50,325 research outputs found
Homodyne-based quantum random number generator at 2.9 Gbps secure against quantum side-information
Quantum random number generators promise perfectly unpredictable random numbers. A popular approach to quantum random number generation is homodyne measurements of the vacuum state, the ground state of the electro-magnetic field. Here we experimentally implement such a quantum random number generator, and derive a security proof that considers quantum side-information instead of classical side-information only. Based on the assumptions of Gaussianity and stationarity of noise processes, our security analysis furthermore includes correlations between consecutive measurement outcomes due to finite detection bandwidth, as well as analog-to-digital converter imperfections. We characterize our experimental realization by bounding measured parameters of the stochastic model determining the min-entropy of the system’s measurement outcomes, and we demonstrate a real-time generation rate of 2.9 Gbit/s. Our generator follows a trusted, device-dependent, approach. By treating side-information quantum mechanically an important restriction on adversaries is removed, which usually was reserved to semi-device-independent and device-independent schemes
How much randomness can be generated from a quantum black-box device?
Quantum theory allows for randomness generation in a device-independent
setting, where no detailed description of the experimental device is required.
Here we derive a general upper bound on the amount of randomness that can be
generated in such a setting. Our bound applies to any black-box scenario, thus
covering a wide range of scenarios from partially characterised to completely
uncharacterised devices. Specifically, we prove that the number of random bits
that can be generated is limited by the number of different input states that
enter the measurement device. We show explicitly that our bound is tight in the
simplest case. More generally, our work indicates that the prospects of
generating a large amount of randomness by using high-dimensional (or even
continuous variable) systems will be extremely challenging in practice
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
Random Numbers Certified by Bell's Theorem
Randomness is a fundamental feature in nature and a valuable resource for
applications ranging from cryptography and gambling to numerical simulation of
physical and biological systems. Random numbers, however, are difficult to
characterize mathematically, and their generation must rely on an unpredictable
physical process. Inaccuracies in the theoretical modelling of such processes
or failures of the devices, possibly due to adversarial attacks, limit the
reliability of random number generators in ways that are difficult to control
and detect. Here, inspired by earlier work on nonlocality based and device
independent quantum information processing, we show that the nonlocal
correlations of entangled quantum particles can be used to certify the presence
of genuine randomness. It is thereby possible to design of a new type of
cryptographically secure random number generator which does not require any
assumption on the internal working of the devices. This strong form of
randomness generation is impossible classically and possible in quantum systems
only if certified by a Bell inequality violation. We carry out a
proof-of-concept demonstration of this proposal in a system of two entangled
atoms separated by approximately 1 meter. The observed Bell inequality
violation, featuring near-perfect detection efficiency, guarantees that 42 new
random numbers are generated with 99% confidence. Our results lay the
groundwork for future device-independent quantum information experiments and
for addressing fundamental issues raised by the intrinsic randomness of quantum
theory.Comment: 10 pages, 3 figures, 16 page appendix. Version as close as possible
to the published version following the terms of the journa
Source-independent quantum random number generation
Quantum random number generators can provide genuine randomness by appealing
to the fundamental principles of quantum mechanics. In general, a physical
generator contains two parts---a randomness source and its readout. The source
is essential to the quality of the resulting random numbers; hence, it needs to
be carefully calibrated and modeled to achieve information-theoretical provable
randomness. However, in practice, the source is a complicated physical system,
such as a light source or an atomic ensemble, and any deviations in the
real-life implementation from the theoretical model may affect the randomness
of the output. To close this gap, we propose a source-independent scheme for
quantum random number generation in which output randomness can be certified,
even when the source is uncharacterized and untrusted. In our randomness
analysis, we make no assumptions about the dimension of the source. For
instance, multiphoton emissions are allowed in optical implementations. Our
analysis takes into account the finite-key effect with the composable security
definition. In the limit of large data size, the length of the input random
seed is exponentially small compared to that of the output random bit. In
addition, by modifying a quantum key distribution system, we experimentally
demonstrate our scheme and achieve a randomness generation rate of over
bit/s.Comment: 11 pages, 7 figure
Source-device-independent heterodyne-based quantum random number generator at 17 Gbps
For many applications, quantum random number generation should be fast and independent from assumptions on the apparatus. Here, the authors devise and implement an approach which assumes a trusted detector but not a trusted source, and allows random bit generations at ~17 Gbps using off-the-shelf components
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