A fringe visibility of zero has now been achieved, a feat previously impossible with superconducting qubit systems, demonstrating a complete transition from wave-like to particle-like behaviour of a microwave photon. This result directly addresses the core tenets of wave-particle duality, a foundational concept in quantum mechanics positing that every quantum entity exhibits both wave-like and particle-like properties, though not simultaneously. Mach-Zehnder interferometry, implemented on a quantum processor containing 16 frequency-tunable transmon qubits, enabled precise control over the which-way measurement strength. The Mach-Zehnder interferometer functions by splitting a single photon into a superposition of states, sending them along two distinct paths, and then recombining them to create an interference pattern. The visibility of these interference fringes is directly related to the degree of coherence between the two paths; complete destruction of coherence results in zero visibility, signifying purely particle-like behaviour. Detailed analysis revealed that increasing measurement strength progressively destroys entanglement between qubits in the interferometer’s two paths, concurrently increasing the von Neumann entropy and reducing system purity; these interconnected effects were quantified through newly derived complementarity relations. Entanglement, a uniquely quantum phenomenon, describes a correlation between two or more particles, regardless of the distance separating them. Its destruction signifies a loss of quantum coherence and a transition towards classical behaviour.
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