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
06
Mä
2013
Nonequilibrium quasiparticle relaxation in superconducting qubits
In a superconducting qubit the lifetime of qubit state is restricted by nonequilibrium quasiparticle tunneling. We calculate the rate of these tunnelings using the nonequilibrium effects
they induce on the condensate chemical potential of leads and islands. We show that by decreasing temperature below a crossover, the quasiparticle relaxation rate changes from exponential to much slower suppression and saturates to a finite value at zero temperature. This prediction is consistent with recent experiments. Our model also indicates a striking modification to qubit transitions: the matrix element of an energy transition in qubit strongly depends on coupling between tunneling quasiparticles and the environment. This features important fabrication hints to improve quantum state efficiency.
04
Mä
2013
A scanning transmon qubit for strong coupling circuit quantum electrodynamics
Like a quantum computer designed for a particular class of problems, a quantum simulator enables quantitative modeling of quantum systems that is computationally intractable with a
classical computer. Quantum simulations of quantum many-body systems have been performed using ultracold atoms and trapped ions among other systems. Superconducting circuits have recently been investigated as an alternative system in which microwave photons confined to a lattice of coupled resonators act as the particles under study with qubits coupled to the resonators producing effective photon-photon interactions. Such a system promises insight into the nonequilibrium physics of interacting bosons but new tools are needed to understand this complex behavior. Here we demonstrate the operation of a scanning transmon qubit and propose its use as a local probe of photon number within a superconducting resonator lattice. We map the coupling strength of the qubit to a resonator on a separate chip and show that the system reaches the strong coupling regime over a wide scanning area.
03
Mä
2013
Detecting non-Abelian geometric phase in circuit QED
We propose a scheme for detecting noncommutative feature of the non-Abelian geometric phase in circuit QED, which involves three transmon qubits capacitively coupled to an one-dimensional
transmission line resonator. By controlling the external magnetic flux of the transmon qubits, we can obtain an effective tripod interaction of our circuit QED setup. The noncommutative feature of the non-Abelian geometric phase is manifested that for an initial state undergo two specific loops in different order will result in different final states. Our numerical calculations show that this difference can be unambiguously detected in the proposed system.
27
Feb
2013
Catch-Disperse-Release Readout for Superconducting Qubits
We analyze single-shot readout for superconducting qubits via controlled catch, dispersion, and release of a microwave field. A tunable coupler is used to decouple the microwave resonator
from the transmission line during the dispersive qubit-resonator interaction, thus circumventing damping from the Purcell effect. We show that if the qubit frequency tuning is sufficiently adiabatic, a fast high-fidelity qubit readout is possible even in the strongly nonlinear dispersive regime. Interestingly, the Jaynes-Cummings nonlinearity leads to the quadrature squeezing of the resonator field below the standard quantum limit, resulting in a significant decrease of the measurement error.
26
Feb
2013
Analyzing Toffoli Gate in Disordered Circuit QED
We study the effects of imperfections on the fidelity of the Toffoli gate
recently realized in a circuit QED setup. The noise is introduced to the
interqubits interactions hence the
couplings no longer remain constant but
belong to the interval [1-d,1+d]J, where J is the average coupling constant and
parameter d quantifies the noise level. We consider both static and dynamical
imperfections. We also obtain a more robust gate by solving the quantum
optimization problem with a modified objective functional including an interval
of coupling values rather than a single ideal value.
22
Feb
2013
Realization of Deterministic Quantum Teleportation with Solid State Qubits
Transferring the state of an information carrier from a sender to a receiver
is an essential primitive in both classical and quantum communication and
information processing. In a quantum
process known as teleportation the unknown
state of a quantum bit can be relayed to a distant party using shared
entanglement and classical information. Here we present experiments in a
solid-state system based on superconducting quantum circuits demonstrating the
teleportation of the state of a qubit at the macroscopic scale. In our
experiments teleportation is realized deterministically with high efficiency
and achieves a high rate of transferred qubit states. This constitutes a
significant step towards the realization of repeaters for quantum communication
at microwave frequencies and broadens the tool set for quantum information
processing with superconducting circuits.
Multimode circuit QED with hybrid metamaterial transmission lines
Quantum transmission lines are a central to superconducting and hybrid
quantum computing. Parallel to these developments are those of left-handed
meta-materials. They have a wide variety
of applications in photonics from the
microwave to the visible range such as invisibility cloaks and perfect flat
lenses. For classical guided microwaves, left-handed transmission lines have
been proposed and studied on the macroscopic scale. We combine these ideas in
presenting a left-handed/right-handed hybrid transmission line for applications
in quantum optics on a chip. The resulting system allows circuit QED to reach a
new regime: multi-mode ultra-strong coupling. Out of the many potential
applications of this novel device, we discuss two; the preparation of
multipartite entangled states and its use as a quantum simulator for the
spin-boson model where a quantum phase transition is reached up to finite
size-effects.
Generation of non-classical photon states in superconducting quantum metamaterials
We report a theoretical study of diverse non-classical photon states that can
be realized in superconducting quantum metamaterials. As a particular example
of superconducting quantum
metamaterials an array of SQUIDs incorporated in a
low-dissipative transmission line (resonant cavity) will be studied. This
system will be modeled as a set of two-levels systems (qubits) strongly
interacting with resonant cavity photons. We predict and analyze {a
second(first)-order phase transition} between an incoherent (the
high-temperature phase) and coherent (the low-temperatures phase) states of
photons. In equilibrium state the partition function $Z$ of the electromagnetic
field (EF) in the cavity is determined by the effective action
$S_{eff}{P(tau)}$ that, in turn, depends on imaginary-time dependent
momentum of photon field $P(tau)$. We show that the order parameter of this
phase transition is the $P_{0}(tau)$ minimizing the effective action of a
whole system. In the incoherent state the order parameter $P_{0}(tau)=0$ but
at low temperatures we obtain various coherent states characterized by non-zero
values of $P_{0}(tau)$. This phase transition in many aspects resembles the
Peierls metal-insulator and the metal-superconductor phase transitions. The
critical temperature of such phase transition $T^star$ is determined by the
energy splitting of two-level systems $Delta$, a number of SQUIDs in the array
$N$, and the strength of the interaction $eta$ between SQUIDs and photons in
cavity.
20
Feb
2013
Partial-measurement back-action and non-classical weak values in a superconducting circuit
We realize indirect partial measurement of a transmon qubit in circuit
quantum electrodynamics by interaction with an ancilla qubit and projective
ancilla measurement with a dedicated
readout resonator. Accurate control of the
interaction and ancilla measurement basis allows tailoring the measurement
strength and operator. The tradeoff between measurement strength and qubit
back-action is characterized through the distortion of a qubit Rabi oscillation
imposed by ancilla measurement in different bases. Combining partial and
projective qubit measurements, we provide the solid-state demonstration of the
correspondence between a non-classical weak value and the violation of a
Leggett-Garg inequality.
19
Feb
2013
Directional amplification with a Josephson circuit
Non-reciprocal devices, which have different transmission coefficients for
propagating waves in opposite directions, are crucial components in many low
noise quantum measurements. In
most schemes, magneto-optical effects provide
the necessary non-reciprocity. In contrast, the proof-of-principle device
presented here, consists of two on-chip coupled Josephson parametric converters
(JPCs), which achieves directionality by exploiting the non-reciprocal phase
response of the JPC in the trans-gain mode. The non-reciprocity of the device
is controlled in-situ by varying the amplitude and phase difference of two
independent microwave pump tones feeding the system. At the desired working
point and for a signal frequency of 8.453 GHz, the device achieves a forward
power gain of 15 dB within a dynamical bandwidth of 9 MHz, a reverse gain of -6
dB and suppression of the reflected signal by 8 dB. We also find that the
amplifier adds a noise equivalent to less than one and a half photons at the
signal frequency (referred to the input). It can process up to 3 photons at the
signal frequency per inverse dynamical bandwidth. With a directional amplifier
operating along the principles of this device, qubit and readout preamplifier
could be integrated on the same chip.