248 research outputs found
Observability of relative phases of macroscopic quantum states
After a measurement, to observe the relative phases of macroscopically
distinguishable states we have to ``undo'' a quantum measurement. We generalise
an earlier model of Peres from two state to N-state quantum system undergoing
measurement process and discuss the issue of observing relative phases of
different branches. We derive an inequality which is satisfied by the relative
phases of macroscopically distinguishable states and consequently any desired
relative phases can not be observed in interference setups. The principle of
macroscopic complementarity is invoked that might be at ease with the
macroscopic world. We illustrate the idea of limit on phase observability in
Stern-Gerlach measurements and the implications are discussed.Comment: Latex file, no figures, 12 pages, submitted to Phys. Lett.
Distinguishing two preparations for same pure state leads to signalling
Pure state of a physical system can be prepared in an infinite number of
ways. Here, we prove that given a pure state of a quantum system it is
impossible to distinguish two preparation procedures. Further, we show that if
we can distinguish two preparation procedures for the same pure state then that
can lead to signalling. This impossibility result is different than the no
measurement without disturbance and the no-cloning. Extending this result for a
pure bipartite entangled state entails that the impossibility of distinguishing
two preparation procedures for a mixed state follows from the impossibility of
distinguishing two preparations for a pure bipartite state.Comment: Two and half pages, Comments welcom
Fast quantum search algorithm and Bounds on it
We recast Grover's generalised search algorithm in a geometric language even
when the states are not approximately orthogonal. We provide a possible search
algorithm based on an arbitrary unitary transformation which can speed up the
steps still further. We discuss the lower and upper bounds on the transition
matrix elements when the unitary operator changes with time, thereby implying
that quantum search process can not be too fast or too slow. This is a
remarkable feature of quantum computation unlike classical one. Quantum
mechanical uncertainty relation puts bounds on search process. Also we mention
the problems of perturbation and other issues in time-dependent search
operation.Comment: Latex file, Two column, 4 pages, no figure
Probabilistic exact cloning and probabilistic no-signalling
We show that non-local resources cannot be used for probabilistic signalling
even if one can produce exact clones with the help of a probabilistic quantum
cloning machine (PQCM). We show that PQCM cannot help to distinguish two
statistical mixtures at a remote location. Thus quantum theory rules out the
possibility of sending superluminal signals not only deterministically but also
probabilistically. We give a bound on the success probability of producing
multiple clones in an entangled system.Comment: Latex file, 6 pages, minor correction
Remote state preparation and measurement of single photon
Quantum information theory has revolutionized the way in which information is
processed using quantum resources such as entangled states, local operations
and classical communications. Two important protocols in quantum communications
are quantum teleportation and remote state preparation. In quantum
teleportation neither the sender nor the receiver know the identity of a state.
In remote state preparation the sender knows the state which is to be remotely
prepared without ever physically sending the object or the complete classical
description of it. Using one unit of entanglement and one classical bit Alice
can remotely prepare a photon (from special ensemble) of her choice at Bob's
laboratory. In remote state measurement Alice asks Bob to simulate any single
particle measurement statistics on an arbitrary photon. In this talk we will
present these ideas and discuss the latest developments and future open
problems.Comment: Latex file, 7 pages, Invited talk in Sixth International Conference
on PHOTONICS-2002, held at TIFR, Mumbai, India from Dec-16-18, 200
Violation of Invariance of Entanglement Under Local PT Symmetric Unitary
Entanglement is one of the key feature of quantum world and any entanglement
measure must satisfy some basic laws. Most important of them is the invariance
of entanglement under local unitary operations. We show that this is no longer
true with local symmetric unitary operations. If two parties
share a maximally entangled state, then under local symmetric
unitary evolution the entropy of entanglement for pure bipartite states does
not remain invariant. Furthermore, we show that if one of the party has access
to -symmetric quantum world, then a maximally entangled state in
usual quantum theory appears as a non-maximally entangled states for the other
party. This we call as the "entanglement mismatch" effect which can lead to the
violation of the no-signaling condition.Comment: 5 pages, Latex, No fi
Infinitesimal change is a Non-local Operation: Hidden power of Quantum Entanglement
We show that the global infinitesimal change in the multi-particle pure
product state gives rise to an entangled state. This suggests that even if
there is no interaction present between the subsystems, i.e., at each time
instant the state is non-entangled, the tangent vector is typically entangled.
Since the tangent space vectors tell the state-space vectors how to change this
implies that quantum entanglement is necessary for motion or change in general.
This is truly a `hidden power' of quantum entanglement. During quantum
computation even though at each time instant the state is not entangled,
quantum entanglement guides the process of computation. This observation
applies to multi-particle pure, pseudo-pure and mixed states as well.Comment: Latex, 4 pages, No figures (Comments welcome
Measuring Electromagnetic Vector Potential via Weak Value
Electromagnetic vector potential has physical significance in quantum
mechanics as revealed by the Aharonov-Bohm effect for charged particles.
However, till date it is thought that we cannot measure the vector potential
directly as this is not a gauge invariant quantity. Contrary to this belief,
here we show that one can indeed measure the electromagnetic vector potential
using the notion of weak value. We show that it is simply the difference
between the weak value of the canonical momentum of a charged particle in the
presence and absence of magnetic field. This suggests that the vector potential
is not only a physical entity but can be measured directly in the experiment.Comment: Latex, 5 pages, Comments and suggestions welcom
Potent Value and Potent Operator with Pre- and Post-selected Quantum Systems
We introduce a novel concept which we call as potent value of system
observable for pre- and post-selected quantum states. This describes, in
general, how a quantum system affects the state of the apparatus during the
time between two strong measurements corresponding to pre- and post-selections.
The potent value can be realized for any interaction strength and for arbitrary
coupling between the system and the apparatus observables. Most importantly,
potent values generalize and unify the notion of the weak values and modular
values of observables in quantum theory. Furthermore, we define a potent
operator which describes the action of one system on the another and show that
superposition of time-evolutions and time-translation machines are potent
operators. These concepts may find useful applications in quantum information
processing and can lead to technological benefits
Entropy decrease in Quantum Zeno Effect
If a measurement process is regarded as an irreversible process, then by
Second law of thermodynamics the entropy should increase after any measurement
process. By the same spirit a quantum system undergoing repeated measurement
should show strong irreversibility leading to entropy production. On the
contrary we show that in quantum Zeno effect setting the entropy of a quantum
system decreases and goes to zero after a large number of measurements. We
discuss the entropy change under continuous measurement model and show that
entropy can decrease if we use a more accurate measuring apparatus.Comment: Latex file, 6 pages, no figure
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