Tuesday, October 16, 2012

Kudos to Nobel Physics Prize Winners for Laying the Basis for Quantum Computing

 Physics Nobel announcement
The two Nobel Physics Prize winners: Serge Haroche and Dave Wineland, displayed on a screen at Stockholm.

Kudos go out to a French and an American physicist who shared  the Nobel Prize last Tuesday for finding ways to measure quantum particles without destroying them, which could make it possible to build a "quantum computer"  far more powerful than any seen before. It is almost impossible to overestimate the importance.

Repeated papers and articles, e.g. in Physics Today, Scientific American, etc. have noted the formidable difficulty in overcoming a phenomenon known as "quantum entanglement" - when two or more quantum states are shared by separated particles, e.g. photons, electrons or atoms. Consider the illustration below where two photons are separated and diverge to analyzers A1 and A2:

 (A1)<---o---------  []----------o--->(A2) 

The results of repeated experiments disclose apparent and instantaneous connections between the photons at A1 and A2, such that although the photons are separated each member can be described by the same quantum mechanical state, including position, momentum, spin and polarization. If this is so, then the particles (.e.g. photons) are said to be in an "indefinite" state, or "entangled".

For a quantum computer to really work, and in this case we'd see qubits processed, as opposed to 'bits', the states would have to be resolved to the point a definite state can be explicated for each. A qubit denotes a quantum bit or a superposition of the two ordinary bits, 1 and 0. So:

U =  U(1) + U (0)

To give an idea of how powerful this concept is, in 1982, the Nobel-winning physicist Richard Feynman made a notable (and as it turns out, prescient) observation concerning a “universal quantum simulator”. Feynman speculated that to obtain 300 nuclear spins, the quantum simulator would need only 300 quantum bits or qubits. So long as one could program the interactions between qubits so that they emulated the interactions between the 300 nuclear spins, the dynamics of an entire simulated cosmos would be simulated.

Was Feynman off his rocker? Not really! As it turns out one of the best ways to generate simulations via a quantum computer is to use nuclear spins. To see how this could work, one could take a 'spin up' nuclear spin denoted by 0> and combine it in superposition to a 'spin down'  version denoted by 1> to obtain a combined state: 0> + 1>.. (The last is a state of spin along the axis perpendicular to the spin-axis).

More to the point, the latter superposition illustrates the property of quantum parallelism: the ability to compute or register two states simultaneously, which is impossible for normal computers (which register 0 OR 1, never both at one time – unless they are in glitch mode!) Thus, a quantum computer can manipulate two bits (qubits) at one time. If there are a trillion such qubits, each can potentially register 1 and 0 in combined wave bracket states, simultaneously. It should also be easily seen that this combined wave state is the analog of the superposition of states seen in the electron double slit experiment, e.g. for: U(1) + U(2).

Much more fascinating (and to the point) is that if another qubit is introduced into the scene - say with the same wave state 0> its presence can “flip” the original qubit , effectively producing a quantum-controlled NOT operation which acts like a classical NOT gate. It is billions and billions of such logic gates which form the basis of computing. Given that qubits also hold much more information than ordinary bits, it is easy to see that if such nuclear spins can be used in the sort of logic gate manner described, one can have the basis for a quantum computer.

In the case of the two recent Nobel prize winners, they're making no claim that we are within a few years of quantum computing, but for most of those who've followed this area, they've at least taken a mammoth step in that direction. (I'd say 10-15 years) Wineland has described his own work as a "parlor trick" that performed the seemingly magical feat of putting an object in two places at once, but he doesn't go on to the next step of resolving the eigenstates for those two "places" (for the same object) - which would yield the reason why he's received the prize.

According to The Royal Swedish Academy of Sciences:


"The Nobel laureates have opened the door to a new era of experimentation with quantum physics by demonstrating the direct observation of individual quantum particles without destroying them,"

By 'direct' obsevation of the individual particles they mean that enough information was obtained to at least separate them by one of more quantum states. The Academy goes on to state:

"Perhaps the quantum computer will change our everyday lives in this century in the same radical way as the classical computer did in the last century."

I can say when and if a working quantum computer comes onstream it will make the  most advanced present type look like a 'steam engine' compared to the most advanced rocket.  Researchers, of course,  have long dreamed of building "quantum computers" that would operate as I showed earlier, but they  all conceded this could only be done if the behavior of individual particles could be observed.  The Nobel Committee put it thusly:

"Single particles are not easily isolated from their surrounding environment, and they lose their mysterious quantum properties as soon as they interact with the outside world. Through their ingenious laboratory methods Haroche and Wineland, together with their research groups, have managed to measure and control very fragile quantum states, which were previously thought inaccessible for direct observation. The new methods allow them to examine, control and count the particles."

Those readers interested in a much detailed examination of quantum computing, and how superposition, interference and entanglement enter into it - as well as quantum data compression - are encouraged to try to get hold of the article 'Quantum Information and Computation' by Charles H. Bennett in Physics Today (Oct. 1995). Bennett shows using excellent illustrations and quantum principles the magnitude of the task as it was seen at that time, barely 17 years ago.

How far we have come with Wineland and Haroche's work!


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