49 research outputs found
Entangled collective-spin states of atomic ensembles under non-uniform atom-light interaction
We consider the optical generation and verification of entanglement in atomic
ensembles under non-uniform interaction between the ensemble and an optical
mode. We show that for a wide range of parameters a system of non-uniformly
coupled atomic spins can be described as an ensemble of uniformly coupled spins
with a reduced effective atom-light coupling and a reduced effective atom
number, with a reduction factor of order unity given by the ensemble-mode
geometry. This description is valid even for complex entangled states with
arbitrary phase-space distribution functions as long as the detection does not
resolve single spins. Furthermore, we derive an analytic formula for the
observable entanglement in the case, of relevance in practice, where the
ensemble-mode coupling differs between state generation and measurement.Comment: 5 pages, 3 figure
Vacuum spin squeezing
We investigate the generation of entanglement (spin squeezing) in an
optical-transition atomic clock through the coupling to a vacuum
electromagnetic field that is enhanced by an optical cavity. We show that if
each atom is prepared in a superposition of the ground state and a long-lived
electronic excited state, and viewed as a spin-1/2 system, then the collective
vacuum light shift entangles the atoms, resulting in a squeezed distribution of
the ensemble collective spin. This scheme reveals that even a vacuum field can
be a useful resource for entanglement and quantum manipulation. The method is
simple and robust since it requires neither the application of light nor
precise frequency control of the ultra-high-finesse cavity. Furthermore, the
scheme can be used to implement two-axis twisting by rotating the spin
direction while coupling to the vacuum, resulting in stronger squeezing
Creation of a Bose-condensed gas of rubidium 87 by laser cooling
We demonstrate direct laser cooling of a gas of rubidium 87 atoms to quantum
degeneracy. The method does not involve evaporative cooling, is fast, and
induces little atom loss. The atoms are trapped in a two-dimensional optical
lattice that enables cycles of cloud compression to increase the density,
followed by degenerate Raman sideband cooling to decrease the temperature.
Light-induced loss at high atomic density is substantially reduced by using far
red detuned optical pumping light. Starting with 2000 atoms, we prepare 1400
atoms in 300 ms at quantum degeneracy, as confirmed by the appearance of a
bimodal velocity distribution as the system crosses over from a classical gas
to a Bose-condensed, interacting one-dimensional gas with a macroscopic
population of the quantum ground state. The method should be broadly applicable
to many bosonic and fermionic species, and to systems where evaporative cooling
is not possible.Comment: 5 pages, 3 figures (main text
Topological Waveguide Quantum Sensors
We present an efficient and robust protocol for quantum-enhanced sensing
using a single-spin qubit in the topological waveguide system. Our method
relies on the topological-paired bound states, which are localized near the
spin and can be effectively regarded as a two-level system. Through the lens of
Bayesian inference theory, we show the sensitivity can reach the Heisenberg
limit across a large field range. Inheriting from the topological robustness of
the waveguide, our sensing protocol is robust against local perturbations. The
advantages of our protocol are multifold as it allows for sensing various
parameters and uses a product initial state, which can be easily prepared in
experiments. We expect this approach would pave the way towards robust
topological quantum sensors based on near term quantum platforms such as
topological photonics and Rydberg arrays.Comment: 4.5 + 3 pages, 3 + 3 figure
Entanglement generation via single-qubit rotations in a teared Hilbert space
We propose an efficient yet simple protocol to generate arbitrary symmetric
entangled states with only global single-qubit rotations in a teared Hilbert
space. The system is based on spin-1/2 qubits in a resonator such as atoms in
an optical cavity or superconducting qubits coupled to a metal microwave
resonator. By sending light or microwave into the resonator, it induces AC
Stark shifts on particular angular-momentum eigenstates (Dicke states) of
qubits. Then we are able to generate barriers that hinder transitions between
adjacent Dicke states and tear the original Hilbert space into pieces.
Therefore, a simple global single-qubit rotation becomes highly non-trivial,
and thus generates entanglement among the many-body system. By optimal control
of energy shifts on Dicke states, we are able to generate arbitrary symmetric
entangled states. We also exemplify that we can create varieties of useful
states with near-unity fidelities in only one or very few steps, including W
states, spin-squeezed states (SSS), and Greenberger-Horne-Zeilinger (GHZ)
states. Particularly, the SSS can be created by only one step with a squeezing
parameter approaching the Heisenberg limit (HL). Our
finding establishes a way for universal entanglement generations with only
single-qubit drivings where all the multiple-qubit controls are integrated into
simply switching on/off microwave. It has direct applications in the
variational quantum optimizer which is available with existing technology.Comment: 12 pages, 10 figure
Strictly nonclassical behavior of a mesoscopic system
We experimentally demonstrate the strictly nonclassical behavior in a
many-atom system using a recently derived criterion [E. Kot et al., Phys. Rev.
Lett. 108, 233601 (2012)] that explicitly does not make use of quantum
mechanics. We thereby show that the magnetic moment distribution measured by
McConnell et al. [R. McConnell et al., Nature 519, 439 (2015)] in a system with
a total mass of atomic mass units is inconsistent with
classical physics. Notably, the strictly nonclassical behavior affects an area
in phase space times larger than the Planck quantum .Comment: 5 page