QUANTUM LIGHT SOURCES
DFM will develop quantum light sources based on conventional lasers, where quantum fluctuations in light properties (e.g. intensity or phase) are manipulated in so-called optical parametric oscillators and/or Rb atoms. The special properties of quantum light sources can only be described by the laws of quantum physics and have great potential in ultra-sensitive optical measurements. As an example, a high signal-to-noise ratio can be maintained by measurements with very low optical power. This is important in optical measurements biological samples, with optically induced damage being reduced by lower power. Furthermore, quantum light sources can contribute to sharper imaging of micro- and nanoscale items, as the quantum light allows to overcome the classical diffraction limitation in microscopy. Similarly, quantum light sources can improve optical interferometers, which are used, for example, for distance measurements.
The need for research into the development of quantum light sources and metrology applications are supported by specific comments on BedreInnovation.dk from DTU Photonics (Jesper Mørk) and DTU Physics (Jonas Neergaard-Nielsen & Ulrich Hoff), as well as in FORSK2025 p. 59.
Different types of non-classical light sources will be developed. The sources are based on parametric down-conversion in second order nonlinear crystals positioned in a low finesse cavity. In the single photon regime, the down-conversion process converts a pump photon with high energy into a pair of photons with lower energies in the signal and idler intracavity modes. This leads to quantum correlations between the intensities and phases of signal and idler fields. OPOs (Optical Parametric Oscillators) can be operated both above and below the self-oscillating threshold.
Spatial multimode entanglement with above threshold OPO:
The above threshold OPO for the generation of spatial multimode entanglement is based on a type II OPO with a KTP crystal pump by 532 nm laser. The down-converted beams (signal/idler) have a wavelength of approximately 1064 nm, which is due to energy conservation. Type II optical parametric oscillators are well-known to generate highly quantum correlated bright twin beams, and noise suppressions down to -10 dB relative to the shot-noise level have been observed. The OPO can also emit spatial multimode modes in the OAM bases. We pump the OPO a few percent above threshold. The number different squeezing is measured using self-homodyne detection. The OPO is part of the developments for the EMPIR project BeCome.
Generation of two-color entanglement in the below threshold OPO:
The OPO for the Q-GWD project will be operated in a non-degenerated mode generating light at 1064 nm and 852 nm. The goal of the project is to establish a novel ‘plug-and-play’ non-classical laser technology for ultra-precise measurements in gravitational wave detectors (GWD). We implement a novel quantum measurement technique that will go significantly beyond the currently envisioned performance of GWDs and will exhaust their ultimate sensitivity limits set by quantum physics. The idea is laid out by A. Kuzmich and E.S. Polzik (PRL 85, 5639 (2000)) and K. Hammerer et al, (PRL. 102, 020501 (2009)). The goal can be achieved by utilizing a joint optical measurement on the GWD and on an atomic vapor using two entangled beams, one at the wavelength of the GWD laser, 1064nm, and the other tuned to the atomic resonance at 852nm. However in general the OPO can be tuned to any wavelength of interest in the 800-1100 nm range, thus generating entangled beams in this range, making the OPO system versatile for many different applications.
Despite OPOs remarkable capabilities, OPOs have so far not found widespread use in commercial products. Only a few companies develop and sell OPOs for spectroscopy and other classical applications, no commercial OPO for quantum applications exist today. Several Universities and research institutes are developing OPOs and using OPOs as sources for non-classical light generation. These systems are large bulky devices, which are fragile and not suitable for commercial exploitation.