Can Quantum Ghosts Cheat Heisenberg?

ResearchBlogging.orgIt is theoretically impossible to observe all of the different aspects of state of matter at the subatomic “quantum” level. This means that at the tiniest level of spacetime, bits and pieces of stuff and action can only be vaguely known, and therefore, if you wanted to build a quantum computer you would have some interesting challenges.

A solution to this problem would be a key step in quantum engineerig. According to Anthony Lang, of Bristol Universtiy, “Apart from providing insight into the fundamentals of quantum physics, [such] work may be crucial for future quantum technologies. How else could a future quantum engineer build a quantum computer if they can’t tell which circuits they have?”

A paper in Physical Review Letters that came out a few days ago pruports to use entanglement and a few other tricks to overcome this limitation.


Here is the abstract:

Discrimination between unknown processes chosen from a finite set is experimentally shown to be possible even in the case of nonorthogonal processes. We demonstrate unambiguous deterministic quantum process discrimination of nonorthogonal processes using properties of entanglement, additional known unitaries, or classical communication. Single qubit measurement and unitary processes and multipartite unitaries (where the unitary acts nonseparably across two distant locations) acting on photons are discriminated with a confidence of >=97% in all cases.

That may be a bit thick for the average non physicist.

Distant particles can exchange information, a phenomenon referred to by Einstein as “spooky action at a distance.” This phenomenon, aka entanglement, is being used here to triangulate on quantum states. From a press release:

In the everyday world any process can be considered as a black box device with an input and an output; if you wish to identify the device you simply apply inputs, measure the outputs and determine what must have happened in between.

But quantum black boxes are different. Distinguishing between them is impossible using only single particle inputs because the outputs are not distinguishable: a fundamental consequence of the laws of quantum mechanics is that only very few states of a quantum particle can be reliably distinguished from one another.

The Bristol-Imperial team has shown how to get around this problem using ‘spooky action’.

I know, it sounds like cheating to me too.

_____________________________

Related:

Review of Black Bodies and Quantum Cats

Gloves, Mittens, Socks, Quarks and Alternative Universes. It all makes so much sense…

Laing, A., Rudolph, T., & O’Brien, J. (2009). Experimental Quantum Process Discrimination Physical Review Letters, 102 (16) DOI: 10.1103/PhysRevLett.102.160502

Comments

  1. #1 NewEnglandBob
    April 30, 2009

    Can’t we just get the quantum computer to build itself? Who would know better how to do it?

  2. #2 Ritchie Annand
    April 30, 2009

    I’m going to have to track down that paper.

    I’ve often wondered, after reading Nick Herbert’s Quantum Reality book (before he wandered off into the wasteland of ‘quantum consciousness’ and the like), whether they might someday be able to determine things a little more tightly than the Uncertainty Principle would have it by figuring out a way of determining the phase of some of the waveforms, somewhat like we can manage with normal radiation using comb filters.

    The position wave is a spike, and probably not amenable to such treatment, but the momentum wave is a continuous wave.

    I wonder how the entanglement “cheat” works…

  3. #3 MPM
    May 1, 2009

    If they build a device to do this, any word if they’ll call it a “Heisenberg compensator”?

    http://memory-alpha.org/en/wiki/Heisenberg_compensator