Showing papers by "Stefano Pirandola published in 2008"

Quantum Illumination with Gaussian States

Abstract: An optical transmitter irradiates a target region containing a bright thermal-noise bath in which a low-reflectivity object might be embedded. The light received from this region is used to decide whether the object is present or absent. The performance achieved using a coherent-state transmitter is compared with that of a quantum-illumination transmitter, i.e., one that employs the signal beam obtained from spontaneous parametric down-conversion. By making the optimum joint measurement on the light received from the target region together with the retained spontaneous parametric down-conversion idler beam, the quantum-illumination system realizes a 6 dB advantage in the error-probability exponent over the optimum reception coherent-state system. This advantage accrues despite there being no entanglement between the light collected from the target region and the retained idler beam.

Continuous-variable quantum cryptography using two-way quantum communication

Abstract: A class of quantum-cryptographic protocols is proposed that involves back-and-forth communication between two parties. The approach is shown to provide enhanced security and should tolerate higher levels of noise and loss than conventional ‘one-way’ protocols.

Characterization of collective Gaussian attacks and security of coherent-state quantum cryptography.

Abstract: 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.

Computable bounds for the discrimination of Gaussian states

Abstract: By combining the Minkowski inequality and the quantum Chernoff bound, we derive easy-to-compute upper bounds for the error probability affecting the optimal discrimination of Gaussian states. In particular, these bounds are useful when the Gaussian states are unitarily inequivalent, i.e., they differ in their symplectic invariants.

Minimal qudit code for a qubit in the phase damping channel

Abstract: Using the stabilizer formalism we construct the minimal code into a $D$-dimensional Hilbert space (qudit) to protect a qubit against phase damping. The effectiveness of this code is then studied by means of input-output fidelity.

Quantum direct communication with continuous variables

Abstract: We show how continuous-variable systems can allow the direct communication of messages with an acceptable degree of privacy. This is possible by combining a suitable phase-space encoding of the plain message with real-time checks of the quantum communication channel. The resulting protocol works properly when a small amount of noise affects the quantum channel. If this noise is non-tolerable, the protocol stops leaving a limited amount of information to a potential eavesdropper.

Symmetric collective attacks for the eavesdropping of symmetric quantum key distribution

Abstract: We consider the collective eavesdropping of the BB84 and six-state protocols. Since these protocols are symmetric in the eigenstates of conjugate bases, we consider collective attacks having the same kind of symmetry. We then show how these symmetric collective attacks are sufficiently strong in order to minimize the Devetak–Winter rates. In fact, it is quite easy to construct simple examples able to reach the unconditionally secure key rates of these protocols.

Symmetric collective attacks for the eavesdropping of symmetric quantum key distribution

Abstract: We consider the collective eavesdropping of the BB84 and six-state protocols. Since these protocols are symmetric in the eigenstates of conjugate bases, we consider collective attacks having the same kind of symmetry. We then show how these symmetric collective attacks are sufficiently strong in order to minimize the Devetak-Winter rates. In fact, it is quite easy to construct simple examples able to reach the unconditionally-secure key-rates of these protocols.

Security of two-way quantum cryptography against asymmetric attacks

Abstract: Recently, we have shown the advantages of two-way quantum communications in continuous variable quantum cryptography. Thanks to this new approach, two honest users can achieve a non-trivial security enhancement as long as the Gaussian interactions of an eavesdropper are independent and identical. In this work, we consider asymmetric strategies where the Gaussian interactions can be different and classically correlated. For several attacks of this kind, we prove that the enhancement of security still holds when the two-way protocols are used in direct reconciliation.

Security of two-way quantum cryptography against asymmetric Gaussian attacks

Abstract: Computer Science, University of York, York YO10 5DD, United Kingdom(Dated: September 18, 2008)Recently, we have shown the advantages of two-way quantum communications in continuousvariable quantum cryptography Thanks to this new approach, two honest users can achieve a non-trivial security enhancement as long as the Gaussian interactions of an eavesdropper are independentand identical In thiswork, we consider asymmetric strategies where theGaussian interactions can bedifferent and classically correlated For several attacks of this kind, we prove that the enhancementof security still holds when the two-way protocols are used in direct reconciliationINTRODUCTION TO CONTINUOUS VARIABLEQUANTUM CRYPTOGRAPHY

Security of two-way quantum cryptography against asymmetric Gaussian attacks

Abstract: Recently, we have shown the advantages of two-way quantum communications in continuous variable quantum cryptography. Thanks to this new approach, two honest users can achieve a non-trivial security enhancement as long as the Gaussian interactions of an eavesdropper are independent and identical. In this work, we consider asymmetric strategies where the Gaussian interactions can be different and classically correlated. For several attacks of this kind, we prove that the enhancement of security still holds when the two-way protocols are used in direct reconciliation.