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
26
Sep
2014
Development of a broadband reflective T-filter for voltage biasing high-Q superconducting microwave cavities
We present the design of a reflective stop-band filter based on quasi-lumped elements that can be utilized to introduce large dc and low-frequency voltage biases into a low-loss superconducting
coplanar waveguide (CPW) cavity. Transmission measurements of the filter are seen to be in good agreement with simulations and demonstrate insertion losses greater than 20dB in the range of 3to10GHz. Moreover, transmission measurements of the CPW’s fundamental mode demonstrate that loaded quality factors exceeding 105 can be achieved with this design for dc voltages as large as 20V and for the cavity operated in the single-photon regime. This makes the design suitable for use in a number of applications including qubit-coupled mechanical systems and circuit QED.
Hybrid quantum circuit with implanted erbium ions
We report on hybrid circuit QED experiments with focused ion beam implanted Er3+ ions in Y2SiO5 coupled to an array of superconducting lumped element microwave resonators. The Y2SiO5
crystal is divided into several areas with distinct erbium doping concentrations, each coupled to a separate resonator. The coupling strength is varied from 5 MHz to 18.7 MHz, while the linewidth ranges between 50 MHz and 130 MHz. We confirm the paramagnetic properties of the implanted spin ensemble by evaluating the temperature dependence of the coupling. The efficiency of the implantation process is analyzed and the results are compared to a bulk doped Er:Y2SiO5 sample. We demonstrate the successful integration of these engineered erbium spin ensembles with superconducting circuits.
25
Sep
2014
Creation of Quantum Error Correcting Codes in the Ultrastrong Coupling Regime
We propose to construct large quantum graph codes by means of superconducting circuits working at the ultrastrong coupling regime. In this physical scenario, we are able to create a
cluster state between any pair of qubits within a fraction of a nanosecond. To exemplify our proposal, creation of the five-qubit and Steane codes are demonstrated. We also provide optimal operating conditions with which the graph codes can be realized with state-of-the-art superconducting technologies.
21
Sep
2014
Coherence and Decay of Higher Energy Levels of a Superconducting Transmon Qubit
We present measurements of coherence and successive decay dynamics of higher energy levels of a superconducting transmon qubit. By applying consecutive π-pulses for each sequential
transition frequency, we excite the qubit from the ground state up to its fourth excited level and characterize the decay and coherence of each state. We find the decay to proceed mainly sequentially, with relaxation times in excess of 20 μs for all transitions. We also provide a direct measurement of the charge dispersion of these levels by analyzing beating patterns in Ramsey fringes. The results demonstrate the feasibility of using higher levels in transmon qubits for encoding quantum information.
19
Sep
2014
Multiplexed Readout of Transmon Qubits with Josephson Bifurcation Amplifiers
Achieving individual qubit readout is a major challenge in the development of scalable superconducting quantum processors. We have implemented the multiplexed readout of a four transmon
qubit circuit using non-linear resonators operated as Josephson bifurcation amplifiers. We demonstrate the simultaneous measurement of Rabi oscillations of the four transmons. We find that multiplexed Josephson bifurcation is an high-fidelity readout method, the scalability of which is not limited by the need of a large bandwidth nearly quantum-limited amplifier as is the case with linear readout resonators.
High-gain weakly nonlinear flux-modulated Josephson parametric amplifier using a SQUID-array
We have developed and measured a high-gain quantum-limited microwave parametric amplifier based on a superconducting lumped LC resonator with the inductor L including an array of 8
superconducting quantum interference devices (SQUIDs). This amplifier is parametrically pumped by modulating the flux threading the SQUIDs at twice the resonator frequency. Around 5 GHz, a maximum gain of 31 dB, a product amplitude-gain x bandwidth above 60 MHz, and a 1 dB compression point of -123 dBm at 20 dB gain are obtained in the non-degenerate mode of operation. Phase sensitive amplification-deamplification is also measured in the degenerate mode and yields a maximum gain of 37 dB. The compression point obtained is 18 dB above what would be obtained with a single SQUID of the same inductance, due to the smaller nonlinearity of the SQUID array.
15
Sep
2014
Motion and gravity effects in the precision of quantum clocks
We show that motion and gravity affect the precision of quantum clocks. We consider a localised quantum field as a fundamental model of a quantum clock moving in spacetime and show
that its state is modified due to changes in acceleration. By computing the quantum Fisher information we determine how relativistic motion modifies the ultimate bound in the precision of the measurement of time. While in the absence of motion the squeezed vacuum is the ideal state for time estimation, we find that it is highly sensitive to the motion-induced degradation of the quantum Fisher information. We show that coherent states are generally more resilient to this degradation and that in the case of very low initial number of photons, the optimal precision can be even increased by motion. These results can be tested with current technology by using superconducting resonators with tunable boundary conditions.
12
Sep
2014
Reexamining classical and quantum models for the D-Wave One processor
We revisit the evidence for quantum annealing in the D-Wave One device (DW1) based on the study of random Ising instances. Using the probability distributions of finding the ground
states of such instances, previous work found agreement with both simulated quantum annealing (SQA) and a classical rotor model. Thus the DW1 ground state success probabilities are consistent with both models, and a different measure is needed to distinguish the data and the models. Here we consider measures that account for ground state degeneracy and the distributions of excited states, and present evidence that for these new measures neither SQA nor the classical rotor model correlate perfectly with the DW1 experiments. We thus provide evidence that SQA and the classical rotor model, both of which are classically efficient algorithms, do not satisfactorily explain all the DW1 data. A complete model for the DW1 remains an open problem. Using the same criteria we find that, on the other hand, SQA and the classical rotor model correlate closely with each other. To explain this we show that the rotor model can be derived as the semiclassical limit of the spin-coherent states path integral. We also find differences in which set of ground states is found by each method, though this feature is sensitive to calibration errors of the DW1 device and to simulation parameters.
10
Sep
2014
Copper waveguide cavities with reduced surface loss for coupling to superconducting qubits
Significant improvements in superconducting qubit coherence times have been achieved recently with three-dimensional microwave waveguide cavities coupled to transmon qubits. While many
of the measurements in this direction have utilized superconducting aluminum cavities, other recent work has involved qubits coupled to copper cavities with coherence times approaching 0.1 ms. The copper provides a good path for thermalizing the cavity walls and qubit chip, although the substantial cavity loss makes conventional dispersive qubit measurements challenging. We are exploring various approaches for improving the quality factor of three-dimensional copper cavities, including electropolishing and coating with superconducting layers of tin. We have characterized these cavities on multiple cooldowns and found the tin-plating to be robust. In addition, we have performed coherence measurements on transmon qubits in these cavities and observed promising performance.
03
Sep
2014
Nanometric constrictions in superconducting coplanar waveguide resonators
We report on the design, fabrication and characterization of superconducting coplanar waveguide resonators with nanometric constrictions. By reducing the size of the center line down
to 50 nm, the RF currents are concentrated into a small cross section and the magnetic field in its vicinity is increased. The device characteristics are only slightly modified by the constrictions, with changes in resonance frequency lower than 1% and changes in transmission and Q-factor lower than 20%. These devices could enable the achievement of higher couplings to small magnetic samples or even to single molecular spins and have applications in circuit quantum electrodynamics, quantum computing and electron paramagnetic resonance.