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
21
Jan
2016
A Schrodinger Cat Living in Two Boxes
Quantum superpositions of distinct coherent states in a single-mode harmonic oscillator, known as „cat states“, have been an elegant demonstration of Schrodinger’s
famous cat paradox. Here, we realize a two-mode cat state of electromagnetic fields in two microwave cavities bridged by a superconducting artificial atom, which can also be viewed as an entangled pair of single-cavity cat states. We present full quantum state tomography of this complex cat state over a Hilbert space exceeding 100 dimensions via quantum non-demolition measurements of the joint photon number parity. The ability to manipulate such multi-cavity quantum states paves the way for logical operations between redundantly encoded qubits for fault-tolerant quantum computation and communication.
Single microwave-photon detector using an artificial Λ-type three-level system
Single photon detection is a requisite technique in quantum-optics experiments in both the optical and the microwave domains. However, the energy of microwave quanta are four to five
orders of magnitude less than their optical counterpart, making the efficient detection of single microwave photons extremely challenging. Here, we demonstrate the detection of a single microwave photon propagating through a waveguide. The detector is implemented with an „impedance-matched“ artificial Λ system comprising the dressed states of a driven superconducting qubit coupled to a microwave resonator. We attain a single-photon detection efficiency of 0.66±0.06 with a reset time of ∼400~ns. This detector can be exploited for various applications in quantum sensing, quantum communication and quantum information processing.
20
Jan
2016
Unexpectedly allowed transition in two inductively coupled transmons
We present experimental results in which the unexpected zero-two transition of a circuit composed of two inductively coupled transmons is observed. This transition shows an unusual
magnetic flux dependence with a clear disappearance at zero magnetic flux. In a transmon qubit the symmetry of the wave functions prevents this transition to occur due to selection rule. In our circuit the Josephson effect introduces strong couplings between the two normal modes of the artificial atom. This leads to a coherent superposition of states from the two modes enabling such transitions to occur.
Polariton states in circuit QED for electromagnetically induced transparency
Electromagnetically induced transparency (EIT) has been extensively studied in various systems. However, it is not easy to observe in superconducting quantum circuits (SQCs), because
the Rabi frequency of the strong controlling field corresponding to EIT is limited by the decay rates of the SQCs. Here, we show that EIT can be achieved by engineering decay rates in a superconducting circuit QED system through a classical driving field on the qubit. Without such a driving field, the superconducting qubit and the cavity field are approximately decoupled in the large detuning regime, and thus the eigenstates of the system are approximately product states of the cavity field and qubit states. However, the driving field can strongly mix these product states and so-called polariton states can be formed. The weights of the states for the qubit and cavity field in the polariton states can be tuned by the driving field, and thus the decay rates of the polariton states can be changed. We choose a three-level system with Λ-type transitions in such a driven circuit QED system, and demonstrate how EIT and ATS can be realized in this compound system. We believe that this study will be helpful for EIT experiments using SQCs.
19
Jan
2016
Fock-state stabilization and emission in superconducting circuits using dc-biased Josephson junctions
In this article we propose a protocol to autonomously stabilize a Fock state within a circuit QED architecture that doesn’t recourse to any ac-drive nor pulse. This protocol is
realized with a microwave cavity driven by a Josephson junction and coupled to quantum engineered reservoir. It relies on two important ingredients. First, a high-impedance resonator to enable photon-dependent driving used as the cavity. Such device has recently been implemented in circuit QED platforms and removes the need for periodic driving. Second, on an auxiliary, low-quality factor cavity used as a engineered environment. Its damping rate must be scaled with respect to the Josephson energy and the main cavity quality factor in order to stabilize a quantum state. A second protocol, based on the same platform, is used to produce a flying qubit.
14
Jan
2016
A strict experimental test of macroscopic realism in a superconducting flux qubit
Macroscopic realism is the name for a class of modifications to quantum theory that allow macroscopic objects to be described in a measurement-independent fashion, while largely preserving
a fully quantum mechanical description of the microscopic world. Objective collapse theories are examples which attempt to provide a physical mechanism for wavefunction collapse, and thereby aim to solve the quantum measurement problem. Here we describe and implement an experimental protocol capable of constraining theories of this class, and show how it is related to the original Leggett-Garg test, yet more noise tolerant and conceptually transparent. By conducting a set of simple ‚prepare, shuffle, measure‘ tests in a superconducting flux qubit, we rule out (by over 77 standard deviations) those theories which would deny coherent superpositions of 170 nA currents over a 9 ns timescale. Further, we address the ‚clumsiness loophole‘ by determining classical disturbance in a set of control experiments.
13
Jan
2016
High-fidelity resonator-induced phase gate with single-mode squeezing
We propose to increase the fidelity of two-qubit resonator-induced phase gates in circuit QED by the use of narrowband single-mode squeezed drive. We show that there exists an optimal
squeezing angle and strength that erases qubit ‚which-path‘ information leaking out of the cavity and thereby minimizes qubit dephasing during these gates. Our analytical results for the gate fidelity are in excellent agreement with numerical simulations of a cascaded master equation that takes into account the dynamics of the source of squeezed radiation. With realistic parameters, we find that it is possible to realize a controlled-phase gate with a gate time of 200 ns and average infidelity of 10−5.
28
Dez
2015
Hidden correlations in indivisible qudits as a resource for quantum technologies on examples of superconducting circuits
We show that the density-matrix states of noncomposite qudit systems satisfy entropic and information relations like the subadditivity condition, strong subadditivity condition, and
Araki–Lieb inequality, which characterize hidden quantum correlations of observables associated with these indivisible systems. We derive these relations employing a specific map of the entropic inequalities known for density matrices of multiqudit systems to the inequalities for density matrices of single-qudit systems. We present the obtained relations in the form of mathematical inequalities for arbitrary Hermitian NxN-matrices. We consider examples of superconducting qubits and qudits. We discuss the hidden correlations in single-qudit states as a new resource for quantum technologies analogous to the known resource in correlations associated with the entanglement in multiqudit systems.
An Atomtronic Flux Qubit: A ring lattice of Bose-Einstein condensates interrupted by three weak links
We study a physical system consisting of a Bose-Einstein condensate confined to a ring shaped lattice potential interrupted by three weak links. The system is assumed to be driven by
an effective flux piercing the ring lattice. By employing path integral techniques, we explore the effective quantum dynamics of the system in a pure quantum phase dynamics regime. Complementarily, the effects of the density’s quantum fluctuations are studied through exact diagonalization analysis of the spectroscopy of the Bose-Hubbard model. We demonstrate that a clear two-level system emerges by tuning the magnetic flux at degeneracy. The lattice confinement, platform for the condensate, is realized experimentally employing a spatial light modulator.
26
Dez
2015
Phase- and frequency-controlled interference nonlinear optics in superconducting circuits
We present a new type of phase- and frequency-sensitive amplification and attenuation in a cyclically driven three-level superconducting Josephson system. Different from the previous
linear theory of pure phase-sensitive amplification, a new physical mechanism−combined action of nonlinear wave mixing and wave interference−is developed and leads to not only amplification but also attenuation. This is referred to as interference nonlinear optics. Our results show the sudden output signal transition from large gain to deep suppression by tuning the relative phase and in this case the system can act as a phase-controlled amplitude modulator. We also show the continuous change from output enhancement to attenuation by adjusting the driving-field frequency and in this situation the system behaves as a frequency-controlled amplitude modulator. Our study opens up a new perspective for its widespread applications in quantum information science.