On the next Q+ Hangout

The next Q+ hangout is all set to run on 22nd April, 14:00 UTC+1. Surprisingly, the topic this time “On the Uncertainty of the Ordering of Nonlocal Wavefunction Collapse when Relativity is Considered”, which I had earlier read through and found to be highly interesting and no less entangled, no pun.

In the EPR experiment, if Alice makes a measurement on her particle then the state of Bob’s particle collapses to the result anti-correlated to Alice’s measurement. This process is said to be  happen instantaneously.

This ‘instantaneous’ gives rise to a paradox. For example, if in one reference frame Alice measures first then Bob’s state collapses. In a different inertial frame, an observer might say that Bob measured first leading to the collapse of Alice’s state. This leads to the identity paradox for who collapsed whose first!

This paper uses a type of clock device that functions on the laws of quantum-mechanics. This device in the experiment keeps the above paradox from occurring.

The bottom line being that in the experiment, Alice and Bob’s measurements cannot be made with infinite precision, rather they are constrained due to the energy-time uncertainty principle. Since energy and time are not relativistic invariant quantities, different observers in different reference frames must transform their uncertainty principles accordingly.

Concluding the paper rightfully claims the uncertainty principle in time always outruns the time difference induced by the change in reference frames. Neither Alice nor Bob will ever, with certainty, observe the two measurements swap temporal order. Furthermore, it can be said that  if a time measurement performed an entangled biphoton is simultaneous in one shared reference frame then it can be considered simultaneous to all measuring observers who do not share a reference frame.

On a personal note, it was only while going through the paper I thought about the time it takes for a EPR photon to collapse when measurement taken on its pair. People have already calculated it experimentally. This hangout already sounds like exciting, fingers crossed that I can attend it uninterrupted this time, have a couple of questions for the presenter.

Further Reading

On the Uncertainty of the Ordering of Nonlocal Wavefunction Collapse when Relativity is Considered arXiv:1310.4956 [quant-ph]

The Uncertainty Relation Between Energy and Time in Non-relativistic Quantum Mechanics DOI: 10.1007/978-3-642-74626-0_8

Experimental test of relativistic quantum state collapse with moving reference frames DOI: 10.1088/0305-4470/34/35/334

Q+ Hangout: Francesco Buscemi

Details of upcoming Q+ Hangout

Date: 28th August 2012

Time: 2pm British Summer Time; 6.30pm Indian Standard Time

Speaker: Francesco Buscemi (Nagoya University)

Title: All entangled quantum states are nonlocal: equivalence between locality and separability in quantum theory

Abstract:

In this talk I will show how, by slightly modifying the rules of nonlocal games, one can prove that all entangled states violate local realism.

As it is well known, Bell inequalities, which are used to test the violation of local realism, can be equivalently reformulated in terms of nonlocal games (namely, cooperative games with incomplete information) played between one referee and two (or more) players, these latter being separated so to make any form of communication between them impossible during the game. Quantum nonlocality is that property of quantum states that allows players sharing them to win nonlocal games more frequently than the assumption of local realism would imply.

However, as Werner proved in 1989, not all quantum states enable such a violation of local realism. In particular, Werner showed the existence of quantum states that cannot be created locally (the so-called “entangled” states) and, yet, do not allow any violation of local realism in nonlocal games. This fact has been since then considered an unsatisfactory gap in the theory, attracting a considerable amount of attentions in the literature.

In this talk I will present a simple proof of the fact that all entangled states indeed violate local realism. This will be done by considering a new larger class of nonlocal games, which I call “semiquantum,” differing from the old ones merely in that the referee can now communicate with the players through quantum channels, rather than being restricted to use classical ones, as it was tacitly assumed before. I will then prove that one quantum state always provides better payoffs than another quantum state, in semiquantum nonlocal games, if and only if the latter can be obtained from the former, by local operations and shared randomness (LOSR). The main claim will then follow as a corollary.

The new approach not only provides a clear theoretical picture of the relation between locality and separability, but also suggests, thanks to its simplicity, new experimental tests able in principle to verify the violation of local realism in situations where previous experiments would fail.

Based on http://arxiv.org/abs/1106.6095

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