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
Okt
2012
Hopf bifurcation in a flux qubit coupled to a nanomechanical oscillator
We study the nonlinear semiclassical dynamics of a driven flux qubit coupled
to a nanomechanical oscillating beam. While both of these systems are
dissipative, their mutual coupling
can effectively suppress the dissipation.
This can lead to a loss of stability and to an emergence of synchronized
self-excited oscillations of the system as a whole, at a time scale set by the
mechanical beam. In this article we argue this by obtaining a set of
semiclassical, nonlinear equations of motion for the two coupled subsystems,
and showing that this system undergoes a supercritical Hopf bifurcation as the
coupling is increased. We derive analytical expressions for the critical
coupling coefficient and the renormalized mechanical dissipation coefficient
and frequency, and give a complete characterization of the limit cycle behavior
when the qubit and the oscillator are near resonance.
16
Okt
2012
Microwave Quantum Optics with an Artificial Atom
We address the recent advances on microwave quantum optics with artificial
atoms. This field relies on the fact that the coupling between a
superconducting artificial atom and propagating
microwave photons in a 1D open
transmission line can be made strong enough to observe quantum coherent
effects, without using any cavity to confine the microwave photons. We
investigate the scattering properties in such a system with resonant coherent
microwaves. We observe the strong nonlinearity of the artificial atom and under
strong driving we observe the Mollow triplet. By applying two resonant tones,
we also observe the Autler-Townes splitting. By exploiting these effects, we
demonstrate two quantum devices at the single-photon level in the microwave
regime: the single-photon router and the photon-number filter. These devices
provide essential steps towards the realization of an on-chip quantum network.
Path Entanglement of Continuous-Variable Quantum Microwaves
Path entanglement constitutes an essential resource in quantum information
and communication protocols. Here, we demonstrate frequency-degenerate
entanglement between continuous-variable
quantum microwaves propagating along
two spatially separated paths. We combine a squeezed and a vacuum state using a
microwave beam splitter. Via correlation measurements, we detect and quantify
the path entanglement contained in the beam splitter output state. Our
experiments open the avenue to quantum teleportation, quantum communication, or
quantum radar with continuous variables at microwave frequencies.
15
Okt
2012
Gradiometric flux qubits with tunable gap
For gradiometric three-Josephson-junction flux qubits, we perform a
systematic study on the tuning of the minimal transition frequency, the
so-called qubit gap. By replacing one of
the qubit’s Josephson junctions by a
dc SQUID, the critical current of this SQUID and, in turn, the qubit gap can be
tuned in situ by a control flux threading the SQUID loop. We present
spectroscopic measurements demonstrating a well-defined controllability of the
qubit gap between zero and more than 10 GHz. In the future, this enables one to
tune the qubit into and out of resonance with other superconducting quantum
circuits, while operating the qubit at its symmetry point with optimal
dephasing properties. The experimental data agree very well with model
calculations based on the full qubit Hamiltonian. From a numerical fit, we
determine the Josephson coupling and the charging energies of the qubit
junctions. The derived values agree well with those measured for other
junctions fabricated on the same chip. We also demonstrate the biasing of
gradiometric flux qubits near the symmetry point by trapping an odd number of
flux quanta in the gradiometer loop. In this way, we study the effect of the
significant kinetic inductance, thereby obtaining valuable information for the
qubit design.
12
Okt
2012
Quantum Heating of a nonlinear resonator probed by a superconducting qubit
We measure the quantum fluctuations of a pumped nonlinear resonator, using a
superconducting artificial atom as an in-situ probe. The qubit excitation
spectrum gives access to the frequency
and temperature of the intracavity field
fluctuations. These are found to be in agreement with theoretical predictions;
in particular we experimentally observe the phenomenon of quantum heating.
08
Okt
2012
Scattering of coherent states on a single artificial atom
In this work we theoretically analyze a circuit QED design where propagating
quantum microwaves interact with a single artificial atom, a single Cooper pair
box. In particular, we derive
a master equation in the so-called transmon
regime, including coherent drives. Inspired by recent experiments, we then
apply the master equation to describe the dynamics in both a two-level and a
three-level approximation of the atom. In the two-level case, we also discuss
how to measure photon antibunching in the reflected field and how it is
affected by finite temperature and finite detection bandwidth.
Thermal emission in the ultrastrong coupling regime
We study thermal emission of a cavity quantum electrodynamic system in the
ultrastrong-coupling regime where the atom-cavity coupling rate becomes
comparable the cavity resonance
frequency. In this regime, the standard
descriptions of photodetection and dissipation fail. Following an approach that
was recently put forward by Ridolfo et al.[arXiv:1206.0944], we are able to
calculate the emission of systems with arbitrary strength of light matter
interaction, by expressing the electric field operator in the cavity-emitter
dressed basis. Here we present thermal photoluminescence spectra, calculated
for given temperatures and for different couplings in particular for available
circuit QED parameters.
04
Okt
2012
Coherent Josephson phase qubit with a single crystal silicon capacitor
We have incorporated a single crystal silicon shunt capacitor into a
Josephson phase qubit. The capacitor is derived from a commercial
silicon-on-insulator wafer. Bosch reactive ion
etching is used to create a
suspended silicon membrane; subsequent metallization on both sides is used to
form the capacitor. The superior dielectric loss of the crystalline silicon
leads to a significant increase in qubit energy relaxation times. T1 times up
to 1.6 micro-second were measured, more than a factor of two greater than those
seen in amorphous phase qubits. The design is readily scalable to larger
integrated circuits incorporating multiple qubits and resonators.
03
Okt
2012
Sub-μm Josephson Junctions for Superconducting Quantum Devices
For high-performance superconducting quantum devices based on Josephson
junctions (JJs) decreasing lateral sizes is of great importance. Fabrication of
sub-mu m JJs is challenging due
to non-flat surfaces with step heights of up
to several 100 nm generated during the fabrication process. We have refined a
fabrication process with significantly decreased film thicknesses, resulting in
almost flat surfaces at intermediate steps during the JJ definition. In
combination with a mix-&-match process, combining electron-beam lithography
(EBL) and conventional photolithography, we can fabricate JJs with lateral
dimensions down to 0.023 mu m^2. We propose this refined process as an
alternative to the commonly used chemical-mechanical polishing (CMP) procedure.
We present transport measurements of JJs at 4.2 K that yield critical-current
densities in the range from 50 to 10^4 A/cm^2. Our JJ process yields excellent
quality parameters, Rsg/Rn up to ~50 and Vgap up to 2.81 mV, and also allows
the fabrication of high-quality sub-mu m wide long JJs (LJJs) for the study of
Josephson vortex behavior. The developed technique can also be used for similar
multilayer processes and is very promising for fabricating sub-mu m JJs for
quantum devices such as SQUIDs, qubits and SIS mixers.
Breakdown of the cross-Kerr scheme for Photon Counting
We show, in the context of single photon detection, that an atomic
three-level model for a transmon in a transmission line does not support the
predictions of the nonlinear polarisability
model known as the cross-Kerr
effect. We show that the induced displacement of a probe in the presence or
absence of a single photon in the signal field, cannot be resolved above the
quantum noise in the probe. This strongly suggests that cross-Kerr media are
not suitable for photon counting or related single photon applications. Our
results are presented in the context of a transmon in a one dimensional
microwave waveguide, but the conclusions also apply to optical systems.