'schrödinger's hat' could spy on quantum particles - Vehicle Wrap Printing - Outdoor Advertising Fl

In the early 1990s, physicists Avshalom Elitzur and Lev Vaidman atTel Aviv University in Israel pointed out that it is not alwaysnecessary to observe particles directly to learn something of theirnature. The researchers imagined a pile of bombs, each of which isdesigned to be triggered by the absorption of a single photon. Someof the bombs are duds; through these, photons pass unimpeded. Onecould check whether a bomb is working by firing a photon at it but,if the bomb were indeed to be working, it would be destroyed in theprocess. Would there be a way to weed out some of the working bombswithout destroying them? Interaction-free measurements The answer is yes, say Elitzur and Vaidman.

They considered aninterferometer: a device through which a photon's path is splitinto two arms, only to recombine at a set of detectors somedistance away. To test a bomb, it would have to be placed in onearm of the interferometer. A dud bomb would have no effect on thephoton, and the photon would pass through both arms, generating aninterference pattern at the detectors. A working bomb, on the otherhand, would force the photon to "choose" through which arm itpasses.

If it took the bomb's arm, the bomb would, regrettably, betriggered. If the photon took the empty arm, it would reach thedetectors unimpeded – but since the other arm was blocked,there would be no interference pattern. This lack of aninterference pattern would reveal the existence of the working bombwithout having triggered it. In 1994 Anton Zeilinger of the University of Innsbruck, Austria,and colleagues demonstrated in a real experiment that such"interaction-free" measurements are indeed possible.

Now, however,mathematician Gunther Uhlmann at the University of Washington inSeattle and colleagues may have come up with an easier way toperform such measurements – with a little help from thescience of invisibility cloaks. First demonstrated in 2006, invisibility cloaks can be understoodthrough an analogy with Einstein's general theory of relativity.This theory shows how very massive objects distort the underlyingfabric of the universe, space–time. In the same way, certainman-made structures known as metamaterials distort an equivalentfabric, a virtual "optical space". Metamaterials distort opticalspace through a spatially varying refractive index, the propertythat governs how light bends as it goes from one medium to another.By stretching out a hole in optical space, invisibility cloaks canshield a small object from light; the light rays pass aroundsmoothly, as though the object were not there.

Unleashing a quasmon In practice, however, not all the light passes around invisibilitycloaks – often, a small amount will leak in. If the inside ofthe cloak had almost the same resonant frequency as that of theincoming light, say Uhlmann and colleagues, that wave's energywould build up, forming a localized excitation. This excitationbehaves much like a particle, which the group has dubbed a"quasmon". This quasmon could then be released by making a slightalteration to the cloak's resonant frequency, perhaps through theapplication of a weak magnetic field.

Matti Lassas, a member of Uhlmann's group based at the Universityof Helsinki, explains that the team calls the modified invisibilitycloak a Schrödinger's hat because tiny "parts" of waves orwavefunctions can be secretly stored, rather like a magician's hat,and detected. And the trick is that the rest of the wave would bescarcely changed. Outside the Schrödinger's hat, says Lassas,the wavefunction would be "the old wavefunction multiplied by aconstant, [which] may be very small". The potential of a Schrödinger's hat can be seen in theexample of an electron in a box.

Although the electron'swavefunction is spread throughout the box, a scientist may be ableto guess the location of areas where it drops to zero. Thatscientist could then position a Schrödinger's hat at such alocation, with no fear of the electron "noticing" the sensor'spresence and collapsing into a definite state. If the experimentwere to be repeated several times, the scientist might be able tomap out where the electron definitely is not – and in doingso, learn something about where it actually is. Useful, but difficult to make Igor Smolyaninov at the University of Maryland, College Park, US,describes such a measurement as "an interesting proposal".Smolyaninov – who was not involved in the research –says that "Measuring the quantum wavefunction without muchperturbation would find important applications in many fields ofbasic science and, in particular, quantum computing." He adds,however, that a Schrödinger hat will be difficult to make,since it will need properties that vary wildly in a very narrowregion. Ulf Leonhardt, a member of Uhlmann's group based at the Universityof St Andrews in the UK, says that a device that works formicrowaves could be made using circuit-board materials.

A devicefor plasmons – waves of electrons in metals – could bemade from metal and plastic rings. He thinks a Schrödinger hatcould even be developed for sound – allowing its users toeavesdrop on sound without disturbing it. The work will be described in an forthcoming paper in the Proceedings of the National Academy of Sciences .

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