Single photon is one of the most valuable resources in quantum science and technology. As being the single particle of light, it shows the behaviour of true quantumness, which is used in various quantum mechanical applications, including quantum information, communication, metrology, etc. Photons traverse at the speed of light and do not interact with the external electromagnetic field, as being chargeless and massless. These characteristics make it a suitable resource for information processing. Photons are also easily manipulated with various linear optics like waveplates, beamsplitters, phase modulators, etc., which makes the system apparatus simpler and less resource-intensive. Various quantum mechanical states (qubit, qutrit, etc.) can be realized by using different degrees of freedom of the single photon state, including polarization, momentum, temporal modes, spatial modes, etc. There are various ways to prepare single photons, among which spontaneous parametric down-conversion (SPDC) based photon sources are the most prevalent, widely used, and researched sources in the quantum community. They are simpler in design compared to other sources but at the same time produce single photons at a high rate and with good quality. Also, another interesting property of an SPDC source is the generation of heralded photons where, single photons are always generated in pairs, and detection of one photon in the pair heralds the other photon of the same pair.

In this talk, I will discuss three experimental projects where we explore various advantages of SPDC based single photons in research areas, covering quantum fundamentals as well as technologies. In the first experiment, we have demonstrated a modified Hong-Ou-Mandel (HOM) setup that also verifies wave-particle-like complementarity relation along with measuring near-100% coincidence visibility dip. Such a modification allows us to check for the degree of indistinguishability as well as the degree of quantumness. We show that the photon pairs generated from the SPDC source maintain both these conditions. Next, we have demonstrated a quantum key distribution (QKD) protocol (B92) using heralded single photons from an SPDC source, where we implement few novel strategies to enhance the security against eavesdropping attacks such as intercept-resend attack, photon number splitting attack, Trojan horse attack, time-shift attack, etc. Another domain of study is related to a purely fundamental aspect of research, again using single photons. We show ‘loophole’ free violations of different ‘macrorealist’ inequalities, like the Leggett-Garg inequality (LGI) and its variant called the Wigner form of Leggett-Garg inequality (WLGI) using an experimental setup comprising two Mach-Zehnder interferometers in tandem. We have adopted strategies to tackle various loopholes like the clumsiness loophole, detection efficiency loophole, multiphoton emission loophole, coincidence loophole, and preparation state loophole which lack sufficient work and discussion in the literature; in the context of macrorealism.