"Robust continuous time crystal in an electron–nuclear spin system" (2024) https://www.nature.com/articles/s41567-023-02351-6 :
> The time crystal state enables fundamental studies of nonlinear interactions and has potential applications as a precise on-chip frequency standard.
Re: Retrocausality: https://news.ycombinator.com/item?id=38047149
Does retrocausality of time crystals affect quantum circuit measurement at t>0?
"Double-slit time diffraction at optical frequencies" (2023), "Incandescent Temporal Metamaterials" (2023) https://news.ycombinator.com/item?id=39162149
(Edit) /? Indefinite causal order: https://www.google.com/search?q=indefinite+causal+order
"Admissible causal structures and correlations" ( https://journals.aps.org/prxquantum/abstract/10.1103/PRXQuan...