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