Squeezed states metrology for THz time-domain spectroscopy.

Abstract. The project I have discussed with Dr Clerici targets the use of quantum optics to improve the sensitivity of time-resolved measurements of electromagnetic radiation. Specifically, it aims at developing quantum-enhanced metrology tools to detect weak Terahertz-frequency (THz) waveforms. The THz region of the electromagnetic spectrum is technologically important for applications in spectroscopy (for instance hazardous gasses and explosives), security (imaging trough packaging materials) and pharmaceutics (quality control). This spectral region is, however, hard to access due to the limited availability of sensitive detectors. In my PhD project I will investigate techniques to use quantum optics to improve the state of the art of THZ time-domain spectroscopy[1]. Background. Electro-Optical Sampling (EOS) is a commonly employed method where a short laser pulse is used to measure the change in refractive index caused by the unknown field in a second-order nonlinear crystal (electro-optical effect)[2]. This technique allows to record THz radiation directly in the time domain and delivers the radiation spectrum by obtaining the Fourier transform. The sensitivity of this technique is limited by the standard quantum limit and cannot be further improved without the use of non-classical radiation. When measuring a quantity using an average number of photons N, in the best case, i.e. if no other sources of noise are present, our measurement will be affected by an error that is proportional to the sensitivity. Classical metrology is limited by this bound, known as the standard quantum limit. Increasing the number of photons, and hence the probe pulse intensity, improves the sensitivity of the measurement. In the case of EOS-based THz detection, the probe intensity can only be increased up to a limit that is imposed by the onset of nonlinear noise. To further increase the sensitivity, quantum correlated states can be utilised to reduce the error proportionality to,which is the Heisenberg limit. In my PhD project I will aim at attaining a Heisenberg-limited THz detection using squeezed states of light. Methodology. EOS is a phase measurement where the electric field is measured at a specific temporal coordinate by recording the phase shift induced on the probe pulse, via a nonlinear interaction in a second-order nonlinear crystal. A primary goal of my PhD project will be to test an experimental approach inTHz EOS that uses twin-beams[3].These are quantum states of light characterised by two beams with a perfect correlation in their photon number. They are one of the possible realisations of squeezed light and will be tested as a tool to overcome the sensitivity limit of THz EOS. To generate and detect twin-beams, I will expand on the work that I have implemented during my final year MEng project. I will use a periodically poled Lithium Niobate crystal provided by Covesion Ltd. designed to work in a type-II phase matched condition. The improvement in sensitivity acquired with the twin-beam configuration, shall be quantified thanks to the sub-shot-noise balanced detector that I designed and started to characterise during my MEng final year project. A method to classically enhance the sensitivity of phase measurements, comprises of using a laser cavity. Intracavity phase interferometry (IPI) is a cavity enhanced version of standard interferometry that provides direct access to the phase shift occurring on one comb of a cavity by beating it with a second, unperturbed comb. Specifically, the phase measurement is performed by recording the beat note obtained by interfering the cavity combs. This technique has never been used to measure THz radiation, and a further goal of my PhD project will be to develop an IPI-based THz detection scheme. To achieve this goal, I will follow be supervised by Dr Clerici and the UNO group, and I will follow