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
27
Feb
2017
Revealing the nonlinear response of a two-level system ensemble using coupled modes
Atomic sized two-level systems (TLSs) in dielectrics are known as a major source of loss in superconducting devices, particularly due to frequency noise. However, the induced frequency
shifts on the devices, even by far off-resonance TLSs, is often suppressed by symmetry when standard single-tone spectroscopy is used. We introduce a two-tone spectroscopy on the normal modes of a pair of coupled superconducting coplanar waveguide resonators to uncover this effect by asymmetric saturation. Together with an appropriate generalized saturation model this enables us to extract the average single-photon Rabi frequency of dominant TLSs to be Ω0/2π≈79 kHz. At high photon numbers we observe an enhanced sensitivity to nonlinear kinetic inductance when using the two-tone method and estimate the value of the Kerr coefficient as K/2π≈−1×10−4 Hz/photon. Furthermore, the life-time of each resonance can be controlled (increased) by pumping of the other mode as demonstrated both experimentally and theoretically.
A simplify proposal for realizing multiqubit tunable phase gate in circuit QED
We propose a scheme to realize multiqubit tunable phase gate in a circuit QED setup where two resonators each coupling with a qudit are interconnected to a common qudit (d = 4). In
this proposal, only two levels of each qudit serve as the logical states and other two levels are used for the gate realization. The proposal is efficient and simple because it requires only one step, no adjustments of the qudit level spacings and resonator mode frequency are needed during the whole process. Only a classical microwave pulse is needed to drive the central coupler qudit, no matter how many qudits involve, which significantly reduces experimental difficulty. In non-resonant case, the tunable phase gate can be achieved readily, while under the resonant condition a {\pi}-phase gate can be realized after a full cycle of Rabi oscillation where the gate speed is rather fast due to the resonant interaction. We have shown that the resulting effective dynamics allows for the creation of high fidelity phase gate. The influence of various decoherence processes such as the decay of the resonator mode, and the relaxation of the qudits is investigated. Moreover, the proposed scheme can be easily generalized to realize N-qubit phase gate.
26
Feb
2017
Correlators in simultaneous measurement of non-commuting qubit observables
We consider the simultaneous and continuous measurement of qubit observables σz and σzcosφ+σxsinφ, focusing on the temporal correlations of the two output signals. Using quantum
Bayesian theory, we derive analytical expressions for the correlators, which we find to be in very good agreement with experimentally measured output signals. We further discuss how the correlators can be applied to parameter estimation, and use them to infer a small residual qubit Hamiltonian arising from calibration inaccuracy in the experimental data.
25
Feb
2017
Quantum eigenstate tomography with qubit tunneling spectroscopy
Measurement of the energy eigenvalues (spectrum) of a multi-qubit system has recently become possible by qubit tunneling spectroscopy (QTS). In the standard QTS experiments, an incoherent
probe qubit is strongly coupled to one of the qubits of the system in such a way that its incoherent tunneling rate provides information about the energy eigenvalues of the original (source) system. In this paper, we generalize QTS by coupling the probe qubit to many source qubits. We show that by properly choosing the couplings, one can perform projective measurements of the source system energy eigenstates in an arbitrary basis, thus performing quantum eigenstate tomography. As a practical example of a limited tomography, we apply our scheme to probe the eigenstates of a kink in a frustrated transverse Ising chain.
20
Feb
2017
Nonclassical photon number distribution in a superconducting cavity under a squeezed drive
A superconducting qubit in the strong dispersive regime of a circuit quantum electrodynamics system is a powerful probe for microwave photons in a cavity mode. In this regime, a qubit
spectrum is split into multiple peaks, with each peak corresponding to an individual photon number in the cavity (discrete ac Stark shift). Here, we measure the qubit spectrum in the cavity that is driven continuously with a squeezed vacuum field generated by a Josephson parametric amplifier. By fitting the qubit spectrum with a model which takes into account the finite qubit excitation power, the photon number distribution, which is dissimilar from the apparent peak area ratio in the spectrum, is determined. The photon number distribution shows the even-odd photon number oscillation and quantitatively fulfills Klyshko’s criterion for the nonclassicality.
16
Feb
2017
Observing a quantum Maxwell demon at work
In apparent contradiction to the laws of thermodynamics, Maxwell’s demon is able to cyclically extract work from a system in contact with a thermal bath exploiting the information
about its microstate. The resolution of this paradox required the insight that an intimate relationship exists between information and thermodynamics. Here, we realize a Maxwell demon experiment that tracks the state of each constituent both in the classical and quantum regimes. The demon is a microwave cavity that encodes quantum information about a superconducting qubit and converts information into work by powering up a propagating microwave pulse by stimulated emission. Thanks to the high level of control of superconducting circuits, we directly measure the extracted work and quantify the entropy remaining in the demon’s memory. This experiment provides an enlightening illustration of the interplay of thermodynamics with quantum information.
15
Feb
2017
Casimir forces in transmission-line circuits: QED and fluctuation-dissipation formalisms
Transmission-line waveguides can mediate long-range fluctuation-induced forces between neutral objects. We present two approaches for the description of these forces between electric
components embedded in transmission-line circuits. The first, following ordinary quantum electrodynamics (QED), consists of the quantization and scattering theory of voltage and current waves inside transmission lines. The second approach relies on a simple circuit analysis with additional noisy current sources due to resistors in the circuit, as per the fluctuation-dissipation theorem (FDT). We apply the latter approach to derive a general formula for the Casimir force induced by circuit fluctuations between any two impedances. The application of this formula, considering the sign of the resulting force, is discussed. While both QED and FDT approaches are equivalent, we conclude that the latter is simpler to generalize and solve.
08
Feb
2017
Tunable Superconducting Qubits with Flux-Independent Coherence
We have studied the impact of low-frequency magnetic flux noise upon superconducting transmon qubits with various levels of tunability. We find that qubits with weaker tunability exhibit
dephasing that is less sensitive to flux noise. This insight was used to fabricate qubits where dephasing due to flux noise was suppressed below other dephasing sources, leading to flux-independent dephasing times T2* ~ 15 us over a tunable range of ~340 MHz. Such tunable qubits have the potential to create high-fidelity, fault-tolerant qubit gates and fundamentally improve scalability for a quantum processor.
07
Feb
2017
Prospective two orders of magnitude enhancement in direct magnetic coupling of a single-atom spin to a circuit resonator
We report on the challenges and limitations of direct coupling of the magnetic field from a circuit resonator to an electron spin bound to a donor potential. We propose a device consisting
of a lumped-element superconducting resonator and a single donor implanted in enriched 28Si. The resonator, in contrast to coplanar waveguide resonators, includes a nano-scale spiral inductor to spatially focus the magnetic field from the photons within. The design promises approximately two orders of magnitude increase in the local magnetic field, and thus the spin to photon coupling rate g, compared to the estimated coupling rate to coplanar transmission-line resonators. We show that by using niobium (aluminum) as the resonator’s superconductor and a single phosphorous (bismuth) atom as the donor, a coupling rate of g/2π=0.24 MHz (0.39 MHz) can be achieved in the small photon limit. For this truly linear cavity quantum electrodynamic system, such enhancement in g is sufficient to enter the strong coupling regime.
06
Feb
2017
Realizing and manipulating space-time inversion symmetric topological semimetal bands with superconducting quantum circuits
We have experimentally realized novel space-time inversion (P-T) invariant Z2-type topological semimetal-bands, via an analogy between the momentum space and a controllable parameter
space in superconducting quantum circuits. By measuring the whole energy spectrum of system, we imaged clearly an exotic tunable gapless band structure of topological semimetals. Two topological quantum phase transitions from a topological semimetal to two kinds of insulators can be manipulated by continuously tuning the different parameters in the experimental setup, one of which captures the Z2 topology of the PT semimetal via merging a pair of nontrivial Z2 Dirac points. Remarkably, the topological robustness was demonstrated unambiguously, by adding a perturbation that breaks only the individual T and P symmetries but keeps the joint PT symmetry. In contrast, when another kind of PT -violated perturbation is introduced, a topologically trivial insulator gap is fully opened.