18 research outputs found
Secure -dimensional Simultaneous Dense Coding and Applications
Simultaneous dense coding guarantees that Bob and Charlie simultaneously
receive their respective information from Alice in their respective processes
of dense coding. The idea is to use the so-called locking operation to "lock"
the entanglement channels, thus requiring a joint unlocking operation by Bob
and Charlie in order to simultaneously obtain the information sent by Alice. We
present some new results on simultaneous dense coding: (1) We propose three
simultaneous dense coding protocols, which use different -dimensional
entanglement (Bell state, W state and GHZ state). (2) Besides the quantum
Fourier transform, two new locking operators are introduced (the double
controlled-NOT operator and the SWAP operator). (3) In the case that spatially
distant Bob and Charlie have to finalise the protocol by implementing the
unlocking operation through communication, we improve our protocol's fairness,
with respect to Bob and Charlie, by implementing the unlocking operation in
series of steps. (4) We improve the security of simultaneous dense coding
against the intercept-resend attack. (5) We show that simultaneous dense coding
can be used to implement a fair contract signing protocol. (6) We also show
that the -dimensional quantum Fourier transform can act as the locking
operator in simultaneous teleportation of -level quantum systems.Comment: 22 pages, comments are welcom
Fair and optimistic quantum contract signing
We present a fair and optimistic quantum contract signing protocol between
two clients that requires no communication with the third trusted party during
the exchange phase. We discuss its fairness and show that it is possible to
design such a protocol for which the probability of a dishonest client to cheat
becomes negligible, and scales as N^{-1/2}, where N is the number of messages
exchanged between the clients. Our protocol is not based on the exchange of
signed messages: its fairness is based on the laws of quantum mechanics. Thus,
it is abuse-free, and the clients do not have to generate new keys for each
message during the Exchange phase. We discuss a real-life scenario when the
measurement errors and qubit state corruption due to noisy channels occur and
argue that for real, good enough measurement apparatus and transmission
channels, our protocol would still be fair. Our protocol could be implemented
by today's technology, as it requires in essence the same type of apparatus as
the one needed for BB84 cryptographic protocol. Finally, we briefly discuss two
alternative versions of the protocol, one that uses only two states (based on
B92 protocol) and the other that uses entangled pairs, and show that it is
possible to generalize our protocol to an arbitrary number of clients.Comment: 11 pages, 2 figure
Identical particles are indistinguishable but..
It is shown that quantum systems of identical particles can be treated as if
they were different when they are in well differentiated states. This
simplifying assumption allows the consideration of quantum systems isolated
from the rest of the universe and justifies many intuitive statements about
identical systems. However, it is shown that this simplification may lead to
wrong results in the calculation of the entropy.
Keywords: quantum mechanics, identical systems, entrop
Fidelity Between Partial States as Signature of Quantum Phase Transitions
We introduce a partial state fidelity approach to quantum phase transitions.
We consider a superconducting lattice with a magnetic impurity inserted at its
centre, and look at the fidelity between partial (either one-site or two-site)
quantum states. In the vicinity of the point of the quantum phase transition,
we observe a sudden drop of the fidelity between two one-site partial states
corresponding to the impurity location and its close vicinity. In the case of
two-site states, the fidelity reveals the transition point as long as one of
the two electron sites is located at the impurity, while the other lies
elsewhere in the lattice. We also determine the Uhlmann mixed state geometric
phase, recently introduced in the study of the structural change of the system
state eigenvectors in the vicinity of the lines of thermal phase transitions,
and find it to be trivial, both for one- and two-site partial states, except
when an electron site is at the impurity. This means that the system partial
state eigenvectors do not contribute significantly to the enhanced state
distinguishability around the point of this quantum phase transition. Finally,
we use the fidelity to analyze the total amount of correlations contained
within a composite system, showing that, even for the smallest two-site states,
it features an abrupt quantitative change in the vicinity of the point of the
quantum phase transition.Comment: 11 pages, 5 figure
Entanglement Concentration Using Quantum Statistics
We propose an entanglement concentration scheme which uses only the effects
of quantum statistics of indistinguishable particles. This establishes the fact
that useful quantum information processing can be accomplished by quantum
statistics alone. Due to the basis independence of statistical effects, our
protocol requires less knowledge of the initial state than most entanglement
concentration schemes. Moreover, no explicit controlled operation is required
at any stage.Comment: 2 figure