We demonstrate phase sensing with undetected photons for the first time with integrated photonics. The undetected photon circuit is fully integrated on chip, performing phase sensing at 1972 nm by detecting only 1291 nm light.
We experimentally demonstrate undetected photon phase sensing on a silicon photonic chip. We use a novel design, implementing an innovative integrated SU(1,1) interferometer based on transverse waveguide modes, that leverages intermodal spontaneous four-wave mixing to generate highly non-degenerate idler–signal photons. In our experiment, we retrieve the phase applied to the signal photons at wavelength 1972 nm by detecting only the correlated idler photons at 1291 nm, demonstrating the first on-chip phase sensing based on undetected photons. This result paves the way for novel generations of quantum refractometric sensors.
Interference measurements with undetected photons employ entangled photon pairs in order to overcome classical limitations in measurement sensitivity. With the undetected photon technique, one photon (signal) interacts with an object, and the signature of the interaction is stored in its entangled counterpart (idler): the interacting photon remains undetected, while the detection is performed on the photon that did not interact with the object. While the signal photon can be chosen in any spectral region suitable for the interaction, the idler photon is generated in a spectral region where detectors are efficient, scalable, and cheap. To date, the configurations proposed are in bulk setups. In this work, we propose a novel configuration on an integrated device, with the advantages of the reduced dimension, the lower cost and the robustness to alignment. In our experiment, we pumped a silicon-on-silica integrated circuit with a classical beam at a wavelength of 1.568 μm. Via intermodal spontaneous four-wave mixing, we generated highly non-degenerate time-energy entangled signal and idler photons at 1.99 μm and 1.29 μm, respectively. As we have integrated two identical sources of entangled photons in series, the photon pairs generated in the two sources are indistinguishable, and controlling the phase of the pump beam and the phase of the signal photons after the first nonlinear source, we observed interference patterns with 24% maximum visibility in the idler photon counts. We successfully measured the dephasing induced on the signal photons by measuring only the idler ones.
Quantum interference with undetected photons has gained significant attention in the past ten years due to its potential to decouple the probing and the detection of an active sensing system. The technique is based on correlated photon pairs: one photon interacts with the analyte, and the signature of the interaction is stored in its correlated counterpart. The probing photon remains undetected, while the other photon is measured. When the photons are highly non-degenerate, the spectral separation of the probing and detected photons offers significant advantages in terms of ease of operation and enhanced sensitivity, enabling more precise measurements and analysis with respect to other sensing techniques. This possibility enables a wide range of applications, particularly in fields such as imaging, spectroscopy, and optical coherence tomography, among others. This thesis describes experiments for imaging and sensing based on undetected photons, both in a free-space optical setup and in an integrated photonic circuit. The free-space configuration was used to perform undetected three-dimensional imaging reconstruction of diffusive targets. This validation of imaging with undetected photons paves the way to further development to in vivo biomedical imaging, covert imaging, and remote sensing. On the other hand, the integrated configuration overcomes the limitation of alignment tolerance and scalability of a free-space optical setup, enabling the development of integrated sensors based on undetected photons.
The ability to generate and detect quantum states of light on a single integrated photonic device is essential to scale quantum photonics into useful quantum technologies. Integrating the required capabilities into complementary-metal-oxide-semiconductor compatible monolithic chips can reduce cost and unlock new functionality through miniaturisation. In this work we demonstrate a single silicon-on-insulator photonic integrated circuit for the monolithic generation and detection of quantum light on a commercially available platform that operates entirely at room temperature. Specifically, we leverage spontaneous four-wave mixing in silicon waveguides to produce squeezed light which is subsequently detected by photodiodes operating in a pulsed homodyne detector configuration on the same chip as the source. We directly measure 0.25(1) dB of squeezing, including contributions from waveguide propagation loss and detection inefficiency, and include a detailed analysis of the impact of nonlinear loss on the squeezing levels achievable using this platform.