Quantum optics at Terahertz frequencies

Research questions: Measuring THz radiation is essential for many engineering challenges, such as the detection of concealed weapons, the quality control in the pharmaceutical industry and for the identification of hazards such as explosives. Currently, one of the main approaches to detect THz radiation relies on measuring the signal that originates from the nonlinear interaction between the unknown THz radiation and a short laser pulse into specific crystals. With this project, we aim to investigate a way to overcome the current limitation of this detection scheme relying on the properties of quantum states of light. Specifically, squeezed radiation can increase the detect insensitivity by providing a lower noise probe pulse for THz detection.

Approach:
For this project, we have three main activities.
1) Novel THz antennas
2) Generation and detection of squeezed states
3) Quantification of the signal-to-noise and sensitivity enhancement due to the use of nonclassical states
For part 1), the student will design a new THz emitter that is optimised on the characteristic of the available laser. This activity includes numerical simulation and the study of previous literature. As a backup plan, the student will have access to an already functioning, but not optimised THz generation antenna.
Part 2 will see the student developing both the source of squeezed radiation and the relevant detection tools. The source will be a standard parametric down-conversion in type II nonlinear crystal. The detection, instead, will require to design a specific electronic amplifier to drive two, high-quantum efficiency, InGaAs photodiodes. The design of the amplifier should be such that it will enable detection of sub-shot-noise signals at ~100kHz.
Part 3 will require understanding how the signal to noise ratio and dynamic range are in a standard THz time-resolved detection and will include the characterisation of the setup developed by the student. Hence, these parameters will be recorded using a two-mode squeezed probe pulse and compared to the classical case.

Novel engineering and physical sciences content
The novelty of this research program is in the use of quantum technologies, such as squeezed state metrology, to improve the capabilities of a well-established and broad impact technique, that is, time-domain THz spectroscopy. New technical tools will need to be developed, such as a non-classical scheme for time-domain spectroscopy, along with the required components, such as THz antenna arrays and low-noise current amplifiers.