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
01
Sep
2014
Prediction and retrodiction for a continuously monitored superconducting qubit
The quantum state of a superconducting transmon qubit inside a three-dimensional cavity is monitored by reflection of a microwave field on the cavity. The information inferred from
the measurement record is incorporated in a density matrix ρt, which is conditioned on probe results until t, and in an auxiliary matrix Et, which is conditioned on probe results obtained after t. Here, we obtain these matrices from experimental data and we illustrate their application to predict and retrodict the outcome of weak and strong qubit measurements.
28
Aug
2014
Entanglement dynamics in superconducting qubits affected by local bistable impurities
We study the entanglement dynamics for two independent superconducting qubits each affected by a bistable impurity generating random telegraph noise (RTN) at pure dephasing. The relevant
parameter is the ratio g between qubit-RTN coupling strength and RTN switching rate, that captures the physics of the crossover between Markovian and non-Markovian features of the dynamics. For identical qubit-RTN subsystems, a threshold value gth of the crossover parameter separates exponential decay and onset of revivals; different qualitative behaviors also show up by changing the initial conditions of the RTN. We moreover show that, for different qubit-RTN subsystems, when both qubits are very strongly coupled to the RTN an increase in entanglement revival amplitude may occur during the dynamics.
Microwave degenerate parametric down-conversion with a single cyclic three-level system in circuit QED
With the assistance of a single cyclic three-level system, which can be realized by a superconducting flux qubit, we study theoretically the degenerate microwave parametric down-conversion
(PDC) in the superconducting transmission line resonator with the fundamental and second harmonic modes involved. By adiabatically eliminating the excited states of the three-level system, we obtain an effective microwave PDC Hamiltonian for the two modes in the resonator. In our system, the PDC efficiency can be much larger than that in the case of two-level system interacting with two-mode transmission line resonator [K. Moon and S. M. Girvin, Phys. Rev. Lett. {\bf 95}, 140504 (2005)]. With the effective coupling between those two resonator modes, a coherent driving of the second harmonic mode can lead to the squeezing and bunching effect of the fundamental one.
26
Aug
2014
Optimal Control of Quantum Measurement
Pulses to steer the time evolution of quantum systems can be designed with optimal control theory. In most cases it is the coherent processes that can be controlled and one optimizes
the time evolution towards a target unitary process, sometimes also in the presence of non-controllable incoherent processes. Here we show how to extend the GRAPE algorithm in the case where the incoherent processes are controllable and the target time evolution is a non-unitary quantum channel. We perform a gradient search on a fidelity measure based on Choi matrices. We illustrate our algorithm by optimizing a phase qubit measurement pulse. We show how this technique can lead to large measurement contrast close to 99%. We also show, within the validity of our model, that this algorithm can produce short 1.4 ns pulses with 98.2% contrast.
Solid state optical interconnect between distant superconducting quantum chips
We propose a design for a quantum interface exploiting the electron spins in crystals to swap the quantum states between the optical and microwave. Using sideband driving of a superconducting
flux qubit and a combined cavity/solid-state spin ensemble Raman transition, we demonstrate how a stimulated Raman adiabatic passage (STIRAP)-type operation can swap the quantum state between a superconducting flux qubit and an optical cavity mode with a fidelity higher than 90%. We further consider two distant superconducting qubits with their respective interfaces joined by an optical fiber and show a quantum transfer fidelity exceeding 90% between the two distant qubits.
25
Aug
2014
A novel protection layer of superconducting microwave circuits toward a hybrid quantum system
We propose a novel multilayer structure based on Bragg layers that can protect a superconducting microwave resonator from photons and blackbody radiation and have little effect on its
quality factor. We also discuss a hybrid quantum system exploiting a superconducting microwave circuit and a two-color evanescent field atom trap, where surface-scattered photons and absorption-induced broadband blackbody radiation might deteriorate the system.
Atomic delocalisation as a microscopic origin of two-level defects in Josephson junctions
Identifying the microscopic origins of decoherence sources prevalent in Josephson junction based circuits is central to their use as functional quantum devices. Focussing on so called
„strongly coupled“ two-level defects, we construct a theoretical model using the atomic position of the oxygen which is spatially delocalised in the oxide forming the Josephson junction barrier. Using this model, we investigate which atomic configurations give rise to two-level behaviour of the type seen in experiments. We compute experimentally observable parameters for phase qubits and examine defect response under the effects of applied electric field and strain.
Inductance of Circuit Structures for MIT LL Superconductor Electronics Fabrication Process with 8 Niobium Layers
Inductance of superconducting thin-film inductors and structures with linewidth down to 250 nm has been experimentally evaluated. The inductors include various striplines and microstrips,
their 90-degree bends and meanders, interlayer vias, etc., typically used in superconducting digital circuits. The circuits have been fabricated by a fully planarized process with 8 niobium layers, developed at MIT Lincoln Laboratory for very large scale superconducting integrated circuits. Excellent run-to-run reproducibility and inductance uniformity of better than 1% across 200-mm wafers have been found. It has been found that the inductance per unit length of stripline and microstrip line inductors continues to grow as the inductor linewidth is reduced deep into the submicron range to the widths comparable to the film thickness and magnetic field penetration depth. It is shown that the linewidth reduction does not lead to widening of the parameter spread due to diminishing sensitivity of the inductance to the linewidth and dielectric thickness. The experimental results were compared with numeric inductance extraction using commercial software and freeware. Simulations using InductEx inductance calculator were found to give an excellent agreement with the experimental results. Methods of further miniaturization of circuit inductors for achieving circuit densities > 10^6 Josephson junctions per cm^2 are discussed.
11
Aug
2014
Quantum Phase Transition in A Multi-connected Superconducting Jaynes-Cummings Lattice
The connectivity and tunability of superconducting qubits and resonators provide us with an appealing platform to study the many-body physics of microwave excitations. Here we present
a multi-connected Jaynes-Cummings lattice model which is symmetric with respect to the qubit-resonator couplings. Our calculation shows that this model exhibits a Mott insulator-superfluid-Mott insulator phase transition, featured by a reentry to the Mott insulator phase, at commensurate filling. The phase diagrams in the grand canonical ensemble are also derived, which confirm the incompressibility of the Mott insulator phase. Different from a general-purposed quantum computer, it only requires two operations to demonstrate this phase transition: the preparation and the detection of the commensurate many-body ground state. We discuss the realization of these operations in the superconducting circuit
10
Aug
2014
Fast universal quantum gates on microwave photons with all-resonance operations in circuit QED
Quantum stark effect on superconducting qubits in circuit quantum electrodynamics (QED) has been used to construct universal quantum entangling gates on superconducting resonators in
previous works. It is a second-order coupling effect between the resonator and the qubit in the dispersive regime, which leads to a long-time state-selective rotation on the qubit. Here, we use the quantum resonance operations to construct the fast universal quantum gates on superconducting resonators in a microwave-photon quantum processor composed of some superconducting resonators coupled to a superconducting transmon, phase, or Xmon qutrit assisted by circuit QED in the dispersive regime, including the controlled-phase (c-phase) gate on two microwave-photon resonators and the controlled-controlled phase (cc-phase) gates on three microwave-photon resonators. Compared with previous works, our universal quantum gates have the higher fidelities and shorter operation times. The fidelity of our c-phase gate is 98.7% within the operation time of 40.1 ns and that of our cc-phase gate is 94.7% within 60 ns. Moreover, they do not require any drive field.