16,917 research outputs found
Parametric resonance in tunable superconducting cavities
We develop a theory of parametric resonance in tunable superconducting
cavities. The nonlinearity introduced by the SQUID attached to the cavity, and
damping due to connection of the cavity to a transmission line are taken into
consideration. We study in detail the nonlinear classical dynamics of the
cavity field below and above the parametric threshold for the degenerate
parametric resonance, featuring regimes of multistability and parametric
radiation. We investigate the phase-sensitive amplification of external signals
on resonance, as well as amplification of detuned signals, and relate the
amplifier performance to that of linear parametric amplifiers. We also discuss
applications of the device for dispersive qubit readout. Beyond the classical
response of the cavity, we investigate small quantum fluctuations around the
amplified classical signals. We evaluate the noise power spectrum both for the
internal field in the cavity and the output field. Other quantum statistical
properties of the noise are addressed such as squeezing spectra, second order
coherence, and two-mode entanglement.Comment: 25 pages, 17 figure
Digitally-Enhanced Software-Defined Radio Receiver Robust to Out-of-Band Interference
A software-defined radio (SDR) receiver with improved robustness to out-of-band interference (OBI) is presented. Two main challenges are identified for an OBI-robust SDR receiver: out-of-band nonlinearity and harmonic mixing. Voltage gain at RF is avoided, and instead realized at baseband in combination with low-pass filtering to mitigate blockers and improve out-of-band IIP3. Two alternative “iterative” harmonic-rejection (HR) techniques are presented to achieve high HR robust to mismatch: a) an analog two-stage polyphase HR concept, which enhances the HR to more than 60 dB; b) a digital adaptive interference cancelling (AIC) technique, which can suppress one dominating harmonic by at least 80 dB. An accurate multiphase clock generator is presented for a mismatch-robust HR. A proof-of-concept receiver is implemented in 65 nm CMOS. Measurements show 34 dB gain, 4 dB NF, and 3.5 dBm in-band IIP3 while the out-of-band IIP3 is + 16 dBm without fine tuning. The measured RF bandwidth is up to 6 GHz and the 8-phase LO works up to 0.9 GHz (master clock up to 7.2 GHz). At 0.8 GHz LO, the analog two-stage polyphase HR achieves a second to sixth order HR > dB over 40 chips, while the digital AIC technique achieves HR > 80 dB for the dominating harmonic. The total power consumption is 50 mA from a 1.2 V supply
High performance photonic reservoir computer based on a coherently driven passive cavity
Reservoir computing is a recent bio-inspired approach for processing
time-dependent signals. It has enabled a breakthrough in analog information
processing, with several experiments, both electronic and optical,
demonstrating state-of-the-art performances for hard tasks such as speech
recognition, time series prediction and nonlinear channel equalization. A
proof-of-principle experiment using a linear optical circuit on a photonic chip
to process digital signals was recently reported. Here we present a photonic
implementation of a reservoir computer based on a coherently driven passive
fiber cavity processing analog signals. Our experiment has error rate as low or
lower than previous experiments on a wide variety of tasks, and also has lower
power consumption. Furthermore, the analytical model describing our experiment
is also of interest, as it constitutes a very simple high performance reservoir
computer algorithm. The present experiment, given its good performances, low
energy consumption and conceptual simplicity, confirms the great potential of
photonic reservoir computing for information processing applications ranging
from artificial intelligence to telecommunicationsComment: non
Transient dynamics of a superconducting nonlinear oscillator
We investigate the transient dynamics of a lumped-element oscillator based on
a dc superconducting quantum interference device (SQUID). The SQUID is shunted
with a capacitor forming a nonlinear oscillator with resonance frequency in the
range of several GHz. The resonance frequency is varied by tuning the Josephson
inductance of the SQUID with on-chip flux lines. We report measurements of
decaying oscillations in the time domain following a brief excitation with a
microwave pulse. The nonlinearity of the SQUID oscillator is probed by
observing the ringdown response for different excitation amplitudes while the
SQUID potential is varied by adjusting the flux bias. Simulations are performed
on a model circuit by numerically solving the corresponding Langevin equations
incorporating the SQUID potential at the experimental temperature and using
parameters obtained from separate measurements characterizing the SQUID
oscillator. Simulations are in good agreement with the experimental
observations of the ringdowns as a function of applied magnetic flux and pulse
amplitude. We observe a crossover between the occurrence of ringdowns close to
resonance and adiabatic following at larger detuning from the resonance. We
also discuss the occurrence of phase jumps at large amplitude drive. Finally,
we briefly outline prospects for a readout scheme for superconducting flux
qubits based on the discrimination between ringdown signals for different
levels of magnetic flux coupled to the SQUID.Comment: 15 pages, 9 figure
Information Transmission using the Nonlinear Fourier Transform, Part III: Spectrum Modulation
Motivated by the looming "capacity crunch" in fiber-optic networks,
information transmission over such systems is revisited. Among numerous
distortions, inter-channel interference in multiuser wavelength-division
multiplexing (WDM) is identified as the seemingly intractable factor limiting
the achievable rate at high launch power. However, this distortion and similar
ones arising from nonlinearity are primarily due to the use of methods suited
for linear systems, namely WDM and linear pulse-train transmission, for the
nonlinear optical channel. Exploiting the integrability of the nonlinear
Schr\"odinger (NLS) equation, a nonlinear frequency-division multiplexing
(NFDM) scheme is presented, which directly modulates non-interacting signal
degrees-of-freedom under NLS propagation. The main distinction between this and
previous methods is that NFDM is able to cope with the nonlinearity, and thus,
as the the signal power or transmission distance is increased, the new method
does not suffer from the deterministic cross-talk between signal components
which has degraded the performance of previous approaches. In this paper,
emphasis is placed on modulation of the discrete component of the nonlinear
Fourier transform of the signal and some simple examples of achievable spectral
efficiencies are provided.Comment: Updated version of IEEE Transactions on Information Theory, vol. 60,
no. 7, pp. 4346--4369, July, 201
Nonlinearities and Parametric Amplification in Superconducting Coplanar Waveguide Resonators
Experimental investigations of the nonlinear properties of superconducting
niobium coplanar waveguide resonators are reported. The nonlinearity due to a
current dependent kinetic inductance of the center conductor is strong enough
to realize bifurcation of the nonlinear oscillator. When driven with two
frequencies near the threshold for bifurcation, parametric amplification with a
gain of +22.4 dB is observed.Comment: 4 pages, 5 figures, 1 table. In version 2: Added appendix with model
description and fits to measurements. Minor corrections and rephrasin
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