Wignera

Wignera's friend measured the qubit inside the lab. To Wignera, outside, the friend is still in superposition. Who is right?

The Physics

A friend measures a qubit inside a sealed lab. Treating the whole lab with quantum mechanics, an outside observer assigns an entangled state of qubit and friend, while the friend has seen a definite result. Recent no-go theorems show that some reasonable assumptions about observers cannot all hold together.

|Ψ⟩ = (|0⟩|F₀⟩ + |1⟩|F₁⟩)/√2

• |0⟩, |1⟩: qubit states
• |F₀⟩, |F₁⟩: friend having seen 0 or 1
• |Ψ⟩: joint state of qubit and friend

Eugene Wigner posed it in 1961; Frauchiger and Renner sharpened it in 2018, and Bong et al. (2020) proved a stronger no-go theorem with a proof-of-principle photon experiment.

Honest caveat: Which assumption fails is still debated, and neither observer is simply "right". In the 2020 experiment a photon's path stood in for the observer.

On the shirt

Front: Wignera, the pixel-art character, with the title. Back: the entangled lab state, the two viewpoints, the no-go theorems and the 1961, 2018 and 2020 sources. Printed on a unisex tee.