Hot vapor OPMs2 are employed in a wide variety of applications, including medical imaging3, geophysical surveying4, and space science5. Recent advances in MEMS cells, and integration with micro-optics, miniaturized field coils6, and opto-electronics, make possible miniaturized or “chip-scale” OPMs, with greatly reduced SWaP relative to traditional OPM implementations. Miniaturization also reduces the number of atoms and photons participating in OPM signal generation, which makes quantum noise management more critical to OPM performance. Recent experiments, using traditional cm-scale vapor cells, have shown that polarization-squeezed light can improve both OPM sensitivity and measurement bandwidth in high-sensitivity OPMs7–10. The evasion of measurement backaction (MBA) is also important in this scenario, as it allows quantum enhancement by squeezing over the entire bandwidth of the sensor8. Objective 2 is to make such quantum advantage available to miniaturized OPMs through: Sub-Objective (SOb) 2.1 development of polarization-squeezed light sources based on the photonic integrated squeezer described in Objective 1; SOb 2.2 development of squeezed-light-compatible MEMS cells for OPM application; SOb 2.3 development and demonstration of quantum-enhancement protocols that optimally exploit the properties of the photonic integrated squeezer and MEMS cells for application-derived performance targets.

We will exploit μTP process to combine a tunable narrow-linewidth laser in SiN and frequency conversion stages in LNOI in a single nanophotonic chip producing phasecoherent laser and squeezed vacuum outputs. We will develop free-space, mode-matched polarization combiners to generate polarization-squeezed light from these PIC outputs.

We will employ MEMS processes (wafer-scale lithography, gas and alkali filling, anodic bonding, optical coating, surface metallization) to develop miniaturized atomic vapor cells with characteristics (multiple vapor chambers, low thermal magnetic noise, reduced optical losses, surface functionalization) tailored simultaneously for squeezed-light compatibility and high OPM performance in miniaturized implementations.

We will leverage recent benchtop success in quantum enhancement of OPM sensitivity and bandwidth, to design protocols providing quantum enhancements for OPMs employing photonic integrated squeezers and MEMS cells. The protocols will be tailored for applications in Earth field conditions, with consequent adaptations of optical pumping, probing and signal recovery methods. Protocols will be demonstrated and refined with photonic integrated squeezer of SOb 2.1 and OPM-specific MEMS cells of SOb 2.2.

Hot- vapor based technologies are ideal for deploying accurate frequency standards outside the laboratory, such as for network synchronization, secure communication, Global Navigation Satellite System GNSS applications and space missions11. While microwave atomic clocks were for decades the gold standard, a race towards the development of optical clocks (OCs) with promising high performance, lower complexity and potential for miniaturization has begun. Two-photon optical clocks (TPOC) belong to the favoured candidates. In QUANTIFY, we will tackle the TPOC challenge both by maturing the technology related to system miniaturization (SOb 3.1) and by investigating quantum enhanced strategies to further increase the clock performances (SOb 3.2). By leveraging quantum light generation in integrated photonics, QUANTIFY has the final objective of combining such breakthroughs in a compact quantum-enhanced physical package.

We will leverage nano-photonic and MEMS processes to significantly advance the miniaturization of the key building blocks of TPOC. We will exploit μTP to combine a tunable narrow-linewidth laser in SiN with a LNOI modulator in a single PIC and we will further advance wafer-scale MEMS Rb cell technology to achieve lower residual pressure and reduced contamination.

Probing with shot-noise limited lasers, as we aim in SOb 3.1, brings the system to the standard quantum limit. In QUANTIFY we will go one step further, and we will experimentally investigate the quantum enhancement due to the use of squeezed states of light for spectroscopy of the two-photon Rb transition. The benefits are expected to be twofold: reduce the phase noise with respect to a coherent state, suppressing the intermodulation effect known to affect the short-term stability of the clock and exploit the time frequency entanglement of the photon pairs underlying the squeezed state to increase the TPA of Rb at low power. This will reduce the pump power with respect to a coherent state, reducing the DC stark shift and improving the long-term stability. Eventually, we will integrate QUANTIFY’s PICSq (Ob 1) and MEMS cell technology into the quantumenhanced TPOC system.

This objective is to demonstrate the use of a single nano-optomechanical system combining different physical effects: photonic thermometry, mechanical noise thermometry and quantum thermometry. Thanks to this combination we will: extend the temperature range (from 4 K to 300 K), provide a self-calibrating device and demonstrate a novel primary sensor for absolute in-situ calibration. The optomechanical sensor will be characterized in terms of its performance (extended temperature range, response time, resolution/sensitivity) and metrological characteristics (repeatability and reproducibility).

QUANTIFY will develop photonic and phononic crystal cavities of different dimensionality to simultaneously engineer the optical and mechanical degrees of freedom and achieve high optical and mechanical Q factors, efficient thermalization and good opto-mechanical coupling. QUANTIFY will use μTP to transfer such OMO onto a SiN motherboard, to increase its robustness, ease the light coupling and enable further on-chip integration of optical circuitry. The same hybrid integration will be used to miniaturize the optical interrogation, which will consist in a TriplexTM low-noise, tunable PIC laser at 1550 nm with LNOI electro-optic modulation.

We will develop thermometry read-out protocols for the optomechanical sensors based on three complementary optical measurements (photonics, noise and quantum thermometry) and working on different but overlapping temperature ranges. They all will be realized on the same device with the same interrogating interface and optical and mechanical degrees of freedom. Below 10 K we will develop a readout method to target a primary quantum measurement exploiting quantum correlations and providing an absolute measurement.

Noise thermometry, based on monitoring the thermal noise of a harmonic oscillator will bridge the gap between Photonic (>50 K) and Quantum thermometry (<10 K) as it theoretically presents a perfect linearity on the full temperature range.

Sensitivity, accuracy and stability are essential properties that must be known to bring the devices that are intended to be developed outside the laboratory. QUANTIFY will develop three different quantum enhanced sensors and will build ad hoc metrological set-ups for each of them. Sensor’s performance will be deeply investigated through highly qualified measurement methods following the International Metrological Standard and best practice with traceability to the International System (SI) of units to achieve a complete knowledge of their capabilities and implementing them in a real metrological chain. Moreover, a comparison between measurements on the same sensor by different QUANTIFY’s partners will also be performed to strengthen the results.