112 research outputs found
Probing wave function collapse models with a classically driven mechanical oscillator
We show that the interaction of a pulsed laser light with a mechanical
oscillator through the radiation pressure results in an opto-mechanical
entangled state in which the photon number is correlated with the oscillator
position. Interestingly, the mechanical oscillator can be delocalized over a
large range of positions when driven by an intense laser light. This provides a
simple yet sensitive method to probe hypothetic post-quantum theories including
an explicit wave function collapse model, like the Diosi and Penrose model. We
propose an entanglement witness to reveal the quantum nature of this
opto-mechanical state as well as an optical technique to record the decoherence
of the mechanical oscillator. We also report on a detailed feasibility study
giving the experimental challenges that need to be overcome to confirm or rule
out predictions from explicit wave function collapse models.Comment: 11 pages, 2 figures. Corrections, and added appendi
Optimal Photon Generation from Spontaneous Raman Processes in Cold Atoms
Spontaneous Raman processes in cold atoms have been widely used in the past
decade for generating single photons. Here, we present a method to optimize
their efficiencies for given atomic coherences and optical depths. We give a
simple and complete recipe that can be used in present-day experiments,
attaining near-optimal single photon emission while preserving the photon
purity.Comment: 6+6 pages, 3 figures, 1 tabl
Witnessing single-photon entanglement with local homodyne measurements: analytical bounds and robustness to losses
Single-photon entanglement is one of the primary resources for quantum
networks, including quantum repeater architectures. Such entanglement can be
revealed with only local homodyne measurements through the entanglement witness
presented in [Morin et al. Phys. Rev. Lett. 110, 130401 (2013)]. Here, we
provide an extended analysis of this witness by introducing analytical bounds
and by reporting measurements confirming its great robustness with regard to
losses. This study highlights the potential of optical hybrid methods, where
discrete entanglement is characterized through continuous-variable
measurements
Enabling secure communications: theoretical tools for quantum repeater systems
As children, whispering into the ear of a friend in the presence of others allows us to pass a secret without interception, and forms one of the simplest attempts at secret communications we can employ. However, sending secret messages becomes deeply nontrivial over long distances.
A solution for two parties to communicate securely is to encrypt and decrypt a message with two identical strings of bits, one for each party. In this case, the security of the encrypted message is provable and does not rely on assumptions on computational power. Quantum theory provides a clear solution for the initial distribution of these identical bit strings through Quantum Key Distribution. However, once long distances are involved, the corresponding loss involved in direct transmission ruins the effectiveness of quantum key distribution by reducing the effective rate exponentially with the distance.
To circumvent the losses involved in direct transmission, quantum repeater architectures have been proposed. We present our contributions towards three aspects of quantum repeater systems in this thesis. We ensure conditions for implementing quantum repeaters with atomic ensembles, explore the option of optomechanical systems for implementing quantum repeaters and verify the success of completed quantum repeater protocols.
In the first part of this thesis, we show how we can ensure conditions for the successful implementation of quantum repeater systems with atomic ensembles. These quantum repeater systems are formed with 1-dimensional networks, where the nodes are made up of quantum memories connected by means of single photons. This requires memories that are highly efficient. Also, if quantum repeater systems are implemented with hybrid resources, tunable photon waveforms will be desirable. We propose a protocol to implement quantum memories with atomic ensembles using a clear recipe to optimise the efficiency. We also demonstrate that a cold ensemble of Rubidium-87 can act as an efficient tunable source of single photons, along with flexibility in the produced temporal shapes.
Next, we show how we can explore alternative options for the nodes of quantum repeater systems. We focus on optomechanical oscillators, and recognise that they can also be used as quantum memories. We present a witness to certify that this memory successfully operates in the quantum regime.
Finally, we focus on the verification of successfully implemented quantum repeater protocols. This verification will be essential for certifying that quantum repeater systems operate as instructed. We use only local homodyne measurements to witness the success of the network, and find that the witness is robust to loss.
We thus present distinct contributions towards three important aspects of quantum repeater systems. As far as a full-fledged quantum repeater system might seem to be right now, we have faith that our work brings the field of quantum-enabled secure communications forward
Enabling Psychiatrists to Explore the Full Potential of E-Health
10.3389/fpsyt.2015.00177Frontiers in Psychiatry6DEC17
Witnessing Opto-Mechanical Entanglement with Photon-Counting
The ability to coherently control mechanical systems with optical fields has
made great strides over the past decade, and now includes the use of photon
counting techniques to detect the non-classical nature of mechanical states.
These techniques may soon be used to perform an opto-mechanical Bell test,
hence highlighting the potential of cavity opto-mechanics for
device-independent quantum information processing. Here, we propose a witness
which reveals opto-mechanical entanglement without any constraint on the global
detection efficiencies in a setup allowing one to test a Bell inequality. While
our witness relies on a well-defined description and correct experimental
calibration of the measurements, it does not need a detailed knowledge of the
functioning of the opto-mechanical system. A feasibility study including
dominant sources of noise and loss shows that it can readily be used to reveal
opto-mechanical entanglement in present-day experiments with photonic crystal
nanobeam resonators.Comment: 5+7 pages, 4 figure
Witnessing trustworthy single-photon entanglement with local homodyne measurements
Single-photon entangled states, i.e. states describing two optical paths
sharing a single photon, constitute the simplest form of entanglement. Yet they
provide a valuable resource in quantum information science. Specifically, they
lie at the heart of quantum networks, as they can be used for quantum
teleportation, swapped and purified with linear optics. The main drawback of
such entanglement is the difficulty in measuring it. Here, we present and
experimentally test an entanglement witness allowing one not only to say
whether a given state is path-entangled but also that entanglement lies in the
subspace where the optical paths are each filled with one photon at most, i.e.
refers to single-photon entanglement. It uses local homodyning only and relies
on no assumption about the Hilbert space dimension of the measured system. Our
work provides a simple and trustful method for verifying the proper functioning
of future quantum networks.Comment: published versio
Device-independent certification of entangled measurements
We present a device-independent protocol to test if a given black-box
measurement device is entangled, that is, has entangled eigenstates. Our scheme
involves three parties and is inspired by entanglement swapping; the test uses
the Clauser-Horne-Shimony-Holt (CHSH) Bell inequality, checked between each
pair of parties. Also, focusing on the case where all particles are qubits, we
characterize quantitatively the deviation of the measurement device from a
perfect Bell state measurement.Comment: 5 pages, 2 figure
Molecular Tracers of the Central 12 pc of the Galactic Center
We have used the BIMA array to image the Galactic Center with a 19-pointing
mosaic in HCN(1-0), HCO+(1-0), and H 42-alpha emission with 5 km/s velocity
resolution and 13'' x 4'' angular resolution. The 5' field includes the
circumnuclear ring (CND) and parts of the 20 and 50 km/s clouds. HCN(1-0) and
HCO+ trace the CND and nearby giant molecular clouds while the H 42-alpha
emission traces the ionized gas in Sgr A West. We find that the CND has a
definite outer edge in HCN and HCO+ emission at ~45'' radius and appears to be
composed of two or three distinct streams of molecular gas rotating around the
nucleus. Outside the CND, HCN and HCO+ trace dense clumps of high-velocity gas
in addition to optically thick emission from the 20 and 50 km/s clouds. A
molecular ridge of compressed gas and dust, traced in NH3 emission and
self-absorbed HCN and HCO+, wraps around the eastern edge of Sgr A East. Just
inside this ridge are several arcs of gas which have been accelerated by the
impact of Sgr A East with the 50 km/s cloud. HCN and HCO+ emission trace the
extension of the northern arm of Sgr A West which appears to be an independent
stream of neutral and ionized gas and dust originating outside the CND. Broad
line widths and OH maser emission mark the intersection of the northern arm and
the CND. Comparison to previous NH3 and 1.2mm dust observations shows that HCN
and HCO+ preferentially trace the CND and are weaker tracers of the GMCs than
NH3 and dust. We discuss possible scenarios for the emission mechanisms and
environment at the Galactic center which could explain the differences in these
images.Comment: 24 pages, including 17 figures; to appear in The Astrophysical
Journa
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