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
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.
19
Jun
2017
In situ characterization of qubit control lines: a qubit as a vector network analyzer
We present a technique to measure the transfer function of a control line using a qubit as a vector network analyzer. Our method requires coupling the line under test to the the longitudinal
component of the Hamiltonian of the qubit and the ability to induce Rabi oscillations through simultaneous driving of the transversal component. We used this technique to characterize the ‚flux‘ control of a superconducting Transmon qubit in the range of 8 to 400\,MHz. Our method can be used for the qubit ‚flux‘ line calibration to increase the fidelity of entangling gates for the quantum processor. The qubit can be also used as a microscopic probe of the electro-magnetic fields on a chip.
Scalable quantum computing model in the circuit-QED lattice with circulator function
We propose a model for a scalable quantum computing in the circuit-quantum electrodynamics(QED) architecture. In the Kagome lattice of qubits three qubits are connected to each other
through a superconducting three-junction flux qubit at the vertices of the lattice. By controlling one of the three Josephson junction energies of the intervening flux qubit we can achieve the circulator function that couples arbitrary pair of two qubits among three. This selective coupling enables the interaction between two nearest neighbor qubits in the Kagome lattice, and further the two-qubit gate operation between any pair of qubits in the whole lattice by performing consecutive nearest neighbor two-qubit gates.
18
Jun
2017
Achieving optimal quantum acceleration of frequency estimation using adaptive coherent control
Precision measurements of frequency are critical to accurate timekeeping, and are fundamentally limited by quantum measurement uncertainties. While for time-independent quantum Hamiltonians,
the uncertainty of any parameter scales at best as 1/T, where T is the duration of the experiment, recent theoretical works have predicted that explicitly time-dependent Hamiltonians can yield a 1/T2 scaling of the uncertainty for an oscillation frequency. This quantum acceleration in precision requires coherent control, which is generally adaptive. We experimentally realize this quantum improvement in frequency sensitivity with superconducting circuits, using a single transmon qubit. With optimal control pulses, the theoretically ideal frequency precision scaling is reached for times shorter than the decoherence time. This result demonstrates a fundamental quantum advantage for frequency estimation.