Thomas Walther

Technische Universität Darmstadt | TU · Institut für Angewandte Physik

Recently, we implemented a basic star-shaped entanglement-based quantum key distribution network without trusted nodes. It enables simultaneous pairwise exchange of quantum keys between multiple users. In this paper, we demonstrate its flexibility by focusing on several aspects. We show continuous system operation over a partially deployed 100 km f...

We demonstrate a method to reduce the number of single-photon detectors (SPDs) required in multi-party quantum key distribution (QKD) networks by a factor of two by using detector time multiplexing (DTM). We implement the DTM scheme for an entanglement-based time-bin protocol and compare QKD results with and without DTM in our QKD network with four...

An improved moveable in vacuo XUV fluorescence detection system was employed for the laser cooling of bunched relativistic (β = 0.47) carbon ions at the Experimental Storage Ring (ESR) of GSI Helmholtzzentrum Darmstadt, Germany. Strongly Doppler boosted XUV fluorescence (∼90 nm) was emitted from the ions in a forward light cone after laser excitati...

Advances in photonics require photon-number resolved simulations of quantum optical experiments with Gaussian states. We demonstrate a simple and versatile method to simulate the photon statistics of general multimode Gaussian states. The derived generating functions enable simulations of the photon number distribution, cumulative probabilities, mo...

Many common photonic states are so-called Gaussian states. In a recent manuscript, we have shown how the photon statistics of such states can be obtained by constructing and differentiating generating functions [1]. In this technical report, we demonstrate the straightforward application of the framework PyTorch to compute the required multivariate...

Lasing without inversion is a compelling method based on the generation of coherences between atomic levels in order to produce UV radiation. While the proof-of-principle of this scheme has been shown for several decades, so far no radiation at a significant shorter wavelength than the necessary drive fields has been observed. In a recent publicati...

A simple and versatile method to simulate the photon statistics of multimode Gaussian states based on automatic differentiation of generating functions is presented. The generating functions for the photon number distribution, cumulative probabilities, moments, and factorial moments of the photon statistics are derived. Related expressions for mult...

Protein hydration shell dynamics plays a pivotal role in biochemical processes such as protein folding, enzyme function, molecular recognition and interaction with biological membranes. Thus, it is crucial to understand the mobility of the solvation shell at the surface of biomolecules. Triplet state solvation dynamics can reveal the slow dynamics...

We present a scalable star-shaped quantum-key-distribution (QKD) optical-fiber network. We use wavelength-division demultiplexing (WDM) of broadband photon pairs to establish key exchange between multiple pairs of participants simultaneously. Our QKD system is the first entanglement-based network of four participants using BBM92 time-bin coding and...

RAdiation-Detected Resonance Ionization Spectroscopy (RADRIS) is a versatile method for highly sensitive laser spectroscopy studies of the heaviest actinides. Most of these nuclides need to be produced at accelerator facilities in fusion-evaporation reactions and are studied immediately after their production and separation from the primary beam du...

Doppler-free three-photon coherence effects at a wavelength of 253.7nm were observed in thermal mercury vapor. The experimental results are compared to simulations based on a detailed theoretical model reproducing the measured effects. Based on these results, we show by simulations that amplification without inversion in our setup is feasible, pavi...

We report on the first realization of time-dependent quantum detector tomography for commercially available InGaAs avalanche photo detectors. For the construction of appropriate time-dependent POVMs from experimentally measured data, we introduce a novel scheme to calculate the weight of the regularization term based on the amount of measured data....

We present entanglement-based quantum key distribution in a star-shaped network with time-bin coding. We demonstrate key exchange under typical operation conditions for metropolitan-scale networks. Without much efforts, it is scalable to more than 100 users.

Doppler-free three-photon coherence effects at a wavelength of 253.7 nm have been observed in thermal mercury vapor. The experimental results are compared to simulations based on a detailed theoretical model reproducing the measured effects. Based on these results, we show by simulations that amplification without inversion in our setup is feasible...

We present simultaneous quantum key distribution (QKD) between any two pairs of participants in a 4-party star-shaped optical fiber network without a trusted node. Our QKD system is the first to extend an entanglement-based time-bin coding protocol to multiple pairs of participants. We demonstrate key exchange over distances of more than 75 km and...

We report on the first realization of time-dependent quantum detector tomography for commercially available InGaAs avalanche photo detectors. For the construction of appropriate time-dependent POVMs from experimentally measured data, we introduce a novel scheme to calculate the weight of the regularization term based on the amount of measured data....

Quantum cryptography allows one to transmit secret information securely, based on the laws of quantum physics. It consists of (1) the transmission of physical particles like photons and (2) the software-based processing of measurements during the transmission. Quantum key distribution (QKD), e.g., transmits material for establishing a shared crypto...

Glass-forming liquids are well known to have significant dynamic heterogeneities, leading to spatially grossly varying elastic properties throughout the system. In this paper, we compare the local elastic response of supercooled 1-propanol monitored by triplet state solvation dynamics to the macroscopic dynamic shear modulus measured by a piezo-ele...

One of the unsolved problems of dynamics in supercooled liquids are the differences in spectral shape of the structural relaxation observed among different methods and substances, and a possible generic line shape has long been debated. We show that the light scattering spectra of very different systems, e.g., hydrogen bonding, van der Waals liquid...

In our laboratory, we employ two biphoton sources for quantum key distribution. The first is based on cw parametric down-conversion of photons at 404 nm in PPKTP waveguide chips, while the second is based on the pulsed parametric down-conversion of 775 nm photons in PPLN waveguides. The spectral characterization is important for the determination o...

Triplet state solvation dynamics (TSD) is a truly local measurement technique, where a dye molecule is dissolved as a probe at low concentration in a solvent. Depending on the dye molecule, local information on mechanical or dielectric solvation can be obtained. So far, this method has mainly been used to investigate topics such as fundamentals of...

Simultaneous pairwise quantum key distribution between four parties was demonstrated by implementing an entanglement-based phase-time coding protocol with a type-0 SPDC photon source. The star-shaped network is scalable to dozens of users.

The $^2S_{1/2} - ^2P_{1/2}$ and $^2S_{1/2} - ^2P_{3/2}$ transitions in Li-like carbon ions stored and cooled at a velocity of $\beta \approx 0.47$ in the Experimental Storage Ring (ESR) at the GSI Helmholtz Centre in Darmstadt have been investigated in a laser spectroscopy experiment. Resonance wavelengths have been obtained using a new continuous-...

This Letter presents a pulsed, Fourier transform limited 1030 nm laser with a variable pulse duration between 47 and 733 ps resulting in a spectral bandwidth of roughly 1 to 10 GHz. The laser system is based on ytterbium-doped fiber amplifiers and acousto-optic and electro-optic modulation technology. The repetition rate can be set arbitrarily betw...

One of the hallmarks of molecular dynamics in deeply supercooled liquids is the non-exponential character of the relaxation functions. It has been a long standing issue if 'universal' features govern the lineshape of glassy dynamics independent of any particular molecular structure or interaction. In the paper, we elucidate this matter by giving a...

We present latest results on our Brillouin Scattering LIDAR to remotely measure the temperature profile in the ocean, i.e. accuracy, improvements on various components as well as a method to independently determine temperature and salinity.

Long-term laser operation in the UV by frequency doubling in BBO is often limited due to degradation effects. We theoretically investigate the impact of elliptical focusing on the fundamental peak intensity to reduce the risk of crystal damage. After choosing a suitable elliptical Gaussian mode allowing for significantly lower fundamental peak inte...

The dielectric Debye relaxation in monohydroxy alcohols has been subject of long-standing scientific interest and is presently believed to arise from the relaxation of transiently H-bonded supramolecular structures. Therefore, its manifestation might be expected to differ from a local dielectric probe as compared to the standard macroscopic dielect...

The dielectric Debye relaxation in monohydroxy alcohols has been subject of long-standing scientific interest and is presently believed to arise from the relaxation of transiently H-bonded supramolecular structures. Therefore, its manifestation might be expected to differ from a local dielectric probe as compared to the standard macroscopic dielect...

Dynamic frequency tuning of the 40.67 GHz intermode beat frequency of a 1255 nm emitting 1 mm long monolithic self mode-locked single section optical frequency comb InAs/InGaAs quantum dot laser across 70 MHz is experimentally demonstrated by fine-delay dual-cavity controlled all optical self-injection. Fiber-based macroscopic optical delay lengths...

With a combination of an elevated temperature for the β-barium borate (BBO) crystal and an elliptical focusing cavity we succeeded in eliminating the degradation in BBO during second harmonic generation into the ultraviolet (UV). We were capable of generating over 600 mW continuous-wave UV light at 257 nm with an off-the-shelf crystal for over 8 h...

Laser cooling and precision laser spectroscopy of highly charged ions are considered as frontiers of atomic physics research at heavy ion storage rings. A brief overview of the fundamentals of these powerful methods, applied to relativistic stored ion beams, is given. Preliminary results from laser cooling of lithium-like ¹⁶O⁵⁺ ion beams at a relat...

Laser spectroscopy enables the determination of fundamental atomic and nuclear properties with high precision. In view of the low production rates of the heaviest elements, a high total efficiency is a crucial requirement for any experimental setup to be used in on-line experiments. The setup requires the use of gas stopping techniques to slow down...

A large amount of sensitive data must remain accessible for decades or even centuries (e.g, electronic health records, governmental documents). Communicating such data over the Internet requires long-term secure communication channels, which, in turn, require robust key distribution protocols. Currently used key distribution protocols, however, are...

Microfabrication: Channels and needles built by laser Using laser light to manufacture microchannels and microneedles offers a way to greatly simplify and reduce the costs of creating these structures for drug delivery and diagnostic sampling of body fluids. Microneedle arrays and microfluidic channels are increasingly used in patches and ‘lab-on-c...

In this work, the impact of fine-delay tunable dual-cavity optical self-feedback (DC-OFB) on the pulse timing stability and pulse repetition rate (RR) frequency of a 1 mm long single-section self-mode-locked quantum-dot (QD) laser emitting at 1255 nm is studied experimentally and by simulation. By tuning both microscopic dual-cavity delay lengths,...

We report on the design of a novel cavity with cylindrical focusing to overcome crystal degradation effects in the generation of UV radiation using evolutionary algorithms. Experimental results show the advantages of the new set-up.

We report on a laser system combining one-photon and two-photon polymerization for precise and fast fabrication of macroscopic three-dimensional structures featuring microscale and nanoscale characteristics. This single-stage process significantly reduces the production time as demonstrated by scaffolds in a shell application. Porous scaffolds with...

One of the most important atomic properties governing an element’s chemical behavior is the energy required to remove its least-bound electron, referred to as the first ionization potential. For the heaviest elements, this fundamental quantity is strongly influenced by relativistic effects which lead to unique chemical properties. Laser spectroscop...

Until recently, ground-state nuclear moments of the heaviest nuclei could only be inferred from nuclear spectroscopy, where model assumptions are required. Laser spectroscopy in combination with modern atomic structure calculations is now able to probe these moments directly, in a comprehensive and nuclear-model-independent way. Here we report on u...

We report on cellular adhesion of human primary fibroblasts on scaffolds fabricated by laser-induced two-photon polymerization using 515 nm femtosecond pulses. The scaffolds are large scaled with a dimension in the range of several millimeters and consist of a periodic reproducible structure. A minimum process duration of 6.3 min is achieved by an...

We present a 1028-nm master oscillator power amplifier system generating transform limited pulses with adjustable duration (250 and 735 ps) and repetition rate (1 and 10 MHz).

We present a fiber-based laser system generating transform limited pulses with adjustable duration (240-735 ps) and repetition rate (1-10 MHz). The pulses are frequency quadrupled to the ultraviolet at 257 nm.

We present a novel design for an efficient enhancement cavity for frequency doubling into the UV. Radiation of 150 mW has been generated at 257 nm and remained stable for more than 15 hours.

We have developed a flexible experimental setup to conduct triplet solvation dynamics (TSD) experiments. The setup is capable of exciting dyes at 355, 320 and 266 nm. Phosphorescence spectra can be recorded up to a 10 ns-resolution usually covering three decades using a grating spectrograph and a CCD camera. In this contribution, we describe the ex...

Sensitive digital data, such as health information or governmental archives, are often stored for decades or centuries. The processing of such data calls for long-term security. Secure channels on the Internet require robust key establishment methods. Currently used key distribution protocols are either vulnerable to future attacks based on Shor's...

We report on a highly stable laser system at a wavelength of 436 nm for the coherent excitation of the transition in mercury based on efficient second harmonic generation in a potassium niobate crystal. The system delivers up to 220 mW of blue light with a peak-to-peak stability of 2.3%. This stability is achieved by setting the whole build-up cavi...

Using the sensitive Radiation Detected Resonance Ionization Spectroscopy (RADRIS) technique an optical transition in neutral nobelium (No, Z = 102) was identified. A remnant signal when delaying the ionizing laser indicated the influence of a strong buffer gas induced de-excitation of the optically populated level. A subsequent investigation of the...

Recently, the technique of `spatially modulated emission' has been introduced which improves the signal-to-noise ratio (SNR) for detecting biological cells in the field of flow cytometry. Based on this concept, we developed two advanced signal processing methods which further enhance the SNR and selectivity for cell detection. The improvements are...

We report on a frequency stabilized laser system with a variable linewidth at a wavelength of 404.7 nm used as an incoherent repump on the 63 P0 ↔73 S1 transition in mer- cury. By directly modulating the laser diode current with Gaussian white noise, the laser linewidth can be broadened up to 68 MHz. A Doppler-free dichroic atomic vapor laser lock...

We report on an experiment that was conducted in preparation of laser cooling experiments at the heavy-ion storage ring CSRe. The lifetimes of ion beams made up of ¹²C³⁺ and ¹⁶O⁴⁺ ions stored at an energy of 122 MeV/u in the CSRe were determined by two independent methods, firstly via a DC current transformer (DCCT) and secondly via a Schottky reso...

We investigate the glassy dynamics of glycerol in the confinement of a microemulsion system, which is stable on cooling down to the glass transition of its components. By changing the composition, we vary the viscosity of the matrix, while keeping the confining geometry intact, as is demonstrated by small angle X-ray scattering. By means of ²H NMR,...

In this contribution we discuss the use of optics in remote sensing. The first remote sensing applications were passive in nature, i.e., used the light reflected or scattered from objects, etc. With the advent of the laser a perfect light source for active remote sensing became available; specifically, the light output of a laser has a high intensi...

Optical spectroscopy of a primordial isotope has traditionally formed the basis for understanding the atomic structure of an element. Such studies have been conducted for most elements and theoretical modelling can be performed to high precision, taking into account relativistic effects that scale approximately as the square of the atomic number. H...

We report on a solid-state-based laser system at 1028 nm. The light is generated by a diode laser seeded ytterbium fiber amplifier. In two build-up cavities, its frequency is doubled and quadrupled to 514 nm and 257 nm, respectively. At 514 nm, the system delivers up to 4.7 W of optical power. In the fourth harmonic, up to 173 mW are available limi...

The experimental determination of atomic levels and the first ionization potential of the heaviest elements ( ) is key to challenge theoretical predictions and to reveal changes in the atomic shell structure. These elements are only artificially produced in complete-fusion evaporation reactions at on-line facilities such as the GSI in Darmstadt at...

Narrowband cw radiation at 940 and 960 nm is used to seed a Nd:YAG-pumped regenerative Ti:Sapphire amplifier. A Pockels cell traps a slice of the seed radiation inside the amplifier cavity, generating two synchronized pulses and releasing them after amplification. The total pulse energy is as high as 9 mJ at a pulse duration of 6.5 ns and a Fourier...

Dieses Buchprojekt ist aus einem Autorenwettbewerb im Rahmen der Veranstaltungsreihe „Saturday Morning Physics“ an der TU Darmstadt hervorgegangen. Die ursprüngliche Vorlesungsreihe über aktuelle Fragestellungen der Physik richtet sich in erster Linie an interessierte Schülerinnen und Schüler der Oberstufe und bringt ihnen auf interessante und schü...

Laser cooling is a powerful technique to reduce the longitudinal momentum spread of stored relativistic ion beams. Based on successful experiments at the experimental storage ring at GSI in Darmstadt, of which we show some important results in this paper, we present our plans for laser cooling of relativistic ion beams in the future heavy-ion synch...

We present a fiber amplifier based laser at 1028 nm frequency converted to 257 nm. A pulse expansion currently set up for the system is also presented. The systems are used in cooling of highly relativitsic ion beams at storage rings.

We have performed a test run for laser cooling experiments with ??C?? ion beams at an energy of 122 MeV/u at the CSRe with a pulsed laser. During this beamtime a lot of progress have been made. This was the first time we could successfully separate the ??C?? ions and ??O?? ions in the Schottky spectrum with the help of electron cooling. And our new...

We present a method of frequency stabilizing a broadband etalon that can serve as a high-precision wavelength calibrator for an echelle spectrograph. Using a laser to probe the Doppler-free saturated absorption of the rubidium D2 line, we stabilize one etalon transmission peak directly to the rubidium frequency. The rubidium transition is an establ...

Flow cytometry is a well-established diagnostic tool for cell counting and characterization. It utilizes fluorescence and scattered excitation light simultaneously emitted from cells passing an excitation laser focus to discriminate various cell types and estimate cell size. Here, we apply the principle of spatially modulated emission (SME) to fluo...

Currently, we are implementing a lasing without inversion (LWI) scheme in mercury based on calculations by Fry et al. [1]. A recent detailed analysis pr