I am going to post here all newly submitted articles on the arXiv related to superconducting circuits. If your article has been accidentally forgotten, feel free to contact me
19
Mai
2018
Strongly driven quantum Josephson circuits
Radio Frequency driven Josephson circuits provide a rich platform to engineer a variety of nonlinear Hamiltonians for superconducting quantum circuits. While Josephson junctions mediate
strong interactions between microwave photons, some particular types of interaction Hamiltonians can only be obtained through the application of microwave drives (pumps) at well-chosen frequencies. For various applications, it is important to increase the pump strength without introducing undesired couplings and interferences that limit the fidelity of the operations. In this Letter, we analyze these limitations through the theoretical study of the steady state behavior of the driven-dissipative systems. Our general analysis, based on the Floquet-Markov theory, indicates that the ubiquitous circuit consisting of a transmon coupled to a harmonic oscillator suffers from strong limitations in this regard. In accordance with a parallel experimental study, we find that above a fairly low critical pump power the transmon state escapes the Josephson potential confinement and is sent to a statistical mixture of free-particle like states. Next, we illustrate that by diluting the non-linearity of the Josephson junction through a parallel inductive shunt, the picture changes significantly and one achieves very large dynamic ranges in the pump power. This theoretical study provides the ground for drastic modifications in Josephson circuit designs to be used in parametric Hamiltonian engineering experiments.
17
Mai
2018
Quantum Zeno effect: A possible resolution to the leakage problem in superconducting quantum computing architectures
We propose to use the continuous version of the quantum Zeno effect to eliminate leakage to higher energy states in superconducting quantum computing architectures based on Josephson
phase and flux qubits. We are particularly interested in the application of this approach to the single-step Greenberger-Horne-Zeilinger (GHZ) state protocol described in [A. Galiautdinov and J. M. Martinis, Phys. Rev. A 78, 010305(R) (2008)]. While being conceptually appealing, the protocol was found to be plagued with a number of spectral crowding and leakage problems. Here we argue that by coupling the qubits to a measuring device which continuously monitors leakage to higher energy states (say, to a very lossy resonator of frequency omega = E3(qubit)-E1(qubit), with 1 labeling the ground state of the qubit), we could potentially restrict the multi-qubit system’s evolution to its computational subspace, thus circumventing the above mentioned problems.
16
Mai
2018
Gauge ambiguities in ultrastrong-coupling QED: the Jaynes-Cummings model is as fundamental as the Rabi model
Ultrastrong-coupling between an effective two-level system and radiation offers immense potential for advancing both fundamental and applied quantum electrodynamics (QED). Such regimes
cannot be treated using the rotating-wave approximation, which applied to the quantum Rabi model yields the apparently less fundamental Jaynes-Cummings model. However, the implications of gauge-freedom for ultrastrong-coupling QED are yet to be recognised. Here we show that when truncating the material system to only two levels, each gauge gives a different effective description whose predictions can vary significantly for ultrastrong-coupling. Rabi models are obtained through specific gauge choices, but so too is a Jaynes-Cummings model without needing the rotating-wave approximation. Analysing a circuit QED setup, we find that this Jaynes-Cummings model is not only valid well beyond the rotating-wave approximation, it is often more accurate than the Rabi model. Among the many implications of this finding is that the system’s ground state is not necessarily highly entangled, which is usually considered a hallmark of ultrastrong-coupling light-matter physics.
14
Mai
2018
Breakdown of gauge invariance in ultrastrong-coupling cavity QED
We revisit the derivation of Rabi- and Dicke-type models, which are commonly used for the study of quantum light-matter interactions in cavity and circuit QED. We demonstrate that the
validity of the two-level approximation, which is an essential step in this derivation, depends explicitly on the choice of gauge once the system enters the ultrastrong coupling regime. In particular, while in the electric dipole gauge the two-level approximation can be performed as long as the Rabi frequency remains much smaller than the energies of all higher-lying levels, it can dramatically fail in the Coulomb gauge, even for systems with an extremely anharmonic spectrum. We extensively investigate this phenomenon both in the single-dipole (Rabi) and multi-dipole (Dicke) case, and considering the specific examples of dipoles confined by double-well and by square-well potentials, and of circuit QED systems with flux qubits coupled to an LC resonator.
Dynamics of an off-resonantly pumped superconducting qubit in a cavity
Strong microwave drives, referred to as pumps, are widely applied to superconducting circuits incorporating Josephson junctions in order to induce couplings between electromagnetic
modes. This offers a variety of applications, from quantum-limited amplification, to quantum state and manifold stabilization. These couplings scale with the pump power, therefore, seeking stronger couplings requires a detailed understanding of the behavior of such circuits in the presence of stronger pumps. In this work, we probe the dynamics of a transmon qubit in a 3D cavity, for various pump powers and frequencies. For all pump frequencies, we find a critical pump power above which the transmon is driven into highly excited states, beyond the first seven states which we individually resolve through cavity spectroscopy. This observation is compatible with our theory describing the escape of the transmon state out of its Josephson potential well, into states resembling those of a free particle which does not induce any non-linear couplings.
Current and Magnetic Field Dependences of a Superconducting Coplanar Waveguide Resonator
We fabricated superconducting coplanar waveguide resonator with leads for dc bias, which enables the ac conductivity measurement under dc bias. The current and the magnetic field dependences
of resonance properties were measured, and hysteretic behavior was observed as a function of the dc driving current. The observed shift in the inverse of the quality factor and the center frequency were understood by considering both the motion of vortices and the suppression of the order parameter with dc current. Our investigation revealed that the strongly pinned vortices have little infuluence on the change in the center frequency, while it still affects that of the quality factor. Our results indicate that an accurate understanding of the dynamics of driven vortices is indispensable when we attempt to control the resonance properties with high precision.
11
Mai
2018
Superconducting cavity electro-optics: a platform for coherent photon conversion between superconducting and photonic circuits
Leveraging the quantum information processing ability of superconducting circuits and long-distance distribution ability of optical photons promises the realization of complex and large-scale
quantum networks. In such a scheme, a coherent and efficient quantum transducer between superconducting and photonic circuits is critical. However, such quantum transducer is still challenging since the use of intermediate excitations in current schemes introduces extra noise and limits bandwidth. Here we realize direct and coherent transduction between superconducting and photonic circuits based on triple-resonance electro-optics principle, with integrated devices incorporating both superconducting and optical cavities on the same chip. Electromagnetically induced transparency is observed, indicating the coherent interaction between microwave and optical photons. Internal conversion efficiency of 25.9\pm0.3\% has been achieved, with 2.05\pm0.04\% total efficiency. Superconducting cavity electro-optics offers broad transduction bandwidth and high scalability, and represents a significant step towards the integrated hybrid quantum circuits and distributed quantum computation.
07
Mai
2018
The Geometry of a Quantum Circuit and its Impact on Electromagnetic Noise
Here we show that to quantize any lumped element circuit, the circuit geometry must be included in a mathematical model of either the circuit fluxes or the circuit charges. By geometry
of the circuit, we refer to the so-called parasitic inductances and capacitances that arise from the details of the circuit layout, which are well known to create noise in classical circuits. In contrast, the classical lumped element model describes only the topology of the circuit, which defines how different finite element variables are connected to one another by circuit components. By geometry we also refer to the fact that the quantum variables define the circuit geometry – some are outside the wire, some are inside the wire, and some are at boundary of the wire. Just as with classical circuits, these effects create noise; this noise arises in the form of high frequency components in the Hamiltonian that are difficult to accurately simulate using a lumped element model. The presentation is appropriate for undergraduate electrical and computer engineering students learning about quantum computing and physicists learning about electrical circuits.
03
Mai
2018
Maxwell’s demon in superconducting circuits
This paper provides an overview of the first experimental realizations of quantum-mechanical Maxwell’s demons based on superconducting circuits. The principal results of these
experiments are recalled and put into context. We highlight the versatility offered by superconducting circuits for studying quantum thermodynamics.
02
Mai
2018
Nanowire Superinductance Fluxonium Qubit
Disordered superconducting materials provide a new capability to implement novel circuit designs due to their high kinetic inductance. Here, we realize a fluxonium qubit in which a
long NbTiN nanowire shunts a single Josephson junction. We explain the measured fluxonium energy spectrum with a nonperturbative theory accounting for the multimode structure of the device in a large frequency range. Making use of multiphoton Raman spectroscopy, we address forbidden fluxonium transitions and observe multilevel Autler-Townes splitting. Finally, we measure lifetimes of several excited states ranging from T1=620 ns to T1=20 μs, by applying consecutive π-pulses between multiple fluxonium levels. Our measurements demonstrate that NbTiN is a suitable material for novel superconducting qubit designs.