Quantum Experiments Confirm How Particles Switch Between Wave and Particle States

Shiyu Wang and colleagues at RIKEN observe a transition between wave-like and particle-like behaviour of a photon within a Mach-Zehnder interferometer. Their experiments reveal how which-way measurements disrupt entanglement and coherence, causing information leakage, and quantify the relationships between entropy and fringe visibility. Furthermore, the research observes the quantum Zeno effect, partially blocking interferometer paths and influencing purity and von Neumann entropy, thereby offering detailed insight into quantum foundations and highlighting the potential of superconducting quantum processors for high-precision testing.

Demonstrating complete wave-particle transition via controlled entanglement destruction

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.

Quantum state tomography on two qubits situated along the interferometer’s separate paths further detailed the dynamics of this quantum system. Quantum state tomography is a process used to reconstruct the quantum state of a system by performing a series of measurements. Actively determining which path a photon takes destroys the entanglement and coherence between these paths, enabling information leakage into the surrounding environment. This leakage is a consequence of the measurement process itself, which inevitably interacts with the quantum system and introduces decoherence. To quantify these observations, the team derived new complementarity relations linking entropy and fringe visibility, confirming the inverse relationship between knowing the path and observing wave-like interference. These relations build upon Bohr’s principle of complementarity, which states that certain properties, such as position and momentum, or in this case, path and interference, cannot be simultaneously known with perfect precision. A continuous which-way measurement also induced the quantum Zeno effect, partially blocking one path and causing non-monotonic changes in both system purity and von Neumann entropy. The quantum Zeno effect, named after the ancient Greek philosopher Zeno, describes the suppression of quantum transitions due to frequent observation. Careful calibration of the 16 frequency-tunable transmon qubits ensured accurate measurements of idle frequency, anharmonicity, and coherence times. Transmon qubits are a type of superconducting qubit favoured for their relatively long coherence times and ease of control; precise calibration of these parameters is crucial for maintaining the fidelity of the experiment.

Wave-particle duality controlled and characterised in a superconducting processor

Our understanding of quantum behaviour is steadily refining, pushing beyond theoretical predictions towards practical applications. Achieving precise control over delicate quantum states remains a formidable challenge, and this work shows the tension between fully characterising a system’s evolution and the very act of observing it. The inherent fragility of quantum states makes them susceptible to environmental noise and decoherence, requiring sophisticated techniques for isolation and control. Continuous measurement, while revealing dynamics and demonstrating the quantum Zeno effect where observation can effectively freeze a system’s change, introduces unavoidable disturbance. This disturbance is not merely a technical limitation but a fundamental aspect of quantum mechanics, reflecting the probabilistic nature of quantum phenomena. Detailed characterisation of these dynamics, alongside insights into entanglement and information leakage, advances the potential for building and testing quantum technologies. Specifically, this research contributes to the development of more robust quantum sensors and communication protocols, as understanding and mitigating decoherence is paramount for maintaining quantum information. The experiment confirms the fundamental principle of complementarity, showing how a photon can transition between acting as a wave and a particle depending on how it is measured. This controlled demonstration of wave-particle duality using a superconducting quantum processor provides a platform for exploring other fundamental aspects of quantum mechanics and developing novel quantum devices. The ability to precisely manipulate and measure quantum states opens avenues for investigating complex quantum phenomena and potentially harnessing them for technological advancements, including quantum computing and secure quantum communication networks. The use of a 16-qubit system represents a significant step towards scaling up quantum processors and performing more complex quantum simulations.

The research successfully demonstrated the transition of a photon from wave-like to particle-like behaviour using a two-dimensional superconducting quantum processor. This confirms wave-particle duality and highlights the fundamental relationship between quantum information and how a system is observed. Measurements designed to determine a particle’s path break entanglement and cause information loss from the quantum system, a key aspect of quantum mechanics. The authors performed quantum state tomography on two qubits and derived complementarity relations between entropy and fringe visibility to characterise these dynamics.