505 research outputs found
Cavity Nonlinear Optics at Low Photon Numbers from Collective Atomic Motion
We report on Kerr nonlinearity and dispersive optical bistability of a
Fabry-Perot optical resonator due to the displacement of ultracold atoms
trapped within. In the driven resonator, such collective motion is induced by
optical forces acting upon up to Rb atoms prepared in the lowest
band of a one-dimensional intracavity optical lattice. The longevity of atomic
motional coherence allows for strongly nonlinear optics at extremely low cavity
photon numbers, as demonstrated by the observation of both branches of optical
bistability at photon numbers below unity.Comment: 4 pages, 3 figures. Modifed following reviewer comment
Observing Parity Time Symmetry Breaking in a Josephson Parametric Amplifier
A coupled two-mode system with balanced gain and loss is a paradigmatic
example of an open quantum system that can exhibit real spectra despite being
described by a non-Hermitian Hamiltonian. We utilize a degenerate parametric
amplifier operating in three-wave mixing mode to realize such a system of
balanced gain and loss between the two quadrature modes of the amplifier. By
examining the time-domain response of the amplifier, we observe a
characteristic transition from real-to-imaginary energy eigenvalues associated
with the Parity-Time-symmetry-breaking transition.Comment: 6 pages, 4 figure
Quantum jumps in the non-Hermitian dynamics of a superconducting qubit
We study the dynamics of a driven non-Hermitian superconducting qubit which
is perturbed by quantum jumps between energy levels, a purely quantum effect
with no classical correspondence. The quantum jumps mix the qubit states
leading to decoherence. We observe that this decoherence rate is enhanced near
the exceptional point, owing to the cube-root topology of the non-Hermitian
eigenenergies. Together with the effect of non-Hermitian gain/loss, quantum
jumps can also lead to a breakdown of adiabatic evolution under the
slow-driving limit. Our study shows the critical role of quantum jumps in
generalizing the applications of classical non-Hermitian systems to open
quantum systems for sensing and control.Comment: 11 pages, 9 figure
Floquet exceptional contours in Lindblad dynamics with time-periodic drive and dissipation
The dynamics of an isolated quantum system is coherent and unitary. Weak
coupling to the environment leads to decoherence, which is traditionally
modeled with a Lindblad equation for the system's density matrix. Starting from
a pure state, such a system approaches a steady state (mixed or otherwise) in
an underdamped or overdamped manner. This transition occurs at an eigenvalue
degeneracy of a Lindblad superoperator, called an exceptional point (EP), where
corresponding eigenvectors coalesce. Recent years have seen an explosion of
interest in creating exceptional points in a truly quantum domain, driven by
the enhanced sensitivity and topological features EPs have shown in their
classical realizations. Here, we present Floquet analysis of a prototypical
qubit whose drive or dissipator strengths are varied periodically. We consider
models with a single dissipator that generate global loss (phase damping) or
mode-selective loss (spontaneous emission). In all cases, we find that periodic
modulations lead to EP lines at small dissipator strengths, and a rich EP
structure in the parameter space. Our analytical and numerical results show
that extending Lindblad Liouvillians to the Floquet domain is a new,
potentially preferred route to accessing exceptional points in the transient
dynamics towards the Lindblad steady state.Comment: 4 figures, 7 page
Collimated, single-pass atom source from a pulsed alkali metal dispenser for laser-cooling experiments
We have developed an improved scheme for loading atoms into a magneto-optical
trap (MOT) from a directed alkali metal dispenser in < 10^-10 torr ultra-high
vacuum conditions. A current-driven dispenser was surrounded with a cold
absorbing "shroud" held at < 0 C, pumping rubidium atoms not directed into the
MOT. This nearly eliminates background alkali atoms and reduces the detrimental
rise in pressure normally associated with these devices. The system can be
well-described as a current-controlled, rapidly-switched, two-temperature
thermal beam, and was used to load a MOT with 3 x 10^8 atoms.Comment: 5 pages, 4 figure
Efficiently Fuelling a Quantum Engine with Incompatible Measurements
We propose a quantum harmonic oscillator measurement engine fueled by
simultaneous quantum measurements of the non-commuting position and momentum
quadratures of the quantum oscillator. The engine extracts work by moving the
harmonic trap suddenly, conditioned on the measurement outcomes. We present two
protocols for work extraction, respectively based on single-shot and
time-continuous quantum measurements. In the single-shot limit, the oscillator
is measured in a coherent state basis; the measurement adds an average of one
quantum of energy to the oscillator, which is then extracted in the feedback
step. In the time-continuous limit, continuous weak quantum measurements of
both position and momentum of the quantum oscillator result in a coherent
state, whose coordinates diffuse in time. We relate the extractable work to the
noise added by quadrature measurements, and present exact results for the work
distribution at arbitrary finite time. Both protocols can achieve unit work
conversion efficiency in principle.Comment: 13 pages, 5 figure
Speeding up entanglement generation by proximity to higher-order exceptional points
Entanglement is a key resource for quantum information technologies ranging
from quantum sensing to quantum computing. Conventionally, the entanglement
between two coupled qubits is established at the time scale of the inverse of
the coupling strength. In this work, we study two weakly coupled non-Hermitian
qubits and observe entanglement generation at a significantly shorter time
scale by proximity to a higher-order exceptional point. We establish a
non-Hermitian perturbation theory based on constructing a biorthogonal complete
basis and further identify the optimal condition to obtain the maximally
entangled state. Our study of speeding up entanglement generation in
non-Hermitian quantum systems opens new avenues for harnessing coherent
nonunitary dissipation for quantum technologies.Comment: 6+18 pages, 4+12 figures. Zeng-Zhao Li and Weijian Chen contributed
equally to this wor
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