Experimental and Theoretical Investigation of Quantum Communication Resources

Abstract

Quantum entanglement is an essential resource in quantum information, communication and metrology. In this thesis we explore different ways of using continuous variable quantum entanglement for quantum communication which can offer security that cannot be broken by any means, even by quantum computers. Quantum key distribution is a famous example of this. Another interesting area is secret sharing where people can exchange classical or quantum information with the security offered by quantum mechanics. In this thesis we experimentally investigate the role of entanglement and how entanglement connects the field of quantum metrology and secure quantum communications using a secret sharing protocol. We perform the security analysis of these protocols using the Holevo Cramér-Rao bound and calculate the probability of detecting an untrusted party. We follow up this investigation with the interesting problem of simulating Gaussian channels using a quantum teleporter. With the help of entanglement, we were able to simulate several Gaussian channels of interest with some having no classical analogue. We demonstrate error correction on Gaussian states by simulating a channel that is less decohering than the original thermal channel through the use of a measurement based noiseless amplifier. Beyond the popular channels like pure loss or thermal channels, we simulate amplifier channels that are useful in quantum tele-amplification. Unlike conventional classical amplifiers, the simulated amplifier channels increases the signal-to-noise ratio. The measurement based noiseless linear amplifier also removes stringent requirements on resource entanglement for simulating Gaussian channels at the expense of determinism. Being a probabilistic protocol, we can also simulate channels that are otherwise unphysical in deterministic protocols. Quantum technology offers many promises, however, fails to deliver at scale because of quantum decoherence. The squeezed states that are used to generate entanglement are very sensitive to losses and system stability. We investigated the engineering of a system that by design is accessible to the community at large. The main aspects under focus were the high duty cycle operation and maximum attainable squeezing levels. We also focus on other points like the quantum control and automation of control sequences. The squeezed light source offered very high duty cycles over several tens of hours with maximum measured squeezing levels as high as 12.6 dB below the shot noise limit. At telecommunication wavelength, this is one among the highest reported so far. Previously presented experiments will benefit from this squeezer in-terms of improving the overall quality of results. The results both experimental and theoretical until now, centres around entanglement and how protocols benefit from it. However, there exist non-classical correlations beyond the framework of entanglement. Though we cannot immediately draw connections between such measures and applications in quantum communications. We envision the study to inspire new protocols. Entropic accord is a non-classical correlation that establishes a correlation through an adversarial game. A correlation that is established through conflict between the involved parties is intriguing trait. We investigate the behaviour of the entropic accord under decoherence and we also compare entropic accord with entanglement and discord for several interesting quantum states.