Detecting single photons in the optical frequency band is a well-established practice; however, this capability is just emergent at microwave frequencies. Single microwave photon detectors (SMPDs) are instrumental for efficiently detecting weak signals from incoherent emitters, with applications in axion searches, hybrid quantum systems, and superconducting quantum computing. At microwave frequencies, SMPDs rely on superconducting qubits to encode the presence or absence of an itinerant photon. Such quantum interaction provide a quantum non-demolition (QND) measurement of the photon state, in constrast with absorptive optical Single Photon detectors (SPDs). In this work, we leverage the QND nature of this interaction to repeatedly measure the photon state in a cascaded manner. By encoding the information on multiple qubits, we mitigate the intrinsic local noise of individual qubits, achieving a two-order-of-magnitude reduction in intrinsic detector noise at the cost of a slight reduction in efficiency. The photon detector concatenates two four wave mixing process coherently on a single chip. This scheme ensures fully quantum coherent dynamic of photon detection, enabling dynamical tuning of the detector’s bandwidth — a critical feature for practical use in setups affected by thermal photons.

We show how to balance the strengths of the parametric processes to maximize sensitivity and evaluate the core metrics of such a device. We demonstrate an intrinsic sensitivity of (8+-1)x10^{-24} W.Hz^{-1/2} at 8.798 GHz, with the detector noise entirely dominated by the thermal noise of the input resonator. The bandwidth tunability is 100 kHz. Limitations are discussed and addressed in recent work with a detector at 11.7 GHz.

Synchronized spectroscopy measurements for acetylene and molecular iodine reference transitions performed with an ultrastable laser signal may be ex-ploited to constrain jointly couplings of dark matter to many fundamental constants.

Quantum technology holds the potential to revolutionize fields like computing and communication by leveraging quantum properties from quantum systems. In photonic systems, there has been significant progress in quantum technology research that extensively exploited quantum optical states (photon sources) to utilize their unique statistical properties. For example, two induced states from coherent state (CS); Photon Added state (PA) and Displaced Fock state (DF) which possess quadrature-squeezing, anti-bunching and sub-Poisson were proven to improve the performance of quantum gates, boson sampling, and BB84/B92 cryptographic protocols. Recently, a new approach to deriving the photon statistics of PA, particularly its photon-number distribution, was introduced [1]. This approach employs a modified moment generating function (MGF) to manipulate the initial CS’s photon-number distribution, offering an alternative to conventional methods. While traditional derivations require performing a quantum operation first and then applying state projection (Born’s rule) to obtain the resulting photon-number distribution, the modified MGF method instead tracks the transformation of the photon-number distribution directly, bypassing the need for quantum operations like the creation operator. Nevertheless, despite its advantages, the new method is strictly limited to PA, overlooking DF. Therefore, we proposed a generalized MGF, which complements the modified MGF. This work paves the way for extending the MGF framework to a broader class of quantum state transformations, offering new insights into quantum optical state engineering and its applications in photonic quantum technology.

Rydberg atoms are highly promising for microwave electric field sensing owing to their large dipole matrix elements. While most experimental developments have focused on room-temperature vapors so far, utilizing cold atoms in this context could open new possibilities for applications where accuracy, long term stability and high-resolution at large integration times are required, such as calibrating blackbody shifts in state-of-the-art optical clocks or measuring the cosmic MW background.

At ONERA, we demonstrate a novel approach for the metrology of microwave fields with cold 87 Rubidium Rydberg atoms based on trap-loss-spectroscopy in a magneto-optical trap (MOT). This technique stands out for its simplicity, relying solely on fluorescence measurements. With a scale factor linearity at the 1% level and a long-term frequency stability equivalent to a resolution of 5 μV/cm at 2500 s and no noticeable drift over this time period, this new measurement technique appears to be particularly well-suited for metrology experiments. This method will allow in principle to reconstruct the amplitude and the frequency of the microwave field simultaneously without the need for an external reference field.