29 research outputs found
Limits of atomic entanglement by cavity-feedback : from weak to strong coupling
We theoretically investigate the entangled states of an atomic ensemble that
can be obtained via cavity-feedback, varying the atom-light coupling from weak
to strong, and including a systematic treatment of decoherence. In the strong
coupling regime for small atomic ensembles, the system is driven by cavity
losses into a long-lived, highly-entangled many-body state that we characterize
analytically. In the weak coupling regime for large ensembles, we find
analytically the maximum spin squeezing that can be achieved by optimizing both
the coupling and the atom number. This squeezing is fundamentally limited by
spontaneous emission to a constant value, independent of the atom number.
Harnessing entanglement in many-body systems is of fundamental interest [1] and
is the key requirement for quantum enhanced technologies, in particular quantum
metrology [2]. In this respect, many efforts have been devoted to prepare
entangled states in atomic ensembles because of their high degree of coherence
and their potential for precision measurement. Spin squeezed states as well as
number states have been produced following methods based either on coherent
evolution in the presence of a non-linearity in the atomic field [3--5], or on
quantum non-demolition measurement [6--8]. Among methods of the first kind,
cavity feedback [5, 9] is one of the most promising: it has already allowed for
the creation of highly squeezed states [5] and the effective non-linearity
introduced by the atom-cavity coupling can be easily switched off, making it
very attractive for metrol-ogy applications. In this Letter, we analyze the
entangled states that can be produced by cavity feedback in different coupling
regimes from weak to strong, and derive the ultimate limits of the metrology
gain, extending the optimization of squeezing to unexplored domains of
parameters values. After optimization of both the coupling strength and the
atom number, we find a maximum squeezing limit that depends only on the atomic
structure
Enhanced and reduced atom number fluctuations in a BEC splitter
We measure atom number statistics after splitting a gas of ultracold 87Rb
atoms in a purely magnetic double-well potential created on an atom chip. Well
below the critical temperature for Bose-Einstein condensation T_c, we observe
reduced fluctuations down to -4.9dB below the atom shot noise level.
Fluctuations rise to more than +3.8dB close to T_c, before reaching the shot
noise level for higher temperatures. We use two-mode and classical field
simulations to model these results. This allows us to confirm that the
super-shot noise fluctuations directly originate from quantum statistics
Quantum bath engineering of a high impedance microwave mode through quasiparticle tunneling
We demonstrate a new approach to dissipation engineering in microwave quantum
optics. For a single mode, dissipation usually corresponds to quantum jumps,
where photons are lost one by one. Here, we are able to tune the minimal number
of lost photons per jump to be two (or more) with a simple dc voltage. As a
consequence, different quantum states experience different dissipation.
Causality implies that the states must also experience different energy shifts.
Our measurements of these Lamb shifts are in good agreement with the
predictions of the Kramers-Kronig relations for single quantum states in a
regime of highly non-linear bath coupling. This work opens new possibilities
for quantum state manipulation in circuit QED, without relying on the Josephson
effect
Observation of the Unconventional Photon Blockade in the Microwave Domain
We have observed the unconventional photon blockade effect for microwave
photons using two coupled superconducting resonators. As opposed to the
conventional blockade, only weakly nonlinear resonators are required. The
blockade is revealed through measurements of the second order correlation
function of the microwave field inside one of the two resonators.
The lowest measured value of is 0.4 for a resonator population of
approximately photons. The time evolution of exhibits an
oscillatory behavior, which is characteristic of the unconventional photon
blockade
Cavity-based single atom preparation and high-fidelity hyperfine state readout
We prepare and detect the hyperfine state of a single 87Rb atom coupled to a
fiber-based high finesse cavity on an atom chip. The atom is extracted from a
Bose-Einstein condensate and trapped at the maximum of the cavity field,
resulting in a reproducibly strong atom-cavity coupling. We use the cavity
reflection and transmission signal to infer the atomic hyperfine state with a
fidelity exceeding 99.92% in a read-out time of 100 microseconds. The atom is
still trapped after detection.Comment: 5 pages, 4 figure
Towards a monolithic optical cavity for atom detection and manipulation
We study a Fabry-Perot cavity formed from a ridge waveguide on a AlGaAs
substrate. We experimentally determined the propagation losses in the waveguide
at 780 nm, the wavelength of Rb atoms. We have also made a numerical and
analytical estimate of the losses induced by the presence of the gap which
would allow the interaction of cold atoms with the cavity field. We found that
the intrinsic finesse of the gapped cavity can be on the order of F ~ 30,
which, when one takes into account the losses due to mirror transmission,
corresponds to a cooperativity parameter for our system C ~ 1