A quantum delayed choice experiment (1205.4926v2)

Abstract: Quantum systems exhibit particle-like or wave-like behaviour depending on the experimental apparatus they are confronted by. This wave-particle duality is at the heart of quantum mechanics, and is fully captured in Wheeler's famous delayed choice gedanken experiment. In this variant of the double slit experiment, the observer chooses to test either the particle or wave nature of a photon after it has passed through the slits. Here we report on a quantum delayed choice experiment, based on a quantum controlled beam-splitter, in which both particle and wave behaviours can be investigated simultaneously. The genuinely quantum nature of the photon's behaviour is tested via a Bell inequality, which here replaces the delayed choice of the observer. We observe strong Bell inequality violations, thus showing that no model in which the photon knows in advance what type of experiment it will be confronted by, hence behaving either as a particle or as wave, can account for the experimental data.

• The paper demonstrates a new variant of Wheeler's delayed choice experiment with a quantum-controlled beam-splitter enabling concurrent observation of particle and wave behaviors.
• The experiment employs a Mach-Zehnder interferometer and superposition states, achieving a maximal Bell’s inequality violation (S=2.45 ± 0.03) that supports quantum indeterminacy.
• The findings refine quantum measurement theory by proposing a continuous complementarity model, paving the way for advanced quantum computing and secure communication.

A Quantum Delayed Choice Experiment

The quantum delayed choice experiment presented by Peruzzo et al. offers a new variant of the foundational wave-particle duality principle. Their investigation explores a sophisticated adaptation of Wheeler's delayed choice experiment through the implementation of a quantum-controlled beam-splitter within an interferometer, testing both particle and wave behaviour of photons concurrently. This approach is rooted in a departure from the traditional delayed choice by an observer to a model in which the choice is governed by entanglement measured against Bell's inequalities, thereby bypassing classical predictions.

Experiment Overview

The researchers address wave-particle duality, a cornerstone of quantum mechanics, where a quantum system such as a photon exhibits both particle-like and wave-like characteristics depending on the experimental parameters. Their setup incorporates a Mach-Zehnder interferometer with a quantum beam-splitter controlled by an ancillary quantum system—another photon—programmed into a superposition state. This construction allows the experiment to examine the wave and particle nature of photons without a priori knowledge of which experiment is being conducted, a significant deviation from Wheeler's original thought experiment.

Methodological Approach

Their methodology involved a reconfigurable integrated quantum photonic device, creating a superposition state of present and absent beam-splitters, leading to intermediate quantum behaviours. The dual consideration of wave and particle behaviour is certified by a Bell-CHSH test, which the authors implement to substantiate that their observations cannot be mimicked by classical models that presume a pre-defined measurement setting.

The experiment achieved a maximal violation of Bell’s inequality ($S=2.45 \pm 0.03$), implying significant discrepancies with local hidden variable theories and providing evidence for the quantum superposition state. Such violations advocate for the quantum nature of the beam-splitter’s operation, affirming the indivisibility of quantum complementarity and providing an implicit endorsement of the broader Copenhagen interpretation.

Results and Implications

The strong Bell inequality violations observed confirm that no classical model, where the photon possesses prior knowledge of the measurement context, can account for the results. This strengthens the postulation that quantum systems inherently do not possess a definite state prior to measurement, fundamentally supporting quantum indeterminacy.

The outcomes of this experiment not only reinforce the principles of quantum mechanics but also propose a novel conceptual perspective on the Copenhagen complementarity principle. The contemporary emphasis on a quantum-controlled apparatus potentially streamlines complementarity into a continuous parameterized model rather than a binary one, advancing our understanding of quantum measurement theory.

Future Directions

Further exploration could involve enhancing the setup towards delivering a loophole-free test of Bell’s inequalities, eliminating the need for additional assumptions concerning detector alignment and efficiency. Continued experimentation might extend this approach to other quantum systems and could inform the development of quantum technologies that leverage such a continuous transition between quantum states for computation and secure communication systems.

Integrating this experimental design within broader quantum systems could assist in refining theoretical models of quantum interaction, especially focusing on the correlation between classical intuition and quantum aftereffects, allowing for advancements in quantum theory's interpretative structures. As implementations become more sophisticated, the role of quantum control mechanisms is expected to escalate in both theoretical explorations and practical applications of quantum mechanics.