Network quantum steering enables randomness certification without seed randomness

TL;DR

This paper introduces swap-steering, a minimal quantum network scenario where quantum nonlocality and steering can be observed without input randomness, enabling device-independent randomness certification.

Contribution

It demonstrates the existence of swap-steering in minimal networks, constructs a linear witness for it, and shows how it enables randomness certification without initial seed randomness.

Findings

Swap-steering can be observed with two sources and one trusted party.

A linear witness for swap-steering is constructed.

Two bits of randomness can be certified without seed randomness.

Abstract

Quantum networks with multiple sources allow the observation of quantum nonlocality without inputs. Consequently, the incompatibility of measurements is not a necessity for observing quantum nonlocality when one has access to multiple quantum sources. Here we investigate the minimal scenario without inputs where one can observe any form of quantum nonlocality. We show that even two parties with two sources that might be classically correlated can witness a form of quantum nonlocality, in particular quantum steering, in networks without inputs if one of the parties is trusted, that is, performs a fixed known measurement. We term this effect as swap-steering. The scenario presented in this work is minimal to observe such an effect. Consequently, a scenario exists where one can observe quantum steering but not Bell non-locality. We further construct a linear witness to observe swap-steering. Interestingly, this witness enables self-testing of the quantum states generated by the sources and the local measurement of the untrusted party. This in turn allows certifying two bits of randomness that can be obtained from the measurement outcomes of the untrusted device without the requirement of initially feeding the device with randomness.