900 research outputs found

### Quantum dynamics of a dc-SQUID coupled to an asymmetric Cooper pair transistor

We present a theoretical analysis of the quantum dynamics of a
superconducting circuit based on a highly asymmetric Cooper pair transistor
(ACPT) in parallel to a dc-SQUID. Starting from the full Hamiltonian we show
that the circuit can be modeled as a charge qubit (ACPT) coupled to an
anharmonic oscillator (dc-SQUID). Depending on the anharmonicity of the SQUID,
the Hamiltonian can be reduced either to one that describes two coupled qubits
or to the Jaynes-Cummings Hamiltonian. Here the dc-SQUID can be viewed as a
tunable micron-size resonator. The coupling term, which is a combination of a
capacitive and a Josephson coupling between the two qubits, can be tuned from
the very strong- to the zero-coupling regimes. It describes very precisely the
tunable coupling strength measured in this circuit and explains the
'quantronium' as well as the adiabatic quantum transfer read-out.Comment: 20 page

### Measurement of the Current-Phase Relation in Josephson Junctions Rhombi Chains

We present low temperature transport measurements in one dimensional
Josephson junctions rhombi chains. We have measured the current phase relation
of a chain of 8 rhombi. The junctions are either in the classical phase regime
with the Josephson energy much larger than the charging energy, $E_{J}\gg
E_{C}$, or in the quantum phase regime where $E_{J}/E_{C}\approx 2$. In the
strong Josephson coupling regime ($E_{J}\gg E_{C} \gg k_{B}T$) we observe a
sawtooth-like supercurrent as a function of the phase difference over the
chain. The period of the supercurrent oscillations changes abruptly from one
flux quantum $\Phi_{0}$ to half the flux quantum $\Phi_{0}/2$ as the rhombi are
tuned in the vicinity of full frustration. The main observed features can be
understood from the complex energy ground state of the chain. For
$E_{J}/E_{C}\approx 2$ we do observe a dramatic suppression and rounding of the
switching current dependence which we found to be consistent with the model
developed by Matveev et al.(Phys. Rev. Lett. {\bf 89}, 096802(2002)) for long
Josephson junctions chains.Comment: to appear in Phys. Rev.

### A V-shape superconducting artificial atom based on two inductively coupled transmons

Circuit quantum electrodynamics systems are typically built from resonators
and two-level artificial atoms, but the use of multi-level artificial atoms
instead can enable promising applications in quantum technology. Here we
present an implementation of a Josephson junction circuit dedicated to operate
as a V-shape artificial atom. Based on a concept of two internal degrees of
freedom, the device consists of two transmon qubits coupled by an inductance.
The Josephson nonlinearity introduces a strong diagonal coupling between the
two degrees of freedom that finds applications in quantum non-demolition
readout schemes, and in the realization of microwave cross-Kerr media based on
superconducting circuits.Comment: 5 pages, 3 figure

### Phase-charge duality in Josephson junction circuits: Role of inertia and effect of microwave irradiation

We investigate the physics of coherent quantum phase slips in two distinct
circuits containing small Josephson junctions: (i) a single junction embedded
in an inductive environment and (ii) a long chain of junctions. Starting from
the standard Josephson Hamiltonian, the single junction circuit can be analyzed
using quasi-classical methods; we formulate the conditions under which the
resulting quasi-charge dynamics is exactly dual to the usual phase dynamics
associated with Josephson tunneling. For the chain we use the fact that its
collective behavior can be characterized by one variable: the number $m$ of
quantum phase slips present on it. We conclude that the dynamics of the
conjugate quasi-charge is again exactly dual to the standard phase dynamics of
a single Josephson junction. In both cases we elucidate the role of the
inductance, essential to obtain exact duality. These conclusions have profound
consequences for the behavior of single junctions and chains under microwave
irradiation. Since both systems are governed by a model exactly dual to the
standard resistively and capacitively shunted junction model, we expect the
appearance of current-Shapiro steps. We numerically calculate the corresponding
current-voltage characteristics in a wide range of parameters. Our results are
of interest in view of a metrological current standard

### Experimental demonstration of Aharonov-Casher interference in a Josephson junction circuit

A neutral quantum particle with magnetic moment encircling a static electric
charge acquires a quantum mechanical phase (Aharonov-Casher effect). In
superconducting electronics the neutral particle becomes a fluxon that moves
around superconducting islands connected by Josephson junctions. The full
understanding of this effect in systems of many junctions is crucial for the
design of novel quantum circuits. Here we present measurements and quantitative
analysis of fluxon interference patterns in a six Josephson junction chain. In
this multi-junction circuit the fluxon can encircle any combination of charges
on five superconducting islands, resulting in a complex pattern. We compare the
experimental results with predictions of a simplified model that treats fluxons
as independent excitations and with the results of the full diagonalization of
the quantum problem. Our results demonstrate the accuracy of the fluxon
interference description and the quantum coherence of these arrays

### Kerr coefficients of plasma resonances in Josephson junction chains

We present an experimental and theoretical analysis of the self- and
cross-Kerr effect of extended plasma resonances in Josephson junction chains.
We calculate the Kerr coefficients by deriving and diagonalizing the
Hamiltonian of a linear circuit model for the chain and then adding the
Josephson non-linearity as a perturbation. The calculated Kerr-coefficients are
compared with the measurement data of a chain of 200 junctions. The Kerr effect
manifests itself as a frequency shift that depends linearly on the number of
photons in a resonant mode. By changing the input power on a low signal level,
we are able to measure this shift. The photon number is calibrated using the
self-Kerr shift calculated from the sample parameters. We then compare the
measured cross-Kerr shift with the theoretical prediction, using the calibrated
photon number.Comment: 10 pages, 9 figure

### Phase-Charge Duality of a Josephson junction in a fluctuating electromagnetic environment

We have measured the current-voltage characteristics of a single Josephson
junction placed in a high impedance environment. The transfer of Cooper pairs
through the junction is governed by overdamped quasicharge dynamics, leading to
Coulomb blockade and Bloch oscillations. Exact duality exists to the standard
overdamped phase dynamics of a Josephson junction, resulting in a dual shape of
the current-voltage characteristic, with current and voltage changing roles. We
demonstrate this duality with experiments which allow for a quantitative
comparison with a theory that includes the effect of fluctuations due to finite
temperature of the electromagnetic environment

### Fast high fidelity quantum non-demolition qubit readout via a non-perturbative cross-Kerr coupling

Qubit readout is an indispensable element of any quantum information
processor. In this work, we experimentally demonstrate a non-perturbative
cross-Kerr coupling between a transmon and a polariton mode which enables an
improved quantum non-demolition (QND) readout for superconducting qubits. The
new mechanism uses the same experimental techniques as the standard QND qubit
readout in the dispersive approximation, but due to its non-perturbative
nature, it maximizes the speed, the single-shot fidelity and the QND properties
of the readout. In addition, it minimizes the effect of unwanted decay channels
such as the Purcell effect. We observed a single-shot readout fidelity of 97.4%
for short 50 ns pulses, and we quantified a QND-ness of 99% for long
measurement pulses with repeated single-shot readouts

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