118 research outputs found
Noiseless Linear Amplifiers in Entanglement-Based Continuous-Variable Quantum Key Distribution
We propose a method to improve the performance of two entanglement-based
continuous-variable quantum key distribution protocols using noiseless linear
amplifiers. The two entanglement-based schemes consist of an entanglement
distribution protocol with an untrusted source and an entanglement swapping
protocol with an untrusted relay. Simulation results show that the noiseless
linear amplifiers can improve the performance of these two protocols, in terms
of maximal transmission distances, when we consider small amounts of
entanglement, as typical in realistic setups.Comment: Special issue on Quantum Cryptograph
Improvement of two-way continuous-variable quantum key distribution with virtual photon subtraction
We propose a method to improve the performance of two-way continuous-variable
quantum key distribution protocol by virtual photon subtraction. The Virtual
photon subtraction implemented via non-Gaussian post-selection not only
enhances the entanglement of two-mode squeezed vacuum state but also has
advantages in simplifying physical operation and promoting efficiency. In
two-way protocol, virtual photon subtraction could be applied on two sources
independently. Numerical simulations show that the optimal performance of
renovated two-way protocol is obtained with photon subtraction only used by
Alice. The transmission distance and tolerable excess noise are improved by
using the virtual photon subtraction with appropriate parameters. Moreover, the
tolerable excess noise maintains a high value with the increase of distance so
that the robustness of two-way continuous-variable quantum key distribution
system is significantly improved, especially at long transmission distance.Comment: 15 pages, 6 figure
Continuous Variable Optimisation of Quantum Randomness and Probabilistic Linear Amplification
In the past decade, quantum communication protocols based on
continuous variables (CV) has seen considerable development in
both theoretical and experimental aspects.
Nonetheless, challenges remain in both the practical security and
the operating range for CV systems, before such systems may be
used extensively. In this thesis, we present
the optimisation of experimental parameters for secure randomness
generation and propose a non-deterministic approach to enhance
amplification of CV quantum state.
The first part of this thesis examines the security of quantum
devices: in particular, we investigate quantum random number
generators (QRNG) and quantum key distribution
(QKD) schemes. In a realistic scenario, the output of a quantum
random number generator is inevitably tainted by classical
technical noise, which potentially compromises
the security of such a device. To safeguard against this, we
propose and experimentally demonstrate an approach that produces
side-information independent randomness. We present a method for
maximising such randomness contained in a number sequence
generated from a given quantum-to-classical-noise ratio. The
detected photocurrent
in our experiment is shown to have a real-time random-number
generation rate of 14 (Mbit/s)/MHz.
Next, we study the one-sided device-independent (1sDI) quantum
key distribution scheme in the context of continuous variables.
By exploiting recently proven entropic
uncertainty relations, one may bound the information leaked to an
eavesdropper. We use such a bound to further derive the secret
key rate, that depends only upon the
conditional Shannon entropies accessible to Alice and Bob, the
two honest communicating parties. We identify and experimentally
demonstrate such a protocol, using only
coherent states as the resource. We measure the correlations
necessary for 1sDI key distribution up to an applied loss
equivalent to 3.5 km of fibre transmission.
The second part of this thesis concerns the improvement in the
transmission of a quantum state. We study two approximate
implementations of a probabilistic noiseless
linear amplifier (NLA): a physical implementation that truncates
the working space of the NLA or a measurement-based
implementation that realises the truncation
by a bounded postselection filter. We do this by conducting a
full analysis on the measurement-based NLA (MB-NLA), making
explicit the relationship between its various
operating parameters, such as amplification gain and the cut-off
of operating domain. We compare it with its physical counterpart
in terms of the Husimi Q-distribution and
their probability of success.
We took our investigations further by combining a probabilistic
NLA with an ideal deterministic linear amplifier (DLA). In
particular, we show that when NLA gain is strictly lesser than
the DLA gain, this combination can be realised by integrating an
MB-NLA in an optical DLA setup. This results in a hybrid device
which we refer to as the heralded hybrid quantum amplifier. A
quantum cloning machine based on this hybrid amplifier is
constructed through an amplify-then-split method. We perform
probabilistic cloning of arbitrary coherent states, and
demonstrate the production of up to five clones, with the
fidelity of each clone clearly exceeding the corresponding
no-cloning limit
Distributing Secret Keys with Quantum Continuous Variables: Principle, Security and Implementations
The ability to distribute secret keys between two parties with
information-theoretic security, that is, regardless of the capacities of a
malevolent eavesdropper, is one of the most celebrated results in the field of
quantum information processing and communication. Indeed, quantum key
distribution illustrates the power of encoding information on the quantum
properties of light and has far reaching implications in high-security
applications. Today, quantum key distribution systems operate in real-world
conditions and are commercially available. As with most quantum information
protocols, quantum key distribution was first designed for qubits, the
individual quanta of information. However, the use of quantum continuous
variables for this task presents important advantages with respect to qubit
based protocols, in particular from a practical point of view, since it allows
for simple implementations that require only standard telecommunication
technology. In this review article, we describe the principle of
continuous-variable quantum key distribution, focusing in particular on
protocols based on coherent states. We discuss the security of these protocols
and report on the state-of-the-art in experimental implementations, including
the issue of side-channel attacks. We conclude with promising perspectives in
this research field.Comment: 21 pages, 2 figures, 1 tabl
Quantum-based security in optical fibre networks
Electronic communication is used everyday for a number of different applications.
Some of the information transferred during these communications can be private
requiring encryption and authentication protocols to keep this information secure.
Although there are protocols today which provide some security, they are not
necessarily unconditionally secure. Quantum based protocols on the other hand, can
provide unconditionally secure protocols for encryption and authentication.
Prior to this Thesis, only one experimental realisation of quantum digital signatures had
been demonstrated. This used a lossy photonic device along with a quantum memory
allowing two parties to test whether they were sent the same signature by a single
sender, and also store the quantum states for measurement later. This restricted the
demonstration to distances of only a few metres, and was tested with a primitive
approximation of a quantum memory rather than an actual one. This Thesis presents an
experimental realisation of a quantum digital signature protocol which removes the
reliance on quantum memory at the receivers, making a major step towards practicality.
By removing the quantum memory, it was also possible to perform the swap and
comparison mechanism in a more efficient manner resulting in an experimental
realisation of quantum digital signatures over 2 kilometres of optical fibre.
Quantum communication protocols can be unconditionally secure, however the
transmission distance is limited by loss in quantum channels. To overcome this loss in
conventional channels an optical amplifier is used, however the added noise from these
would swamp the quantum signal if directly used in quantum communications.
This Thesis looked into probabilistic quantum amplification, with an experimental
realisation of the state comparison amplifier, based on linear optical components and
single-photon detectors. The state comparison amplifier operated by using the wellestablished
techniques of optical coherent state comparison and weak subtraction to
post-select the output and provide non-deterministic amplification with increased
fidelity at a high repetition rate. The success rates of this amplifier were found to be
orders of magnitude greater than other state of the art quantum amplifiers, due to its lack
of requirement for complex quantum resources, such as single or entangled photon
sources, and photon number resolving detectors
Continuous Variable Quantum Key Distribution over Long Distances
Quantum key distribution (QKD) is fundamentally different from most classical key distribution schemes, such as Diffie-Hellman key exchange, in the sense that no computational complexity assumption is required on the power of adversaries to prove its security. QKD relies on basic laws of quantum physics and it is proven that it can enable highly secure data communication. Such achievements, however, are facing technological problems that have to be resolved in order to provide a viable solution to a large group of customers. While there are discrete-variable QKD schemes, which rely on encoding data in discrete degrees of freedom, such as polarization of single photons, in this thesis, we focus on the continuous-variable QKD (CV-QKD) protocols, in which data is encoded on the quadratures of light. Currently, one of the major drawbacks of CV-QKD is its poor performance at long distances. Nevertheless, such a limitation in CV-QKD can be overcome with the assistance of quantum repeaters that rely on entanglement distillation via noiseless linear amplifiers (NLAs). Such systems can, in principle, offer large secret key rates over long distances. In this thesis, we aim to provide a realistic analysis of a CV-QKD protocol running over quantum scissors (QSs) as realistic NLAs. We will report the obstacles that one could face in realizing CV-QKD in such a scenario. A review of CV-QKD and QS-based NLAs will be given, based on which QS-assisted CV-QKD is proposed. We, particularly, focus on the modelling of the QSs' structure and their effect on the secret key rate aiming to find operational regimes where the performance of the QKD scheme is enhanced. This study paves the way for implementing long-distance CV-QKD protocols that rely on QS/NLA devices over CV quantum repeaters.
In this thesis, we also consider and account for a realistic analysis of a CV-QKD protocol with non-Gaussian modulation, which is assisted by the means of QSs. We will show that, while we have to deal with similar obstacles as in the Gaussian modulation, we can potentially improve performance of the non-Gaussian modulation protocol.
As an alternative approach to extend the secure distance of CV-QKD protocols, the last part of this thesis is devoted to presenting realistic threat models for satellite QKD, wherein we consider several eavesdropping scenarios by limiting eavesdroppers' access to the trusted ground and/or satellite stations. In such scenarios, the eavesdropper has only limited access to the sender and/or receiver stations.
For example, we will explore the case where an eavesdropper can only receive an attenuated version of the transmitted signals. As well, we will focus on the case where Eve's signals would reach the receiver via a lossy channel inaccessible to the eavesdropper. We show that, in the case of both Gaussian and non-Gaussian protocols, this limitation would allow trusted parties to achieve higher key rates than what can be achieved when unrestricted eavesdropping is possible
Post-selection-based continuous variable quantum information processing
Quantum communication and computation harness the intriguing and bewildering nature of quantum mechanics to realize information processing tasks that have no classical analog. Nonetheless, this supremacy comes with fundamental limits that, in some scenarios, pose undesirable bounds on the performance of these quantum technologies. One such example is the well-known quantum no-cloning theorem imposed by the Heisenberg uncertainty principle. It states that an unknown quantum state cannot be duplicated with arbitrarily high accuracy. Very recently, however, post-selection was proposed as a way out: it was demonstrated that in various quantum information tasks, deterministic bounds can be overcome by forgoing determinism. In this thesis, we investigate post-selection as a novel approach to enhance the performance of versatile continuous-variable (CV) quantum information processing and envisage it to become a useful component of the general Gaussian toolbox.
The first part of this thesis examines applications of post-selection in purely linear systems. In particular, two implementations of the noiseless linear amplifier (NLA), the measurement-based NLA and the physical NLA, are investigated and compared in terms to their abilities to preserve the state Gaussianity and their success probability.
We show that the inevitable signal-to-noise ratio (SNR) degradation accompanying a linear quantum amplifier can be circumvented by resorting to a probabilistic scheme. Amplification with a signal transfer coefficient of Ts>1 is realised by combining a measurement-based NLA with a deterministic linear amplifier. We also construct a quantum cloning machine based on this hybrid amplifier for arbitrary coherent input states. We demonstrate a production of multiple clones (up to five) with fidelity of each clone exceeding the corresponding no-cloning limit.
We then consider employing the post-selection algorithm in information protocols involving nonlinearity.
First, we develop two squeezers as optical parametric amplifiers, each producing fairly pure squeezed output field up to 11.2dB (after correcting the detection loss). The squeezers are served as the nonlinear source in the remaining part of this thesis.
We demonstrate a high fidelity quantum squeezing gate which is one indispensible building block for constructing a universal CV quantum computer. An inverse-Gaussian filter is incorporated into the feedforward line, leading to an enhancement in precision of the inline dual-homodyne measurement and therefore combats efficiently the correlation degradation due to loss and noise introduced during feedforward. As one example, we show that a fidelity of 98.49% for a target squeezing of -2.3dB is obtained with only -6dB ancilla squeezing, which would otherwise require -20.5dB initial squeezing using a conventional deterministic setup.
Additionally, we introduce a CV quantum teleportation scheme using post-selection, allowing for a significantly improved fidelity against the conventional deterministic CV teleporter. The intuition behind this improvement is that post-selection effectively distilled the accessible entanglement and therefore a high fidelity only originally achievable with a higher amount of initial squeezing is now obtainable with only modest amount of squeezing, coming at an expense of finite success probability
Gaussian Quantum Information
The science of quantum information has arisen over the last two decades
centered on the manipulation of individual quanta of information, known as
quantum bits or qubits. Quantum computers, quantum cryptography and quantum
teleportation are among the most celebrated ideas that have emerged from this
new field. It was realized later on that using continuous-variable quantum
information carriers, instead of qubits, constitutes an extremely powerful
alternative approach to quantum information processing. This review focuses on
continuous-variable quantum information processes that rely on any combination
of Gaussian states, Gaussian operations, and Gaussian measurements.
Interestingly, such a restriction to the Gaussian realm comes with various
benefits, since on the theoretical side, simple analytical tools are available
and, on the experimental side, optical components effecting Gaussian processes
are readily available in the laboratory. Yet, Gaussian quantum information
processing opens the way to a wide variety of tasks and applications, including
quantum communication, quantum cryptography, quantum computation, quantum
teleportation, and quantum state and channel discrimination. This review
reports on the state of the art in this field, ranging from the basic
theoretical tools and landmark experimental realizations to the most recent
successful developments.Comment: 51 pages, 7 figures, submitted to Reviews of Modern Physic
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