71 research outputs found
Circuit QED with a Nonlinear Resonator : ac-Stark Shift and Dephasing
We have performed spectroscopic measurements of a superconducting qubit
dispersively coupled to a nonlinear resonator driven by a pump microwave field.
Measurements of the qubit frequency shift provide a sensitive probe of the
intracavity field, yielding a precise characterization of the resonator
nonlinearity. The qubit linewidth has a complex dependence on the pump
frequency and amplitude, which is correlated with the gain of the nonlinear
resonator operated as a small-signal amplifier. The corresponding dephasing
rate is found to be close to the quantum limit in the low-gain limit of the
amplifier.Comment: Paper : 4 pages, 3 figures; Supplementary material : 1 page, 1 figur
Spectral measurement of the thermal excitation of a superconducting qubit
We report the measurement of the fluctuations of a transmon qubit through the
noise spectrum of the microwave signal that measures its state. The amplitude
of the Lorentzian noise power spectrum allows to determine the average qubit
excitation, in agreement with the estimated thermal radiation reaching the
sample. Its width yields the qubit energy relaxation rate which decreases with
temperature, contrary to the predictions for a two-level system solely coupled
to thermal radiation. This indicates the existence of another non-radiative
energy relaxation channel for the qubit
Tunable resonators for quantum circuits
We have designed, fabricated and measured high-Q coplanar
waveguide microwave resonators whose resonance frequency is made tunable with
magnetic field by inserting a DC-SQUID array (including 1 or 7 SQUIDs) inside.
Their tunability range is 30% of the zero field frequency. Their quality factor
reaches up to 3. We present a model based on thermal fluctuations
that accounts for the dependance of the quality factor with magnetic field.Comment: subm. to JLTP (Proc. of LTD12 conference
Quantum information processing using quasiclassical electromagnetic interactions between qubits and electrical resonators
Electrical resonators are widely used in quantum information processing, by engineering an electromagnetic interaction with qubits based on real or virtual exchange of microwave photons. This interaction relies on strong coupling between the qubits' transition dipole moments and the vacuum fluctuations of the resonator in the same manner as cavity quantum electrodynamics (QED), and has consequently come to be called 'circuit QED' (cQED). Great strides in the control of quantum information have already been made experimentally using this idea. However, the central role played by photon exchange induced by quantum fluctuations in cQED does result in some characteristic limitations. In this paper, we discuss an alternative method for coupling qubits electromagnetically via a resonator, in which no photons are exchanged, and where the resonator need not have strong quantum fluctuations. Instead, the interaction can be viewed in terms of classical, effective 'forces' exerted by the qubits on the resonator, and the resulting resonator dynamics used to produce qubit entanglement are purely classical in nature. We show how this type of interaction is similar to that encountered in the manipulation of atomic ion qubits, and we exploit this analogy to construct two-qubit entangling operations that are largely insensitive to thermal or other noise in the resonator, and to its quality factor. These operations are also extensible to larger numbers of qubits, allowing interactions to be selectively generated among any desired subset of those coupled to a single resonator. Our proposal is potentially applicable to a variety of physical qubit modalities, including superconducting and semiconducting solid-state qubits, trapped molecular ions, and possibly even electron spins in solids.United States. Dept. of Defense. Assistant Secretary of Defense for Research & Engineering (United States. Air Force Contract FA8721-05-C-0002
Parameter identification of the STICS crop model, using an accelerated formal MCMC approach
This study presents a Bayesian approach for the parameters’ identification of the STICS crop model based on the recently developed Differential Evolution Adaptive Metropolis (DREAM) algorithm. The posterior distributions of nine specific crop parameters of the STICS model were sampled with the aim to improve the growth simulations of a winter wheat (Triticum aestivum L.) culture. The results obtained with the DREAM algorithm were initially compared to those obtained with a Nelder-Mead Simplex algorithm embedded within the OptimiSTICS package. Then, three types of likelihood functions implemented within the DREAM algorithm were compared, namely the standard least square, the weighted least square, and a transformed likelihood function that makes explicit use of the coefficient of variation (CV). The results showed that the proposed CV likelihood function allowed taking into account both noise on measurements and heteroscedasticity which are regularly encountered in crop modellingPeer reviewe
A scattering quantum circuit for measuring Bell's time inequality: a nuclear magnetic resonance demonstration using maximally mixed states
In 1985, Leggett and Garg (1985 Phys. Rev. Lett. 54 857) proposed a Bell-like
inequality to test (in)compatibility between two fundamental concepts of
quantum mechanics. The first concept is 'macroscopic realism', which is the
quality of a physical property of a quantum system being independent of
observation at the macroscopic level. The second concept is 'noninvasive
measurability', which is the possibility of performing a measurement without
disturbing the subsequent evolution of a system. One of the key requirement for
testing the violation of the Leggett-Garg inequality, or Bell's time
inequality, is the ability to perform noninvasive measurements over a qubit
state. In this paper, we present a quantum scattering circuit that implements
such a measurement for maximally mixed states. The operation of the circuit is
demonstrated using liquid-state nuclear magnetic resonance (NMR) in chloroform,
in which the time correlations of a qubit are measured on a probe (ancillary)
qubit state. The results clearly show a violation region and are in excellent
agreement with the predictions of quantum mechanics
Experimental violation of the Leggett-Garg inequality under decoherence
Despite the great success of quantum mechanics, questions regarding its application still exist and the boundary between quantum and classical mechanics remains unclear. Based on the philosophical assumptions of macrorealism and noninvasive measurability, Leggett and Garg devised a series of inequalities (LG inequalities) involving a single system with a set of measurements at different times. Introduced as the Bell inequalities in time, the violation of LG inequalities excludes the hidden-variable description based on the above two assumptions. We experimentally investigated the single photon LG inequalities under decoherence simulated by birefringent media. These generalized LG inequalities test the evolution trajectory of the photon and are shown to be maximally violated in a coherent evolution process. The violation of LG inequalities becomes weaker with the increase of interaction time in the environment. The ability to violate the LG inequalities can be used to set a boundary of the classical realistic description
Parametric amplification with weak-link nonlinearity in superconducting microresonators
Nonlinear kinetic inductance in a high Q superconducting coplanar waveguide
microresonator can cause a bifurcation of the resonance curve. Near the
critical pumping power and frequency for bifurcation, large parametric gain is
observed for signals in the frequency band near resonance. We show experimental
results on signal and intermodulation gain which are well described by a theory
of the parametric amplification based on a Kerr nonlinearity. Phase dependent
gain, or signal squeezing, is verified with a homodyne detection scheme.Comment: Submitted to Physica Scripta, topical issue: Nobel Symposium on
Quantum Bits, 2009. 10 pages, 5 figures. Version 2 contains a few new
sentences about the current-phase relation of weak link
HESS Opinions: Incubating deep-learning-powered hydrologic science advances as a community
Recently, deep learning (DL) has emerged as a revolutionary and
versatile tool transforming industry applications and generating new and
improved capabilities for scientific discovery and model building. The
adoption of DL in hydrology has so far been gradual, but the field is now
ripe for breakthroughs. This paper suggests that DL-based methods can open up a
complementary avenue toward knowledge discovery in hydrologic sciences. In
the new avenue, machine-learning algorithms present competing hypotheses that
are consistent with data. Interrogative methods are then invoked to interpret
DL models for scientists to further evaluate. However, hydrology presents
many challenges for DL methods, such as data limitations, heterogeneity
and co-evolution, and the general inexperience of the hydrologic field with
DL. The roadmap toward DL-powered scientific advances will require the
coordinated effort from a large community involving scientists and citizens.
Integrating process-based models with DL models will help alleviate data
limitations. The sharing of data and baseline models will improve the
efficiency of the community as a whole. Open competitions could serve as the
organizing events to greatly propel growth and nurture data science education
in hydrology, which demands a grassroots collaboration. The area of
hydrologic DL presents numerous research opportunities that could, in turn,
stimulate advances in machine learning as well.</p
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