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
30
Dez
2016
Towards Measuring Vacuum Polarization of Quantum Electrodynamics with Superconducting Junctions
In this proposal, we present an experimental setup based on superconducting circuits and Josephson junctions to explore the modification of Josephson coefficient in the presence of
external magnetic field due to vacuum polarization of quantum electrodynamics. This robust experiment can be considered as one of the few possible chances to observe the fine quantum field theory corrections in the low energy regimes in condensed matter systems. It can also be a new check for the universality of Josephson constant which is important in metrology. We will expect the signal to noise ratio of the read-out signal to increases quadratically by running time of the experiment. This characteristic of the output signal the will guarantee the feasibility of measurements with desired precision
Fast microwave-driven three-qubit gates for cavity-coupled superconducting qubits
Although single and two-qubit gates are sufficient for universal quantum computation, single-shot three-qubit gates greatly simplify quantum error correction schemes and algorithms.
We design fast, high-fidelity three-qubit entangling gates based on microwave pulses for transmon qubits coupled through a superconducting resonator. We show that when interqubit frequency differences are comparable to single-qubit anharmonicities, errors occur primarily through a single unwanted transition. This feature enables the design of fast three-qubit gates based on simple analytical pulse shapes that are engineered to minimize such errors. We show that a three-qubit ccz gate can be performed in 260 ns with fidelities exceeding 99.38%, or 99.99% with numerical optimization.
29
Dez
2016
Ultrastrong-coupling QPT phenomena in few-qubit circuit QED system
We consider the system of a few qubits interacting with a single mode resonator in superconducting circuit QED system. We get an effective low-energy Hamiltonian in the limit where
the ratio of the qubit frequency to the resonator frequency tends to infinity. In the one-qubit case, three secondorder superradiant phases occur and Goldstone mode emerges under the condition of ultrastrong coupling strength. Moreover, a first-order phase transition occurs between two different superradiant phases. In the two-qubit case, a two-qubit Hamiltonian with qubit-qubit interactions is analyzed fully quantum mechanically. We show that quantum phase transition is inhibited even in the ultrastrong coupling regime in this model. In addition, in the three qubits model, the superradiant quantum phase transition is retrieved in the ultrastrong coupling regime. Furthermore, the N-qubit model with U(1) symmetry is studied. We find that the superradiant phase transition is inhibited or restored with the qubit-number parity.
27
Dez
2016
Suppressing relaxation in superconducting qubits by quasiparticle pumping
Dynamical error suppression techniques are commonly used to improve coherence in quantum systems. They reduce dephasing errors by applying control pulses designed to reverse erroneous
coherent evolution driven by environmental noise. However, such methods cannot correct for irreversible processes such as energy relaxation. In this work, we investigate a complementary, stochastic approach to reducing errors: instead of deterministically reversing the unwanted qubit evolution, we use control pulses to shape the noise environment dynamically. In the context of superconducting qubits, we implement a pumping sequence to reduce the number of unpaired electrons (quasiparticles) in close proximity to the device. We report a 70% reduction in the quasiparticle density, resulting in a threefold enhancement in qubit relaxation times, and a comparable reduction in coherence variability.
24
Dez
2016
Scalable quantum circuit and control for a superconducting surface code
We present a scalable scheme for executing the error-correction cycle of a monolithic surface-code fabric composed of fast-flux-tuneable transmon qubits with nearest-neighbor coupling.
An eight-qubit unit cell forms the basis for repeating both the quantum hardware and coherent control, enabling spatial multiplexing. This control uses three fixed frequencies for all single-qubit gates and a unique frequency detuning pattern for each qubit in the cell. By pipelining the interaction and readout steps of ancilla-based X- and Z-type stabilizer measurements, we can engineer detuning patterns that avoid all second-order transmon-transmon interactions except those exploited in controlled-phase gates, regardless of fabric size. Our scheme is applicable to defect-based and planar logical qubits, including lattice surgery.
Nonreciprocal and reconfigurable microwave transmission using dissipative optomechanical pathways
Devices that achieve nonreciprocal microwave transmission are ubiquitous in radar and radio-frequency communication systems, and commonly rely on magnetically biased ferrite materials.
Such devices are also indispensable in the readout chains of superconducting quantum circuits as they protect sensitive quantum systems from the noise emitted by readout electronics. Since ferrite-based nonreciprocal devices are bulky, lossy, and re- quire large magnetic fields, there has been significant interest in magnetic-field-free on-chip alternatives, such as those recently implemented using Josephson junctions. Here we realize reconfigurable nonreciprocal transmission between two microwave modes using purely optomechanical interactions in a superconducting electromechanical circuit. The scheme relies on purposely breaking the symmetry between two mechanically-mediated dissipative coupling pathways. This enables reconfigurable nonreciprocal isolation on-chip without any external magnetic field, rendering it fully compatible with superconducting quantum circuits. All-optomechanically- mediated nonreciprocity demonstrated here can be extended to implement other types of devices such as directional amplifiers and circulators, and it forms the basis towards realizing topological states of light and sound.
23
Dez
2016
Characterizing the attenuation of coaxial and rectangular microwave-frequency waveguides at cryogenic temperatures
Low-loss waveguides are required for quantum communication at distances beyond the chip-scale for any low-temperature solid-state implementation of quantum information processors. We
measure and analyze the attenuation constant of commercially available microwave-frequency waveguides down to millikelvin temperatures and single photon levels. More specifically, we characterize the frequency-dependent loss of a range of coaxial and rectangular microwave waveguides down to 0.005dB/m using a resonant-cavity technique. We study the loss tangent and relative permittivity of commonly used dielectric waveguide materials by measurements of the internal quality factors and their comparison with established loss models. The results of our characterization are relevant for accurately predicting the signal levels at the input of cryogenic devices, for reducing the loss in any detection chain, and for estimating the heat load induced by signal dissipation in cryogenic systems.
21
Dez
2016
Multiplying microwave photons by inelastic Cooper-pair tunneling
The interaction between propagating microwave fields and Cooper-pair tunneling across a DC voltage-biased Josephson junction can be highly nonlinear. We show theoretically that this
nonlinearity can be used to convert an incoming single microwave photon into an outgoing n-photon Fock state in a different mode. In this process the Coulomb energy released by Cooper-pair tunneling is transferred to the outgoing Fock state, providing energy gain. The conversion can be made reflectionless (impedance-matched) so that all incoming photons are converted to n-photon states. With realistic parameters multiplication ratios n>2 can be reached. By cascading two to three such multiplication stages, the outgoing Fock-states can be sufficiently large to accurately discriminate them from vacuum with linear post-amplification and classical power measurement, implying that our scheme can be used as single-photon detector for itinerant microwave photons without dead time.
20
Dez
2016
Superluminal Physics with Superconducting Circuit Technology
We introduce a toolbox for the quantum simulation of superluminal motion with superconducting circuits. We show that it is possible to simulate the motion of a superconducting qubit
at constant velocities that exceed the speed of light in the electromagnetic medium and the subsequent emission of Ginzburg radiation. We consider as well possible setups for simulating the superluminal motion of a mirror, finding a link with the superradiant phase transition of the Dicke model.
16
Dez
2016
Quantum simulation of ultrastrongly coupled bosonic modes using superconducting circuits
The ground state of a pair of ultrastrongly coupled bosonic modes is predicted to be a two-mode squeezed vacuum. However, the corresponding quantum correlations are currently unobservable
in condensed matter where such a coupling can be reached, since it cannot be extracted from these systems. Here, we show that superconducting circuits can be used to perform an analog simulation of a system of two bosonic modes in regimes ranging from strong to ultrastrong coupling. More importantly, our quantum simulation set-up enables to detect output excitations that are related to the ground state properties of the bosonic modes. We compute the emission spectra of this physical system and show that the produced state presents single and two-mode squeezing simultaneously.