Wednesday, 7 November 2012

If you can't explain it simply, you don't understand it well enough.

Vacuum fluctuations are a funny thing. You take an ordinary mirror and place it into a spring so that the system has a certain resonant frequency. We know that temperature is just energy and in this case movement of the mirror. This movement can be measured by firing a laser (which is basically a single frequency [sine] electromagnetic wave) at the mirror. The thermal movement of the mirror will introduce frequency shifts into the reflected laser beam (as depicted in the picture below). This can be detected by observing the spectrum (splitting the laser beam into different frequency components by a prism for example). More or less symmetric movement (thermal energy) of the mirror back and forth will introduce sidebands below and above the laser frequency. However when the temperature of the mirror system is cooled close to absolute zero, there is no thermal energy present in the system anymore and the upper sideband will vanish. However what is paradoxial is that there is still a small, but finite lower sideband present in the signal, this sideband is due to vacuum fluctuations and cannot be cooled away by any amount of cooling, in some sense the mirror is still moving. Vacuum fluctuations, however, cannot give energy (which would be the case if a higher frequency sideband would be generated) and hence only the lower sideband is present, as if the mirror was only moving away from the laser (only absorbing energy, but not emitting any).

[Physics 5, 8 (2012)]

The imbalance is explained of course by the fact that part of the photons energy is absorbed by the cool environment before it is re-emitted. What's interesting though is that the amplitude of the lower sideband below which one cannot cool down the mirror is determined by the resonant frequency of the mirror. The higher the frequency, the higher the amplitude below which one cannot go. This all is of course due to the quantized nature of energy. The effect is very real though and has many consequences, one is that vacuum fluctuations prevent helium from solidifying at any temperature, even at absolute zero (in normal pressure ranges, if the pressure is very high then it can solidify due to the fact that vacuum fluctuations become small relative to the pressure).

I'm sure most people are familiar with the idea that light consists of photons and know that when a single photon is fired at a 50:50 beam splitter, it will be detected either in one branch of the apparatus or the other with equal probability. (This sort of light is called antibunched and it is not emitted by normal light sources such as lamps, however single atoms exited by a laser do emit this sort of light. Bunched light emitted by a regular lamp on the other hand can (and will sometimes) trigger both detectors simultaneously.)

Now instead of using a single photon source, if we have a source which gives two entangled photons (it's not important how they are made, it is sufficient to know we can) something strange happens. We know that the entangled photons have opposite polarization and that they are always generated in pairs. If photon A has vertical polarization then photon B has horizontal and vice versa. This can be detected by a polarizing beam splitter. When the angle between the polarizing beam splitter at left and right are equal, nothing strange happens, if we detect vertical left, we will detect horizontal right and vice versa  However if one of the polarizing beam splitters is rotated by 45 degrees then something strange happens. At 45 degrees the transmission probability of vertically and horizontally polarized photon is 50% (or if you consider the process as continuous then you might say that the power is split 50:50). What you see is that each time no photons are lost, we always detect 2 photons, one at the left side of the apparatus, the other at the right side of the apparatus. However if one counts the correlations between the photons, say a photon which at the left side was detected having a horizontal polarization and at the right side was detected having vertical polarization, we get roughly 70% (sqrt(2)/2), same if reverse order is detected, but how could this be? A polarizing beam splitter has 50% transmission for both polarization states so one might classically expect no more to be possible.

The quantum mechanical prediction is different and consistent with the experiments. Somehow the other photon knows which way the other went (in certain sense).

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