457 research outputs found
Measuring the Decoherence of a Quantronium Qubit with the Cavity Bifurcation Amplifier
Dispersive readouts for superconducting qubits have the advantage of speed
and minimal invasiveness. We have developed such an amplifier, the Cavity
Bifurcation Amplifier (CBA) [10], and applied it to the readout of the
quantronium qubit [2]. It consists of a Josephson junction embedded in a
microwave on-chip resonator. In contrast with the Josephson bifurcation
amplifier [17], which has an on-chip capacitor shunting a junction, the
resonator is based on a simple coplanar waveguide imposing a pre-determined
frequency and whose other RF characteristics like the quality factor are easily
controlled and optimized. Under proper microwave irradiation conditions, the
CBA has two metastable states. Which state is adopted by the CBA depends on the
state of a quantronium qubit coupled to the CBA's junction. Due to the MHz
repetition rate and large signal to noise ratio we can show directly that the
coherence is limited by 1/f gate charge noise when biased at the sweet spot - a
point insensitive to first order gate charge fluctuations. This architecture
lends itself to scalable quantum computing using a multi-resonator chip with
multiplexed readouts.Comment: 6 pages, 5 figures To be published in Physical Review
Circuit QED and engineering charge based superconducting qubits
The last two decades have seen tremendous advances in our ability to generate
and manipulate quantum coherence in mesoscopic superconducting circuits. These
advances have opened up the study of quantum optics of microwave photons in
superconducting circuits as well as providing important hardware for the
manipulation of quantum information. Focusing primarily on charge-based qubits,
we provide a brief overview of these developments and discuss the present state
of the art. We also survey the remarkable progress that has been made in
realizing circuit quantum electrodynamics (QED) in which superconducting
artificial atoms are strongly coupled to individual microwave photons.Comment: Proceedings of Nobel Symposium 141: Qubits for Future Quantum
Informatio
Introduction to Quantum Noise, Measurement and Amplification
The topic of quantum noise has become extremely timely due to the rise of
quantum information physics and the resulting interchange of ideas between the
condensed matter and AMO/quantum optics communities. This review gives a
pedagogical introduction to the physics of quantum noise and its connections to
quantum measurement and quantum amplification. After introducing quantum noise
spectra and methods for their detection, we describe the basics of weak
continuous measurements. Particular attention is given to treating the standard
quantum limit on linear amplifiers and position detectors using a general
linear-response framework. We show how this approach relates to the standard
Haus-Caves quantum limit for a bosonic amplifier known in quantum optics, and
illustrate its application for the case of electrical circuits, including
mesoscopic detectors and resonant cavity detectors.Comment: Substantial improvements over initial version; include supplemental
appendices
The reconfigurable Josephson circulator/directional amplifier
Circulators and directional amplifiers are crucial non-reciprocal signal
routing and processing components involved in microwave readout chains for a
variety of applications. They are particularly important in the field of
superconducting quantum information, where the devices also need to have
minimal photon losses to preserve the quantum coherence of signals.
Conventional commercial implementations of each device suffer from losses and
are built from very different physical principles, which has led to separate
strategies for the construction of their quantum-limited versions. However, as
recently proposed theoretically, by establishing simultaneous pairwise
conversion and/or gain processes between three modes of a Josephson-junction
based superconducting microwave circuit, it is possible to endow the circuit
with the functions of either a phase-preserving directional amplifier or a
circulator. Here, we experimentally demonstrate these two modes of operation of
the same circuit. Furthermore, in the directional amplifier mode, we show that
the noise performance is comparable to standard non-directional superconducting
amplifiers, while in the circulator mode, we show that the sense of circulation
is fully reversible. Our device is far simpler in both modes of operation than
previous proposals and implementations, requiring only three microwave pumps.
It offers the advantage of flexibility, as it can dynamically switch between
modes of operation as its pump conditions are changed. Moreover, by
demonstrating that a single three-wave process yields non-reciprocal devices
with reconfigurable functions, our work breaks the ground for the development
of future, more-complex directional circuits, and has excellent prospects for
on-chip integration
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