Temporal correlations in the simplest measurement sequences

Lucas B. Vieira1 and Costantino Budroni1,2

1Institute for Quantum Optics and Quantum Information (IQOQI), Austrian Academy of Sciences, Boltzmanngasse 3, 1090 Vienna, Austria
2Faculty of Physics, University of Vienna, Boltzmanngasse 5, 1090 Vienna, Austria

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We investigate temporal correlations in the simplest measurement scenario, i.e., that of a physical system on which the same measurement is performed at different times, producing a sequence of dichotomic outcomes. The resource for generating such sequences is the internal dimension, or $memory$, of the system. We characterize the minimum memory requirements for sequences to be obtained deterministically, and numerically investigate the probabilistic behavior below this memory threshold, in both classical and quantum scenarios. A particular class of sequences is found to offer an upper-bound for all other sequences, which suggests a nontrivial universal upper-bound of $1/e$ for the classical probability of realization of any sequence below this memory threshold. We further present evidence that no such nontrivial bound exists in the quantum case.

An information processing task can be thought of as sequential operations, possibly conditioned on inputs, producing an output at each step. As these operations occur one at a time, outputs are correlated in time. From a quantum mechanical perspective, these operations can be interpreted as measurements of some properties of a physical system. Any such operation on a physical system modifies (or store information in) it, but up to a limit, given by its internal memory.

Here, we consider the simplest form of temporal correlations: A single finite-dimensional system subjected to the same two-outcome (0 or 1) measurement at different times, producing a binary sequence. We investigate the minimal dimension d necessary to generate a sequence with certainty, which we call its deterministic complexity (DC), and provide an efficient algorithm to compute it. This quantity is central to investigating quantum advantages, since a difference between classical and quantum temporal correlations may arise only below this threshold, for d < DC, when sequences can only be produced probabilistically.

In this scenario, we numerically optimize probabilities for a large number of sequences, confined to various memory sizes, for classical and quantum models. Our results suggest that the probability of each sequence, for any d < DC, can be upper bounded by a directly computable expression associated with the probability of a special sequence, called one-tick sequence, with the same DC and same dimension. If proven, this conjecture would imply a universal classical bound of 1/e for the probabilistic realization of any sequence. We introduce a method to optimize quantum models that shows a quantum advantage for the vast majority of sequences and dimensions considered. Finally, we show that a universal (nontrivial) bound does not exist for quantum models.

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