Experimental device-independent certified randomness generation with an instrumental causal structure

Abstract

The intrinsic random nature of quantum physics offers novel tools for the generation of random numbers, a central challenge for a plethora of fields. Bell non-local correlations obtained by measurements on entangled states allow for the generation of bit strings whose randomness is guaranteed in a device-independent manner, i.e. without assumptions on the measurement and state-generation devices. Here, we generate this strong form of certified randomness on a new platform: the so-called instrumental scenario, which is central to the field of causal inference. First, we theoretically show that certified random bits, private against general quantum adversaries, can be extracted exploiting device-independent quantum instrumental-inequality violations. Then, we experimentally implement the corresponding randomness-generation protocol using entangled photons and active feed-forward of information. Moreover, we show that, for low levels of noise, our protocol offers an advantage over the simplest Bell-nonlocality protocol based on the Clauser-Horn-Shimony-Holt inequality.

Experimental test of quantum causal influences

“…Our results offer a novel way to detect the presence of quantum correlations, without the need of violating Bell inequalities, which are often very demanding from the experimental perspective [38][39][40]. We thus show that the incompatibility of quantum predictions with classical concepts can go beyond the paradigmatic Bell's theorem, opening a venue of research that might lead to new insights in quantum causality [31,32,41] and practical applications [21].…”

“…In particular, notice that the instrumental causal structure underlies the remote state preparation [47] and teleportation [48] protocols, hinting at the possibility of revisiting paradigmatic quantum tasks from the causal perspective. On the more applied side, it is known that the instrumental scenario can also be employed in cryptography protocols [21], but the role of causal effects and interventions on such protocols remains, to our knowledge, completely unexplored. We hope our finding might trigger future developments along these and other promising lines of research.…”

“…This causal inference framework, however, breaks down when quantum effects come into play. As recently discovered and experimentally demonstrated [18][19][20][21], with quantum entanglement acting as the common source, instrumental inequalities can be violated even by a perfect instrument. This not only shows that fundamental results in causality theory have to be reevaluated but also displays the value of moving beyond the paradigmatic Bell's theorem [22,23], since considering different causal structures [18,[24][25][26][27][28][29] leads to new forms of nonclassical correlations and a broader understanding of the role of causality in quantum theory [30][31][32][33][34].…”

Causal inference with imperfect instrumental variables

“…As shown in Refs. [23,[25][26][27], even if one imposes the instrumental causal structure to a quantum experiment, still some instrumental inequalities can be violated. This can be seen as a stronger version of Bell's theorem [20], showing that correlations mediated via quantum entanglement can fail to have a description in terms of standard causal models.…”

Quantum violation of local causality in urban network with hybrid photonic technologies

“…At the basis of many of these quantum communication protocols is the phenomenon of Bell nonlocality [9][10][11], arguably the most radical departure between classical and quantum descriptions of nature. Besides its profound foundational implications, generating nonlocal correlations has become of crucial importance for a variety of quantum technologies, ranging from distributed computing [12], quantum cryptography [13][14][15][16][17][18][19] and quantum key distribution [20,21] to randomness generation [22][23][24] and selftesting [25,26].…”