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
25
Okt
2016
Josephson junction microwave modulators for qubit control
We demonstrate Josephson junction based double-balanced mixer and phase shifter circuits operating at 6-10 GHz, and integrate these components to implement both a monolithic amplitude/phase
vector modulator and a quadrature mixer. The devices are actuated by flux signals, dissipate no power on chip, exhibit input saturation powers in excess of 1 nW, and provide cryogenic microwave modulation solutions for integrated control of superconducting qubits.
24
Okt
2016
Mesoscopic fluctuations in biharmonically driven flux qubits
We investigate flux qubits driven by a biharmonic magnetic signal, with a phase lag that acts as an effective time reversal broken parameter. The driving induced transition rate between
the ground and the excited state of the flux qubit can be thought as an effective transmitance, profiting from a direct analogy between interference effects at avoided level crossings and scattering events in disordered electronic systems. For time scales prior to full relaxation but large compared to the decoherence time, this characteristic rate has been accessed experimentally and its sensitivity with both the phase lag and the dc flux detuning explored. In this way signatures of Universal Conductance Fluctuations-like effects have recently been analized in flux qubits and compared with a phenomenological model that only accounts for decoherence, as a classical noise. We here solve the full dynamics of the driven flux qubit in contact with a quantum bath employing the Floquet Markov Master equation. Within this formalism relaxation and decoherence rates result strongly dependent on both the phase lag and the dc flux detuning. Consequently, the associated pattern of fluctuations in the characteristic rates display important differences with those obtained within the mentioned phenomenological model. In particular we demonstrate the Weak Localization-like effect in the averages values of the relaxation rate. Our predictions can be tested for accessible, but longer time scales than the current experimental times.
20
Okt
2016
Robust quantum state transfer between two superconducting qubits via partial measurement
We develop a potentially practical proposal for robust quantum state transfer (QST) between two superconducting qubits coupled by a coplanar waveguide (CPW) resonator. We show that
the partial measurement could drastically enhance the fidelity even when the dissipation of qubits and CPW is considered. Unlike many other schemes for QST, our proposal does not require the couplings between the qubits and the CPW resonator to be strong. In fact, our method works much more efficiently in the weak coupling regime. The underlying mechanism is attributed to the probabilistic nature of partial measurement.
19
Okt
2016
Transferring arbitrary d-dimensional quantum states of a superconducting qudit in circuit QED
A qudit (d-level quantum systems) has a large Hilbert space and thus can be used to achieve many quantum information and communication tasks. Here, we propose a method to transfer arbitrary
d-dimensional quantum states (known or unknown) between two superconducting qudits coupled to a single cavity. The state transfer can be performed fast because of employing resonant interactions only. In addition, quantum states can be deterministically transferred without measurement. Numerical simulations show that high-fidelity transfer of quantum states between two superconducting transmon qudits (d≤5) is feasible with current circuit QED technology. This proposal is quite general and can be applied to accomplish the same task with various superconducting qudits, quantum dots, or natural atoms coupled to a cavity or resonator.
18
Okt
2016
Circuit Quantum Electrodynamics Architecture for Gate-Defined Quantum Dots in Silicon
We demonstrate a hybrid device architecture where the charge states in a double quantum dot (DQD) formed in a Si/SiGe heterostructure are read out using an on-chip superconducting microwave
cavity. A quality factor Q = 5,400 is achieved by selectively etching away regions of the quantum well and by reducing photon losses through low-pass filtering of the gate bias lines. Homodyne measurements of the cavity transmission reveal DQD charge stability diagrams. These measurements indicate that electrons trapped in a Si DQD can be effectively coupled to microwave photons, potentially enabling coherent electron-photon interactions in silicon.
14
Okt
2016
Quantum entanglement for two qubits in a nonstationary cavity
The quantum entanglement and the probability of the dynamical Lamb effect for two qubits caused by non-adiabatic fast change of the boundary conditions are studied. The conditional
concurrence of the qubits for each fixed number of created photons in a nonstationary cavity is obtained as a measure of the dynamical quantum entanglement due to the dynamical Lamb effect. We discuss the physical realization of the dynamical Lamb effect, based on superconducting qubits.
11
Okt
2016
Introduction to Quantum Electromagnetic Circuits
The article is a short opinionated review of the quantum treatment of electromagnetic circuits, with no pretension to exhaustiveness. This review, which is an updated and modernized
version of a previous set of Les Houches School lecture notes, has 3 main parts. The first part describes how to construct a Hamiltonian for a general circuit, which can include dissipative elements. The second part describes the quantization of the circuit, with an emphasis on the quantum treatment of dissipation. The final part focuses on the Josephson non-linear element and the main linear building blocks from which superconducting circuits are assembled. It also includes a brief review of the main types of superconducting artificial atoms, elementary multi-level quantum systems made from basic circuit elements.
Magnetic resonance with squeezed microwaves
Although vacuum fluctuations appear to represent a fundamental limit to the sensitivity of electromagnetic field measurements, it is possible to overcome them by using so-called squeezed
states. In such states, the noise in one field quadrature is reduced below the vacuum level while the other quadrature becomes correspondingly more noisy, as required by Heisenberg’s uncertainty principle. Squeezed optical fields have been proposed and demonstrated to enhance the sensitivity of interferometric measurements beyond the photon shot-noise limit, with applications in gravitational wave detection. They have also been used to increase the sensitivity of atomic absorption spectroscopy, imaging, atom-based magnetometry, and particle tracking in biological systems. At microwave frequencies, cryogenic temperatures are required for the electromagnetic field to be in its vacuum state. Squeezed microwaves have been produced, used for fundamental studies of light-matter interaction and for enhanced sensing of a mechanical resonator, and proposed to enhance the sensitivity of the readout of superconducting qubits. Here we report the use of squeezed microwave fields to enhance the sensitivity of magnetic resonance spectroscopy of an ensemble of electronic spins. Our scheme consists in sending a squeezed vacuum state to the input of a cavity containing the spins while they are emitting an echo, with the phase of the squeezed quadrature aligned with the phase of the echo. We demonstrate a total noise reduction of 1.2\,dB at the spectrometer output due to the squeezing. These results provide a motivation to examine the application of the full arsenal of quantum metrology to magnetic resonance detection.
10
Okt
2016
Observing Topological Invariants Using Quantum Walk in Superconducting Circuits
The direct measurement of topological invariants in both engineered and naturally occurring quantum materials is a key step in classifying quantum phases of matter. Here we motivate
a toolbox based on time-dependent quantum walks as a method to digitally simulate single-particle topological band structures. Using a superconducting qubit dispersively coupled to a microwave cavity, we implement two classes of split-step quantum walks and directly measure the topological invariant (winding number) associated with each. The measurement relies upon interference between two components of a cavity Schr\“odinger cat state and highlights a novel refocusing technique which allows for the direct implementation of a digital version of Bloch oscillations. Our scheme can readily be extended to higher dimensions, whereby quantum walk-based simulations can probe topological phases ranging from the quantum spin Hall effect to the Hopf insulator.
Otto refrigerator based on a superconducting qubit – classical and quantum performance
We analyse a quantum Otto refrigerator based on a superconducting qubit coupled to two LC-resonators each including a resistor acting as a reservoir. We find various operation regimes:
nearly adiabatic (low driving frequency), ideal Otto cycle (intermediate frequency), and non-adiabatic coherent regime (high frequency). In the nearly adiabatic regime, the cooling power is quadratic in frequency, and we find substantially enhanced coefficient of performance ϵ, as compared to that of an ideal Otto cycle. Quantum coherent effects lead invariably to decrease in both cooling power and ϵ as compared to purely classical dynamics. In the non-adiabatic regime we observe strong coherent oscillations of the cooling power as a function of frequency. We investigate various driving waveforms: compared to the standard sinusoidal drive, truncated trapezoidal drive with optimized rise and dwell times yields higher cooling power and efficiency.