A regular TV camera wouldn’t be able to detect anything at the low-photon flux we need to ensure only one photon passes the slits at a time. An “image-intensifier tube”—like those used by soldiers to see at night—is needed to make the image visible to a conventional camera element (e.g., a CCD camera).
In our single-photon experimental system (Figure 90), the photons that pass through the double slit strike the photocathode of one of these image-intensifier tubes. Just as with a PMT, these photons cause the release of electrons via the photoelectric effect, and are multiplied and accelerated within the intensifier tube. However, instead of finally hitting an anode, the secondary electrons are focused on a phosphor screen. Each incident photon that strikes the photocathode surface causes the release of many photons from the fluorescent screen, making it possible for a standard video camera to record the weak flux of photons passing through the double slit.
The gain and noise of the intensifier tube determine how low a photon flux can be reached and still yield an image of the interference pattern. So-called “Generation I” image-intensifier tubes are simple in design. As shown in Figure 92a, they utilize only a single potential difference to accelerate electrons from the photocathode to the anode (screen). Therefore, they achieve only moderate gain (a few hundred times), but provide high image resolution, a wide dynamic range, and low noise. In contrast, Generation II and Generation III tubes employ true electron multipliers to boost gain. That is, not only the energy but also the number of electrons between the photocathode and the screen is significantly increased. Multiplication is achieved by use of very thin plates of conductive glass containing scores of minute holes (each a few micrometers in diameter), inside which a cascade of secondary electron emission occurs. As shown in Figure 92, each hole in a microchannel plate (MCP) acts as a miniature PMT! The difference between the second and third generations resides in the type of material used in the photocathode. The “multialkali” photocathodes in second generation tubes yield a current of around 300 μA per lumen, while the gallium-arsenide photocathodes in third generation tubes have a luminous sensitivity of approximately 1,200 μA per lumen, meaning that “Gen III” intensifier tubes can reach higher gains (up to 107 photons from the screen for every photon hitting the photocathode) than Gen IIs. Gain doesn’t come for free though—in general, Gen II and Gen III intensifier tubes have lower resolution and produce more noise than Gen I tubes.
Figure 92 Each incident photon that strikes the photocathode surface of an image-intensifier tube causes the release of many photons from the fluorescent screen. (a) Generation I tubes utilize a potential difference to accelerate electrons from the photocathode to the anode (screen) to achieve moderate gains. (b) Generation II and III tubes employ MCP electron multipliers to boost gain. (c) MCPs are very thin plates of conductive glass containing scores of micrometer-sized diameter holes, in each of which (d) a cascade of secondary electron emission occurs, thus acting as a miniature PMT.
There are a number of options when acquiring or building a camera with sufficient sensitivity. The easiest way is to purchase a camera with a built-in intensifier, commonly used in fluorescence microscopy, astronomy, nighttime surveillance, and other applications that require image acquisition under very-low-level illumination. Depending on the technology, these go by the name of “ISIT Camera” (intensified silicon-intensified target), “IST Camera” (intensified silicon target), “ICCD” (intensified CCD), and “Photon-Counting Camera.” They are expensive—upward of $8,000. However, they can often be found for just a few hundred dollars on eBay and at surplus laboratory supply houses.
We opted instead for building our own intensified camera using components that we purchased on the surplus market. Our setup, shown in Figure 93, uses a surplus Gen III image-intensifier tube (an MX-10160 Gen III intensifier tube used in the helmet-mounted AN/AVS-6 “ANVIS” aviation night vision imaging system, which we purchased on eBay) followed by a pinhole-lens black-and-white CCD camera. The small plastic box contains two AA cells to supply 3 V to the intensifier tube. For convenience, we built our optical tube using Thorlabs’ SM1 and SM2 optical tubes and components. We fabricated the housing for the image-intensifier tube and light shields from surplus telescope-to-camera adapter tubes. The CCD camera looks directly at the intensifier’s phosphor screen. We shielded all potential photon leakage gaps with thick black electrician’s tape.
Figure 93 In our single-photon interference setup, a black-and-white CCD camera looks directly at the phosphor screen of a surplus Gen III image-intensifier tube. The interference pattern is projected directly onto the intensifier tube’s photocathode.
We found that the performance of our Gen III intensifier is superb, since it allows us to use D > 10 attenuators and see the interference pattern build up one photon at a time. With this setup, one can truly see single photons arriving to the detector, slowly building up the interference pattern from seemingly random hits. Watching this as it happens is a very moving experience for anyone who understands that under mundane, commonsense rules we should see two lines of light aligned with the slits. Instead, somehow, the single photons interfere with themselves! That means that each photon behaves as if it somehow flies through both slits at the same time!
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