This Spotlight aims to provide a general introduction to linear and nonlinear optical components that are frequently used to implement the protocols for quantum computation and communication. The role of each optical element is described briefly, and it is shown that these optical elements can be combined to constitute quantum circuits for performing various quantum computing and communication tasks. Present challenges and future scopes are also mentioned briefly. Specifically, after introducing the qubit, it is stated that a quantum gate is a device that transforms a quantum state into another quantum state. Thus, most of the optical components (e.g., a beamsplitter, a wave plate, a mirror, a polarizing beamsplitter) can be viewed as quantum gates because each of these optical elements can transform a quantum state into another quantum state. This fact is the basis for constructing useful quantum circuits using optical elements. Finally, a few proposals for optical realizations of schemes for quantum communication and computation are described.
When we accepted the invitation of Dr. Robert D. Fiete, Editor of the Spotlight series, we did not realize how challenging it would be. As it was supposed to be 20 to 40 pages long, we thought it would not take much time. Later, when we started organizing all of the materials, we recognized that it is not that simple to describe the vast area of optical quantum computation and communication within that page limit. Naturally, we could not cover all of the important aspects that we initially thought to cover, but we still think that readers will find this Spotlight very useful in understanding both theoretical and experimental optical quantum information. Further, this Spotlight covers the interdisciplinary subject of optical quantum information, and it is expected to be of use for communication engineers and computer scientists.
To bring this Spotlight to its present shape, we have benefited from the support and feedback of many colleagues and students. Specifically, Prof. Pathak wishes to gratefully thank his colleagues Dr. Karel Lemr, Dr. Antonín Cˇ ernoch, and Prof. Jan Soubusta from the Joint Laboratory of Optics, Olomouc, Czech Republic, for carefully reading the manuscript and providing their feedback, suggestions, and also a few very nice photographs of real experimental setups used in their lab. He also wishes to thank Prof. Marco Genovese, INRIM, Turin, Italy, for his comments and permission to use a few photos of their experiment. Prof. Pathak also thanks Prof. Ajoy Ghatak, Prof. Debabrata Goswami, Dr. R. Srikanth, and Dr. Chitra Shukla for their feedback and comments on this Spotlight. Finally, a big thank you is due to Mr. Kishore Thapliyal, who has helped us in more than one way to prepare this book. He checked proofs, he prepared high-resolution figures, and most importantly he often criticized what we wrote, which helped us to improve the text. This Spotlight was written at the Jaypee Institute of Information Technology (JIIT), Noida, and Bose Institute, Kolkata. During the writing process we used the infrastructural facilities of these institutes and the resources occupied through several DST, India, funded projects. So, we thank JIIT, Bose and DST, India. Further, we gratefully acknowledge the support of our family members and students received during this project.