Quantum Photonics & Sensing

Research Area

Photonics is one of the main enablers for quantum technology. In this context non-classical states of light, in particular photon pairs, allow novel concepts for imaging and sensing beyond classical limitations. Moreover, as quantum information carriers, photons are the essential building block for photonic quantum simulation and computing.
Dr. Gräfe's research focusses on entanglement, correlation, and interferometer based quantum imaging techniques as well as on integrated quantum photonics including quantum walks of photon pairs and their non-classical correlations. His research topics include:

• quantum imaging and sensing with undetected photons
• microscopy with non-classical states of light
• photon-pair and multi-photon source development
• quantum walks of correlated photon pairs and multiphoton states
• entropy sources and quantum random number generation

Quantum Optics Research Methodes

The following methods and equipment are harnessed in the group:

photon pair sources based on spontaneous parametric down conversion

• non-linear interferometers for induced coherence
• cw and pulsed laser systems at various wavelengths
• single photon detectors and correlation electronics
• linear and non-linear waveguide characterization
• entangled two-photon absorption

Recent Research Results

Harnessing photon pairs from spontaneous parametric down conversion allows to make use of nonclassical properties of such quantum states. This can be of particular interest in the fields of imaging, microscopy, and sensing, since it allows new modalities extending the possibilities of classical techniques [1]. In doing so, it becomes possible to spectrally separate illumination of a specimen and the actual detection on the camera by so-called imaging with undetected photons. One major step is advancing this technique towards a portable system capable of video rate imaging at long-term stability [2]. Moreover, demonstrating holography with undetected light brings threedimensional imaging in exotic spectral ranges within reach [3]. In addition, two-photon fluorescence can be driven by photon pairs with the advantage of having a linear absorption response with respect to a quadratic one in the classical domain. This beneficial behavior (more fluorescence light at same excitation level) allows longer observation of light-sensitive biosamples. The pioneering demonstration of the linear absorption rate for convenient fluorophores was demonstrated [4].
Further, the fundamental limits of the quantum random number generation rate using the phase noise of gainswitched pulsed lasers was investigated. An essential conclusion is an upper limit of the repetition rate for independent, and thus, high-quality random numbers [5].
Quantum walks of photonic states in integrated networks are the implementation tool for quantum simulation and quantum gate operations [6]. In this field many contributions have been made, such as the recent demonstration of an on-chip quantum state tomography tool [7], a heralded CNOT quantum gate [8], and the investigation of quantum coherence endurance in noisy environments [9]. Some of this works did benefit from an advanced fabrication technique of quantum photonic chips that allows better mode matching, and thus lower loss, for interconnecting with optical
fibers [10].