Quantum Nature of Quasi-Classical States and Highest Possible Single-photon Rate

Observation of the purely quantum mechanical effects of quasi-classical states is of utmost importance since these states are realistic sources of radiation and do not have any shortage in photon numbers. Therefore, they do not face the scalability problem as much as other single-photon sources do, which makes them much more robust against photon loss. Moreover, these states define the standard quantum limit. Hence, finding their quantum signature hints to the highest possible single-photon rate. In this manuscript, we attempt to demonstrate this idea theoretically using known dynamics and then present supporting experimental results. Through our experiment, we realize two-photon bunching from the transfer of quantum information using such states with the projection of orbital angular momentum from a continuous wave source. Our work is a step forward towards a more diverse and practical use of quasi-classical states in the domain of quantum optics and quantum information.Comment: 7 pages, 5 figure

Violating Bell inequality using weak coherent states

We present an experimental investigation of two-photon interference using a continuous-wave laser. We demonstrate the violation of the CHSH inequality using the phase randomized weak coherent states from a continuous wave laser. Our implementation serves as an approach to reveal the quantum nature of a source that is considered to be a classical source.Comment: 6 pages, 2 figure

Probing the Quantum Nature of Coherent States

Radiation from a continuous wave (CW) source will be described by a set of states known as coherent or quasi-classical states. The common thought regarding these states is that they are essentially quantum states with random behavior obeying the Poisson probability distribution. According to the anticorrelation test, sources represented by these states do not behave as single-photon sources. However, we have shown that it is possible to look for purely quantum mechanical manifestations of such states in the weak limit and by randomizing their relative temporal phase in an interferometric setup. We call such signals Phase Randomized Weak Coherent States (PRWCSs). First, we demonstrated such dynamics through experimental realization of the two-photon bunching and the Hong-Ou-Mandel (HOM) effect. Then we continued our search by probing the concept of entanglement through the test of Bell inequality. The result confirmed the prediction of quantum mechanics, violating the Clauser, Horne, Shimony and Holt (CHSH) form of the Bell inequality. In the third project, we tried to add the Orbital Angular Momentum (OAM) of light to this toolbox and examine if our method could be used to transfer quantum information using this spatial degree of freedom. This implementation was successful, and we realized the two-photon bunching again with interfering beams carrying OAM. The distinct feature of our approach is the use of CW lasers as a source of coherent states that can overcome the low photon rate from single-photon sources based on non-linearity. This feature is much more robust against photon loss and does not face the scalability problem as much as other photon sources do. In addition, our approach is a practical method for the realization of the entangled based Quantum Key Distribution (QKD) which is closely related to the Bell inequality. Finally, by observation of the quantum nature of coherent states, our work paves the way for the realization of multi-photon interference which is in the heart of many modern quantum sciences and quantum technologies.</p