Generalized quantum measurement and its applications in quantum information

Abstract

Quantum physics, which began in the early 20th century, has revolutionized the way we understand the world. Blessing of quantum physical notions made our understanding of nature more complete in atomic electronic structure, photoelectric effect, periodic table, chemical interaction, electronic semiconductor physics, and many more. Although the quantum physics and the following semiconductor physics has driven the computer chip industry and the information age, most of the conventional modern technologies are based on or explained by classical physics. Recently, however, attempts to directly utilize quantum physical properties for information technology gave birth to a newly emerging field of quantum information science. Quantum measurement is not only at the heart of deep questions in quantum physics, but also plays an essential role in realizing quantum information science. In this thesis, many aspects of generalized quantum measurement and some other quantum resources for the application in quantum information are studied.

The biggest obstacle in realizing quantum information science to a practical level is decoherence, i.e. environment induced loss of quantum coherence. Here, I experimentally study a method to suppress decoherence using weak quantum measurements. Quantum measurements are typically understood as destructive to quantum coherence. By carefully designing a protocol with a more generalized form of quantum measurement, however, the decoherence could be effectively tackled.

Decoherence is also detrimental to quantum entanglement, which is an essential resource to many quantum information protocols. In some cases, the entanglement can be completely lost due to decoherence: the phenomenon called entanglement sudden death. It is experimentally demonstrated in the thesis that the entanglement loss and sudden death can be circumvented with weak measurements.

Next, delayed-choice decoherence suppression scheme is proposed and experimentally demonstrated. Using entangled pair of qubits, it is possible to postpone the decision to suppress decoherence even after the decoherence itself. This protocol can be an important addition to methods for suppressing Markovian noise.

Quantum measurement in the position and momentum variables are also studied. Specifically, I propose and analyze the method to probe the commutation relation of position and momentum operators with interference.

One of the most important photonic quantum resource is the quantum memory. For fully utilizing spatial multimode capacity of quantum memories, spatial quantum coherence should be preserved during write-read process. In this thesis, the preservation of spatial coherence of an optical pulse in an atomic vapor quantum memory is studied.

Additionally, the methods to prepare photonic quantum resources, such as quantum light source and atom-photon interface, are discussed in the text. The effect of pump focusing in spontaneous parametric down-conversion is studied in detail.