitize fermionic models of molecular structure with single-qubit and two-qubit gates, by means of Trotter-Suzuki decomposition and Jordan-Wigner transformation. Furthermore, we address the modelling of system-environment interactions of biomolecules involving bosonic degrees of freedom with a digital-analog approach. Finally, we consider gate-truncated quantum algorithms to allow the study of environmental effects.

Algorithmic Quantum Simulation of Memory Effects

We propose a method for the algorithmic quantum simulation of memory effects described by integro-differential evolution equations. It consists in the systematic use of perturbation theory techniques and a Markovian quantum simulator. Our method aims to efficiently simulate both completely positive and non-positive dynamics without the requirement of engineering non-Markovian environments. Finally, we find that small error bounds can be reached with polynomially scaling resources, evaluated as the number of performed measurements.

Biomimetic Cloning of Quantum Observables

The authors acknowledge funding from Basque Government BFI-2012-322 and IT472-10 grants, Spanish MINECO FIS2012-36673-C03-02, Ramon y Cajal Grant RYC-2012-11391, UPV/EHU UFI 11/55, CCQED, PROMISCE, and SCALEQIT European projects.We propose a bio-inspired sequential quantum protocol for the cloning and preservation of the statistics associated to quantum observables of a given system. It combines the cloning of a set of commuting observables, permitted by the no-cloning and no-broadcasting theorems, with a controllable propagation of the initial state coherences to the subsequent generations. The protocol mimics the scenario in which an individual in an unknown quantum state copies and propagates its quantum information into an environment of blank qubits. Finally, we propose a realistic experimental implementation of this protocol in trapped ions.