Juan José García-Ripoll

Circuit quantum electrodynamics in the ultrastrong-coupling regime

T. Niemczyk, F. Deppe, 2 H. Huebl, E. P. Menzel, F. Hocke, M. J. Schwarz, J. J. Garcia-Ripoll, D. Zueco, T. Hümmer, E. Solano, 7 A. Marx, and R. Gross 2 Walther-Meißner-Institut, Bayerische Akademie der Wissenschaften, D-85748 Garching, Germany∗ Physik-Department, Technische Universität München, D-85748 Garching, Germany Instituto de F́ısica Fundamental, CSIC, Serrano 113-bis, 28006 Madrid, Spain Instituto de Ciencia de Materiales de Aragón y Departamento de F́ısica de la Materia Condensada, CSIC-Universidad de Zaragoza, E-50012 Zaragoza, Spain. Institut für Physik, Universität Augsburg, Universitätsstraße 1, D-86135 Augsburg, Germany Departamento de Qúımica F́ısica, Universidad del Páıs Vasco Euskal Herriko Unibertsitatea, Apdo. 644, 48080 Bilbao, Spain IKERBASQUE, Basque Foundation for Science, Alameda Urquijo 36, 48011 Bilbao, Spain (Dated: December 24, 2010)

Observation of the Bloch-Siegert Shift in a Qubit-Oscillator System in the Ultrastrong Coupling Regime

We measure the dispersive energy-level shift of an LC resonator magnetically coupled to a superconducting qubit, which clearly shows that our system operates in the ultrastrong coupling regime. The large mutual kinetic inductance provides a coupling energy of ≈ 0.82 GHz, requiring the addition of counter-rotating-wave terms in the description of the Jaynes-Cummings model. We find a 50 MHz Bloch-Siegert shift when the qubit is in its symmetry point, fully consistent with our analytical model.

Matrix product density operators: Simulation of finite-temperature and dissipative systems

We show how to simulate numerically the evolution of 1D quantum systems under dissipation as well as in thermal equilibrium. The method applies to both finite and inhomogeneous systems, and it is based on two ideas: (a) a representation for density operators which extends that of matrix product states to mixed states; (b) an algorithm to approximate the evolution (in real or imaginary time) of matrix product states which is variational.

Deep Strong Coupling Regime of the Jaynes-Cummings Model

We study the quantum dynamics of a two-level system interacting with a quantized harmonic oscillator in the deep strong coupling regime (DSC) of the Jaynes-Cummings model, that is, when the coupling strength g is comparable or larger than the oscillator frequency ω (g/ω≳1). In this case, the rotating-wave approximation cannot be applied or treated perturbatively in general. We propose an intuitive and predictive physical frame to describe the DSC regime where photon number wave packets bounce back and forth along parity chains of the Hilbert space, while producing collapse and revivals of the initial population. We exemplify our physical frame with numerical and analytical considerations in the qubit population, photon statistics, and Wigner phase space.

Speed optimized two-qubit gates with laser coherent control techniques for ion trap quantum computing

Trapped ions constitute one of the most promising systems to implement scalable quantum computation. [1] In an ion trap quantum computer qubits are stored in long-lived internal atomic states. A universal set of single and two qubit gates is obtained by manipulating the internal states with lasers, and entangling the ions via the motional states [2]. During the last years a remarkable experimental progress in building an ion trap quantum computer has allowed to realize two–qubit gates [3, 4, 5, 6] and also to prepare entangled states [7, 8, 9]. The ultimate challenge is now the development of scalable ion trap quantum computing. Scalability is based on storing a set of ions, and moving ions independently, in particular to bring together pairs of ions to perform a two-qubit gate [10, 11]. Basic steps towards this goal have already been demonstrated experimentally [12].

Quantum simulation of the Klein paradox with trapped ions

We report on quantum simulations of relativistic scattering dynamics using trapped ions. The simulated state of a scattering particle is encoded in both the electronic and vibrational state of an ion, representing the discrete and continuous components of relativistic wave functions. Multiple laser fields and an auxiliary ion simulate the dynamics generated by the Dirac equation in the presence of a scattering potential. Measurement and reconstruction of the particle wave packet enables a frame-by-frame visualization of the scattering processes. By precisely engineering a range of external potentials we are able to simulate text book relativistic scattering experiments and study Klein tunneling in an analogue quantum simulator. We describe extensions to solve problems that are beyond current classical computing capabilities.

Implementation of spin Hamiltonians in optical lattices.

We propose an optical lattice setup to investigate spin chains and ladders. Electric and magnetic fields allow us to vary at will the coupling constants, producing a variety of quantum phases including the Haldane phase, critical phases, quantum dimers, etc. Numerical simulations are presented showing how ground states can be prepared adiabatically. We also propose ways to measure a number of observables, like energy gap, staggered magnetization, end-chain spins effects, spin correlations, and the string-order parameter.

Optimizing Schrödinger Functionals Using Sobolev Gradients: Applications to Quantum Mechanics and Nonlinear Optics

In this paper we study the application of the Sobolev gradients technique to the problem of minimizing several Schrodinger functionals related to timely and difficult nonlinear problems in quantum mechanics and nonlinear optics. We show that these gradients act as preconditioners over traditional choices of descent directions in minimization methods and show a computationally inexpensive way to obtain them using a discrete Fourier basis and a fast Fourier transform. We show that the Sobolev preconditioning provides a great convergence improvement over traditional techniques for finding solutions with minimal energy as well as stationary states and suggest a generalization of the method using arbitrary linear operators.

Ultrastrong coupling of a single artificial atom to an electromagnetic continuum in the nonperturbative regime

A superconducting artificial atom coupled to a 1D waveguide tests the limits of light–matter interaction in an unexplored coupling regime, which may enable new perspectives for quantum technologies.

Microwave Photon Detector in Circuit QED

In this Letter we design a metamaterial composed of discrete superconducting elements that implements a high-efficiency microwave photon detector. Our design consists of a microwave guide coupled to an array of metastable quantum circuits, whose internal states are irreversibly changed due to the absorption of photons. This proposal can be widely applied to different physical systems and can be generalized to implement a microwave photon counter.