Microwave Photon Characterisation Enables Quantum Network Error Diagnostics and Control.

The reliable transmission of quantum information necessitates precise characterisation of the photons that carry it, particularly as these signals propagate through complex networks. Understanding a photon’s ‘Wigner function’—a quantum mechanical representation of its state—is crucial for both active data transmission and identifying sources of error when photons leak from processing units. However, measuring the Wigner function of travelling microwave photons presents significant technical challenges due to the weak signals and inherent noise at room temperature. Researchers at Aalto University, alongside colleagues from VTT Technical Research Centre of Finland Ltd, now detail a method for performing ‘Wigner function computed tomography’ of these propagating photons, utilising a sensitive bolometer and adapting principles from medical imaging. Qi-Ming Chen, Aarne Keränen, Aashish Sah, and Mikko Möttönen present their findings in a paper entitled ‘Computed tomography of propagating microwave photons’, demonstrating the technique’s ability to reconstruct photon states at the single-photon level, even with a reduced number of measurements achieved through compressed sensing and neural networks.

Researchers have successfully demonstrated cryogenic Wigner function tomography of microwave photons, resolving a persistent challenge in the characterisation of superconducting networks. This technique allows for the complete reconstruction of a photon’s quantum state, offering a pathway to improved performance and error mitigation in quantum systems.

The core innovation lies in a method that circumvents the limitations of amplification noise, a significant impediment in room-temperature quantum measurements. The team employs a superconductor-normal metal-superconductor (SNS) bolometer, a device that directly measures the resistive heating resulting from absorbed radiation. This functions as a sensitive and broadband quadrature detector, capable of discerning the amplitude and phase of the microwave photons. Quadrature refers to the two components defining a signal’s amplitude and phase, essential for fully describing its quantum state.

The process draws a direct analogy to computed tomography (CT) scans used in medical imaging. Instead of reconstructing a three-dimensional image from X-ray projections, the researchers acquire quadrature histograms – graphical representations of the probability distribution of quadrature measurements – at various projection angles. These projections are then used to reconstruct the Wigner function, a mathematical representation that fully describes the quantum state of the photon. The Wigner function is a quasi-probability distribution, meaning it provides a classical analogue to the quantum state, allowing for intuitive visualisation and analysis.

The technique proves particularly effective for characterising Gaussian states, quantum states where probability distributions follow a Gaussian (normal) distribution. These states exhibit predictable symmetries that the method accurately captures. Importantly, the researchers achieve a substantial reduction in experimental complexity by utilising compressed sensing and neural networks to reduce the number of required projections to just three, without compromising the quality of the Wigner function reconstruction. Compressed sensing is a signal processing technique that allows for the accurate reconstruction of a signal from fewer samples than traditionally required, while neural networks provide the computational power to process and interpret the data.

This advancement facilitates real-time error diagnostics and correction within superconducting networks, crucial for maintaining the coherence and fidelity of quantum information. The ability to precisely measure and correct quantum states represents a significant step towards building more robust and reliable quantum technologies. Furthermore, the research extends beyond Gaussian states, opening avenues for characterising more complex quantum states, including squeezed and entangled photons. Squeezed states exhibit reduced noise in one quadrature at the expense of increased noise in the other, while entangled photons exhibit correlations that are stronger than classically possible. These non-classical states of light are fundamental resources for quantum communication and computation.