202 research outputs found
Dependence of a quantum mechanical system on its own initial state and the initial state of the environment it interacts with
We present a unifying framework to the understanding of when and how quantum
mechanical systems become independent of their initial conditions and adapt
macroscopic properties (like temperature) of the environment.By viewing this
problem from an quantum information theory perspective, we are able to simplify
it in a very natural and easy way. We first show that for any interaction
between the system and the environment, and almost all initial states of the
system, the question of how long the system retains memory of its initial
conditions can be answered by studying the temporal evolution of just one
special initial state. This special state thereby depends only on our knowledge
of macroscopic parameters of the system. We provide a simple entropic
inequality for this state that can be used to determine whether mosts states of
the system have, or have not become independent of their initial conditions
after time . We discuss applications of our entropic criterion to
thermalization times in systems with an effective light-cone and to quantum
memories suffering depolarizing noise. We make a similar statement for almost
all initial states of the environment, and finally provide a sufficient
condition for which a system never thermalizes, but remains close to its
initial state for all times.Comment: 9+4 pages, revtex. v2: minor changes in notation; v4: greatly
rewritten, new title, new applications of main results, to appear in PR
Composable Security in the Bounded-Quantum-Storage Model
We present a simplified framework for proving sequential composability in the
quantum setting. In particular, we give a new, simulation-based, definition for
security in the bounded-quantum-storage model, and show that this definition
allows for sequential composition of protocols. Damgard et al. (FOCS '05,
CRYPTO '07) showed how to securely implement bit commitment and oblivious
transfer in the bounded-quantum-storage model, where the adversary is only
allowed to store a limited number of qubits. However, their security
definitions did only apply to the standalone setting, and it was not clear if
their protocols could be composed. Indeed, we first give a simple attack that
shows that these protocols are not composable without a small refinement of the
model. Finally, we prove the security of their randomized oblivious transfer
protocol in our refined model. Secure implementations of oblivious transfer and
bit commitment then follow easily by a (classical) reduction to randomized
oblivious transfer.Comment: 21 page
More non-locality with less entanglement
We provide an explicit example of a Bell inequality with 3 settings and 2
outcomes per site for which the largest violation is not obtained by the
maximally entangled state, even if its dimension is allowed to be arbitrarily
large. This complements recent results by Junge and Palazuelos
(arXiv:1007.3042) who show, employing tools from operator space theory, that
such inequalities do exist. Our elementary example provides arguably the
simplest setting in which it can be demonstrated that even an infinite supply
of EPR pairs is not the strongest possible nonlocal resource.Comment: 9 pages; Added reference to arXiv:1012.151
Asynchronous reference frame agreement in a quantum network
An efficient implementation of many multiparty protocols for quantum networks
requires that all the nodes in the network share a common reference frame.
Establishing such a reference frame from scratch is especially challenging in
an asynchronous network where network links might have arbitrary delays and the
nodes do not share synchronised clocks. In this work, we study the problem of
establishing a common reference frame in an asynchronous network of nodes
of which at most are affected by arbitrary unknown error, and the
identities of the faulty nodes are not known. We present a protocol that allows
all the correctly functioning nodes to agree on a common reference frame as
long as the network graph is complete and not more than nodes are
faulty. As the protocol is asynchronous, it can be used with some assumptions
to synchronise clocks over a network. Also, the protocol has the appealing
property that it allows any existing two-node asynchronous protocol for
reference frame agreement to be lifted to a robust protocol for an asynchronous
quantum network.Comment: 13 pages, revte
Quantum Anonymous Transmissions
We consider the problem of hiding sender and receiver of classical and
quantum bits (qubits), even if all physical transmissions can be monitored. We
present a quantum protocol for sending and receiving classical bits
anonymously, which is completely traceless: it successfully prevents later
reconstruction of the sender. We show that this is not possible classically. It
appears that entangled quantum states are uniquely suited for traceless
anonymous transmissions. We then extend this protocol to send and receive
qubits anonymously. In the process we introduce a new primitive called
anonymous entanglement, which may be useful in other contexts as well.Comment: 18 pages, LaTeX. Substantially updated version. To appear at
ASIACRYPT '0
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