Bringing Young to Life

thesis Bringing Young to life

In my mind — and I suppose this comes Aspden and colleagues fashioned two from some book I read in my youth — I Real experiments slits 100 μm wide and separated centre- see protons as tiny blue spheres. Neutrons showing these effects to-centre by 500 μm, and illuminated are brown spheres of the same size, the slits with down-converted photons electrons smaller and yellow. Photons are don’t ordinarily play a at 710 nm. As a heralding detector, they also yellow, although blurred out a bit. part in undergraduate used a single photon avalanche diode, and, My automatic imagery is completely in for the camera, an Andor iStar Gen III the spirit of pre-quantum physics, taking education. intensified CCD. As a minor point, the ‘particle’ terminology quite literally. they added an optical delay line to the I expect many physicists carry around slow-moving electrons from the surface camera arm to compensate for the time similar intuitive images, even if they know of a nickel crystal. For light itself, the taken by the triggering operation. Except they cannot be taken seriously. two-slit experiment was first performed perhaps for the camera, all the relevant After all, Young’s famous two-slit with feeble light as early as 1909 by British technology, they note, is already available experiment reveals the limitations of the physicist Geoffrey Taylor, who observed an in undergraduate labs at hundreds of particle concept for real quanta. With both interference pattern from single-photon institutions. This is the equivalent of slits open, the quanta within a coherent impacts by running an experiment over a laboratory microscopes or pendulums but beam sample both possible paths to a period of six months. Even today, a similar for basic quantum theory. screen behind. Despite always impacting experiment can’t be done using ordinary The results of the experiment are, the screen at a precise point, they build photographic cameras due to inherent of course, just as expected — although up an interference pattern over time. optical noise — especially during camera presented with remarkable clarity and Close one slit, and the interference is read out — that ends up swamping the simplicity in a set-up easily mastered destroyed. No simple scheme of particles signal, making it impossible to observe the by students. With a coherent beam, following precise trajectories can account single photon impacts as they build up the transmission through the two slits for both outcomes (although non-classical statistical interference pattern. generates a series of crisp interference schemes, such as the one developed by What Aspden and colleagues have done fringes. After a few dozen or even a few David Bohm, show that the notion of is to reduce the noise through careful hundred photons, no clear pattern is yet precise trajectories can be preserved). timing. Their idea is to keep the camera evident. But the fringes become very The strange outcome of this experiment turned off most of the time during the clear after a few thousand single photon is a staple of undergraduate physics experiment, turning it on only for brief detections. (For videos of the results see education. Oddly enough, however, real intervals to include the moment of photon http://go.nature.com/woaz1w). experiments showing these effects don’t arrival. That sounds tricky, but is in fact The authors also show how the set-up ordinarily play a part in that education. fairly easy — conceptually, at least — by can be used to display some of the peculiar You can’t easily just set up a laser, shine it using entangled photons. Two correlated consequences of photon entanglement. on a pair of slits, and see single photons photons can be created at one moment, You wouldn’t normally expect a beam registering the statistical interference with one being directed toward the of photons sent directly to an imaging pattern on a photographic plate, despite two-slit apparatus, and the second going screen — and not encountering any slits many textbook discussions to that effect. elsewhere. This second photon can then at all — to show an interference pattern. At least you couldn’t do this until now. be used as herald, its detection at a known But what if those photons were entangled Physicists Reuben Aspden, Miles Padgett distance away being used to trigger a with others that do encounter some and Gabriel Spalding have devised a switch to turn the camera on, briefly, and slits? The two down-converted photons clever scheme to do the experiment in the just in time for the first photon’s arrival. are correlated not only in time but also undergraduate lab using commercially As the authors describe, this can be in space. Hence, it’s possible to take the available equipment, including a laser and done using parametric down-conversion two-slit set-up and place it into the arm a CCD array detector (preprint at http:// in beta barium borate crystals. The of the herald, rather than the camera. In arxiv.org/abs/1602.05987; 2016). Seeing nonlinear optical properties of such this configuration, Aspden and colleagues the experiment in simple terms probably crystals lead to the absorption of a single show that the photon beam again produces won’t change the images in anyone’s photon, and the subsequent simultaneous an interference pattern even though, in head of what quanta really are, but it’s a emission of two photons of half the this case, none of the photons ever interact beautiful example of essential quantum energy. In their set-up, a pump laser with the two slits. The authors refer to behaviour brought to life. enters the crystal and its photons generate this as ghost interference, as it reflects In the historical development of a stream of pairs of entangled photons. Einstein’s reference to the “spooky action quantum theory, the first experiments A beamsplitter ensures that, from each at a distance” involved in entanglement. revealing the wave character of massive pair, one travels forward toward the two- Again, none of this is really ‘new’ particles relied on crystal diffraction, slit apparatus and the CCD camera, and physics. It’s old physics beautifully rather than close analogies to the two-slit the other — the herald photon — goes displayed. And soon to be enjoyed, I set-up. Famously, Clinton Davisson and elsewhere. A heralding detector stands expect, by students around the world. ❐ Lester Germer, while working at Bell Labs ready to absorb this photon and send a in 1927, observed the diffraction of trigger signal to the camera. MARK BUCHANAN