Causal independence

Louis Osgood Kattsoff

Dengke Qu, Quan Lin, Lei Xiao +2 more

Indefinite causal order can be realized by a quantum switch, yet its experimental certification has so far relied on assumptions about the devices. A device-independent verification of indefinite causal order would show, from the observed correlations alone and without relying on a detailed model of the devices, that they are incompatible with any classical description based on a fixed causal order. Here, we implement a photonic quantum switch in a device-independent scenario and test a Bell-like causal inequality using entangled photons. One photon serves as the control qubit of the quantum switch, while the other is measured by a distant party with rapidly and independently chosen settings. We observe a violation of the causal inequality by 24 standard deviations. These results provide experimental evidence for indefinite causal order, albeit in the presence of remaining loopholes, and mark an important step toward a fully loophole-free device-independent verification of causal indefiniteness in quantum processes.

Kelly Oakes

Roberto Salazar, Michał Kamon, Karol Horodecki +4 more

A fundamental question in device-independent quantum cryptography is to determine the minimal physical principle upon which the security of such a cryptographic protocol (such as for key distribution or for randomness generation) may be based. Since the seminal work on device-independent quantum key distribution by J. Barrett, L. Hardy, and A. Kent [Phys. Rev. Lett. 95, 010503 (2005)], a conjectured candidate for certification of device-independent security has been the principle of relativistic causality, namely the disallowance of causal loops. While this principle has thus far been equated with the no-signaling constraints, it has been shown recently that in multiparty Bell scenarios, the no-signaling constraints are sufficient but not necessary for relativistic causality, and a refined set of constraints has been proposed that more precisely capture the notion of relativistic causality. In this paper, we build upon this finding to show that, in contrast to the no-signaling constraints, the constraints of relativistic causality are not sufficient for certification of device-independent security. More specifically, we show that there exist correlations allowed by the causality principle that allow an adversary to gain complete information about the measurement outcomes of honest parties in any device-independent cryptographic protocol, thereby rendering the protocol completely insecure. As a tool to develop this adversarial attack strategy, we fully characterize the set of correlations allowed by relativistic causality in the tripartite Bell scenario of three parties, each performing two binary measurements, that may be of independent interest. We also demonstrate the difference between the relativistic causal correlations and those allowed by the usual no-signaling constraints by presenting explicit communication tasks wherein the two sets exhibit striking difference in their respective winning probabilities.

Hippolyte Dourdent, Alastair A. Abbott, Ivan Šupić +1 more

Causal nonseparability is the property underlying quantum processes incompatible with a definite causal order. So far it has remained a central open question as to whether any process with a clear physical realisation can violate a causal inequality, so that its causal nonseparability can be certified in a device-independent way, as originally conceived. Here we present a method solely based on the observed correlations, which certifies the causal nonseparability of all the processes that can induce a causally nonseparable distributed measurement in a scenario with trusted quantum input states, as defined in [Dourdent et al., Phys. Rev. Lett. 129, 090402 (2022)]. This notably includes the celebrated quantum switch. This device-independent certification is achieved by introducing a network of untrusted operations, allowing one to self-test the quantum inputs on which the effective distributed measurement induced by the process is performed.

Peter Mittelstaedt