## Making Cesium Atoms Do a Quantum Walk 117

Posted
by
kdawson

from the like-a-perp-walk-only-without-the-cameras dept.

from the like-a-perp-walk-only-without-the-cameras dept.

An anonymous reader recommends an Ars Technica account of a breakthrough in efforts toward quantum computing. German scientists have managed to get cesium atoms in a state called a "quantum walk": basically a superposition of all the possible states of a particle.

*"Quantum walks were first proposed by physicist Richard Feynman and are, in terms of probability, the opposite of a random walk. A random walk might be modeled by a person flipping a coin, and for each flip he steps left for heads and right for tails. In this case, his most probable location is the center, with the probability distribution tapering off in either direction. A quantum walk involves the use of internal states and superpositions, and results in the hypothetical person 'exploring' every possible position simultaneously."*In the abstract of the paper from*Science*(subscription needed for full-text access), the researchers say: "Our system allows the observation of the quantum-to-classical transition and paves the way for applications, such as quantum cellular automata."
## Re:Misunderstanding this, most likely (Score:5, Informative)

Quantum mechanics applies to large particles. Classical mechanics are merely an approximation of quantum mechanics when applied to large particles.

Wikipedia to the rescue

http://en.wikipedia.org/wiki/Correspondence_principle [wikipedia.org]

## Re:Misunderstanding this, most likely (Score:2, Informative)

## Re:Quantum CPU extensions? (Score:3, Informative)

That's what superposition means, just less fancy :)Nope, and this is a good straight line for my futile quest to explain something about quantum weirdness, because it is precisely the difference between "maybe" and "superposition" that makes life interesting for a quantum mechanic.

"Maybe" is a classical concept. If we see a cat get into a box, and then there is a sudden yowling and howling from the box, and you ask me, "Is the cat ok?" and I reply, "Maybe" we are talking about a classical situation, in which the cat "really is" either OK or !OK. There are two possible states and classically they are mutually exclusive and jointly exhaustive, regardless of anything else we do to the system. We don't have to look at the cat or measure the cat, we know that it can only be "OK" or "!OK" (for some sufficiently crisp definition of "OK").

"Superposition" is a quantum concept. If a photon interacts with a double slit apparatus and you ask me, "Did it go through the left slit?" and I say, "Maybe" I've said something incoherent unless I quickly stick an apparatus for measuring which slit it went through into the photon's path, because until a measurement is made that distinguishes a photon that passed through the left slit from one that passed through the right, the photon is in a superposition of both states, which are still jointly exhaustive but no longer mutually exclusive, and there is no "fact of the matter" about which slit the photon "really" went through until we ask it with an appropriate apparatus.

The big question to me, which no one from Copenhagen to Consistent Histories or Decoherence answers, is why the classical world--that is, the world of human experience--arises from the quantum world at all. Which is to say, no one has ever answered Max Born's question, "WHY must I treat the measuring apparatus as classical? What will happen to me if I don't!?"

The standard interpretations all take for granted that there is a classical world in which superposition is unobservable, but this papers over the enormous ontological gap between the classical and quantum worlds. The classical world obeys Aristotelian limits on contradiction and causality and locality: a thing cannot both be and not be the same thing at the same time and in the same respect. The quantum world does not obey these limits: the photon can both be and not be a photon that has passed through the left slit, but the wavefunction pulls off some nonlocal legerdemain to clean up after itself when we try to catch it out.

Various interpretations make arguments about HOW this cleanup happens, but no one says anything about why the classical world exists at all: why we are unaware of all the "extra" components of wavefunctions floating around loose after a measurement has been made. Decoherence comes closest to an answer by simply declaring that interference phenomena are the only means by which we can be aware of these other components, but it still says nothing about why we are privileged to observe the effects of one component and not all the others, when the natural expectation would be that we would be that after a measurement had taken place we would be aware of the measurement apparatus as being in an incoherent superposition of orthogonal states.