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Optimization of entanglement witnesses
An entanglement witness (EW) is an operator that allows to detect entangled states. We give necessary and sufficient conditions for such operators to be optimal, i.e. to detect entangled states in an…
Information-theoretic security proof for quantum-key-distribution protocols (17 pages)
This work investigates a general class of QKD protocols with one-way classical post-processing and gives new lower and upper bounds on the secret-key rate which only involve entropies of two-qubit density operators and which are thus easy to compute.
Optimal creation of entanglement using a two-qubit gate
We consider a general unitary operator acting on two qubits in a product state. We find the conditions such that the state of the qubits after the action is as entangled as possible. We also consider…
Quantum states and phases in driven open quantum systems with cold atoms
An open quantum system, the time evolution of which is governed by a master equation, can be driven into a given pure quantum state by an appropriate design of the coupling between the system and t…
Preparation of entangled states by quantum Markov processes
We investigate the possibility of using a dissipative process to prepare a quantum system in a desired state. We derive for any multipartite pure state a dissipative process for which this state is…
Quantum memory for nonstationary light fields based on controlled reversible inhomogeneous broadening
We propose a method for efficient storage and recall of arbitrary nonstationary light fields, such as, for instance, single photon time-bin qubits or intense fields, in optically dense atomic…
Separability and distillability in composite quantum systems-a primer
This article presents a primer on the current state of knowledge concerning two problems of quantum information theory, and discusses the relation of these problems to one of the most challenging questions of linear algebra: the classification and characterization of positive operator maps.
Trojan-horse attacks on quantum-key-distribution systems (6 pages)
It is shown that all systems must implement active counter measures, including an auxiliary detector that monitors any incoming light, to reduce the maximal information gain that an adversary can gain using Trojan-horse attacks.
Lower and upper bounds on the secret-key rate for quantum key distribution protocols using one-way classical communication.
It is shown that full security can be proven by considering only collective attacks, and computable lower and upper bounds on the secret-key rate of those QKD protocols involving only entropies of two-qubit density operators are derived.
Security of two quantum cryptography protocols using the same four qubit states (18 pages)
The SARG04 protocol is found to perform better than BB84, both in secret-key rate and in maximal achievable distance, for a wide class of Eve's attacks.