39,795 research outputs found
Genuine Counterfactual Communication with a Nanophotonic Processor
In standard communication information is carried by particles or waves.
Counterintuitively, in counterfactual communication particles and information
can travel in opposite directions. The quantum Zeno effect allows Bob to
transmit a message to Alice by encoding information in particles he never
interacts with. The first suggested protocol not only required thousands of
ideal optical components, but also resulted in a so-called "weak trace" of the
particles having travelled from Bob to Alice, calling the scalability and
counterfactuality of previous proposals and experiments into question. Here we
overcome these challenges, implementing a new protocol in a programmable
nanophotonic processor, based on reconfigurable silicon-on-insulator waveguides
that operate at telecom wavelengths. This, together with our telecom
single-photon source and highly-efficient superconducting nanowire
single-photon detectors, provides a versatile and stable platform for a
high-fidelity implementation of genuinely trace-free counterfactual
communication, allowing us to actively tune the number of steps in the Zeno
measurement, and achieve a bit error probability below 1%, with neither
post-selection nor a weak trace. Our demonstration shows how our programmable
nanophotonic processor could be applied to more complex counterfactual tasks
and quantum information protocols.Comment: 6 pages, 4 figure
Imprinting interference fringes in massive optomechanical systems
An interferometric scheme for the creation of momentum superposition states
of mechanical oscillators, using a quantum mirror kicked by free photons is
analyzed. The scheme features ultra-fast preparation with immediate detection
and should allow for the observation of signatures of momentum superpositions
in a massive macroscopic system at non-zero temperatures. It is robust against
thermalized initial states, displacement and movement, mirror imperfections,
and the measurements' back-actions.Comment: 4 pages, 3 figures, 7 subfigure
Generation of a flat-top laser beam for gravitational wave detectors by means of a nonspherical Fabry-Perot resonator
We have tested a new kind of Fabry-Perot long-baseline optical resonator proposed to reduce the thermal noise sensitivity of gravitational wave interferometric detectors--the "mesa beam" cavity--whose flat top beam shape is achieved by means of an aspherical end mirror. We present the fundamental mode intensity pattern for this cavity and its distortion due to surface imperfections and tilt misalignments, and contrast the higher order mode patterns to the Gauss-Laguerre modes of a spherical mirror cavity. We discuss the effects of mirror tilts on cavity alignment and locking and present measurements of the mesa beam tilt sensitivity
Efficient coupling to an optical resonator by exploiting time-reversal symmetry
The interaction of a cavity with an external field is symmetric under time
reversal. Thus, coupling to a resonator is most efficient when the incident
light is the time reversed version of a free cavity decay, i.e. when it has a
rising exponential shape matching the cavity lifetime. For light entering the
cavity from only one side, the maximally achievable coupling efficiency is
limited by the choice of the cavity mirrors' reflectivities. Such an
empty-cavity experiment serves also as a model system for single-photon
single-atom absorption dynamics. We present experiments coupling exponentially
rising pulses to a cavity system which allows for high coupling efficiencies.
The influence of the time constant of the rising exponential is investigated as
well as the effect of a finite pulse duration. We demonstrate coupling 94% of
the incident TEM00 mode into the resonator.Comment: 7 pages, 5 figure
Quantum noise of non-ideal Sagnac speed meter interferometer with asymmetries
The speed meter concept has been identified as a technique that can
potentially provide laser-interferometric measurements at a sensitivity level
which surpasses the Standard Quantum Limit (SQL) over a broad frequency range.
As with other sub-SQL measurement techniques, losses play a central role in
speed meter interferometers and they ultimately determine the quantum noise
limited sensitivity that can be achieved. So far in the literature, the quantum
noise limited sensitivity has only been derived for lossless or lossy cases
using certain approximations (for instance that the arm cavity round trip loss
is small compared to the arm cavity mirror transmission). In this article we
present a generalised, analytical treatment of losses in speed meters that
allows accurate calculation of the quantum noise limited sensitivity of Sagnac
speed meters with arm cavities. In addition, our analysis allows us to take
into account potential imperfections in the interferometer such as an
asymmetric beam splitter or differences of the reflectivities of the two arm
cavity input mirrors. Finally,we use the examples of the proof-of-concept
Sagnac speed meter currently under construction in Glasgow and a potential
implementation of a Sagnac speed meter in the Einstein Telescope (ET) to
illustrate how our findings affect Sagnac speed meters with meter- and
kilometre-long baselines.Comment: 22 pages, 8 figures, 1 table, (minor corrections and changes made to
text and figures in version 2
Quantum vacuum fluctuations
The existence of irreducible field fluctuations in vacuum is an important
prediction of quantum theory. These fluctuations have many observable
consequences, like the Casimir effect which is now measured with good accuracy
and agreement with theory, provided that the latter accounts for differences
between real experiments and the ideal situation considered by Casimir. But the
vacuum energy density calculated by adding field mode energies is much larger
than the density observed around us through gravitational phenomena. This
``vacuum catastrophe'' is one of the unsolved problems at the interface between
quantum theory on one hand, inertial and gravitational phenomena on the other
hand. It is however possible to put properly formulated questions in the
vicinity of this paradox. These questions are directly connected to observable
effects bearing upon the principle of relativity of motion in quantum vacuum.Comment: 8 pages, 2 figures, contribution to a special issue in CRAS (Comptes
rendus de l'Academie des Sciences), corrected typos, added reference
- …