Curator's Take
This research reveals how quantum entangled photon pairs can create distinctive "fingerprints" in quantum wave mixing experiments, offering a new window into understanding the fundamental quantum nature of light-matter interactions. The team's discovery that certain spectral peaks are suppressed when photons arrive in correlated pairs rather than individually provides a powerful new tool for probing and characterizing nonclassical light sources. This work bridges quantum optics and superconducting quantum circuits in a way that could advance both quantum sensing applications and our ability to generate and manipulate exotic quantum states of light. The findings are particularly significant because they demonstrate how carefully designed cascaded qubit systems can serve as sensitive detectors for quantum correlations that would otherwise be difficult to measure directly.
— Mark Eatherly
Summary
We develop a theoretical description of quantum wave mixing (QWM) in a cascaded waveguide-QED system of two superconducting qubits, where the probe is driven by an external coherent tone and by the resonance fluorescence of a strongly driven source qubit. Starting from the field correlation functions of the source emission, we derive an effective master-equation treatment for the probe and identify the regime in which the incident fluorescence is characterized by anomalous correlations. When the coherent Rayleigh component of the source spectrum is suppressed, the probe equations of motion become equivalent to those for a qubit driven by a coherent tone and broadband squeezed light. This equivalence implies a selection rule for the peaks of the QWM spectrum, with a strong suppression of sidebands associated with processes involving an odd number of photons taken from the source field. Numerical simulations of the full cascaded two-qubit model for different ratios of radiative decay rates unambiguously confirm the participation of correlated photon pairs in QWM processes. The current research illustrates that the analysis of peak amplitudes can be used to probe photon statistics in the incident nonclassical field.