Semi-device-independent certification of indefinite causal order

  title={Semi-device-independent certification of indefinite causal order},
  author={Jessica Bavaresco and Mateus Ara'ujo and {\vC}aslav Brukner and Marco T'ulio Quintino},
When transforming pairs of independent quantum operations according to the fundamental rules of quantum theory, an intriguing phenomenon emerges: some such higher-order operations may act on the input operations in an indefinite causal order. Recently, the formalism of process matrices has been developed to investigate these noncausal properties of higher-order operations. This formalism predicts, in principle, statistics that ensure indefinite causal order even in a device-independent scenario… 

Tables from this paper

Device-independent certification of indefinite causal order in the quantum switch
Quantum theory is compatible with scenarios in which the order of operations is indefinite. Experimental investigations of such scenarios, all of which have been based on a process known as the
Quantum processes without a causal order
Less than ten years ago it was realized that quantum theory permits the existence of processes which do not have a defined causal order. To date, several experiments have confirmed the existence of
Consequences of preserving reversibility in quantum superchannels
This paper provides a mathematical characterization for pure superchannels with two slots (also known as bipartite pure processes), which are superch channels preserving the reversibility of quantum operations, and shows that the reversible preserving condition restricts all pure super channels withTwo slots to be either a causally ordered quantum circuit only consisting of unitary operations or a coherent superposition of two pure causally order circuits.
Causal orders, quantum circuits and spacetime: distinguishing between definite and superposed causal orders
It is shown that every circuit can be immersed into a classical spacetime, preserving the compatibility between the two causal structures, and a recently introduced operational notion of an event does allow for superpositions of respective causal orders in flat spacetime quantum switch implementations.
Practical computational advantage from the quantum switch on a generalized family of promise problems
The “query per gate” parameter is introduced and used to prove that the quantum switch provides computational advantage for both the continuous and discrete cases and leads to the conclusion that a continuous variable system is necessary to implement the most general promise problem.
“All-versus-nothing” proof of genuine tripartite steering and entanglement certification in the two-sided device-independent scenario
We consider the task of certification of genuine entanglement of tripartite states. For this purpose, we first present an “all-versus-nothing” proof of genuine tripartite Einstein-Podolsky-Rosen
Reversible dynamics with closed time-like curves and freedom of choice
This work characterisation to an arbitrary number of regions of spacetime finds that there exist several inequivalent processes that can only arise due to non-trivial time travel, which supports the view that complex dynamics is possible in the presence of CTCs, compatible with free choice of local operations and free of inconsistencies.
Unitary channel discrimination beyond group structures: Advantages of sequential and indefinite-causal-order strategies
An absolute upper bound for the maximal probability of successfully discriminating any set of unitary channels with any number of copies for the most general strategies that are suitable for channel discrimination is derived.
Hidden nonmacrorealism: Reviving the Leggett-Garg inequality with stochastic operations
Huan-Yu Ku, 2, ∗ Hao-Cheng Weng, ∗ Yen-An Shih, Po-Chen Kuo, 2 Neill Lambert, Franco Nori, 4 Chih-Sung Chuu, † and Yueh-Nan Chen 2, ‡ Department of Physics and Center for Quantum Frontiers of


Experimental verification of an indefinite causal order
Researchers report the first decisive demonstration of a process with an indefinite causal order, and experimentally determine a causal witness, demonstrating by almost 7 SDs that the experimentally implemented process does not have a definite causal order.
Quantum circuits with classical versus quantum control of causal order
Quantum supermaps are transformations that map quantum operations to quantum operations. It is known that quantum supermaps which respect a definite, predefined causal order between their input
A purification postulate for quantum mechanics with indefinite causal order
A purification postulate is proposed: processes are physical only if they are purifiable, and necessary conditions for a process to be purifiable are derived, and several known processes do not satisfy them.
Witnesses of causal nonseparability: an introduction and a few case studies
This work presents a shorter introduction to the concept of witnesses of causal nonseparability, and focuses on some explicit examples—by considering in particular different noise models for the quantum switch—to show how to construct and use such witnesses in practice.
Experimental Entanglement of Temporal Orders
The study of causal relations, a cornerstone of physics, has recently been applied to the quantum realm, leading to the discovery that not all quantum processes have a definite causal structure.
Quantum correlations with no causal order
It is shown that in a classical limit causal order always arises, which suggests that space-time may emerge from a more fundamental structure in a quantum-to-classical transition.
Experimentally feasible computational advantage from quantum superposition of gate orders
Tasks (1) for which there is a provable computational advantage of a quantum-controlled ordering of gates in the asymptotic case and (2) that require only qubit gates are introduced, suitable to demonstrate this advantage experimentally.
Witnessing causal nonseparability
This work shows that a recently demonstrated resource for quantum computation—the quantum switch—is a genuine example of 'indefinite causal order', and introduces a new tool—the causal witness—which can detect the causal nonseparability of any quantum resource that is incompatible with a definite causal order.
Indefinite Causal Order in a Quantum Switch.
A photonic quantum switch is realized, where two operations A and B act in a quantum superposition of their two possible orders, on the transverse spatial mode of the photons; polarization coherently controls their order.
Quantum Metrology with Indefinite Causal Order.
This work addresses the study of quantum metrology enhanced by indefinite causal order, demonstrating a quadratic advantage in the estimation of the product of two average displacements in a continuous variable system and proves that no setup can have root-mean-square error vanishing faster than the Heisenberg limit 1/N.