Experimental entanglement of temporal order
1Vienna Center for Quantum Science and Technology (VCQ), Faculty of Physics, University of Vienna, Boltzmanngasse 5, Vienna A-1090, Austria
2Quantum Engineering Technology Labs, H. H. Wills Physics Laboratory and Department of Electrical & Electronic Engineering, University of Bristol, Bristol BS8 1FD, United Kingdom
3Institute for Quantum Optics & Quantum Information (IQOQI), Austrian Academy of Sciences, Boltzmanngasse 3, Vienna A-1090, Austria
4Centre for Engineered Quantum Systems, School of Mathematics and Physics, The University of Queensland, St Lucia, QLD 4072, Australia
Published: | 2022-01-11, volume 6, page 621 |
Eprint: | arXiv:1712.06884v5 |
Doi: | https://doi.org/10.22331/q-2022-01-11-621 |
Citation: | Quantum 6, 621 (2022). |
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Abstract
The study of causal relations has recently been applied to the quantum realm, leading to the discovery that not all physical processes have a definite causal structure. While indefinite causal processes have previously been experimentally shown, these proofs relied on the quantum description of the experiments. Yet, the same experimental data could also be compatible with definite causal structures within different descriptions. Here, we present the first demonstration of indefinite temporal order outside of quantum formalism. We show that our experimental outcomes are incompatible with a class of generalised probabilistic theories satisfying the assumptions of locality and definite temporal order. To this end, we derive physical constraints (in the form of a Bell-like inequality) on experimental outcomes within such a class of theories. We then experimentally invalidate these theories by violating the inequality using entangled temporal order. This provides experimental evidence that there exist correlations in nature which are incompatible with the assumptions of locality and definite temporal order.
![](https://quantum-journal.org/wp-content/uploads/2021/12/ExpEntanglement.jpeg)
Featured image: $\textbf{Entangled quantum-switch}$. Our work is based on two quantum-switches (S1 and S2). In each quantum-switch, there are two parties, Alice (${U}_{i_\text{A}}$) and Bob (${U}_{i_\text{B}}$). A target qubit is first sent to one party, and then to the other. The order in which the qubit is sent to the two parties is governed by the state of an additional qubit: if the state of the control qubit is $\vert{0}\rangle_i^c$, the target qubit is sent first to Alice and then Bob (top left panel), and vice versa if the control qubit is in the state $\vert{1}\rangle_i^c$ (top right panel). In our work, we entangle the control qubits (bottom panel). In this case, the order in which the target qubit in quantum-switch S1 passes through ${U}_{1_\text{A}}$ and ${U}_{1_\text{B}}$ is entangled with the order in which the target qubit in quantum-switch S2 passes through ${U}_{2_\text{A}}$ and ${U}_{2_\text{B}}$. The control qubits are measured in the basis $\lbrace\vert{+}\rangle_i^c, \vert{-}\rangle_i^c\rbrace$. If the orders inside the two quantum-switches are entangled, it will be possible to violate a Bell inequality by measuring the target qubits after the quantum-switches (BM). This is possible even if the target qubits start in a separable state and only local operations are applied within each quantum-switch.
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