Saturday, 17 September 2016

Quantum bomb

This is an experiment in which we wish to detect the presence of a bomb B without detonating it. However, the bomb will automatically detonate even if only a single photon hits it. One might intuitively think such detection is impossible, because in order to see something, one needs to shine light at it. However, turns out this is in fact possible and here's how.

A polarised photon is injected into the device and it experiences a rotation of A degrees by the rotator each time it passes through. It is passed through N times such that N*A = 90 degrees before being ejected. After the rotator the light is split into horizontally and vertically polarised components by a polarising beam splitter (PBS). The bomb is placed in the path of the weaker signal and the signals are combined with another PBS afterwards. The photon will remain polarised in the original orientation only if there was a bomb that prevented polarisation from being rotated. The probability of setting off the bomb is sin(A)^(2N) which approaches 0 as N approaches infinity and A approaches 0.

This outcome is a consequence of the fact that insignificantly small amount of light lost in one of the arms will prevent a large amount of light from rotating over a large number of circulations which would detonate the bomb. This works simply due to the fact that the lost light compared to the total in each rotation is insignificant compared to the rotation. While the polarisation is rotated by A, it's not difficult to see that loss becomes insignificant respect to A as A becomes small. Never the less, it is the elimination of this small signal which prevents the rotation from accumulating and consequently makes detection without large interaction possible. It is also closely related to quantum Zeno effect.
Rotation by 1 degree
This works even with a classical signal if the detector simply consists of light level detector which is set off when some threshold is exceeded. The only quantum mechanical aspect of this is that it will work with a single photon as well, implying that the photon didn't simply take one path or the other, but actually took both. One might also say that its wavefunction collapsed into one single polarisation each round because of the presense of the bomb. The experiment would seem to imply that these extremely small subphoton amplitudes are in some sense real even though one can only detect single photons.


This paper [] seems to be of the opinion that the universe is discrete and finite. I'm sure some people find it pleasing.


It is suggested that quantum entanglement emerges from the holographic principle stating that all of the information of a region (bulk bits) can be described by the bits on its boundary surface. There are redundancy and information loss in the bulk bits that lead to the nonlocal correlation among the bulk bits. Quantum field theory overestimates the independent degrees of freedom in the bulk. []

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