Correlated photon pairs key to new quantum mixing effect

Researchers at the Dukhov Research Institute of Automatics, alongside the Moscow Institute of Physics and Technology and Skolkovo Institute of Science and Technology, have detailed a new quantum wave mixing effect achieved within a system of two superconducting qubits. The work demonstrates that analyzing the peaks of the quantum wave mixing spectrum can reveal information about the statistics of incoming light, specifically probing photon correlations.

This theoretical description centers on a “cascaded waveguide-QED system” driven by both an external coherent tone and the resonance fluorescence emitted directly from one of the qubits. Numerical simulations confirm the participation of correlated photon pairs in these quantum wave mixing processes.

Cascaded Qubit System Enables Quantum Wave Mixing

The research details how this configuration allows for the spectral filtering of nonclassical radiation directly within the two-qubit system, eliminating the need for separate devices like displaced parametric amplifiers. The theoretical framework developed by R. D. Ivanovskikh and colleagues focuses on separating the coherent and fluctuating components of the fluorescence triplet emitted by the source qubit, identifying both normal and anomalous correlators within the emitted light.

This separation reveals how the incident fluorescence is characterized by anomalous correlations, a key element in driving the quantum wave mixing process. When the coherent Rayleigh component of the source spectrum is suppressed, the probe qubit’s behavior becomes equivalent to that of a qubit driven by coherent light and broadband squeezed light, despite the source emitting non-Gaussian, antibunched light.

This equivalence implies a selection rule for the peaks of the quantum wave mixing spectrum, with a strong suppression of sidebands associated with processes involving an odd number of photons originating 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 analysis of peak amplitudes can be used to probe photon statistics in the incident nonclassical field.

The cascaded architecture, where the same two-qubit waveguide circuit generates, filters, and reads out the nonclassical radiation, offers a significant simplification for experimental implementations of quantum wave mixing. The team’s calculations show that the probe dynamics are driven by the field emitted by the source, with the probe itself not acting back on the source.

The theoretical framework developed by the team provides a microscopic understanding of quantum wave mixing, offering insights into the fundamental processes governing the interaction of light and matter at the quantum level. The work, originating from a collaboration that includes the Kotel’nikov Institute of Radio Engineering and Electronics, Russian Academy of Sciences and HSE University, illustrates that the analysis of peak amplitudes can be used to probe photon statistics in the incident nonclassical field.

Squeezed Light Equivalence in Probe Qubit Equations

The suppression of specific sidebands in quantum wave mixing (QWM) processes relies on the emission of correlated photon pairs from a cascaded two-qubit system, according to new theoretical work. This observation stems from a detailed analysis of the field correlations originating from the source qubit, which allowed the team to derive an effective master equation for the probe qubit’s dynamics.

The resulting equations reveal that the probe’s behavior is governed by normal and anomalous parameters, denoted as N and M, generated by the source qubit and sampled within the probe’s bandwidth. This term arises from the correlated nature of the photons emitted by the source qubit and is important for understanding the observed equivalence to a squeezed-light driven system.

Photon Statistics Revealed Through QWM Spectrum

The research extends beyond simply observing the effect. It establishes a method for probing the statistical properties of incident nonclassical light through analysis of peak amplitudes within the QWM spectrum. The analysis of the probe qubit interacting with the field produced by the source focuses on the frequency range of the fluorescence triplet central component, using a Hamiltonian that incorporates the photon reservoir and the coherent drive applied to the probe.

A finite-bandwidth occupation, denoted as N, is defined as the mean occupation of the incident radiation averaged over the frequency window selected by the probe, rather than the total photon number in the complete source spectrum.

The team’s work builds upon earlier investigations into wave-mixing effects in multilevel emitters, such as those studied for a three-level atom, and complements alternative routes to nonclassical driving offered by cascaded waveguide-QED systems. In these systems, radiation emitted by the source qubit interacts with the probe qubit, creating a QWM spectrum that reflects the statistics of the field generated by the source.

Correlated Photon Pairs Influence Mixing Processes

This behavior, observed in a cascaded waveguide-QED system using two superconducting qubits, demonstrates a selection rule governing the peaks of the QWM spectrum. The result avoids the need for a separate device, a discrete parametric amplifier, to achieve the same effect. The observed suppression of sidebands linked to odd photon numbers stems from the inherent properties of the emitted photons. The analysis of peak amplitudes can be used to probe photon statistics in the incident nonclassical field.

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