203 research outputs found
For quantum information, two wrongs can make a right
Superactivation is the phenomenon where two quantum channels which
individually have zero-capacity can have positive capacity when used together.
The perspective given here provides an intuitive explanation of this discovery
by Smith and Yard, and gives a protocol to activate any private channel
How to reuse a one-time pad and other notes on authentication, encryption and protection of quantum information
Quantum information is a valuable resource which can be encrypted in order to
protect it. We consider the size of the one-time pad that is needed to protect
quantum information in a number of cases. The situation is dramatically
different from the classical case: we prove that one can recycle the one-time
pad without compromising security. The protocol for recycling relies on
detecting whether eavesdropping has occurred, and further relies on the fact
that information contained in the encrypted quantum state cannot be fully
accessed. We prove the security of recycling rates when authentication of
quantum states is accepted, and when it is rejected. We note that recycling
schemes respect a general law of cryptography which we prove relating the size
of private keys, sent qubits, and encrypted messages. We discuss applications
for encryption of quantum information in light of the resources needed for
teleportation. Potential uses include the protection of resources such as
entanglement and the memory of quantum computers. We also introduce another
application: encrypted secret sharing and find that one can even reuse the
private key that is used to encrypt a classical message. In a number of cases,
one finds that the amount of private key needed for authentication or
protection is smaller than in the general case.Comment: 13 pages, improved rate of recycling proved in the case of rejection
of authenticatio
Hybrid Zero-capacity Channels
There are only two known kinds of zero-capacity channels. The first kind
produces entangled states that have positive partial transpose, and the second
one - states that are cloneable. We consider the family of 'hybrid' quantum
channels, which lies in the intersection of the above classes of channels and
investigate its properties. It gives rise to the first explicit examples of the
channels, which create bound entangled states that have the property of being
cloneable to the arbitrary finite number of parties. Hybrid channels provide
the first example of highly cloneable binding entanglement channels, for which
known superactivation protocols must fail - superactivation is the effect where
two channels each with zero quantum capacity having positive capacity when used
together. We give two methods to construct a hybrid channel from any binding
entanglement channel. We also find the low-dimensional counterparts of hybrid
states - bipartite qubit states which are extendible and possess two-way key
Gibbs-Preserving Maps outperform Thermal Operations in the quantum regime
In this brief note, we compare two frameworks for characterizing possible
operations in quantum thermodynamics. One framework considers Thermal
Operations---unitaries which conserve energy. The other framework considers all
maps which preserve the Gibbs state at a given temperature. Thermal Operations
preserve the Gibbs state; hence a natural question which arises is whether the
two frameworks are equivalent. Classically, this is true---Gibbs-Preserving
Maps are no more powerful than Thermal Operations. Here, we show that this no
longer holds in the quantum regime: a Gibbs-Preserving Map can generate
coherent superpositions of energy levels while Thermal Operations cannot. This
gap has an impact on clarifying a mathematical framework for quantum
thermodynamics.Comment: 4 pages, 1 figur
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