Loophole-Free Test of Quantum Nonlocality and Efficient Noiseless Amplification for Large Optical Fields

Abstract

The concept of quantum nonlocality was used first by Einstein, Podolsky, and Rosen with intention to indicate the paradox of quantum physics. However, quantum nonlocality turns out to be one of the most important resource for quantum information processing, which can be used for quantum teleportation and device-independent quantum key distribution not to mention of violation of local realism assumed by classical physics.

In the beginning of this dissertation, we investigate the hierarchy of quantum nonlocality with emphasis on relation between EPR-steerable states and quantum teleportable states. Efficient ways to demonstrate Bell nonlocality and EPR steering are followed.

Demonstration of Bell nonlocality can rule out local realism assumed by classical physics. The major two loopholes are required to be closed for the perfect demonstration, which are so called detection loophole and locality loophole. Locality loophole was solely closed by using photonic quantum system, while detection loophole could be closed alone by using atomic quantum system with high detection efficiency. Up to now, demonstration which closes both of loopholes does not exist. Taking advantage from both systems, we have proposed a scheme which uses a photon in cavity and a Rydberg atom is suggested and experimental conditions which can close locality loophole is analyzed. If we can inhibit spontaneous emission by using thin cryogenic superconducting cylindrical cavity and enhance photon storage time of the main cavity, the locality loophole can be avoided.

EPR steering is the ability to change the quantum phenomena of space-likely separated system from the measurement on the other system. Demonstration of EPR steering less suffers from detection loophole and locality loophole than Bell nonlocality. We investigate the EPR steering of entangled coherent states using homodyne detection, photon parity detection, and photon on/off detection. The maximum value of EPR steering inequality is achieved by low-efficiency homodyne detection with enough amplitude of entangled coherent state. If entangled coherent state can be successfully generated, the demonstration of EPR steering may be feasible considering the practical efficiency of photon on/off detection.

The remainder of this dissertation is about efficient noiseless linear amplification using photon addition and subtraction. Numerous studies have shown that various quantum information processing can be enhanced by the noiseless amplification of continuous variable quantum states. It has been verified that the photon-addition-then-subtraction operator is efficient noiseless amplifier for small coherent states. We show that the double-photon-addition operator is more effective amplifier for medium and large amplitude coherent states in terms of equivalent input noise, which considers the noise and the gain caused by the amplifier on equal footing. The extension to superpositions of coherent states is also made with analogous results followed, which can be inferred from the optimal phase uncertainty computed from quantum Fisher information.