73 research outputs found
Implementation vulnerabilities in general quantum cryptography
Quantum cryptography is information-theoretically secure owing to its solid
basis in quantum mechanics. However, generally, initial implementations with
practical imperfections might open loopholes, allowing an eavesdropper to
compromise the security of a quantum cryptographic system. This has been shown
to happen for quantum key distribution (QKD). Here we apply experience from
implementation security of QKD to several other quantum cryptographic
primitives. We survey quantum digital signatures, quantum secret sharing,
source-independent quantum random number generation, quantum secure direct
communication, and blind quantum computing. We propose how the eavesdropper
could in principle exploit the loopholes to violate assumptions in these
protocols, breaking their security properties. Applicable countermeasures are
also discussed. It is important to consider potential implementation security
issues early in protocol design, to shorten the path to future applications.Comment: 13 pages, 8 figure
Generating Greenberger-Horne-Zeilinger states using multiport splitters
Symmetric multiport splitters are versatile tools in optical quantum
information processing. They can be used for studying multiparticle scattering,
studying distinguishability and mixedness, and also for the generation of
multipartite entangled quantum states. Here, we show that N-photon N-mode
Greenberger-Horne-Zeilinger (GHZ) states can be generated using symmetric
multiport beam splitters. Varying the input states' internal degrees of freedom
and post-selecting onto certain photon-number distributions allows the
probabilistic generation of GHZ states with arbitrary photon numbers. We
present two novel schemes, one for odd and one for even numbers of photons, to
generate GHZ states using symmetric multiport splitters and compare them to a
strategy utilizing a 2N-port network as well as the standard post-selection
method
Experimental entanglement generation using multiport beam splitters
Multi-photon entanglement plays a central role in optical quantum
technologies. One way to entangle two photons is to prepare them in orthogonal
internal states, for example, in two polarisations, and then send them through
a balanced beam splitter. Post-selecting on the cases where there is one photon
in each output port results in a maximally entangled state. This idea can be
extended to schemes for the post-selected generation of larger entangled
states. Typically, switching between different types of entangled states
require different arrangements of beam splitters and so a new experimental
setup. Here, we demonstrate a simple and versatile scheme to generate different
types of genuine tripartite entangled states with only one experimental setup.
We send three photons through a three-port splitter and vary their internal
states before post-selecting on certain output distributions. This results in
the generation of tripartite W, G and GHZ states. We obtain fidelities of up to
with regard to the respective ideal states, confirming a
successful generation of genuine tripartite entanglement
Bell-state measurement exceeding 50% success probability with linear optics
Bell-state projections serve as a fundamental basis for most quantum
communication and computing protocols today. However, with current Bell-state
measurement schemes based on linear optics, only two of four Bell states can be
identified, which means that the maximum success probability of this vital step
cannot exceed . Here, we experimentally demonstrate a scheme that amends
the original measurement with additional modes in the form of ancillary
photons, which leads to a more complex measurement pattern, and ultimately a
higher success probability of . Experimentally, we achieve a success
probability of , a significant improvement over the
conventional scheme. With the possibility of extending the protocol to a larger
number of ancillary photons, our work paves the way towards more efficient
realisations of quantum technologies based on Bell-state measurements
Experimental Demonstration of Blind Quantum Computing
Quantum computers, besides offering substantial computational speedups, are
also expected to provide the possibility of preserving the privacy of a
computation. Here we show the first such experimental demonstration of blind
quantum computation where the input, computation, and output all remain unknown
to the computer. We exploit the conceptual framework of measurement-based
quantum computation that enables a client to delegate a computation to a
quantum server. We demonstrate various blind delegated computations, including
one- and two-qubit gates and the Deutsch and Grover algorithms. Remarkably, the
client only needs to be able to prepare and transmit individual photonic
qubits. Our demonstration is crucial for future unconditionally secure quantum
cloud computing and might become a key ingredient for real-life applications,
especially when considering the challenges of making powerful quantum computers
widely available
- …