Experimental verification of an indefinite causal order

Area of Science:

• Quantum mechanics
• Foundations of physics

Background:

• Causal relations in quantum mechanics are not always well-defined.
• Theoretical interest in indefinite causal order is growing.

Purpose of the Study:

• To experimentally demonstrate a process with indefinite causal order.
• To quantify the incompatibility of a setup with a definite causal order.

Main Methods:

• Implemented a setup capable of a superposition of causal orders.
• Measured a 'causal witness' to detect deviations from definite causal order.
• Performed measurements within and outside the causally non-ordered process without destroying coherence.

Main Results:

• Successfully demonstrated a process with indefinite causal order.
• Experimentally determined a causal witness.
• Achieved a statistical significance of almost 7 standard deviations, confirming the lack of definite causal order.

Conclusions:

• The experiment provides the first decisive demonstration of indefinite causal order.
• The findings challenge the a priori assumption of well-defined causal relations in quantum mechanics.
• Opens new experimental possibilities for exploring quantum causality.