Quantum theory radically challenges our classical intuitions, and while it seems so abstract, it has already found some useful applications. For instance, quantum cryptographic protocols allow us to send secure messages, employing quantum physics to show immediately if a message has been tampered by an eavesdropper. But can the message be secure even if the communication device itself has been tampered with by eavesdroppers? In the last decades, physicists have devised cryptographic protocols that remain secure even in this case. These protocols employ a “black box” approach: without opening the devices, their correct functioning is certified solely from statistical tests on how they respond to inputs. However, these tests are often difficult to realize in practice, or rely on assumptions that have no direct physical meaning.
Researchers from the Institute for Quantum Optics and Quantum Information of Vienna, part of the Austrian Academy of Sciences (ÖAW), have now introduced a new strategy that may simplify this task. In their paper "Semi-device-independent information processing with spatiotemporal degrees of freedom" just published in Physical Review Research, they consider black boxes with a twist: they can be moved or rotated, or accept inputs corresponding to waiting different lengths of time. This additional feature is naturally satisfied by experiments, where measurements are performed with rotatable polarizers or adjustable time durations. These additional features can reduce the amount of data that needs to be collected to guarantee security, as well as provide firmer physical ground for the underlying assumptions needed to trust the statistics.
In addition to these potential applications, the framework provides interesting insights into the foundations of physics. The researchers show that their approach allows for a better understanding of the infamous Bell correlations in quantum mechanics, and in particular how this quintessential quantum behaviour relates to certain properties of space and time. “We believe that our framework will also lead to new experimental tests of quantum mechanics”, says Andrew J. P. Garner, first author of the work and researcher at IQOQI-Vienna.