261 research outputs found
QuTiP 2: A Python framework for the dynamics of open quantum systems
We present version 2 of QuTiP, the Quantum Toolbox in Python. Compared to the
preceding version [Comput. Phys. Comm. 183 (2012) 1760], we have introduced
numerous new features, enhanced performance, made changes in the Application
Programming Interface (API) for improved functionality and consistency within
the package, as well as increased compatibility with existing conventions used
in other scientific software packages for Python. The most significant new
features include efficient solvers for arbitrary time-dependent Hamiltonians
and collapse operators, support for the Floquet formalism, and new solvers for
Bloch-Redfield and Floquet-Markov master equations. Here we introduce these new
features, demonstrate their use, and give a summary of the important
backward-incompatible API changes introduced in this version.Comment: 9 pages, 5 figure
Josephson parametric phase-locked oscillator and its application to dispersive readout of superconducting qubits
The parametric phase-locked oscillator (PPLO), also known as a parametron, is
a resonant circuit in which one of the reactances is periodically modulated. It
can detect, amplify, and store binary digital signals in the form of two
distinct phases of self-oscillation. Indeed, digital computers using PPLOs
based on a magnetic ferrite ring or a varactor diode as its fundamental logic
element were successfully operated in 1950s and 1960s. More recently, basic bit
operations have been demonstrated in an electromechanical resonator, and an
Ising machine based on optical PPLOs has been proposed. Here, using a PPLO
realized with Josephson-junction circuitry, we demonstrate the demodulation of
a microwave signal digitally modulated by binary phase-shift keying. Moreover,
we apply this demodulation capability to the dispersive readout of a
superconducting qubit. This readout scheme enables a fast and latching-type
readout, yet requires only a small number of readout photons in the resonator
to which the qubit is coupled, thus featuring the combined advantages of
several disparate schemes. We have achieved high-fidelity, single-shot, and
non-destructive qubit readout with Rabi-oscillation contrast exceeding 90%,
limited primarily by the qubit's energy relaxation.Comment: 15 pages, 11 figures, including supplementary materia
Engineering the quantum states of light in a Kerr-nonlinear resonator by two-photon driving
Cat states of the microwave field stored in high-Q resonators show great
promise for robust encoding and manipulation of quantum information. Here we
propose an approach to efficiently prepare such cat states in a Kerr-nonlinear
resonator by the use of a two-photon drive. We show that this preparation is
robust against single-photon loss. We moreover find that it is possible to
remove undesirable phase evolution induced by a Kerr nonlinearity using a
two-photon drive of appropriate amplitude and phase. Finally, we present a
universal set of quantum logical gates that can be performed on the engineered
eigenspace of the two-photon driven Kerr-nonlinear resonator
-Variational Autoencoder as an Entanglement Classifier
We focus on using an architecture similar to the -Variational
Autoencoder (-VAE) to discriminate if a quantum state is entangled or
separable based on measurements. We split the data into two sets, the set of
local and correlated measurements. Using the latent space, which is a low
dimensional representation of the data, we show that restricting ourselves to
the set of local data it is not possible to distinguish between entangled and
separable states. Meanwhile, when considering both correlated and local
measurements, an accuracy of over 80% is attained in the structure of the
latent space.Comment: 5 pages, 4 figure
Adiabatic Quantum Computing for Binary Clustering
Quantum computing for machine learning attracts increasing attention and
recent technological developments suggest that especially adiabatic quantum
computing may soon be of practical interest. In this paper, we therefore
consider this paradigm and discuss how to adopt it to the problem of binary
clustering. Numerical simulations demonstrate the feasibility of our approach
and illustrate how systems of qubits adiabatically evolve towards a solution
High-density quantum sensing with dissipative first order transitions
The sensing of external fields using quantum systems is a prime example of an
emergent quantum technology. Generically, the sensitivity of a quantum sensor
consisting of independent particles is proportional to . However,
interactions invariably occuring at high densities lead to a breakdown of the
assumption of independence between the particles, posing a severe challenge for
quantum sensors operating at the nanoscale. Here, we show that interactions in
quantum sensors can be transformed from a nuisance into an advantage when
strong interactions trigger a dissipative phase transition in an open quantum
system. We demonstrate this behavior by analyzing dissipative quantum sensors
based upon nitrogen-vacancy defect centers in diamond. Using both a variational
method and numerical simulation of the master equation describing the open
quantum many-body system, we establish the existence of a dissipative first
order transition that can be used for quantum sensing. We investigate the
properties of this phase transition for two- and three-dimensional setups,
demonstrating that the transition can be observed using current experimental
technology. Finally, we show that quantum sensors based on dissipative phase
transitions are particularly robust against imperfections such as disorder or
decoherence, with the sensitivity of the sensor not being limited by the
coherence time of the device. Our results can readily be applied to other
applications in quantum sensing and quantum metrology where interactions are
currently a limiting factor.Comment: 6+3 pages, 6+3 figure
Magic trapping of a Rydberg ion with a diminished static polarizability
Highly excited Rydberg states are usually extremely polarizable and
exceedingly sensitive to electric fields. Because of this Rydberg ions confined
in electric fields have state-dependent trapping potentials. We engineer a
Rydberg state that is insensitive to electric fields by coupling two Rydberg
states with static polarizabilities of opposite sign, in this way we achieve
state-independent magic trapping. We show that the magically-trapped ion can be
coherently excited to the Rydberg state without the need for control of the
ion's motion.Comment: 4 pages and 4 figures in main body, 6 pages and 5 figures in tota
Electromagnetically induced transparency in circuit QED with nested polariton states
Electromagnetically induced transparency (EIT) is a signature of quantum
interference in an atomic three-level system. By driving the dressed
cavity-qubit states of a two-dimensional circuit QED system, we generate a set
of polariton states in the nesting regime. The lowest three energy levels are
utilized to form the -type system. EIT is observed and verified by
Akaike's information criterion based testing. Negative group velocities up to
km/s are obtained based on the dispersion relation in the EIT
transmission spectrum.Comment: 5 pages, 5 figure
Reflective amplification without population inversion from a strongly driven superconducting qubit
Amplification of optical or microwave fields is often achieved by strongly
driving a medium to induce population inversion such that a weak probe can be
amplified through stimulated emission. Here we strongly couple a
superconducting qubit, an artificial atom, to the field in a semi-infinite
waveguide. When driving the qubit strongly on resonance such that a Mollow
triplet appears, we observe a 7\% amplitude gain for a weak probe at
frequencies in-between the triplet. This amplification is not due to population
inversion, neither in the bare qubit basis nor in the dressed-state basis, but
instead results from a four-photon process that converts energy from the strong
drive to the weak probe. We find excellent agreement between the experimental
results and numerical simulations without any free fitting parameters. The
device demonstrated here may have applications in quantum information
processing and quantum-limited measurements
Proposal for a quantum delayed-choice experiment with a spin-mechanical setup
We describe an experimentally feasible protocol for performing a variant of
the quantum delayed-choice experiment with massive objects. In this scheme, a
single nitrogen-vacancy (NV) center in diamond driven by microwave fields is
dispersively coupled to a massive mechanical resonator. A double-pulse Ramsey
interferometer can be implemented with the spin-mechanical setup, where the
second Ramsey microwave pulse drives the spin conditioned on the number states
of the resonator. The probability for finding the NV center in definite spin
states exhibits interference fringes when the mechanical resonator is prepared
in a specific number state. On the other hand, the interference is destroyed if
the mechanical resonator stays in some other number states. The wavelike and
particlelike behavior of the NV spin can be superposed by preparing the
mechanical resonator in a superposition of two distinct number states. Thus a
quantum version of Wheeler's delayed-choice experiment could be implemented,
allowing of fundamental tests of quantum mechanics on a macroscopic scale.Comment: To be published in Phys.Rev.
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