PHYSICIST Richard Feynman once said that the double-slit experiment, which clearly shows both the particle and the wave nature of matter, contained the 鈥渉eart of quantum mechanics鈥� and was its 鈥渙nly mystery鈥�. Now it seems matter will display its dual nature even when the slits exist only in time.
The first double-slit experiment was performed by Thomas Young in 1803 and physicists have since replicated the results with ever increasing sophistication. Essentially it involves shining a beam of, say, photons onto an opaque screen that has two parallel slits. Even though the photons can be thought of as particles, they form bright and dark 鈥渋nterference鈥� fringes on a screen behind the slits, as if photons were passing through the slits as waves. If you try and work out which slit each photon travels through, the fringes vanish.
鈥淭he positive peaks of the electric field are the equivalent of the double slit, with each ejecting an electron at a different time鈥�
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But what if these slits were separated by time, rather than space? Gerhard Paulus of Texas A&M University in College Station and his colleagues in Germany, Austria and Bosnia tried to find out. They fired ultra-short pulses of laser light at a cloud of argon atoms. Each pulse could be controlled so that it contained just two positive peaks of the electric field and one negative peak. The electric field at the peaks was strong enough to sometimes rip an electron from an argon atom (see Graphic). An atom ionised by one of the positive peaks shot an electron towards one detector (A), while an electron from an atom ionised by the negative peak sped towards an opposite detector (B).
The two positive peaks are the equivalent of the double slit, with each letting an electron through to detector A at a different point in time. The negative peak behaves like a single slit. When Paulus鈥檚 team plotted the number of electrons seen by detector A as a function of their energy, they found interference fringes, because they could not tell, as in the conventional double-slit experiment, whether the electrons had come from the first positive peak or the second. However, no such fringes were observed by detector B 鈥� all these electrons had been ejected by a single negative peak, the equivalent of a lone slit.
However, if the team tweaked the laser pulse to have two negative peaks and one positive peak, the interference fringes disappeared from detector A and appeared instead at detector B.
Other physicists are impressed. Wolfgang Schleich, at the University of Ulm in Germany says, 鈥淭his experiment should be included in every textbook on quantum mechanics.鈥� Charles Joachain at the Free University of Brussels (ULB) agrees. 鈥淭his beautiful experiment gives another illustration of the subtle quantum mechanical effects that lie at the heart of intense laser-atom interactions.鈥�
