239 research outputs found
Constructing finite dimensional codes with optical continuous variables
We show how a qubit can be fault-tolerantly encoded in the
infinite-dimensional Hilbert space of an optical mode. The scheme is efficient
and realizable with present technologies. In fact, it involves two travelling
optical modes coupled by a cross-Kerr interaction, initially prepared in
coherent states, one of which is much more intense than the other. At the exit
of the Kerr medium, the weak mode is subject to a homodyne measurement and a
quantum codeword is conditionally generated in the quantum fluctuations of the
intense mode.Comment: 7 pages, 5 figure
All-optical generation of states for "Encoding a qubit in an oscillator"
Both discrete and continuous systems can be used to encode quantum
information. Most quantum computation schemes propose encoding qubits in
two-level systems, such as a two-level atom or an electron spin. Others exploit
the use of an infinite-dimensional system, such as a harmonic oscillator. In
"Encoding a qubit in an oscillator" [Phys. Rev. A 64 012310 (2001)], Gottesman,
Kitaev, and Preskill (GKP) combined these approaches when they proposed a
fault-tolerant quantum computation scheme in which a qubit is encoded in the
continuous position and momentum degrees of freedom of an oscillator. One
advantage of this scheme is that it can be performed by use of relatively
simple linear optical devices, squeezing, and homodyne detection. However, we
lack a practical method to prepare the initial GKP states. Here we propose the
generation of an approximate GKP state by using superpositions of optical
coherent states (sometimes called "Schr\"odinger cat states"), squeezing,
linear optical devices, and homodyne detection.Comment: 4 pages, 3 figures. Submitted to Optics Letter
A Quantum Teleportation Game
We investigate a game where a sender (Alice) teleports coherent states to two
receivers (Bob and Charlie) through a tripartite Gaussian state. The aim of the
receivers is to optimize their teleportation fidelities by means of local
operations and classical communications. We show that a non-cooperative
strategy, corresponding to the standard telecloning protocol, can be
outperformed by a cooperative strategy, which gives rise to a novel
(cooperative) telecloning protocol.Comment: Typographic corrections 4 pages, 4 figure
On the security and degradability of Gaussian channels
We consider the notion of canonical attacks, which are the cryptographic
analog of the canonical forms of a one-mode Gaussian channel. Using this
notion, we explore the connections between the degradability properties of the
channel and its security for quantum key distribution. Finally, we also show
some relations between canonical attacks and optimal Gaussian cloners.Comment: Proceeding of TQC2009, 4th Workshop on Theory of Quantum Computation,
Communication, and Cryptography, Waterloo, Canada, 11-13 May 200
Characterization of Collective Gaussian Attacks and Security of Coherent-State Quantum Cryptography
We provide a simple description of the most general collective Gaussian
attack in continuous-variable quantum cryptography. In the scenario of such
general attacks, we analyze the asymptotic secret-key rates which are
achievable with coherent states, joint measurements of the quadratures and
one-way classical communication.Comment: 4 pages, 1 figure + 1 Table, REVteX. More descriptive titl
Gaussian two-mode attacks in one-way quantum cryptography
We investigate the asymptotic security of one-way continuous variable quantum
key distribution against Gaussian two-mode coherent attacks. The one-way
protocol is implemented by arranging the channel uses in two-mode blocks. By
applying symmetric random permutations over these blocks, the security analysis
is in fact reduced to study two-mode coherent attacks and, in particular,
Gaussian ones, due to the extremality of Gaussian states. We explicitly show
that the use of two-mode Gaussian correlations by an eavesdropper leads to
asymptotic secret key rates which are strictly larger than the rate obtained
under standard single-mode Gaussian attacks.Comment: 9 pages, 2 figure
Fundamental limits of repeaterless quantum communications
Quantum communications promises reliable transmission of quantum information, efficient distribution of entanglement and generation of completely secure keys. For all these tasks, we need to determine the optimal point-to-point rates that are achievable by two remote parties at the ends of a quantum channel, without restrictions on their local operations and classical communication, which can be unlimited and two-way. These two-way assisted capacities represent the ultimate rates that are reachable without quantum repeaters. Here, by constructing an upper bound based on the relative entropy of entanglement and devising a dimension-independent technique dubbed ‘teleportation stretching’, we establish these capacities for many fundamental channels, namely bosonic lossy channels, quantum-limited amplifiers, dephasing and erasure channels in arbitrary dimension. In particular, we exactly determine the fundamental rate-loss tradeoff affecting any protocol of quantum key distribution. Our findings set the limits of point-to-point quantum communications and provide precise and general benchmarks for quantum repeaters
Side-channel-free quantum key distribution
Quantum key distribution (QKD) offers the promise of absolutely secure
communications. However, proofs of absolute security often assume perfect
implementation from theory to experiment. Thus, existing systems may be prone
to insidious side-channel attacks that rely on flaws in experimental
implementation. Here we replace all real channels with virtual channels in a
QKD protocol, making the relevant detectors and settings inside private spaces
inaccessible while simultaneously acting as a Hilbert space filter to eliminate
side-channel attacks. By using a quantum memory we find that we are able to
bound the secret-key rate below by the entanglement-distillation rate computed
over the distributed states.Comment: Considering general quantum systems, we extended QKD to the presence
of an untrusted relay, whose measurement creates secret correlations in
remote stations (achievable rate lower-bounded by the coherent information).
This key ingredient, i.e., the use of a measurement-based untrusted relay,
has been called 'measurement-device independence' in another arXiv submission
(arXiv:1109.1473
Quantum cryptography with an ideal local relay
We consider two remote parties connected to a relay by two quantum channels.
To generate a secret key, they transmit coherent states to the relay, where the
states are subject to a continuous-variable (CV) Bell detection. We study the
ideal case where Alice's channel is lossless, i.e., the relay is locally
situated in her lab and the Bell detection is performed with unit efficiency.
This configuration allows us to explore the optimal performances achievable by
CV measurement-device-independent (MDI) quantum key distribution (QKD). This
corresponds to the limit of a trusted local relay, where the detection loss can
be re-scaled. Our theoretical analysis is confirmed by an experimental
simulation where 10^-4 secret bits per use can potentially be distributed at
170km assuming ideal reconciliation.Comment: in Proceedings of the SPIE Security + Defence 2015 conference on
Quantum Information Science and Technology, Toulouse, France (21-24 September
2015) - Paper 9648-4
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