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
28
Jun
2017
Optimal configurations for normal-metal traps in transmon qubits
Controlling quasiparticle dynamics can improve the performance of superconducting devices. For example, it has been demonstrated effective in increasing lifetime and stability of superconducting
qubits. Here we study how to optimize the placement of normal-metal traps in transmon-type qubits. When the trap size increases beyond a certain characteristic length, the details of the geometry and trap position, and even the number of traps, become important. We discuss for some experimentally relevant examples how to shorten the decay time of the excess quasiparticle density. Moreover, we show that a trap in the vicinity of a Josephson junction can reduce the steady-state quasiparticle density near that junction, thus suppressing the quasiparticle-induced relaxation rate of the qubit. Such a trap also reduces the impact of fluctuations in the generation rate of quasiparticles, rendering the qubit more stable.
Josephson parametric reflection amplifier with integrated directionality
A superconducting parametric amplifier in the GHz frequency range without a magnetic nonreciprocal circuit element is presented, suitable for circuit quantum electrodynamics (cQED)
and for ultra low-noise microwave detectors employed in astrophysics. It consists of an only one wavelength small nondegenerate Josephson parametric reflection amplifier with integrated directionality. The device has two Josephson junction oscillators, connected via a tailored impedance to an on-chip passive circuit which directs the in- to the output port. The amplifier provides a gain of 20~dB over a bandwidth of 440~MHz and the total photon shot noise at the input corresponds to maximally ∼1.3 photons per second per Hertz of bandwidth. We predict a factor of four increase in dynamic range compared to conventional Josephson parametric amplifiers.
27
Jun
2017
Manipulating heat flow in quantum devices
There has been significant interest recently in using complex quantum systems to create effective nonreciprocal dynamics. Theoretical proposals have been put forward for the realization
of artificial magnetic fields for photons and even phonons; experimental progress is fast making these proposals a reality. Much work has concentrated on the use of such systems for controlling the flow of signals, e.g., to create isolators or directional amplifiers for optical signals. In this paper, we build on this work but move in a different direction. We develop the theory and discuss a potential realization for the controllable flow of heat in quantum systems. We demonstrate theoretically that the observation of unidirectional flow of heat is possible within quantum cascaded systems. Viewing an optomechanical cavity platform as a cascaded system we show here that one can ultimately control the direction of the heat flow. By appropriately engineering the mechanical resonator, which acts as an artificial reservoir, heat flow can be constrained to a desired direction, yielding a thermal rectifier. The proposed quantum heat rectifier could potentially be used to develop devices such as a thermal modulator, a thermal router, and a thermal amplifier for nanoelectronic devices and superconducting circuits.
26
Jun
2017
Incoherent qubit control using the quantum Zeno effect
The quantum Zeno effect is the suppression of Hamiltonian evolution by repeated observation, resulting in the pinning of the state to an eigenstate of the measurement observable. Using
measurement only, control of the state can be achieved if the observable is slowly varied such that the state tracks the now time-dependent eigenstate. We demonstrate this using a circuit-QED readout technique that couples to a dynamically controllable observable of a qubit. Continuous monitoring of the measurement record allows us to detect an escape from the eigenstate, thus serving as a built-in form of error detection. We show this by post-selecting on realizations with arbitrarily high fidelity with respect to the target state. Our dynamical measurement operator technique offers a new tool for numerous forms of quantum feedback protocols, including adaptive measurements and rapid state purification.
25
Jun
2017
Circuit QED with qutrit: coupling three or more atoms via virtual photon exchange
We present a model to describe a generic circuit QED system which consists of multiple artificial three-level atoms, namely qutrits, strongly coupled to a cavity mode. When the state
transition of the atoms disobey the selection rules the process that does not conserve the number of excitations can happen determinatively. Therefore, we can realize coherent exchange interaction among three or more atoms mediated by the exchange of virtual photons. In addition, we generalize the one cavity mode mediated interactions to the multi-cavity situation, providing a method to entangle atoms located in different cavities. Using experimental feasible parameters, we investigate the dynamics of the model including three cyclic-transition three-level atoms, for which the two lowest-energy levels can be treated as qubits. Hence, we have found that two qubits can jointly exchange excitation with one qubit in a coherent and reversible way. In the whole process, the population in the third level of atoms is negligible and the cavity photon number is far smaller than 1. Our model provides a feasible scheme to couple multiple distant atoms together, which may find applications in quantum information processing.
21
Jun
2017
Universal controlled-phase gate with cat-state qubits in circuit QED
Cat-state qubits (qubits encoded with cat states) have recently drawn intensive attention due to their long lifetimes. We here propose a method to implement a universal controlled-phase
gate of two cat-state qubits, via two microwave resonators coupled to a superconducting transmon qutrit. During the gate operation, the qutrit remains in the ground state; thus decoherence from the qutrit is greatly suppressed. This proposal requires only two basic operations and neither classical pulse nor measurement is needed; therefore the gate realization is simple. Numerical simulations show that high-fidelity implementation of this gate is feasible with current circuit QED technology. The proposal is quite general and can be applied to implement the proposed gate with two microwave resonators or two optical cavities coupled to a single three-level natural or artificial atom.
20
Jun
2017
Demonstration of Universal Parametric Entangling Gates on a Multi-Qubit Lattice
We show that parametric coupling techniques can be used to generate selective entangling interactions for multi-qubit processors. By inducing coherent population exchange between adjacent
qubits under frequency modulation, we implement a universal gateset for a linear array of four superconducting qubits. An average process fidelity of =93% is measured by benchmarking three two-qubit gates with quantum process tomography. In order to test the suitability of these techniques for larger computations, we prepare a six-qubit register in all possible bitstring permutations and monitor the performance of a two-qubit gate on another pair of qubits. Across all these experiments, an average fidelity of =91.6±2.6% is observed. These results thus offer a path to a scalable architecture with high selectivity and low crosstalk.
Analytical modeling of parametrically-modulated transmon qubits
Scaling up quantum machines requires developing appropriate models to understand and verify their complex quantum dynamics. We focus on superconducting quantum processors based on transmons
for which full numerical simulations are already challenging at the level of qubytes. It is thus highly desirable to develop accurate methods of modeling qubit networks that do not rely solely on numerical computations. Using systematic perturbation theory to large orders in the transmon regime, we derive precise analytic expressions of the transmon parameters. We apply our results to the case of parametrically-modulated transmons to study recently-implemented parametrically-activated entangling gates.
Parametrically-Activated Entangling Gates Using Transmon Qubits
We propose and implement a family of entangling qubit operations activated by radio-frequency flux pulses. By parametrically modulating the frequency of a tunable transmon, these operations
selectively actuate resonant exchange of excitations with a statically coupled, but otherwise off-resonant, neighboring transmon. This direct exchange of excitations between qubits obviates the need for mediator qubits or resonator modes, and it allows for the full utilization of all qubits in a scalable architecture. Moreover, we are able to activate three highly-selective resonances, corresponding to two different classes of entangling gates that enable universal quantum computation: an iSWAP and a controlled-Z rotation. This selectivity is enabled by resonance conditions that depend both on frequency and amplitude, and is helpful in avoiding frequency crowding in a scalable architecture. We report average process fidelities of F = 0.93 for a 135 ns iSWAP, and F = 0.92 for 175 ns and 270 ns controlled-Z operations.
Shaped, on-demand microwave single-photon generator
We demonstrate the full functionality of a circuit that generates single microwave photons on-demand, with a wavepacket that can be modulated with a near-arbitrary shape. We achieve
such a high tunability by coupling a superconducting qubit near the end of a semi-infinite transmission line. A DC-SQUID shunts the line to ground and is employed to modify the spatial dependence of the electromagnetic mode structure in the transmission line. This allows us to couple and decouple the qubit from the line, shaping its emission rate on fast-time scales. Our decoupling scheme is applicable to all types of superconducting qubits and other solid state systems and can be generalized to multiple qubits as well as to resonators.