Quantum Erasure: Quantum Interference Revisited

It may be somewhat surprising that Thomas Young’s double-slit experiment a staple in the freshman physics laboratory would be such an invaluable testing ground for the foundations of quantum physics. Yet the quantum version of the double-slit experiment has been at the center of many debates over the fundamentals of quantum physics since the theory was born, nearly a century ago. In fact, Young’s experiment embodies the very nature of quantum physics. Last year, the readers of Physics World magazine voted Young’s double-slit experiment with electrons “the most beautiful experiment” in physics. The significance of Young’s experiment lies in the fact that interference is a phenomenon exhibited only by waves. The puzzle that quantum physics presents is that a particle, which is usually thought of as an indivisible, localized object, can also behave like a classical wave, which interferes and diffracts. In “the most beautiful experiment”, electrons pass through the slits like waves and are detected like particles! This interference behavior is perhaps the greatest mystery in quantum theory. In fact, Nobel Prize-winning physicist Richard Feynman has called quantum interference “the only mystery” in quantum physics. Recently, some progress has been made in the understanding of these interference effects within the foundations of quantum theory. Experiments called quantum erasers modified versions of Young’s experiment have shed light on the foundations of quantum physics. However, before we explain the notion of quantum erasure, we take a detour to explore the concept and the history of classical and quantum interference.