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
18
Feb
2013
Ten Milliseconds for Aluminum Cavities in the Quantum Regime
A promising quantum computing architecture couples superconducting qubits to
microwave resonators (circuit QED), a system in which three-dimensional
microwave cavities have become a
valuable resource. Such cavities have
surface-to-volume ratios, or participation ratios a thousandfold smaller than
in planar devices, deemphasizing potentially lossy surface elements by an equal
amount. Motivated by this principle, we have tested aluminum superconducting
cavity resonators with internal quality factors greater than 0.5 billion and
intrinsic lifetimes reaching 0.01 seconds at single photon power and
millikelvin temperatures. These results are the first to explore the use of
superconducting aluminum, a ubiquitous material in circuit QED, as the basis of
highly coherent (Q~10^7-10^9) cavity resonators. Measurements confirm the
cavities‘ predicted insensitivity to quasiparticles (kinetic inductance
fraction-5ppm) and an absence of two level dielectric fluctuations.
17
Feb
2013
Protected gates for superconducting qubits
We analyze the accuracy of quantum phase gates acting on „zero-pi qubits“ in
superconducting circuits, where the gates are protected against thermal and
Hamiltonian noise
by continuous-variable quantum error-correcting codes. The
gates are executed by turning on and off a tunable Josephson coupling between
an LC oscillator and a qubit or pair of quits; assuming perfect qubits, we show
that the gate errors are exponentially small when the oscillator’s impedance
sqrt{L/C} is large compared to hbar/4e^2 ~ 1 kilo-ohm. The protected gates are
not computationally universal by themselves, but a scheme for universal
fault-tolerant quantum computation can be constructed by combining them with
unprotected noisy operations. We validate our analytic arguments with numerical
simulations.
15
Feb
2013
Tunable electromagnetic environment for superconducting quantum bits
We introduce a setup which realises a tunable engineered environment for
experiments in circuit quantum electrodynamics. We illustrate this concept with
the specific example of a quantum
bit, qubit, in a high-quality-factor cavity
which is separated from a resistor in another cavity by a capacitor. The
temperature of the resistor can be controlled in a well defined manner in order
to provide a hot or cold environment for the qubit, as desired. Furthermore,
introducing superconducting quantum interference devices (SQUIDs) into the
resistor cavity provides control of the coupling strength between this
artificial environment and the qubit. We demonstrate that our scheme allows us
to couple strongly to the environment enabling rapid initialization of the
system, and by subsequent tuning of the magnetic flux of the SQUIDs we may
greatly reduce the resistor-qubit coupling, allowing the qubit to evolve
unhindered.
Ultrafast QND measurements based on diamond-shape artificial atom
We propose a Quantum Non Demolition (QND) read-out scheme for a
superconducting artificial atom coupled to a resonator in a circuit QED
architecture, for which we estimate a very high
measurement fidelity without
Purcell effect limitations. The device consists of two transmons coupled by a
large inductance, giving rise to a diamond-shape artificial atom with a logical
qubit and an ancilla qubit interacting through a cross-Kerr like term. The
ancilla is strongly coupled to a transmission line resonator. Depending on the
qubit state, the ancilla is resonantly or dispersively coupled to the
resonator, leading to a large contrast in the transmitted microwave signal
amplitude. This original method can be implemented with state of the art
Josephson parametric amplifier, leading to QND measurements in a few tens of
nanoseconds with fidelity as large as 99.9 %.
14
Feb
2013
Exploring the Effect of Noise on Geometric Phases using Superconducting Qubits
We make use of a superconducting qubit to study the effects of noise on
adiabatic geometric phases. The state of the system, an effective spin one-half
particle, is adiabatically guided
along a closed path in parameter space and
thereby acquires a geometric phase. By introducing artificial fluctuations in
the control parameters, we measure the geometric contribution to dephasing for
a variety of noise powers and evolution times. Our results clearly show that
only fluctuations which distort the path lead to geometric dephasing. In a
direct comparison with the dynamic phase, which is path-independent, we observe
that the adiabatic geometric phase is less affected by noise-induced dephasing.
This observation directly points towards the potential of geometric phases for
quantum gates or metrological applications.
09
Feb
2013
Photon solid phases in driven arrays of non-linearly coupled cavities
We introduce and study the properties of an array of QED cavities coupled by
non-linear elements, in the presence of photon leakage and driven by a coherent
source. The non-linear
couplings lead to photon hopping and to nearest-neighbor
Kerr terms. By tuning the system parameters, the steady state of the array can
exhibit a photon crystal associated to a periodic modulation of the photon
blockade. In some cases the crystalline ordering may coexist with phase
synchronisation. The class of cavity arrays we consider can be built with
superconducting circuits of existing technology.
04
Feb
2013
Fast microwave beam splitters from superconducting resonators
Coupled superconducting transmission line resonators have applications in
quantum information processing and fundamental quantum mechanics. A particular
example is the realization of
fast beam splitters, which however is hampered by
two-mode squeezer terms. Here, we experimentally study superconducting
microstrip resonators which are coupled over one third of their length. By
varying the position of this coupling region we can tune the strength of the
two-mode squeezer coupling from 2.4% to 12.9% of the resonance frequency of
5.44GHz. Nevertheless, the beam splitter coupling rate for maximally suppressed
two-mode squeezing is 810MHz, enabling the construction of a fast and pure beam
splitter.
Collective Suppression of Linewidths in Circuit QED
We report the experimental observation, and a theoretical explanation, of
collective suppression of linewidths for multiple superconducting qubits
coupled to a good cavity. This demonstrates
how strong qubit-cavity coupling
can significantly modify the dephasing and dissipation processes that might be
expected for individual qubits, and can potentially improve coherence times in
many-body circuit QED.
29
Jan
2013
Scalable universal holonomic quantum computation realized with an adiabatic quantum data bus and potential implementation using superconducting flux qubits
In this paper we examine the use of an adiabatic quantum data transfer protocol to build a universal quantum computer. Single qubit gates are realized by using a bus protocol to transfer
qubits of information down a spin chain with a unitary twist. This twist arises from altered couplings on the chain corresponding to unitary rotations performed on one region of the chain. We show how a controlled NOT gate can be realized by using a control qubit with Ising type coupling. The method discussed here can be extended to non-adiabatic quantum bus protocols. We also examine the potential of realizing such a quantum computer by using superconducting flux qubits.
25
Jan
2013
Stabilizing the trajectory of a superconducting qubit by projective measurement feedback
Making a system state follow a prescribed trajectory despite fluctuations and
errors commonly consists in monitoring an observable (temperature,
blood-glucose level…) and reacting
on its controllers (heater power, insulin
amount …). In the quantum domain, there is a change of paradigm in feedback
since measurements modify the state of the system, most dramatically when the
trajectory goes through superpositions of measurement eigenstates. Here, we
demonstrate the stabilization of an arbitrary trajectory of a superconducting
qubit by measurement based feedback. The protocol benefits from the long
coherence time ($T_2>10 mu$s) of the 3D transmon qubit, the high efficiency
(82%) of the phase preserving Josephson amplifier, and fast electronics
ensuring less than 500 ns delay. At discrete time intervals, the state of the
qubit is measured and corrected in case an error is detected. For Rabi
oscillations, where the discrete measurements occur when the qubit is supposed
to be in the measurement pointer states, we demonstrate an average fidelity of
85% to the targeted trajectory. For Ramsey oscillations, which does not go
through pointer states, the average fidelity reaches 75%. Incidentally, we
demonstrate a fast reset protocol allowing to cool a 3D transmon qubit down to
0.6% in the excited state.