Device-independent certification of indefinite causal order in the quantum switch

Area of Science:

• Quantum physics
• Quantum information science
• Foundations of quantum mechanics

Background:

• Quantum theory permits scenarios with indefinite operational order.
• Previous experiments demonstrated indefinite causal order using the quantum switch, but required device-dependent assumptions.
• Device-independent certification, akin to Bell inequality tests, was previously shown to be impossible for isolated quantum switches.

Purpose of the Study:

• To investigate the possibility of device-independent certification of indefinite causal order in quantum switch scenarios.
• To develop a method for certifying quantum phenomena without relying on assumptions about the internal workings of experimental devices.

Main Methods:

• Introduction of a novel inequality tailored for the quantum switch.
• Inclusion of a spacelike-separated observer to the experimental setup.
• Formulation of an assumption precluding superluminal and retrocausal influences.

Main Results:

• A new inequality has been derived that enables device-independent certification of indefinite causal order.
• The certification is achieved in the presence of an additional spacelike-separated observer.
• The method relies on the assumption that superluminal and retrocausal influences are impossible.

Conclusions:

• Device-independent certification of indefinite causal order in quantum switch experiments is achievable.
• This work extends the paradigm of device-independent verification to causal structures in quantum mechanics.
• The findings pave the way for more robust and trustworthy demonstrations of quantum phenomena.