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 of 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.
Double-resonant sum-frequency generation of blue light with record-high conversion efficiency:
Based on the same technology as our OPOs for two-color entanglement, i.e. a second order nonlinear crystal placed in a low finesse double-resonant cavity, we developed a system for sum-frequency generation (SFG) of blue light that reaches near-unity quantum conversion efficiency (QCE) of mode-matched input photons.
Our system consists of a double-resonant bow-tie cavity containing a 10-mm-long PPKTP crystal with a poling period of 6.1 µm and pumped by two near-infrared lasers at 1064 nm and 852 nm. Tuneability of the crystal temperature and pump laser wavelength around 852 nm enables fulfilling phase matching conditions for SFG wavelengths spanning 5 nm around 473 nm. The two pump beams are maintained simultaneously on cavity resonance by applying two Pound-Drever-Hall locks. First, the cavity length is locked to the 1064 nm pump laser, and afterwards the tuneable laser frequency is stabilized to the cavity resonance at 852 nm. This stabilization technique locks the tuneable laser frequency to the more stable 1064 nm pump laser using the double-resonant cavity as a frequency transfer cavity, and ensures a high frequency stability of the blue light SFG output. Our double-resonant cavity design and operation ensures high frequency stability and high spatial mode quality (M2 value below 1.05) of the SFG output with optical power deviation below 0.8% over the course of an hour. We achieve a very high QCE due to ultra-low intra-cavity losses of about 0.3%, excluding an 8% transmission input coupler. Optimum QCE is obtained when both pump beams reach impedance matching due to depletion via the SFG process.
Our SFG system can find applications as pump source for optical atomic clocks, nonlinear optics and the generation of non-classical twin beams of light in the near-infrared region. High QCE is particularly relevant for the raising field of quantum frequency conversion in order to preserve quantum correlations between beams at different wavelengths. Finally, our SFG source has potential as a light source for various types of microscopes, high spatial resolution scatterometry, and dark-field wafer inspection.
To know more, check out our publication here!
Fig. Schematics of the double-resonant SFG setup. DM: dichroic mirrors, PM: phase modulators, HWP: half-wave plate, L: lenses, PZT: Piezo-electrical element, M: mirrors.