Squeezed Light Mimics Coherent Tones to Control Superconducting Qubits

Quantum wave mixing in a system of superconducting qubits reveals insights into non-classical light interactions. R. D. Ivanovskikh at Dukhov Research Institute of Automatics (VNIIA) and colleagues demonstrate a theoretical framework describing this effect within a cascaded qubit system, driven by both external signals and resonance fluorescence. The framework establishes a key equivalence between the observed phenomena and a qubit driven by coherent tones and squeezed light, leading to a predictable selection rule for the quantum wave mixing spectrum. Numerical simulations confirm the involvement of correlated photon pairs in these processes, highlighting a new method for characterising the statistical properties of incident non-classical fields. Spectral peak mapping reveals light properties via cascaded qubit interactions Analysing the peaks within a quantum wave mixing (QWM) spectrum has emerged as a central technique for probing subtle light characteristics, akin to mixing two colours of light to create new shades but occurring at a quantum level. This process relies on careful examination of how two light sources interact with a quantum system, specifically a cascaded two-qubit system where one qubit emits radiation to another. A spectrum is carefully mapped, identifying specific peaks corresponding to photon exchange between the light sources and the qubits, which allows deduction of information about the incoming light. The significance of this approach lies in its ability to characterise non-classical states of light, which are crucial resources for quantum information processing and quantum communication protocols. Conventional methods often struggle to fully capture the nuances of these states, making QWM spectroscopy a valuable addition to the quantum optics toolkit. The experiment investigated a cascaded system of two superconducting qubits interacting via a one-dimensional waveguide. Superconducting qubits are particularly well-suited for this type of experiment due to their long coherence times and strong coupling to electromagnetic fields. A ‘probe’ qubit was driven by both an external coherent tone and resonance fluorescence emitted from a strongly driven ‘source’ qubit. The coherent tone provides a well-defined input signal, while the resonance fluorescence, a spontaneous emission process, introduces quantum correlations. Focusing on the QWM spectrum, the identified peaks correspond to photon exchange, allowing deduction of the incoming light’s characteristics. The waveguide acts as a mediator for the photon exchange, facilitating the interaction between the qubits and the light fields. This setup provides a platform for detailed investigation of light-matter interactions at the quantum level, potentially advancing quantum technologies such as quantum sensors and quantum repeaters. Correlated photon pairs enable enhanced sensitivity in quantum wave mixing spectroscopy Suppression of sidebands in the QWM spectrum reached a factor of two when the coherent Rayleigh component of the source field was diminished; previously, observing QWM relied on multiphoton processes involving at most one photon from the source. This advance, achieved within a cascaded two-qubit system, unlocks the ability to probe photon statistics with greater sensitivity than methods utilising coherent tones or pulse trains. The Rayleigh component represents the classical, coherent part of the emitted light, and its reduction allows the quantum correlations to become more prominent in the QWM signal. A theoretical framework and numerical simulations confirm that correlated photon pairs are central to this process, revealing a direct link between the QWM spectrum and the anomalous correlations of the source emission. Anomalous correlations signify a deviation from the classical Poissonian statistics expected for coherent light, indicating the presence of non-classical correlations such as squeezing or entanglement. Numerical simulations, utilising varying rates of qubit decay, unambiguously confirmed the role of these correlated pairs in generating the QWM signal. The qubit decay rate represents the timescale over which the qubit loses its quantum information, and its influence on the QWM signal provides further validation of the theoretical model. Analysing the strength of these signals allows for sensitive probing of the photon statistics of the incoming light, a key feature for quantum technologies. Specifically, the amplitude and shape of the QWM peaks are directly related to the higher-order correlation functions of the light field, providing a quantitative measure of its non-classicality. However, the current findings do not yet demonstrate sustained coherence or scalability needed to build practical quantum devices, representing a future research direction. Achieving these goals will require further advancements in qubit fabrication, control, and error correction techniques. Revealing photon statistics through refined quantum wave mixing spectral analysis Characterising the subtle quantum properties of light sources is vital for advances in quantum technologies, demanding ever-more-precise diagnostic tools. This work offers a new method, analysing peak amplitudes within the QWM spectrum, to reveal information about the statistical properties of incoming light, a significant refinement over existing techniques. Traditional methods, such as single-photon counting and homodyne detection, often require complex setups and are limited in their ability to fully characterise complex quantum states. The QWM approach provides a complementary technique that is particularly sensitive to photon correlations. Extending this approach to other physical platforms, however, presents a considerable challenge. Adapting the cascaded qubit system to different materials or architectures may require significant modifications to the experimental setup and theoretical model. A direct connection between the amplitudes of peaks in a QWM spectrum and the statistical properties of incoming non-classical light was established. Using the cascaded two-qubit system, scientists demonstrated a suppression of specific spectral sidebands linked to an odd number of photons, confirming the importance of correlated photon pairs in the interaction. This suppression arises from the quantum mechanical interference between different pathways for photon exchange, highlighting the non-classical nature of the process. This refined method offers a new diagnostic tool, allowing for characterisation of light sources beyond reliance on coherent tones or pulse trains, and opens avenues for exploring more complex quantum states of light. Future research could focus on extending this technique to characterise entangled photon pairs or other exotic quantum states, further expanding its applications in quantum information science. The research established a direct link between peak amplitudes in a quantum wave mixing spectrum and the statistical properties of incoming non-classical light. This matters because it provides a new method for characterising light sources, moving beyond techniques reliant on simple coherent tones. Scientists demonstrated this using a cascaded two-qubit system, observing a suppression of spectral sidebands associated with an odd number of photons, which confirms the role of correlated photon pairs. The authors suggest future work could extend this technique to characterise entangled photon pairs and other complex quantum states. 👉 More information 🗞 Photon pairs, squeezed light and the quantum wave mixing effect in a cascaded qubit system 🧠 ArXiv: https://arxiv.org/abs/2604.08139 Tags: