Single Photon Interference

Presentation Slides: 

Wave/particle duality observed in Young's double slit experiment with camera sensitive to individual photons.

What it shows:
In this demonstration we perform the double-slit interference experiment with extremely dim light and show that even when the light intensity is reduced down to several photons/sec, the audience can see the familiar Young's double-slit interference pattern build up over a period of time. This addresses the question of how can single photons interfere with photons that have already gone through the apparatus in the past, or with those that will go through in the future, or with themselves.

The experiment is designed to also make it possible (in principle) to know which of the two slits the photons are passing through. In this so-called which-path case, the Young's double-slit interference pattern does not manifest itself. This can be followed by a quantum eraser (to erase the which-path information) to recover interference. Thus, the act of measurement and the design of the experiment affect what is being measured. Even if not actually measured, the mere possibility that an observer could determine which slit the photon passed through causes the interference pattern to switch to non-interference.

How it works:  The layout of the apparatus is illustrated below.

layout of apparatus

A 4-meter long PVC pipe supports all the optics and acts as a light shield. Light from a blue LED enters through a 5 micron slit. The double-slit (two, 200 micron wide, slits separated 1.0 mm) is located in the middle. The detector is a thermoelectrically cooled EMCCD camera (Andor Luca DL-658M), providing single photon detection sensitivity with a QE of 50%. The image data are downloaded to a PC every 1/2 second and integrated over time. The PC displays the interference pattern as it accumulates and integrates over a period of two minutes. A graph of the intensity distribution as well as the pixel counts is included in the display. The light source and entrance slit are at one end of the PVC pipe and the Luca camera is attached to the other end.

The blue LED, linear polarizer, and 5 micron entrance slit are all mounted in standard lens holders and the entire assembly is attached to the end-cap of the PVC pipe.


The which-path marker consists of two, mutually perpendicular, polarizing filters. When either the vertical or the horizontal filter covers both slits, the double-slit interference pattern is preserved, albeit at a reduced intensity compared to no filter. However, when the vertical filter covers one slit and the horizontal filter covers the other, the double-slit pattern disappears completely. Two superimposed single-slit patterns are all that remain. This new arrangement changes the setup to a which-path experiment in the sense that it is now (in principle) possible to know which slit the photon passed through; this destroys the quantum interference. Introducing a third polarizing filter, the quantum eraser, between the marker and the detector thwarts the which path experiment if it is oriented 45 degrees with respect to the marker filters. Every photon reaching the detector is now polarized in the direction of the third polarizer, and it is no longer possible to know which slit each photon passes through.

The which-path marker consist of vertical and horizontal polarizers. They can be accurately positioned in front of the double-slit by means of a calibrated screw.

A third polarizing filter, oriented 45˚ w.r.t the other filters, acts as the quantum eraser.

The marker and eraser assemblies attached to the PVC pipe. The eraser polarizer is moved into place by simply lowering the lens post. No fine adjustments are necessary.

Setting it up:  Obviously, the experiment will occupy a large portion of the space in front of the lecture benches. A dedicated cart supports the 4-meter long PVC pipe. A second cart accommodates the PC and camera power supply (this is stored in the Phys 191 lab). Camera functionalities as well as display options are software controlled. We use the Andor Solis platform. Full instructions on setting up the camera and running Solis are kept on the PC cart.

Strictly speaking, we are not detecting single photons of light but rather single photoelectrons, liberated by the light impinging on the CCD. Nevertheless, the quantum nature of light is evident. The positions of arrival (of the photons) are random but the probabilities of arriving at certain positions are not. This is beautifully born out in the demonstration. For a full and detailed description of the experiment and apparatus, click on the "single_photon_paper.pdf" link above. The paper includes several images of the PC output display.