Indefinite Causal Order in a Quantum Switch

Area of Science:

• Quantum Physics
• Quantum Information Science

Background:

• Quantum mechanics challenges classical notions of causality, allowing for events without a definite temporal order.
• Verifying quantum phenomena like entanglement relies on specific measurement techniques, such as entanglement witnesses.

Purpose of the Study:

• To experimentally realize a photonic quantum switch enabling operations in a superposition of orders.
• To verify the absence of definite causal order in a quantum system using a causal witness.

Main Methods:

• Implementation of a photonic quantum switch using the transverse spatial mode of photons.
• Control of operation order via quantum superposition, with polarization acting as the control mechanism.
• Construction and measurement of a causal witness to quantify the deviation from definite causal order.

Main Results:

• Successful realization of a quantum switch where two operations (A and B) exist in a superposition of their temporal order.
• Ensured operations are indistinguishable by spatial or temporal position, allowing for qudit encoding.
• Measured a causal witness value significantly beyond the bound for definite causal order (18 standard deviations).

Conclusions:

• The experiment confirms that quantum mechanics permits events to occur without a definite causal order.
• The photonic quantum switch serves as a verifiable platform for studying non-classical causal structures.
• This work opens avenues for exploring quantum computation and information processing with indefinite causal orders.