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
Mai
2012
Electron spin resonance detected by a superconducting qubit
A new method for detecting the magnetic resonance of electronic spins at low
temperature is demonstrated. It consists in measuring the signal emitted by the
spins with a superconducting
qubit that acts as a single-microwave-photon
detector, resulting in an enhanced sensitivity. We implement this new type of
electron-spin resonance spectroscopy using a hybrid quantum circuit in which a
transmon qubit is coupled to a spin ensemble consisting of NV centers in
diamond. With this setup we measure the NV center absorption spectrum at 30mK
at an excitation level of thicksim15,mu_{B} out of an ensemble of 10^{11}
spins.
24
Mai
2012
Quantum feedback control of a superconducting qubit: Persistent Rabi oscillations
The act of measurement bridges the quantum and classical worlds by projecting
a superposition of possible states into a single, albeit probabilistic,
outcome. The time-scale of this
„instantaneous“ process can be stretched using
weak measurements so that it takes the form of a gradual random walk towards a
final state. Remarkably, the interim measurement record is sufficient to
continuously track and steer the quantum state using feedback. We monitor the
dynamics of a resonantly driven quantum two-level system — a superconducting
quantum bit –using a near-noiseless parametric amplifier. The high-fidelity
measurement output is used to actively stabilize the phase of Rabi
oscillations, enabling them to persist indefinitely. This new functionality
shows promise for fighting decoherence and defines a path for continuous
quantum error correction.
22
Mai
2012
Engineering two-mode entangled states between two superconducting resonators by dissipation
We present an experimental feasible scheme to synthesize two-mode
continuous-variable entangled states of two superconducting resonators that are
interconnected by two gap-tunable superconducting
qubits. We show that, with
each artificial atom suitably driven by a bichromatic microwave field to induce
sidebands in the qubit-resonator coupling, the stationary state of the photon
fields in the two resonators can be cooled and steered into a two-mode squeezed
vacuum state via a dissipative quantum dynamical process, while the
superconducting qubits remain in their ground states. In this scheme the qubit
decay plays a positive role and can help drive the system to the target state,
which thus converts a detrimental source of noise into a resource.
14
Mai
2012
Dressed-state amplification by a superconducting qubit
We demonstrate amplification of a microwave signal by a strongly driven
two-level system in a coplanar waveguide resonator. The effect known from
optics as dressed-state lasing is observed
with a single quantum system formed
by a persistent current (flux) qubit. The transmission through the resonator is
enhanced when the Rabi frequency of the driven qubit is tuned into resonance
with one of the resonator modes. Amplification as well as linewidth narrowing
of a weak probe signal has been observed. The laser emission at the resonator’s
fundamental mode has been studied by measuring the emission spectrum. We
analyzed our system and found an excellent agreement between the experimental
results and the theoretical predictions obtained in the dressed-state model.
10
Mai
2012
Non-Markovian qubit decoherence during dispersive readout
We study qubit decoherence under generalized dispersive readout, i.e., we
investigate a qubit coupled to a resonantly driven dissipative harmonic
oscillator. We provide a complete picture
by allowing for arbitrarily large
qubit-oscillator detuning and by considering also a coupling to the square of
the oscillator coordinate, which is relevant for flux qubits. Analytical
results for the decoherence time are obtained by a transformation of the
qubit-oscillator Hamiltonian to the dispersive frame and a subsequent master
equation treatment beyond the Markov limit. We predict a crossover from
Markovian decay to a decay with Gaussian shape. Our results are corroborated by
the numerical solution of the full qubit-oscillator master equation in the
original frame.
Readout of superconducting flux qubit state with a Cooper pair box
We study a readout scheme of superconducting flux qubit state with a Cooper
pair box as a transmon. The qubit states consist of the superpositions of two
degenerate states where the
charge and phase degrees of freedom are entangled.
Owing to the robustness of transmon against external fluctuations, our readout
scheme enables the quantum non-demolition and single-shot measurement of flux
qubit states. The qubit state readout can be performed by using the non-linear
Josephson amplifiers after a $pi/2$-rotation driven by an ac-electric field.
09
Mai
2012
Multi-qubit parity measurement in circuit quantum electrodynamics
We present a concept for performing direct parity measurements on three or more qubits in microwave structures with superconducting resonators coupled to Josephson-junction qubits.
We write the quantum-eraser conditions that must be fulfilled for the parity measurements as requirements for the scattering phase shift of our microwave structure. We show that these conditions can be fulfilled with present-day devices. We present one particular scheme, implemented with two-dimensional cavity techniques, in which each qubit should be coupled equally to two different microwave cavities. The magnitudes of the couplings that are needed are in the range that has been achieved in current experiments. A quantum calculation indicates that the measurement is optimal if the scattering signal can be measured with near single photon sensitivity. A comparison with an extension of a related proposal from cavity optics is presented. We present a second scheme, for which a scalable implementation of the four-qubit parities of the surface quantum error correction code can be envisioned. It uses three-dimensional cavity structures, using cavity symmetries to achieve the necessary multiple resonant modes within a single resonant structure.
08
Mai
2012
Simple quantum error detection and correction for superconducting qubits
We analyze simple quantum error detection and quantum error correction
protocols relevant to current experiments with superconducting qubits. We show
that for qubits with energy relaxation
the repetitive N-qubit codes cannot be
used for quantum error correction, but can be used for quantum error detection.
In the latter case it is sufficient to use only two qubits for the encoding. In
the analysis we demonstrate a useful technique of unraveling the qubit energy
relaxation into „relaxation“ and „no relaxation“ scenarios. Also, we propose
and numerically analyze several two-qubit algorithms for quantum error
detection/correction, which can be readily realized at the present-day level of
the phase qubit technology.
03
Mai
2012
Coherent control of microwave pulse storage in superconducting circuits
Coherent pulse control for quantum memory is viable in the optical domain but
nascent in microwave quantum circuits. We show how to realize coherent storage
and on-demand pulse retrieval
entirely within a superconducting circuit by
exploiting and extending existing electromagnetically induced transparency
technology in superconducting quantum circuits. Our scheme employs a linear
array of superconducting artificial atoms coupled to a microwave transmission
line.
Motional Averaging in a Superconducting Qubit
Superconducting circuits with Josephson junctions are promising candidates
for developing future quantum technologies. Of particular interest is to use
these circuits to study effects
that typically occur in complex
condensed-matter systems. Here, we employ a superconducting quantum bit
(qubit), a transmon, to carry out an analog simulation of motional averaging, a
phenomenon initially observed in nuclear magnetic resonance (NMR) spectroscopy.
To realize this effect, the flux bias of the transmon is modulated by a
controllable pseudo-random telegraph noise, resulting in stochastic jumping of
the energy separation between two discrete values. When the jumping is faster
than a dynamical threshold set by the frequency displacement of the levels, the
two separated spectral lines merge into a single narrow-width,
motional-averaged line. With sinusoidal modulation a complex pattern of
additional sidebands is observed. We demonstrate experimentally that the
modulated system remains quantum coherent, with modified transition
frequencies, Rabi couplings, and dephasing rates. These results represent the
first steps towards more advanced quantum simulations using artificial atoms.