Thursday, August 3, 2017

Chinese Experiment Dramatically Demonstrates Quantum Entanglement

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Artist's conception of Chinese ground station 'teleporting' photons to its Micius satellite


Nearly one year ago I cited work by Morgan  Mitchell and a team from the Institute of Photonic Sciences in Barcelona,  in Physical Review Letters . Their work described a specially prepared light beam that enabled the observation of individual photons in addition to charting the quantum links between them. Basically then, the Mitchell team confirmed the theoretical prediction that all the photons involved  exhibit some degree of entanglement and that the most strongly "entangled" would be pairs of photons striking the detectors at the same time.

According to Mitchell, quoted in Science News, "entanglement should be present in pretty much any situation with a lot of particles interacting with each other."    Most physics' arguments, however, take the view that quantum level entanglement is a bad thing for quantum computing. After all, if the quantum particles that are the basis for one's quantum computer become "entangled" (with quantum particles outside) then it is possible for information leakage and lost security.  However, the flip side of that is the view that entanglement can actually lead to an instantaneous, ultra-secure internet. This would be feasible if information encoded in one set of photons pops up instantaneously (and hence unhackably) in the corresponding set.

Now, Chinese quantum physicists have putatively demonstrated this feasibility in a recent experiment. The team from the University of Science and Technology of China announced barely 7 weeks ago that it had succeeded in splitting pairs of photons.  One of each split pair remained in the lab on Earth while the other member was beamed to China's Micius satellite, orbiting  300 miles above the Earth.  Most importantly, each photon "teleported" to the satellite remained entangled with its partner on Earth. An invisible connection hundreds of miles apart, if you will. (The accompanying diagram portrays this experiment).

A month earlier, in fact, the Chinese physicists showed that the teleportation worked in the opposite direction as they originated the splitting of photons on the Micius before sending them to separate receiving stations on Earth some 745 miles apart.  The word "incredible" doesn't even begin to describe the result.

Let's back up a bit: Einstein, Podolsky and Rosen (E-P-R) originally  imagined a quantum system (atom) which could be 'ruptured' such that two electrons were dispatched to two differing measurement devices. Each electron would carry a property called 'spin'. Since the atom had zero spin, this meant one would have spin (+ 1/2), the other (-1/2). The diagram below illustrates this, the atom being disrupted inside the box, with its opposing spin electrons sent to the left and right.

A1 (+ ½ ) <----------->(- ½ )A2

Orthodox quantum mechanics forbade the simultaneous measurement of a property (say different spin states) for the same system. If you got one, you could not obtain the other. This was a direct outcome of the Heisenberg Indeterminacy Principle which stated that simultaneous quantum measurements could not be made to the same precision.

 E-P-R argued that this showed the incompleteness of quantum mechanics. It was not the 'paragon' of physical theories its apologists claimed, if such indeterminacy was fundamentally embedded within it.  Years later, mathematician John S. Bell asked the question: 'What if the E-P-R experiment could actually be carried out? What sort of mathematical results would be achieved?' In a work referred to as "the most profound discovery in the history of science", Bell then proceeded to place the E-P-R experiment in a rigorous and quantifiable context, which could be checked by actual measurements.

The first ever hints of entanglement (and tests of Bell's theorem) actually emerged in experiments carried out in the early 1980s, for example by Alain Aspect and his colleagues at the University of Paris.  In these experiments, the detection of the polarizations of photons was the key. These were observed with the photons emanating from a Krypton-Dye laser and arriving at two different analyzers, e.g.


(P1) A1 ¯|< ------D------> |­ A2 (P2)

Here, the laser device is D, and the analyzers (polarization detectors) are A1 and A2 along with two representative polarizations given at each, for two photons P1 and P2. The results of these remarkable experiments disclosed apparent and instantaneous connections between the photons at A1 and A2. In the case shown, a photon (P1) in the minimum (0) intensity polarization mode, is anti-correlated with one in the maximum intensity (1) mode.

Aside:  Polarization refers to that property whereby EM-radiation as it propagates can be confined say to one plane, or one rotation plane. If we say "circularly polarized" then the E-vector rotates through a full 360 degrees. If we say "linearly polarized" then it vibrates in one plane, e.g.

(A)
^
!
!
!
!

Or:

(B) < --------------------------------->----------------------------


Here, one might take (A) as the maximum intensity polarization mode and (B) as the minimum. 

Aspect et al on inspection of their data found there was a 100% anti-correlation (i.e. 100% negative correlation) between the two and an apparent nonlocal connection. In practice, the experiment was set out so that four (not two - as shown) different orientation 'sets' were obtained for the analyzers. These might be denoted: (A1,A2)I, (A1,A2)II, (A1,A2,)III, and (A1,A2)IV.

Essentially, the basis for our modern conception of entanglement was laid. It thereby became theoretically possible for two particles to become connected in such a way that even if placed on opposite sides of the universe whatever happened to one of the particles would instantly be reflected in the behavior of the other. 

The photons split by the Chinese weren't on opposite sides of the universe, but they were hundreds of miles apart which is impressive enough. 

Historically, let's note that not all physicists have accepted the notion of "superluminal transport" which is what the instantaneous connections suggest. According to special relativity nothing moves faster than light, at c = 300,000 km/ second. Thus, if a pair of split photons instantaneously displays connections 300 or more miles apart it indicates faster than light transport.  That is, in the "teleportation" scenario,  the split photon leaving - say its twin on Earth- had to arrive at the Micius satellite at the same instant. This would have meant v >  c.

To get away from that inference, physicist David Bohm,  in his book Wholeness and the Implicate Order, proposed that the seemingly instantaneously connected particles - whether photons, atoms or electrons - are actually always connected at a higher dimensionality.  However, after the Chinese experiments, when physicist Alan Steinhardt was asked if such particles could be connected via another dimension, responded:  "Extra dimensions do potentially solve certain quantum quandries, but I don't believe entanglement is one of them."

He may be correct, but more experiments are needed to test that suggestion. Also, we need to ascertain the degree to which entanglement can in fact provide a practical benefit for security using quantum computers.

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