Quantum erasure with causally disconnected choice (1206.6578v2)

Abstract: The counterintuitive features of quantum physics challenge many common-sense assumptions. In an interferometric quantum eraser experiment, one can actively choose whether or not to erase which-path information, a particle feature, of one quantum system and thus observe its wave feature via interference or not by performing a suitable measurement on a distant quantum system entangled with it. In all experiments performed to date, this choice took place either in the past or, in some delayed-choice arrangements, in the future of the interference. Thus in principle, physical communications between choice and interference were not excluded. Here we report a quantum eraser experiment, in which by enforcing Einstein locality no such communication is possible. This is achieved by independent active choices, which are space-like separated from the interference. Our setup employs hybrid path-polarization entangled photon pairs which are distributed over an optical fiber link of 55 m in one experiment, or over a free-space link of 144 km in another. No naive realistic picture is compatible with our results because whether a quantum could be seen as showing particle- or wave-like behavior would depend on a causally disconnected choice. It is therefore suggestive to abandon such pictures altogether.

• The paper demonstrates that quantum erasure with causally disconnected choice validates the complementarity of wave-particle duality using hybrid entangled photon pairs.
• It employs distinct experimental setups over 55 meters and 144 kilometers to ensure space-like separation and enforce Einstein locality during measurements.
• Measured interference visibilities up to 0.951 support theoretical predictions, challenging classical intuitions with non-local quantum behavior.

Quantum Erasure with Causally Disconnected Choice

The paper "Quantum Erasure with Causally Disconnected Choice" presents an experimental investigation that challenges classical intuitions regarding quantum mechanics, particularly in the context of the quantum eraser and wave-particle duality. The authors, including Xiao-song Ma, Johannes Kofler, and Anton Zeilinger, demonstrate a quantum eraser experiment where the choice of whether to erase which-path information is causally disconnected from the system where interference occurs, thus enforcing Einstein locality.

Experimental Setup and Methodology

The experiment utilizes hybrid path-polarization entangled photon pairs, with one photon (the "system photon") traversing an interferometer and the other (the "environment photon") subjected to polarization measurements. Notably, the experiment ensures space-like separation of the choice to erase information and detection events, thus eliminating the possibility of communication between these occurrences through any subluminal signals.

Two distinct setups were realized: a short-distance optical fiber link covering 55 meters and a long-distance free-space link extending 144 kilometers between two Canary Islands. These configurations allow for an examination of the quantum eraser effect under different spatio-temporal conditions, testing the robustness of the observed phenomena against variables such as distance and environmental influences.

Key Findings

1. Wave-Particle Duality and Complementarity: The results confirm that it is possible to choose whether interference (wave behavior) or path information (particle behavior) is observed by measuring the entangled environment photon. This underpins the complementarity principle, demonstrating that acquiring different types of information about quantum systems results in mutually exclusive observational outcomes.
2. Einstein Locality and Its Implications: By demonstrating quantum erasure with causally disconnected settings, the experiment pushes the boundaries of our understanding of quantum non-locality. The authors argue that if the photon behavior were predetermined as either particle-like or wave-like, such behavior would imply faster-than-light interactions, which starkly conflicts with the principles of relativity suggested by Einstein.
3. Numerical Results: The observed visibility of interference patterns, with values up to 0.951, strongly supports the non-classical interpretation of photon behavior dependent on delayed choices. The complementarity inequality parameter ($\mathcal{I}^2 + \mathcal{V}^2 \leq 1$), used to quantify the trade-off between path information and interference visibility, was closely tested, and in all iterations adhered to the theoretical predictions.

Theoretical and Practical Implications

Theoretically, these results challenge naive realistic interpretations of quantum mechanics, where such phenomena might be attributed to classical-like hidden variables affecting particle states. Practically, the findings contribute to the foundational understanding necessary for developing future quantum technologies, potentially impacting areas such as quantum cryptography, quantum communication, and fundamental tests of the quantum-classical boundary.

Future Directions

Further experimentation could explore more extensive separation distances and various entangled particle configurations, testing the limits of quantum correlations and their implications for concepts like causality and time in quantum mechanics. Additionally, advancements in quantum random number generation and fast-switching technology would aid in minimizing potential loopholes in these intriguing quantum phenomena studies.

In summary, this work substantiates the non-classical correlations inherent in quantum systems, emphasizing the intricate interplay of measurement, entanglement, and non-locality, steadfastly challenging our classical intuitions about space, time, and reality.