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.

We propose a method to achieve coherent coupling between nitrogen-vacancy (NV) centers in diamond and superconducting (SC) flux qubits. The resulting coupling can be used to create a coherent interaction between the spin states of distant NV centers mediated by the flux qubit. Furthermore, the magnetic coupling can be used to achieve a coherent transfer of quantum information between the flux qubit and an ensemble of NV centers. This enables a long-term memory for a SC quantum processor and possibly an interface between SC qubits and light.

). Electron-nuclear interaction in 13C nanotube double quantum dots. Nature Physics, 5(5), 321-326. https://doi.org

We show how engineered classical noise can be used to generate constrained Hamiltonian dynamics in atomic quantum simulators of many-body systems, taking advantage of the continuous Zeno effect. After discussing the general theoretical framework, we focus on applications in the context of lattice gauge theories, where imposing exotic, quasi-local constraints is usually challenging. We demonstrate the effectiveness of the scheme for both Abelian and non-Abelian gauge theories, and discuss how engineering dissipative constraints substitutes complicated, non-local interaction patterns by global coupling to laser fields.PACS numbers: 03.65. Xp,37.10.Jk,11.15.Ha Laboratory experiments with atomic quantumdegenerate gases have established a synergetic link between atomic physics and condensed matter [1][2][3], and hold prospects for a similar connection to high-energy physics [4][5][6][7][8][9][10][11][12]. Loaded into optical lattices, cold atoms realize Hubbard models, which can be designed and controlled via external fields to mimic the dynamics of quantum many-body systems in equilibrium and nonequilibrium situations [3, 13]. While a focus of research during the last decade has been the development of a toolbox for designing specific lattice Hamiltonians [1-3], we address below the problem of implementing desired Hubbard dynamics in the presence of constraints, i.e., we wish to keep the system dynamics within a certain subspace of the total Hilbert space. A familiar way of imposing such constraints is to add an energy penalty to the Hamiltonian [14]. Below, we describe an alternative scenario that is based on driving the system with engineered classical noise, exploiting the Zeno effect [15][16][17][18][19][20]. As we will see, 'adding noise' provides a general tool to implement -in an experimentally efficient and accessible way -highly nontrivial constraints in quantum many-body systems.The present work is motivated by the ongoing quest to build a quantum simulator for Abelian and non-Abelian lattice gauge theories (LGTs) with cold atoms in optical lattices [5][6][7][8][9][10][11][12].LGTs play a prominent role in both particle and condensed matter physics: in the standard model, the interaction between constituents of matter are mediated by gauge bosons [21][22][23][24], and in frustrated magnetism, quantum spin liquids are suitably described in the language of gauge theories [14,25,26]. The key feature of a LGT is the presence of local (gauge) symmetries. The generators G a x of these local gauge transformations, with x denoting lattice sites and a a color index, commute with the lattice Hamiltonian, [H 0 , G a x ] = 0 for all x, a, and thus provide local conservation laws. They can be interpreted in analogy to Gauss's law from electrodynamics, as they constrain the dynamics of the system to a physical LGTs consist of fermions ψx living on sites, coupled to gauge fields Ux,x+1 living on links. The dynamics can be constrained to the physical subspace by coupling independent noise sources linearly to each gener...

We discuss how a lattice Schwinger model can be realized in a linear ion trap, allowing a detailed study of the physics of Abelian lattice gauge theories related to one-dimensional quantum electrodynamics. Relying on the rich quantum-simulation toolbox available in state-of-the-art trapped-ion experiments, we show how one can engineer an effectively gauge-invariant dynamics by imposing energetic constraints, provided by strong Ising-like interactions. Applying exact diagonalization to ground-state and time-dependent properties, we study the underlying microscopic model, and discuss undesired interaction terms and other imperfections. As our analysis shows, the proposed scheme allows for the observation in realistic setups of spontaneous parity-and charge-symmetry breaking, as well as false-vacuum decay. Besides an implementation aimed at larger ion chains, we also discuss a minimal setting, consisting of only four ions in a simpler experimental setup, which enables to probe basic physical phenomena related to the full many-body problem. The proposal opens a new route for analog quantum simulation of high-energy and condensed-matter models where gauge symmetries play a prominent role.Some of these limitations can be circumvented in the framework of lattice gauge theories (LGTs) [28][29][30][31]47]. Here, by means of Monte Carlo simulation of the corresponding lattice action, a broad regime of interaction parameters becomes accessible, thus allowing the investigation of non-perturbative effects with controlled numerical techniques [31,46]. Nevertheless, classical simulations are severely limited by the sign problem, which prevents an accurate description of finite-density regimes (as relevant, e.g., for the core of dense neutron stars) and outof-equilibrium dynamics (which is realized in heavy-ion collider experiments). Given these difficulties, it becomes particularly attractive to develop a quantum simulator of arXiv:1306.2162v3 [cond-mat.quant-gas]

We present a formalism to calculate finite-frequency current correlations in interacting nanoscopic conductors. We work within the n-resolved density matrix approach and obtain a multi-time cumulant generating function that provides the fluctuation statistics solely from the spectral decomposition of the Liouvillian. We apply the method to the frequency-dependent third cumulant of the current through a single resonant level and through a double quantum dot. Our results, which show that deviations from Poissonian behaviour strongly depend on frequency, demonstrate the importance of finite-frequency higher-order cumulants in fully characterizing transport.PACS numbers: 73.23. Hk,72.70.+m,03.65.Yz Following the considerable success of shot-noise in the understanding of transport through mesoscopic systems [1], attention is now turning towards the higher-order statistics of electron current. The so-called Full Counting Statistics (FCS) of electron transport yields all moments (or cumulants) of the probability distribution P (n, t) of the number of transferred electrons during time t. Despite their difficulty, measurements of the third moment of voltage fluctuations have been made [2,3], and recent developments in single electron detection [4,5,6] promise to open new horizons on the experimental side.The theory of FCS is now well established in the zerofrequency limit [7,8,9]. However, this is by no means the full picture, since the higher-order current correlators at finite frequencies contain much more information than their zero-frequency counterparts. Already at second order (shot-noise), one can extract valuable information about transport time scales and correlations. When the conductor has various intrinsic time scales like, for example, the charge relaxation time and the dwelling time of a chaotic cavity [10], one needs to go beyond second-order in order to fully characterize electronic transport. Apart from this example, and some other notable exceptions [11,12,13], the behaviour of finite-frequency correlators beyond shot-noise is still largely unexplored.In this Rapid Communication, we develop a theory of frequency-dependent current correlators of arbitrary order in the context of the n-resolved density matrix (DM) approach, -a Quantum Optics technique [14] that has recently found application in mesoscopic transport [15]. Within this approach, the DM of the system, ρ(t), is unravelled into components ρ (n) (t) in which n = n(t) = 0, 1, . . . electrons have been transferred to the collector. Considering a generic mesoscopic system with Hamiltonian H = H S + H L + H T , where H S and H L refer to the system and leads respectively, and provided that the Born-Markov approximation with respect to the tunnelling term H T is fulfilled, the time-evolution of this n-resolved DM can be written quite generally aṡwhere the vector ρ (n) (t) contains the nonzero elements of the DM, written in a suitable many-body basis. The Liouvillian L 0 describes the 'continuous' evolution of the system, whereas L J describes the ...

A quantum simulator of U(1) lattice gauge theories can be implemented with superconducting circuits. This allows the investigation of confined and deconfined phases in quantum link models, and of valence bond solid and spin liquid phases in quantum dimer models. Fractionalized confining strings and the real-time dynamics of quantum phase transitions are accessible as well. Here we show how state-of-the-art superconducting technology allows us to simulate these phenomena in relatively small circuit lattices. By exploiting the strong non-linear couplings between quantized excitations emerging when superconducting qubits are coupled, we show how to engineer gauge invariant Hamiltonians, including ring-exchange and four-body Ising interactions. We demonstrate that, despite decoherence and disorder effects, minimal circuit instances allow us to investigate properties such as the dynamics of electric flux strings, signaling confinement in gauge invariant field theories. The experimental realization of these models in larger superconducting circuits could address open questions beyond current computational capability.

We describe a superconducting-circuit lattice design for the implementation and simulation of dynamical lattice gauge theories. We illustrate our proposal by analyzing a one-dimensional U(1) quantum-link model, where superconducting qubits play the role of matter fields on the lattice sites and the gauge fields are represented by two coupled microwave resonators on each link between neighboring sites. A detailed analysis of a minimal experimental protocol for probing the physics related to string breaking effects shows that, despite the presence of decoherence in these systems, distinctive phenomena from condensed-matter and high-energy physics can be visualized with state-of-the-art technology in small superconducting-circuit arrays.