137 research outputs found
Single-shot qubit readout in circuit Quantum Electrodynamics
The future development of quantum information using superconducting circuits
requires Josephson qubits [1] with long coherence times combined to a
high-fidelity readout. Major progress in the control of coherence has recently
been achieved using circuit quantum electrodynamics (cQED) architectures [2,
3], where the qubit is embedded in a coplanar waveguide resonator (CPWR) which
both provides a well controlled electromagnetic environment and serves as qubit
readout. In particular a new qubit design, the transmon, yields reproducibly
long coherence times [4, 5]. However, a high-fidelity single-shot readout of
the transmon, highly desirable for running simple quantum algorithms or measur-
ing quantum correlations in multi-qubit experiments, is still lacking. In this
work, we demonstrate a new transmon circuit where the CPWR is turned into a
sample-and-hold detector, namely a Josephson Bifurcation Amplifer (JBA) [6, 7],
which allows both fast measurement and single-shot discrimination of the qubit
states. We report Rabi oscillations with a high visibility of 94% together with
dephasing and relaxation times longer than 0:5 \mu\s. By performing two
subsequent measurements, we also demonstrate that this new readout does not
induce extra qubit relaxation.Comment: 14 pages including 4 figures, preprint forma
Experimental violation of a Bell's inequality in time with weak measurement
The violation of J. Bell's inequality with two entangled and spatially
separated quantum two- level systems (TLS) is often considered as the most
prominent demonstration that nature does not obey ?local realism?. Under
different but related assumptions of "macrorealism", plausible for macroscopic
systems, Leggett and Garg derived a similar inequality for a single degree of
freedom undergoing coherent oscillations and being measured at successive
times. Such a "Bell's inequality in time", which should be violated by a
quantum TLS, is tested here. In this work, the TLS is a superconducting quantum
circuit whose Rabi oscillations are continuously driven while it is
continuously and weakly measured. The time correlations present at the detector
output agree with quantum-mechanical predictions and violate the inequality by
5 standard deviations.Comment: 26 pages including 10 figures, preprint forma
Quantum feedback control of a superconducting qubit: Persistent Rabi oscillations
The act of measurement bridges the quantum and classical worlds by projecting
a superposition of possible states into a single, albeit probabilistic,
outcome. The time-scale of this "instantaneous" process can be stretched using
weak measurements so that it takes the form of a gradual random walk towards a
final state. Remarkably, the interim measurement record is sufficient to
continuously track and steer the quantum state using feedback. We monitor the
dynamics of a resonantly driven quantum two-level system -- a superconducting
quantum bit --using a near-noiseless parametric amplifier. The high-fidelity
measurement output is used to actively stabilize the phase of Rabi
oscillations, enabling them to persist indefinitely. This new functionality
shows promise for fighting decoherence and defines a path for continuous
quantum error correction.Comment: Manuscript: 5 Pages and 3 figures ; Supplementary Information: 9
pages and 3 figure
Demonstration of quantum Zeno effect in a superconducting phase qubit
Quantum Zeno effect is a significant tool in quantum manipulating and
computing. We propose its observation in superconducting phase qubit with two
experimentally feasible measurement schemes. The conventional measurement
method is used to achieve the proposed pulse and continuous readout of the
qubit state, which are analyzed by projection assumption and Monte Carlo
wave-function simulation, respectively. Our scheme gives a direct
implementation of quantum Zeno effect in a superconducting phase qubit.Comment: 5 pages, 4 figure
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
Quantum Correlations in NMR systems
In conventional NMR experiments, the Zeeman energy gaps of the nuclear spin
ensembles are much lower than their thermal energies, and accordingly exhibit
tiny polarizations. Generally such low-purity quantum states are devoid of
quantum entanglement. However, there exist certain nonclassical correlations
which can be observed even in such systems. In this chapter, we discuss three
such quantum correlations, namely, quantum contextuality, Leggett-Garg temporal
correlations, and quantum discord. In each case, we provide a brief theoretical
background and then describe some results from NMR experiments.Comment: 21 pages, 7 figure
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
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Towards an empirical test of realism in cognition
We review recent progress in designing an empirical test of (temporal) realism in cognition. Realism in this context is the property that cognitive variables always have well defined (if possibly unknown) values at all times. We focus most of our attention in this contribution on discussing the exact notion of realism that is to be tested, as we feel this issue has not received enough attention to date. We also give a brief outline of the empirical test, including some comments on an experimental realisation, and we discuss what we should conclude from any purported experimental ‘disproof’ of realism. This contribution is based on Yearsley and Pothos (2014)
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
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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