Limits of multimode bunching for boson sampling validation: anomalous bunching induced by time delays

TL;DR

This paper investigates the reliability of multimode bunching as a validation tool for boson sampling, revealing that temporal mode mismatch can cause anomalous bunching, thus challenging its effectiveness.

Contribution

It identifies interferometric configurations where anomalous bunching is excluded and demonstrates that temporal mode mismatch can induce anomalous bunching, clarifying validation conditions.

Findings

Anomalous bunching can occur due to temporal distinguishability.

Certain interferometric setups exclude anomalous bunching.

Time delays can enhance multimode bunching unexpectedly.

Abstract

The multimode bunching probability is expected to provide a useful criterion for validating boson sampling experiments. Its applicability, however, is challenged by the existence of anomalous bunching, namely paradoxical situations in which partially distinguishable particles exhibit a higher bunching probability in two or more modes than perfectly indistinguishable ones. Using multimode bunching as a reliable criterion of genuine indistinguishability, therefore, requires a clear identification of the interferometric configurations in which anomalous bunching can or cannot occur. In particular, since uncontrolled small time delays between single-photon pulses constitute a common source of mode mismatch in current photonic platforms, it is essential to determine whether the resulting photon distinguishability might lead to anomalous bunching. Here, we first identify a broad class of interferometric configurations in which anomalous bunching is rigorously excluded, thereby establishing regimes where multimode bunching-based validation remains valid. Then, we find that, quite unexpectedly, temporal mode mismatch does not belong to this class. We exhibit a specific interferometric setup in which temporal distinguishability enhances multimode bunching, demonstrating that time delays can induce an anomalous behavior. These results help clarify the conditions under which multimode bunching remains a reliable validation tool.