HIGH-PURITY SINGLE-PHOTON SOURCE

Our single-photon experiments (Figure 33, Figure 90, Figure 121, Figure 125, and Figure 132) have all used a highly attenuated laser beam to produce single photons. However, a precise analysis of this method shows that we could be assured that single photons fly through the apparatus only if the source photons are completely independent of each other. But photons sometimes bunch together, allowing pairs and triplets to fly together through the system, resulting in data that does not pass the rigorous scrutiny required for cutting-edge research.

An entangled-photon setup, such as the one shown in Figure 146, is now commonly used in high-level research involving single photons, because the down-conversion process does not produce bunched photons. In addition, one of the two entangled photons is used to “gate” the detection system. Single photons detected after they fly through the experimental apparatus (e.g., double slit, Mach–Zehnder interferometer, etc.) are counted only if their arrival is expected as announced by the gating photon. This technique considerably reduces background counts due to detector noise or stray photons.

Figure 146 An entangled-photon source is often used in advanced single-photon experiments, because the down-conversion process intrinsically produces two beams of individual photons that verify each other’s existence. A photon reaching the experiment’s detector (channel A) is counted only if it coincides with the detection of its partner photon (channel B). This eliminates the possibility of “photon bunching,” which may affect single-photon sources based on strongly attenuated laser beams.

Other single-photon sources are being investigated because of their importance for quantum technology (e.g., in quantum cryptography), as we will see a bit later. Most of these rely on tightly focusing a laser beam onto a sample area containing a very low concentration of quantum dots, such that only one quantum dot becomes excited and emits a photon at any given time. These sources are inherently anti-bunched,⁶⁸ and can produce high-intensity beams of single photons at visible wavelengths.