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
03
Jul
2013
Analog Superconducting Quantum Simulator for Holstein Polarons
We propose an analog quantum simulator for the Holstein molecular-crystal model based on a dispersive superconducting circuit QED system composed of transmon qubits and microwave resonators.
By varying the circuit parameters, one can readily access both the adiabatic and the anti-adiabatic regimes of this model, and realize the coupling strengths required for small-polaron formation. We present a pumping scheme for preparing small-polaron states of arbitrary quasimomentum within time scales much shorter than the qubit decoherence time. The ground state of the system is characterized by anomalous amplitude fluctuation and measurement-based momentum squeezing in the resonator modes.
The Properties and Light-Matter Interaction of a Superconducting Majorana-Transmon Qubit
We analyse the transmon regime Hamiltonian of a Cooper-Pair-Box where the superconducting phase difference is coupled to the zero energy parity states that arise from Majorana quasi-particles.
We investigate the level structure and properties of the transmon qubit in this regime where even a small coupling causes hybridization of different transmon-parity states without compromising the suppression of charge dispersion. We show that the microwave photon-qubit coupling is sensitive to the gate bias and all the energy scales of the Hamiltonian. As well as a probe for topological-superconductor excitations, we propose that this type of device could be used to realise a high coherence tunable four-level system in the superconducting circuits architecture.
28
Jun
2013
Observation of three-state dressed states in circuit quantum electrodynamics
We have investigated the microwave response of a transmon qubit coupled directly to a transmission line. In a transmon qubit, owing to its weak anharmonicity, a single driving field
may generate dressed states involving more than two bare states. We confirmed the formation of three-state dressed states by observing all of the six associated Rabi sidebands, which appear as either amplification or attenuation of the probe field. The experimental results are reproduced with good precision by a theoretical model incorporating the radiative coupling between the qubit and the microwave.
19
Jun
2013
Dynamic parity recovery in a strongly driven Cooper-pair box
We study a superconducting charge qubit coupled to an intensive electromagnetic field and probe changes in the resonance frequency of the formed dressed states. At large driving strengths,
exceeding the qubit energy-level splitting, this reveals the well known Landau-Zener-St\“uckelberg (LZS) interference structure of a longitudinally driven two-level system. For even stronger drives we observe a significant change in the LZS pattern and contrast. We attribute this to photon-assisted quasiparticle tunneling in the qubit. This results in the recovery of the qubit parity, eliminating effects of quasiparticle poisoning and leads to an enhanced interferometric response. The interference pattern becomes robust to quasiparticle poisoning and has a good potential for accurate charge sensing.
17
Jun
2013
Deterministic entanglement of superconducting qubits by parity measurement and feedback
The stochastic evolution of quantum systems during measurement is arguably the most enigmatic feature of quantum mechanics. Measuring a quantum system typically steers it towards a
classical state, destroying any initial quantum superposition and any entanglement with other quantum systems. Remarkably, the measurement of a shared property between non-interacting quantum systems can generate entanglement starting from an uncorrelated state. Of special interest in quantum computing is the parity measurement, which projects a register of quantum bits (qubits) to a state with an even or odd total number of excitations. Crucially, a parity meter must discern the two parities with high fidelity while preserving coherence between same-parity states. Despite numerous proposals for atomic, semiconducting, and superconducting qubits, realizing a parity meter creating entanglement for both even and odd measurement results has remained an outstanding challenge. We realize a time-resolved, continuous parity measurement of two superconducting qubits using the cavity in a 3D circuit quantum electrodynamics (cQED) architecture and phase-sensitive parametric amplification. Using postselection, we produce entanglement by parity measurement reaching 77% concurrence. Incorporating the parity meter in a feedback-control loop, we transform the entanglement generation from probabilistic to fully deterministic, achieving 66% fidelity to a target Bell state on demand. These realizations of a parity meter and a feedback-enabled deterministic measurement protocol provide key ingredients for active quantum error correction in the solid state.
16
Jun
2013
Generating entanglement between microwave photons and qubits in multiple cavities coupled by a superconducting qutrit
We discuss how to generate entangled coherent states of four extrm{microwave} resonators extrm{(a.k.a. cavities)} coupled by a superconducting qubit. We also show extrm{that}
a GHZ state of four superconducting qubits embedded in four different resonators \textrm{can be created with this scheme}. In principle, \textrm{the proposed method} can be extended to create an entangled coherent state of $n$ resonators and to prepare a Greenberger-Horne-Zeilinger (GHZ) state of $n$ qubits distributed over $n$ cavities in a quantum network. In addition, it is noted that four resonators coupled by a coupler qubit may be used as a basic circuit block to build a two-dimensional quantum network, which is useful for scalable quantum information processing.
11
Jun
2013
Sputtered TiN films for superconducting coplanar waveguide resonators
We present a systematic study of the properties of TiN films by varying the deposition conditions in an ultra-high-vacuum reactive magnetron sputtering chamber. By increasing the deposition
pressure from 2 to 9 mTorr while keeping a nearly stoichiometric composition of Ti(1-x)N(x) (x=0.5), the film resistivity increases, the dominant crystal orientation changes from (100) to (111), grain boundaries become clearer, and the strong compressive strain changes to weak tensile strain. The TiN films absorb a high concentration of contaminants including hydrogen, carbon, and oxygen when they are exposed to air after deposition. With the target-substrate distance set to 88 mm the contaminant levels increase from ~0.1% to ~10% as the pressure is increased from 2 to 9 mTorr. The contaminant concentrations also correlate with in-plane distance from the center of the substrate and increase by roughly two orders of magnitude as the target-substrate distance is increased from 88 mm to 266 mm. These contaminants are found to strongly influence the properties of TiN films. For instance, the resistivity of stoichiometric films increases by around a factor of 5 as the oxygen content increases from 0.1% to 11%. These results suggest that the sputtered TiN particle energy is critical in determining the TiN film properties, and that it is important to control this energy to obtain high-quality TiN films. Superconducting coplanar waveguide resonators made from a series of nearly stoichiometric films grown at pressures from 2 mTorr to 7 mTorr show an increase in intrinsic quality factor from ~10^4 to ~10^6 as the magnitude of the compressive strain decreases from nearly 3800 MPa to approximately 150 MPa and the oxygen content increases from 0.1% to 8%. The films with a higher oxygen content exhibit lower loss, but the nonuniformity of the oxygen incorporation hinders the use of sputtered TiN in larger circuits.
10
Jun
2013
Detection and Manipulation of Majorana Fermions in Circuit QED
Motivated by recent experimental progress to measure and manipulate Majorana fermions with superconducting circuits, we propose a device interfacing Majorana fermions with circuit quantum
electrodynamics. The proposed circuit acts as a charge parity detector changing the resonance frequency of a superconducting \lambda/4 – resonator conditioned on the parity of charges on nearby gates. Operating at both charge and flux sweet-spots, this device is highly insensitive to environmental noise and enables high-fidelity single-shot quantum non-demolition readout of the state of a pair of Majorana fermions. Additionally, the interaction permits the realization of an arbitrary phase gate on the topological qubit, closing the loop for computational completeness. Away from the charge sweet-spot, this device can be used as a highly sensitive charge detector with a sensitivity smaller than 10^{-4} e / \sqrt{Hz} and bandwidth larger than 1 MHz.
07
Jun
2013
Superconducting Circuits for Quantum Simulation of Dynamical Gauge Fields
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.
30
Mai
2013
Input-output theory for waveguide QED with an ensemble of inhomogeneous atoms
We study the collective effects that emerge in waveguide quantum electrodynamics where several (artificial) atoms are coupled to a one-dimensional (1D) superconducting transmission
line. Since single microwave photons can travel without loss for a long distance along the line, real and virtual photons emitted by one atom can be reabsorbed or scattered by a second atom. Depending on the distance between the atoms, this collective effect can lead to super- and subradiance or to a coherent exchange-type interaction between the atoms. Changing the artificial atoms transition frequencies, something which can be easily done with superconducting qubits (two levels artificial atoms), is equivalent to changing the atom-atom separation and thereby opens the possibility to study the characteristics of these collective effects. To study this waveguide quantum electrodynamics system, we extend previous work and present an effective master equation valid for an ensemble of inhomogeneous atoms. Using input-output theory, we compute analytically and numerically the elastic and inelastic scattering and show how these quantities reveal information about collective effects. These theoretical results are compatible with recent experimental results using transmon qubits coupled to a superconducting one-dimensional transmission line [A.F. van Loo {\it et al.} (2013)].