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Single photon interference with a Fresnel biprism


Most textbooks on Quantum Mechanics, like Feynman’s lectures, describe a thought-experiment based on the Young interference setup and aimed at exhibiting the essence of wave-particle duality (see Figure 1.a and Figure 1.b). Such interferences have been indeed observed with massive particles : electrons, neutrons, atoms and recently molecules as large as fullerenes.




Figure 1 - Principle of a wave-particle experiment. A source S emits independent particles, one at a time. The particles fall on a screen drilled with two holes T1 and T2. In the experiment (a), a movable detector D measures in a detection plane the detection rate N as a function of its x position. The record of N(x) displays an interference pattern, which can be interpreted by invoking a wave passing through both holes. In the experiment (b), two detectors collect particles that pass either through hole T1 or through T2 and their outputs feed a coincidence circuit. Although the two detectors are fired at the same rate, particle-like behaviour is evidenced by the absence of coincidences.


 


Since the concept of wave-particle duality was first introduced about light by Einstein in 1905, it is natural to look for such experiment with photons. Until the emergence of quantum optics, "single-photon regime" was reached by simply attenuating light pulses, at a level where the probability of having more than one photon became negligeable. Nevertheless, even with faint laser pulses, there is a strong likelihood that two photons are present within the same pulse.
Therefore, in an interference experiment performed with such attenuated light pulses, there is at least some chance that more than one photon may be present between the source and the observation plane of the interference fringes.


We present an experiment (figure 2) realized with single photons and based upon wavefront separation with a Fresnel biprism. We use single photon wavepackets produced by a triggered single photon source (SPS), based on the photoluminescence of a single colour defect in diamond. The results have been obtained with the same emitting colour centre which reliably produced single photons for almost three weeks, the time duration we needed to took all experimental data presented hereafter.



Figure 2 - Wave-particle experiment based on a triggered single photon source (SPS) and a Fresnel biprism.


 


The interference pattern is detected by registering the photons one at a time, using an image-intensified CCD camera. Whereas each single photon seems to have a random spatial distribution
on the detection plane, the interference pattern emerges after a few minutes of integration time. A downlable video movie shows the build-up of the interference fringes photon per photon.


We then look at the photon probability distribution in the two spatially separated wavefronts which appear in the far field. The lack of significant coincidence counts between the
two interference paths gives evidence for the fact that the source is actually producing very good approximates of single photon states, and that we truly observed single photon interference.


Note that a similar experiment has been done by Tonomura et al (Hitachi Research Laboratories) with an electron microscope. In that experimental arrangement, electrons are sent one by one through an "electron biprism", which consists of two parallel plates and a fine filament at the center. An appropriate detector allows the record of the built-up of interference fringes, after each particle detection.


Our experiment was achieved at Laboratoire de Photonique Quantique et Moléculaire in Ecole Normale Supérieure de Cachan, by Vincent JACQUES, E WU and Timothée TOURY.


It is a pleasure to thank Alain Aspect for many enlightning and stimulating discussions. We aknowledge the support of Palais de la Découverte and Institut Universitaire de France.

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