41 research outputs found
Intrication temporelle et communication quantique
La communication quantique est l'art de transférer un état quantique d'un endroit à un
autre et l'étude des tâches que cela permet d'accomplir. Cette thèse présente des avancées
technologiques et théoriques appliquées à la communication quantique dans un contexte
réaliste avec essais sur le terrain. Ceci a été réalisé à l'aide d'une transmission de l'information
quantique par une fibre optique déployée dans un environnement urbain. Les innovations
présentées élargissent le champ d'application de l'intrication temporelle à travers l'élaboration
de nouvelles méthodes pour manipuler l'encodage temporel, d'un nouveau modèle de caract
érisation d'une source de paires de photons, de nouvelles facons d'étudier la non-localité
et de l'élaboration et la première réalisation d'un nouveau protocole de pile ou face quantique
tolérant aux pertes.
Manipulation de l'encodage temporel
Le photon unique est un excellent véehicule avec lequel un qubit, l'unité fondamentale
de l'information quantique, peut être encodée. En particulier, l'encodage temporel de qubits
photoniques est bien adapté à la transmission par fibre optique. Avant les travaux de cette
thèse, le champ d'application de cet encodage était limité par l'absence de méthodes réalisant
opérations et mesures arbitraires. Nous avons éliminé cette restriction et propose les premières
méthodes permettant de réaliser une opération arbitraire et deterministe sur un qubit temporel
ainsi qu'une mesure dans une base arbitraire. Nous avons appliqué ces propositions au
cas spécifique du calcul quantique basé sur la mesure et sur l'optique linéaire et montré comment
réaliser les opérations en aval essentielles à cette approche. Ceci ouvre la voie vers la
création d'un ordinateur quantique basé sur l'optique, mais également à de nouvelles tâches
en communication quantique.
Caractérisation de sources de paires de photons
La communication quantique expérimentale nécessite la création de photons uniques et
de paires de photons intriqués. Ces deux ingrédients peuvent être obtenus à partir d'une
source de paires de photons basée sur un processus non-linéaire spontané. Plusieurs tâches
en communication quantique nécessitent une connaissance précise des propriétés de la source
utilisée. Nous avons développé et démontré expérimentalement une nouvelle méthode simple
et rapide permettant de caractériser une source de paires de photons. Cette méthode est
particulièrement bien adaptée à un contexte de transmission sur le terrain où les conditions
expérimentales, telles que la transmittance d'un canal, peuvent
uctuer, et où la caract
érisation de la source doit être faite en temps réel.----------Abstract: Quantum communication is the art of transferring a quantum state from one place to
another and the study of tasks that can be accomplished with it. This thesis is devoted
to the development of tools and tasks for quantum communication in a real-world setting.
These were implemented using an underground optical bre link deployed in an urban environment.
The technological and theoretical innovations presented here broaden the range of
applications of time-bin entanglement through new methods of manipulating time-bin qubits,
a novel model for characterizing sources of photon pairs, new ways of testing non-locality
and the design and the rst implementation of a new loss-tolerant quantum coin-
ipping
protocol.
Manipulating time-bin qubits
A single photon is an excellent vehicle in which a qubit, the fundamental unit of quantum
information, can be encoded. In particular, the time-bin encoding of photonic qubits is
well suited for optical bre transmission. Before this thesis, the applications of quantum
communication based on the time-bin encoding were limited due to the lack of methods to
implement arbitrary operations and measurements. We have removed this restriction by
proposing the rst methods to realize arbitrary deterministic operations on time-bin qubits
as well as single qubit measurements in an arbitrary basis. We applied these propositions
to the specic case of optical measurement-based quantum computing and showed how to
implement the feedforward operations, which are essential to this model. This therefore
opens new possibilities for creating an optical quantum computer, but also for other quantum
communication tasks.
Characterizing sources of photon pairs
Experimental quantum communication requires the creation of single photons and entangled
photons. These two ingredients can be obtained from a source of photon pairs based on
non-linear spontaneous processes. Several tasks in quantum communication require a precise
knowledge of the properties of the source being used. We developed and implemented a fast
and simple method to characterize a source of photon pairs. This method is well suited for a
realistic setting where experimental conditions, such as channel transmittance, may
uctuate,
and for which the characterization of the source has to be done in real time.
Testing the non-locality of time-bin entanglement
Entanglement is a resource needed for the realization of many important tasks in quantum
communication. It also allows two physical systems to be correlated in a way that canno
Cryptographie quantique à plusieurs participants par multiplexage en longueur d'onde
Mémoire numérisé par la Direction des bibliothèques de l'Université de Montréal
Testing nonlocality over 12.4 km of underground fiber with universal time-bin qubit analyzers
We experimentally demonstrate that the nonlocal nature of time-bin entangled
photonic qubits persists when one or two qubits of the pair are converted to
polarization qubits. This is possible by implementing a novel Universal
Time-Bin Qubit Analyzer (UTBA), which, for the first time, allows analyzing
time-bin qubits in any basis. We reveal the nonlocal nature of the emitted
light by violating the Clauser-Horne-Shimony-Holt inequality with measurement
bases exploring all the dimensions of the Bloch sphere. Moreover, we conducted
experiments where one qubit is transmitted over a 12.4 km underground fiber
link and demonstrate the suitability of our scheme for use in a real-world
setting. The resulting entanglement can also be interpreted as hybrid
entanglement between different types of degrees of freedom of two physical
systems, which could prove useful in large scale, heterogeneous quantum
networks. This work opens new possibilities for testing nonlocality and for
implementing new quantum communication protocols with time-bin entanglement.Comment: 6 pages, 5 figure
Quantum storage of polarization qubits in birefringent and anisotropically absorbing materials
Storage of quantum information encoded into true single photons is an
essential constituent of long-distance quantum communication based on quantum
repeaters and of optical quantum information processing. The storage of
photonic polarization qubits is, however, complicated by the fact that many
materials are birefringent and have polarization-dependent absorption. Here we
present and demonstrate a simple scheme that allows compensating for these
polarization effects. The scheme is demonstrated using a solid-state quantum
memory implemented with an ensemble of rare-earth ions doped into a biaxial
yttrium orthosilicate () crystal. Heralded single photons generated
from a filtered spontaneous parametric downconversion source are stored, and
quantum state tomography of the retrieved polarization state reveals an average
fidelity of , which is significantly higher than what is
achievable with a measure-and-prepare strategy.Comment: 7 pages, 3 figures, 1 table, corrected typos and added ref. 3
Experimental certification of millions of genuinely entangled atoms in a solid
Quantum theory predicts that entanglement can also persist in macroscopic
physical systems, albeit difficulties to demonstrate it experimentally remain.
Recently, significant progress has been achieved and genuine entanglement
between up to 2900 atoms was reported. Here we demonstrate 16 million genuinely
entangled atoms in a solid-state quantum memory prepared by the heralded
absorption of a single photon. We develop an entanglement witness for
quantifying the number of genuinely entangled particles based on the collective
effect of directed emission combined with the nonclassical nature of the
emitted light. The method is applicable to a wide range of physical systems and
is effective even in situations with significant losses. Our results clarify
the role of multipartite entanglement in ensemble-based quantum memories as a
necessary prerequisite to achieve a high single-photon process fidelity crucial
for future quantum networks. On a more fundamental level, our results reveal
the robustness of certain classes of multipartite entangled states, contrary
to, e.g., Schr\"odinger-cat states, and that the depth of entanglement can be
experimentally certified at unprecedented scales.Comment: 11 pages incl. Methods and Suppl. Info., 4 figures, 1 table. v2:
close to published version. See also parallel submission by Zarkeshian et al
(1703.04709
High-detection efficiency and low-timing jitter with amorphous superconducting nanowire single-photon detectors
Recent progress in the development of superconducting nanowire single-photon
detectors (SNSPDs) made of amorphous material has delivered excellent
performances, and has had a great impact on a range of research fields. Despite
showing the highest system detection efficiency (SDE) ever reported with
SNSPDs, amorphous materials typically lead to lower critical currents, which
impacts on their jitter performance. Combining a very low jitter and a high SDE
remains a challenge. Here, we report on highly efficient superconducting
nanowire single-photon detectors based on amorphous MoSi, combining system
jitters as low as 26 ps and a SDE of 80% at 1550 nm. We also report detailed
observations on the jitter behaviour, which hints at intrinsic limitations and
leads to practical implications for SNSPD performance
Enhanced heralded single-photon source with a photon-number-resolving parallel superconducting nanowire single-photon detector
Heralded single-photon sources (HSPS) intrinsically suffer from multiphoton
emission, leading to a trade-off between the source's quality and the heralding
rate. A solution to this problem is to use photon-number-resolving (PNR)
detectors to filter out the heralding events where more than one photon pair is
created. Here, we demonstrate the use of a high-efficiency PNR superconducting
nanowire single-photon detector (SNSPD) as a heralding detector for a HSPS. By
filtering out higher-order heralding detections, we can reduce the
of the heralded single photon by , or alternatively, for a
fixed pump power, increasing the heralding rate by a factor of for a fixed . Additionally, we use the detector to directly
measure the photon-number distribution of a thermal mode and calculate the
unheralded . We show the possibility to perform
measurements with only one PNR detector, with the results in agreement with
those obtained by more common-place techniques which use multiple threshold
detectors. Our work shows that efficient PNR SNSPDs can significantly improve
the performance of HSPSs and can precisely characterize them, making these
detectors a useful tool for a wide range of optical quantum information
protocols
High-efficiency and fast photon-number resolving parallel superconducting nanowire single-photon detector
Photon-number resolving (PNR) single-photon detectors are an enabling
technology in many areas such as photonic quantum computing, non-classical
light source characterisation and quantum imaging. Here, we demonstrate
high-efficiency PNR detectors using a parallel superconducting nanowire
single-photon detector (P-SNSPD) architecture that does not suffer from
crosstalk between the pixels and that is free of latching. The behavior of the
detector is modelled and used to predict the possible outcomes given a certain
number of incoming photons. We apply our model to a 4-pixel P-SNSPD with a
system detection efficiency of 92.5%. We also demonstrate how this detector
allows reconstructing the photon-number statistics of a coherent source of
light, which paves the way towards the characterisation of the photon
statistics of other types of light source using a single detector.Comment: 8 pages, 7 figure
Cluster state quantum computing in optical fibers
A scheme for the implementation of the cluster state model of quantum
computing in optical fibers, which enables the feedforward feature, is
proposed. This scheme uses the time-bin encoding of qubits. Following
previously suggested methods of applying arbitrary one-qubit gates in optical
fibers, two different ways for the realization of fusion gate types I and II
for cluster production are proposed: a fully time-bin based encoding scheme and
a combination of time-bin and polarization based encoding scheme. Also the
methods of measurement in any desired bases for the purpose of the processing
of cluster state computing for both these encodings are explained.Comment: 6 pages, 11 figures, submitted to the Optical Quantum-Information
Science focus issue of JOSA
Fair Loss-Tolerant Quantum Coin Flipping
Coin flipping is a cryptographic primitive in which two spatially separated
players, who in principle do not trust each other, wish to establish a common
random bit. If we limit ourselves to classical communication, this task
requires either assumptions on the computational power of the players or it
requires them to send messages to each other with sufficient simultaneity to
force their complete independence. Without such assumptions, all classical
protocols are so that one dishonest player has complete control over the
outcome. If we use quantum communication, on the other hand, protocols have
been introduced that limit the maximal bias that dishonest players can produce.
However, those protocols would be very difficult to implement in practice
because they are susceptible to realistic losses on the quantum channel between
the players or in their quantum memory and measurement apparatus. In this
paper, we introduce a novel quantum protocol and we prove that it is completely
impervious to loss. The protocol is fair in the sense that either player has
the same probability of success in cheating attempts at biasing the outcome of
the coin flip. We also give explicit and optimal cheating strategies for both
players.Comment: 12 pages, 1 figure; various minor typos corrected in version