Quantum Photonics & Sensing
Research Area
Photonics is one of the main enablers for quantum technology. In this context nonclassical 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 nonclassical correlations. His research topics include:
 quantum imaging and sensing with undetected photons
 microscopy with nonclassical states of light
 photonpair and multiphoton 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
 nonlinear interferometers for induced coherence
 cw and pulsed laser systems at various wavelengths
 single photon detectors and correlation electronics
 linear and nonlinear waveguide characterization
 entangled twophoton 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 socalled imaging with undetected photons. One major step is advancing this technique towards a portable system capable of video rate imaging at longterm stability [2]. Moreover, demonstrating holography with undetected light brings threedimensional imaging in exotic spectral ranges within reach [3]. In addition, twophoton 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 lightsensitive 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, highquality 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 onchip 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].
 Related publications

[1] GILABERTE BASSET ET AL., LASER PHOTON . REV. 13, 1900097 (2019).
[2] GILABERTE BASSET ET AL., LASER PHOTON . REV., ACCEPTED (2021).
[3] T ÖPFER ET AL., IN PREPARATION (2021)
[4] GÄBLER ET AL., IN PREPARATION (2021)
[5] SEPTRIANI ET AL., AIP ADVANCES 10, 055022 (2020).
[6] GRÄFE ET AL., J. PHYS. B 53, 073001 (2019).
[7] TITCHENER ET AL., NPJ QUANTUM INFORMATION 4, 19 (2018).
[8] ZEUNER ET AL., NPJ QUANTUM INFORMATION 4, 13 (2018).
[9] PEREZLEIJA ET AL., NPJ QUANTUM INFORMATION 4, 45 (2018).
[10] HEILMANN ET AL., APPL. OPT., 57, 377 (2018).