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
05
Apr
2012
Geometric phases in superconducting qubits beyond the two-level-approximation
Geometric phases, which accompany the evolution of a quantum system and
depend only on its trajectory in state space, are commonly studied in two-level
systems. Here, however, we study
the adiabatic geometric phase in a weakly
anharmonic and strongly driven multi-level system, realised as a
superconducting transmon-type circuit. We measure the contribution of the
second excited state to the two-level geometric phase and find good agreement
with theory treating higher energy levels perturbatively. By changing the
evolution time, we confirm the independence of the geometric phase of time and
explore the validity of the adiabatic approximation at the transition to the
non-adiabatic regime.
03
Apr
2012
Black-box superconducting circuit quantization
We present a semi-classical method for determining the effective low-energy
quantum Hamiltonian of weakly anharmonic superconducting circuits containing
mesoscopic Josephson junctions
coupled to electromagnetic environments made of
an arbitrary combination of distributed and lumped elements. A convenient
basis, capturing the multi-mode physics, is given by the quantized eigenmodes
of the linearized circuit and is fully determined by a classical linear
response function. The method is used to calculate numerically the low-energy
spectrum of a 3D-transmon system, and quantitative agreement with measurements
is found.
Generating Entangled Microwave Radiation Over Two Transmission Lines
Using a superconducting circuit, the Josephson mixer, we demonstrate the
first experimental realization of spatially separated two-mode squeezed states
of microwave light. Driven by
a pump tone, a first Josephson mixer generates,
out of quantum vacuum, a pair of entangled fields at different frequencies on
separate transmission lines. A second mixer, driven by a $pi$-phase shifted
copy of the first pump tone, recombines and disentangles the two fields. The
resulting output noise level is measured to be lower than for vacuum state at
the input of the second mixer, an unambiguous proof of entanglement. Moreover,
the output noise level provides a direct, quantitative measure of entanglement,
leading here to the demonstration of 6 Mebit.s$^{-1}$ (Mega entangled bits per
second) generated by the first mixer.
Improving the Quality Factor of Microwave Compact Resonators by Optimizing their Geometrical Parameters
Applications in quantum information processing and photon detectors are
stimulating a race to produce the highest possible quality factor on-chip
superconducting microwave resonators.
We have tested the surface-dominated loss
hypothesis by systematically studying the role of geometrical parameters on the
internal quality factors of compact resonators patterned in Nb on sapphire.
Their single-photon internal quality factors were found to increase with the
distance between capacitor fingers, the width of the capacitor fingers, and the
impedance of the resonator. Quality factors were improved from 210,000 to
500,000 at T = 200 mK. All of these results are consistent with our starting
hypothesis.
23
Mä
2012
Low-Disorder Microwave Cavity Lattices for Quantum Simulation with Photons
We assess experimentally the suitability of coupled transmission line
resonators for studies of quantum phase transitions of light. We have measured
devices with low photon hopping
rates t/2pi = 0.8MHz to quantify disorder in
individual cavity frequencies. The observed disorder is consistent with small
imperfections in fabrication. We studied the dependence of the disorder on
transmission line geometry and used our results to fabricate devices with
disorder less than two parts in 10^4. The normal mode spectrum of devices with
a high photon hopping rate t/2pi = 31MHz shows little effect of disorder,
rendering resonator arrays a good backbone for the study of condensed matter
physics with photons.
22
Mä
2012
Identifying capacitive and inductive loss in lumped element superconducting hybrid titanium nitride/aluminum resonators
We present a method to systematically locate and extract capacitive and
inductive losses in superconducting resonators at microwave frequencies by use
of mixed-material, lumped element
devices. In these devices, ultra-low loss
titanium nitride was progressively replaced with aluminum in the
inter-digitated capacitor and meandered inductor elements. By measuring the
power dependent loss at 50 mK as the Al-TiN fraction in each element is
increased, we find that at low electric field, i.e. in the single photon limit,
the loss is two level system in nature and is correlated with the amount of Al
capacitance rather than the Al inductance. In the high electric field limit,
the remaining loss is linearly related to the product of the Al area times its
inductance and is likely due to quasiparticles generated by stray radiation. At
elevated temperature, additional loss is correlated with the amount of Al in
the inductance, with a power independent TiN-Al interface loss term that
exponentially decreases as the temperature is reduced. The TiN-Al interface
loss is vanishingly small at the 50 mK base temperature.
20
Mä
2012
Superconducting Resonators with Parasitic Electromagnetic Environments
Parasitic electromagnetic fields are shown to strongly suppress the quality
(Q)-factor of superconducting coplanar waveguide resonators via non-local
dissipation in the macroscopic
environment. Numerical simulation and low
temperature measurements demonstrate how this parasitic loss can be reduced,
establishing a Lorentzian lineshape in the resonator frequency response and
yielding a loaded Q-factor of 2.4 x 10^5 for niobium devices on sapphire
substrates. In addition, we report the dependence of the Q and resonance
frequency shift Delta f_0 with input power and temperature in the limit where
loss from two-level systems in the dielectric dominate.
12
Mä
2012
Emission spectrum of the driven nonlinear oscillator
Motivated by recent „circuit QED“ experiments we investigate the noise
properties of coherently driven nonlinear resonators. By using Josephson
junctions in superconducting
circuits, strong nonlinearities can be engineered,
which lead to the appearance of pronounced effects already for a low number of
photons in the resonator. Based on a master equation approach we determine the
emission spectrum and observe for typical circuit QED parameters, in addition
to the primary Raman-type peaks, second-order peaks. These peaks describe
higher harmonics in the slow noise-induced fluctuations of the oscillation
amplitude of the resonator and provide a clear signature of the nonlinear
nature of the system.
08
Mä
2012
Nonequilibrium phases in hybrid arrays with flux qubits and NV centers
We propose a startling hybrid quantum architecture for simulating a
localization-delocalization transition. The concept is based on an array of
superconducting flux qubits which are
coupled to a diamond crystal containing
nitrogen-vacancy (NV) centers. The underlying description is a
Jaynes-Cummings-lattice in the strong-coupling regime. However, in contrast to
well-studied coupled cavity arrays the interaction between lattice sites is
mediated here by the qubit rather than by the oscillator degrees of freedom.
Nevertheless, we point out that a transition between a localized and a
delocalized phase occurs in this system as well. We demonstrate the possibility
of monitoring this transition in a non-equilibrium scenario, including
decoherence effects. The proposed scheme allows the monitoring of
localization-delocalization transitions in Jaynes-Cummings-lattices by use of
currently available experimental technology. Contrary to cavity-coupled
lattices, our proposed recourse to stylized qubit networks facilitates (i) to
investigate localization-delocalization transitions in arbitrary dimensions and
(ii) to tune the inter-site coupling in-situ.
06
Mä
2012
Dynamical decoupling of superconducting qubits
We show that two superconducting qubits interacting via a fixed transversal
coupling can be decoupled by appropriately-designed microwave feld excitations
applied to each qubit. This
technique is useful for removing the effects of
spurious interactions in a quantum processor. We also simulate the case of a
qubit coupled to a two-level system (TLS) present in the insulating layer of
the Josephson junction of the qubit. Finally, we discuss the qubit-TLS problem
in the context of dispersive measurements, where the qubit is coupled to a
resonator.