The new paper “Virtual Quantum Broadcasting” with Arthur Parzygnat, James Fullwood and Giulio Chiribella was published a few days ago in Physical Review Letters. Here are my answers to a number of questions posed by phys.org

Q: Did any personal motivation or inspiration drive you to pursue this research?

A: Quantum theory is often said to be the most successful theory ever conceived in physics, and yet its formalism cannot predict the outcome of a measurement: it can only predict averages. It’s as if there is a layer beyond which no one can look. This becomes painfully clear when you try to describe quantum phenomena in terms of trajectories: the formalism of quantum theory simply does not allow it. Nevertheless, there are situations where we would like to follow a quantum system — and the correlations in its physical properties — in time, like following its “trajectory”. So should we just give up? Or should we try to push the limits of quantum formalism a bit and see what we can do? More than a decade ago, I and other co-authors took the latter path and proposed the idea of a quantum “time correlator” (link to arXiv), which would allow direct observation of correlations in physical quantities of the same physical system at different times. Much like what happens when we can look at the trajectory of a classical object. Perhaps ten years ago this idea was premature, but in my eyes this paper finally brings it to fruition, expands its scope, and motivates it even further. I sincerely hope that more researchers in quantum information theory will join us on these still relatively unexplored paths.

Q: In simple terms, could you briefly explain your research and its key findings?

A: This paper is rather technical, but its key finding, loosely speaking, is that it is not necessary to give up on time correlations in quantum theory altogether. In fact, in agreement with our previous paper on the quantum time-correlator, we find that in quantum theory there’s basically a unique, canonical way to describe a quantum system at different times. This is achieved by means of a virtual broadcasting map, which is able to “spread” a quantum state symmetrically between different times, thus allowing the extraction of information that can be used to reconstruct the time correlations typical of a trajectory.

Q: Could you elaborate on any unexpected or particularly interesting findings that emerged during the course of your research?

A: The most appealing, motivating, and intriguing feature of this work, in my opinion, is that the virtual broadcasting map we found is uniquely characterized by a simple set of natural requirements. That’s why we call it “canonical”. Such a uniqueness property in turn seems to point to a whole new part of quantum theory, i.e. its time-like structure, which is still largely unexplored.

Q: How does the concept of virtual quantum broadcasting contribute to our understanding of quantum state manipulation and information processing?

A: For example, consider the situation where one needs to evaluate the accuracy of a measurement of a physical quantity. This is something we encounter all the time in the design of quantum devices. By definition, the measurement is “accurate” if the final result is “highly correlated” with the value of the physical quantity before the measurement. As natural and innocuous as this definition may seem, quantum theory does not allow it in the strict sense. However, with our virtual broadcasting map, we can now directly measure the accuracy of any measurement — something that was previously thought to be, if not impossible, at least “unorthodox”. This is just the first application that comes to mind, but there are many more in quantum communication and quantum computation.

Q: The study mentions the optimization of physical approximations to the canonical virtual broadcasting map. Could you explain the significance of this optimization and how it impacts the practical implementation of virtual quantum broadcasting?

A: Symmetric and antisymmetric cloning are the “optimal” physical approximations to our canonical virtual broadcasting map, in the precise sense that using the former makes it possible to reconstruct the latter with minimal overhead in terms of measurement statistics. It’s all about efficiency of reconstructing time correlations. Moreover, as an added bonus, symmetric and antisymmetric cloning can be easily implemented using simple interferometric schemes.

Q: What avenues do you see for further research in this area, and are there specific aspects you’d like to explore in the future?

A: As mentioned earlier, it seems that we have found enough mathematical and conceptual evidence to justify entering an area of quantum theory that was previously considered “off limits”. Perhaps the answers to many fundamental questions can be found here.