On the structure of mirrored operators obtained from optimal entanglement witnesses.

Entanglement witnesses (EWs) are a versatile tool in the verification of entangled states. The framework of mirrored EW doubles the power of a given EW by introducing its twin-a mirrored EW-whereby two EWs related by mirroring can bound the set of separable states more efficiently. In this work, we investigate the relation between the EWs and its mirrored ones, and present a conjecture which claims that the mirrored operator obtained from an optimal EW is either a positive operator or a decomposable EW, which implies that positive-partial-transpose entangled states, also known as the bound entangled states, cannot be detected.

Quantum vs Noncontextual Semi-Device-Independent Randomness Certification.

We compare the power of quantum and classical physics in terms of randomness certification from devices which are only partially characterized. We study randomness certification based on state discrimination and take noncontextuality as the notion of classicality. A contextual advantage was recently shown to exist for state discrimination.

Nonlocal Network Coding in Interference Channels.

In a network, a channel introduces correlations to the parties that aim to establish a communication protocol. We present a framework of nonlocal network coding by exploiting a Bell scenario and show the usefulness of nonlocal and quantum resources in network coding. Two-sender and two-receiver interference channels are considered, for which network coding is characterized by two-input and four-outcome Bell scenarios.

More Entanglement Implies Higher Performance in Channel Discrimination Tasks.

We show that every entangled state provides an advantage in ancilla-assisted bi- and multichannel discrimination that singles out its degree of entanglement, quantified in terms of the Schmidt number. The Schmidt-number robustness provides a compelling quantification of such an advantage, and, remarkably, the well-known robustness of entanglement exactly provides the largest multiplicative advantage an entangled state can provide compared to the case where no ancilla is used in a channel discrimination task.

Designing quantum information processing via structural physical approximation.

In quantum information processing it may be possible to have efficient computation and secure communication beyond the limitations of classical systems. In a fundamental point of view, however, evolution of quantum systems by the laws of quantum mechanics is more restrictive than classical systems, identified to a specific form of dynamics, that is, unitary transformations and, consequently, positive and completely positive maps to subsystems. This also characterizes classes of disallowed transformations on quantum systems, among which positive but not completely maps are of particular interest as they characterize entangled states, a general resource in quantum information processing.

Operational Characterization of Divisibility of Dynamical Maps.

In this work, we show the operational characterization to the divisibility of dynamical maps in terms of the distinguishability of quantum channels. It is proven that the distinguishability of any pair of quantum channels does not increase under divisible maps, in which the full hierarchy of divisibility is isomorphic to the structure of entanglement between system and environment. This shows that (i) channel distinguishability is the operational quantity signifying (detecting) divisibility (indivisibility) of dynamical maps and (ii) the decision problem for the divisibility of maps is as hard as the separability problem in entanglement theory.

N-dimensional measurement-device-independent quantum key distribution with N + 1 un-characterized sources: zero quantum-bit-error-rate case.

We study N-dimensional measurement-device-independent quantum-key-distribution protocol where one checking state is used. Only assuming that the checking state is a superposition of other N sources, we show that the protocol is secure in zero quantum-bit-error-rate case, suggesting possibility of the protocol. The method may be applied in other quantum information processing.

Discrimination of two-qubit unitaries via local operations and classical communication.

Distinguishability is a fundamental and operational measure generally connected to information applications. In quantum information theory, from the postulates of quantum mechanics it often has an intrinsic limitation, which then dictates and also characterises capabilities of related information tasks. In this work, we consider discrimination between bipartite two-qubit unitary transformations by local operations and classical communication (LOCC) and its relations to entangling capabilities of given unitaries.

No-signaling principle can determine optimal quantum state discrimination.

We provide a general framework of utilizing the no-signaling principle in derivation of the guessing probability in the minimum-error quantum state discrimination. We show that, remarkably, the guessing probability can be determined by the no-signaling principle. This is shown by proving that, in the semidefinite programing for the discrimination, the optimality condition corresponds to the constraint that quantum theory cannot be used for a superluminal communication.

Experimental realization of an approximate partial transpose for photonic two-qubit systems.

We report the first experimental realization of an approximate partial transpose for photonic two-qubit systems. The proposed scheme is based on the local operation on single copies of quantum states and classical communication, and therefore can be easily applied for other quantum information tasks within current technologies. Direct detection of entanglement, i.

Asymptotic quantum cloning is state estimation.

The impossibility of perfect cloning and state estimation are two fundamental results in quantum mechanics. It has been conjectured that quantum cloning becomes equivalent to state estimation in the asymptotic regime where the number of clones tends to infinity. We prove this conjecture using two known results of quantum information theory: the monogamy of quantum correlations and the properties of entanglement breaking channels.