Session A34: Focus Session: AMO Quantum Information Processing and Superconducting Qubits: Exploring Interactions of Photons and Qubits

Coupling ions and photons via an optical cavity

Trapped ions are a key experimental platform for quantum computing and simulation, while photons can transport quantum information over long distances. Optical cavities provide a coherent interface between these two quantum systems, an essential ingredient for future quantum networks. I will describe a cavity-mediated, bichromatic Raman transition using trapped calcium ions which allows us to connect the ions' electronic states with the polarization states of cavity photons. This transition is the basis for two protocols: the transfer of a quantum state from an ion onto a photon, and the generation of ion-photon entanglement. Furthermore, if two ions are coupled to the cavity mode, they can be entangled with one another, an event heralded by the detection of two orthogonally polarized cavity photons. Such entanglement could provide a robust link between remote ion-based quantum computers, and within a single cavity, enables enhanced quantum memories and the generation of photonic cluster states.   

Addressing cavity finesse degradation in ion-cavity systems

High finesse optical cavities are an essential part for achieving strong coupling between single ions and single photons, which offers a strong experimental platform in the pursuit of efficient light-matter interactions. However, degradation of cavity finesse has been repeatedly observed in ultra-high vacuum (UHV) systems, especially in the blue and ultraviolet part of the spectrum. One hypothesis attributes this finesse degradation to oxygen depletion from the mirror top layer coating. We have investigated this decay behavior of optical cavities of different top layer coatings, with resonance at 422nm in UHV conditions. In this talk, I will present our studies of finesse decay with and without baking, and recovery in oxygen at different temperature settings.   

Development of a high-Q superconducting microwave resonator for coupling to trapped laser-cooled atoms

We present progress towards a hybrid quantum system in which microwave quanta may be exchanged between a superconducting qubit and laser-trapped atoms via a magnetic dipole interaction. In initial experiments, we seek to couple a thin-film superconducting LC resonator cooled to 20 mK to the 6.835 GHz hyperfine splitting in an ensemble of optically trapped 87Rb atoms.[1] The atoms will be trapped in the evanescent optical field on the waist of a tapered 500-nm-diameter optical fiber which is moved to within a few microns of the inductor in the LC resonator. Rayleigh scattered light from defects in the optical fiber will impinge on the superconducting device. We describe the resulting effects of absorbed photons and how to minimize optical effects as well as results on positioning the resonator relative to the optical fiber within a dilution refrigerator. [1] Z. Kim et al., AIP ADVANCES 1, 042107 (2011).   

Designing the Environmental Structure for a Giant Artificial Atom

In traditional quantum optics, where the interaction between atoms and light at optical frequencies is studied, the atoms can be approximated as point-like compared to the wavelength of the light. So far, this relation has also been true for artificial atoms made out of superconducting circuits or quantum dots, interacting with microwave radiation. However, recent and ongoing experiments using surface acoustic waves show that one can couple a single artificial atom to a bosonic field at several points wavelengths apart. In this work, we theoretically study this type of system. We find that the multiple coupling points give rise to a frequency dependence for the coupling strength between the atom and its environment, and also for the Lamb shift of the atom. The frequency dependence can be designed, since it is given by the discrete Fourier transform of the coupling point coordinates. We discuss a number of possible applications for this phenomenon, including tunable coupling, single-atom lasing, and other effects that can be achieved by designing the relative coupling strengths of different transitions in a multi-level atom.   

Dynamics of superconducting qubits in open transmission lines

The time and space resolved dynamics of a superconducting qubit with an Ohmic coupling to propagating 1D photons is studied, from weak coupling to the ultrastrong coupling regime (USC). A nonperturbative study based on Matrix Product States (MPS) shows the following results [1]: (i) The ground state of the combined systems contains excitations of both the qubit and the surrounding bosonic field. (ii) An initially excited qubit equilibrates through spontaneous emission to a state, which under certain conditions, is locally close to that ground state, both in the qubit and the field. (iii) The resonances of the combined qubit-photon system match those of the spontaneous emission process and also the predictions of the adiabatic renormalisation [2]. These results set the foundations for future studies and engineering of the interactions between superconducting qubits and propagating photons, as well as the design of photon-photon interactions based on artificial materials built from these qubits. \\[4pt] [1] B. Peropadre, D. Zueco, D. Porras, J. J. G. R., arXiv:1307.3870\\[0pt] [2] A.\ J. Leggett \textit{et al}, Rev. Mod. Phys. 59, 1, (1987)   

Microwave-controlled generation of shaped photons in circuit QED

Techniques for quantum information transfer using photons propagating between distant qubits are often based on the ability to engineer the shape of the emitted photon waveform. For instance, a shape symmetric in time enables a reversal of the emission process leading to efficient reabsorption of the photon by the target qubit [1]. Here, we demonstrate the generation of shaped microwave photons in a superconducting circuit QED system consisting of a standard transmon circuit coupled to a transmission line resonator [2]. We make use of the multi-level structure of the transmon and employ a tunable Raman transition induced by a modulated microwave signal to emit a single shaped photon. This technique known from quantum optics allows us to produce symmetric photons with controllable amplitude and phase using microwave control signals only. The method is easy to implement in standard circuit QED systems because it does not rely on specialized circuit elements [3,4] to tune the transmon-photon coupling. \\[4pt] [1] J.~I.~Cirac {\em et al.}, Phys.~Rev.~Lett.~{\bf 78}, 3221 (1997).\newline [2] M.~Pechal {\em et al.}, arXiv:1308.4094.\newline [3] Y.~Yin {\em et al.}, Phys.~Rev.~Lett.~{\bf 110}, 107001 (2013).\newline [4] S.~J.~Srinivasan {\em et al.}, arXiv:1308.3471.   

Tunable coupling in atom-mirror system

We embedded an artificial atom, a superconducting transmon qubit, at a distance from the end of a transmission line. The distance between the qubit and the end (mirror) determines the electromagnetic (EM) environment coupled to the qubit. By tuning the transition wavelength of the qubit, we can control the coupling. In particular, we show that when the qubit stays a node of the EM field, the coupling is completely off. For finite coupling, we investigate the coherent scattering properties of the system, including both the time dynamic response and the steady state behavior.   

Hybrid quantum magnetism in circuit-QED: from spin-photon waves to many-body spectroscopy

We introduce a model of quantum magnetism induced by the non-perturbative exchange of microwave photons between distant superconducting qubits. By interconnecting qubits and cavities, we obtain a spin-boson lattice model that exhibits a quantum phase transition where both qubits and cavities spontaneously polarise. We present a many-body ansatz that captures this phenomenon all the way, from a the perturbative dispersive regime where photons can be traced out, to the non-perturbative ultra-strong coupling regime where photons must be treated on the same footing as qubits. Our ansatz also reproduces the low-energy excitations, which are described by hybridised spin-photon quasiparticles, and can be probed spectroscopically from transmission experiments in circuit-QED, as shown by simulating a possible experiment by Matrix-Product-State methods.   

Tuning the Dispersive Coupling Rate and Suppressing the Photon Shot Noise in a Superconducting Qubit

We report on measurements of the dispersive coupling between a microwave cavity and a tunable coupling qubit (TCQ). By operating the TCQ in the straddling regime, we can tune $\chi$, the cavity pull of the qubit, from a few MHz to close to zero while maintaining a constant qubit frequency. The vanishing $\chi$ leads to the suppression of the photon shot noise, which is one of the sources of qubit decoherence. Readout of the qubit at small $\chi$ is achieved by transferring the qubit state to a higher energy level. Experimental results of tunable $\chi$ and its impact on qubit coherence will be presented.   

Observation of superradiance in a small ensemble of artificial atoms

Superradiant effects can be efficiently addressed in an experimental setting where the atomic linewidth $\gamma$ is small compared to the coupling rate $g$ and the cavity linewidth $\kappa$. We have realized this parameter regime, known as the bad cavity limit, in a circuit QED architecture, consisting of a coplanar wavequide resonator and two transmon qubits. One main advantage of introducing only a small number of two-level systems is the possibility to prepare well defined initial states by exploiting full quantum control of the individual atom states. For the set of initial states presented here the obtained responses are clearly non-exponential and show strong indication of a correlated behavior. Preparing both qubits in superposition states with relative phase $\phi$ and measuring the amplitude of the emitted signal indicates that the effect arises in an underlying phase locking mechanism. We have further observed the suppression of the emission from one excited qubit due to the presence of a second qubit in its ground state. The coherence properties of the emitted radiation have been analyzed by measuring high order photon correlations and are in good agreement with the theoretical statistics.   

Parametric interactions in circuit-QED

The circuit-QED architecture is a versatile platform for implementing quantum optics at microwave frequencies. Incorporating additional nonlinear coupling elements between linear modes in this architecture provides a mechanism to drive parametric interactions. These interactions are a powerful set of tools that can be used to synthesize complex quantum states, generate entanglement and enhance measurement. We will discuss experimental results utilizing parametric interactions to generate and study quantum states in superconducting circuits.   

The Forgotten Quantum Number: The Radial Modes of Laguerre-Gauss Beams

The orbital angular momentum quantum number of Laguerre-Gauss beams has received an explosively increasing amount of attention over the past twenty years. However, often overlooked is the so-called radial number of these beams. We present a quantum-mechanical operator formalism of this ``forgotten" quantum number. We place an emphasis on the detailed understanding of the physical interpretation of this formalism, including it's connection to concrete physical observables and conjugate variables. We then draw some connections between this new formalism and the effect the radial number has on beam stability with possible application to quantum communication.