arXiv subjects

Joachim von Zanthier

Publications and source records attributed to Joachim von Zanthier.

At least 19 recordsLinked to original sources

arXiv2026

Sub 35-ps Single-Photon Stellar Intensity Interferometry of Vega

SII enables measurements of stellar photospheres by correlating intensity fluctuations, offering a robust alternative to amplitude-based techniques. With the advent of modern single-photon counting detectors, this method has re-emerged as a powerful technique in stellar interferometry exploiting their ultra-high timing resolution. We determine the stellar diameter of Vega at 405 nm using spatial intensity correlations measured at the C2PU facility, part of the Observatoire de la Cote d'Azur. We employed a setup identical to Leopold et al. (2025). The two campaigns in April and August 2024 used hybrid photodetectors (HPDs; 22.7 ps resolution) and new PhotonPix detectors (34.8 ps resolution). A Photonscore TDC integrated photon arrival times and IRIG-B timestamps for synchronization. Our analysis consisted of calculating auto- and cross-correlation histograms from the recorded data, which were then normalized and corrected for optical path differences. The temporal auto-correlations yield a consistent coherence time of 0.21 ps. This confirms the effectiveness of the PhotonPix detectors despite a slight degradation in timing resolution relative to the HPDs. Combining the data from the two measurement campaigns yields a uniform disk diameter of Vega of 2.97 +/- 0.23 mas. This agrees with the 3.047 mas diameter predicted by the stellar atmosphere model at our observation wavelength, validating our technique and the new PhotonPix detectors for precision SII at sub-35 ps resolution. The integration of advanced single-photon detectors like the PhotonPix into existing IACT SII instruments promises substantial enhancements in signal-to-noise ratio and observational capabilities, particularly with CTA LSTs. Although challenges such as night sky background saturation and precise positioning and guiding remain, SII observations at apparent magnitudes up to 7.8 are within reach.

astro-ph.IM↗

arXiv2026

Real-time measurement error mitigation for one-way quantum computation

We propose a quantum error mitigation scheme for single-qubit measurement errors, particularly suited for one-way quantum computation. Contrary to well established error mitigation methods for circuit-based quantum computation that require the circuits to be run several times, our method is capable of mitigating measurement errors in real-time, during the processing measurements of the one-way computation. For that, an ancillary qubit register is entangled with the to-be-measured qubit and additionally measured afterwards. By using a voting protocol on all measurement outcomes, occurring measurement errors can be mitigated in real-time while the one-way computation continues. We provide an analytical expression for the probability to detect a measurement error depending on the error rate and the number of ancilla qubits. From this, we derive an estimate of the ancilla register size for a given measurement error rate and a required success probability to detect a measurement error. Additionally, we also consider the CNOT gate error in our mitigation method and investigate how this influences the probability to detect a measurement error. Finally, we show in proof-of-principle simulations and in experiments executed on IBM quantum hardware that our method is capable of reducing the measurement errors significantly in a one-way quantum computation with only a small number of ancilla qubits.

quant-ph↗

arXiv2026

Quantum Signatures of Two-Electron HBT Interference in Free Space

Understanding how fermionic exchange and Coulomb repulsion jointly shape two-electron correlations is essential for identifying genuine quantum signatures in multi-electron interference experiments. To address this interplay, we investigate Hanbury Brown and Twiss interference of two electrons generated by two independent needle-tip emitters within a full quantum-mechanical framework. In the absence of Coulomb interaction, the approach reproduces the results previously obtained within a quantum path formalism. For Coulomb-interacting electrons, we predict characteristic features absent in a semiclassical description: a pronounced Coulomb-dominated suppression region as well as Coulomb-induced phase offsets and fringe shifts. At the same time, outside of the Coulomb-dominated region, the spatial oscillation frequency is essentially governed by fermionic exchange symmetry. Our results establish quantitative parameter regimes for disentangling Coulomb interaction from fermionic exchange symmetry in such experiments.

quant-ph↗

arXiv2026

Deviations from thermal light statistics in ensembles of independent two-level emitters

We investigate the light statistics of an ensemble of independent motionless two-level atoms in a product state. We identify the conditions under which the cold atomic ensemble emits thermal light statistics characterized by the Gaussian Moment Theorem. For the theorem to hold, we derive for each correlation order two conditions on the atom number and the ratio of coherent to incoherent light emission. We further discuss their validity for atoms either in a pure or mixed state. Our results contribute to the understanding of the generation of thermal light by two-level atoms without interactions among the emitters.

quant-ph↗

arXiv2025

Single-particle incoherent diffractive imaging and amplified spontaneous emission in copper nanocubes

We demonstrate element-specific incoherent diffractive imaging (IDI) of single copper nanocubes using intensity correlations of K$α$ fluorescence at a hard X-ray free-electron laser. Combining single particle diffraction classification with IDI, we retrieve the form factor of 88 nm cubes with 20 nm resolution, extending IDI to the destructive single-particle regime with a large gain in resolution. IDI visibility drops sharply above a fluence of $10^2$ J/cm$^2$, consistent with the assumption of amplified spontaneous emission. Our results reveal fundamental limits for high-fluence nanoimaging towards future single-particle X-ray imaging.

physics.optics↗

arXiv2025

Nonlinear phenomena in X-ray fluorescence from single nanoparticles under extreme conditions

Materials exposed to intense femtosecond X-ray pulses with energies above their K-shell absorption edge can enter an extremely ionized state, which could give rise to nonlinear phenomena, such as saturable absorption and reverse saturable absorption. In this work, we investigate these effects on single copper nanoparticles irradiated by an X-ray free-electron laser pulse. We study the properties of the K$α$ fluorescence for two different short pulse durations and three X-ray incident energies below and above the K-shell absorption edge, and correlate these with incident fluence estimates based on coherent diffraction. We observe that the incident fluence of the pulse and not its duration, is the main factor that modulates the nonlinear response, which leads to an effective shortening of the fluorescence emission. Our findings have implications for fluorescence-based methods for imaging single particles using transiently coherent fluorescence, or diffractive imaging through transient resonances.

physics.optics↗

arXiv2025

Spin-selective coherent light scattering from ion crystals

We study coherent light scattering from linear crystals with up to twelve $^{40}\text{Ca}^+$ ions, acting as single photon emitters. Light-scattering is induced by two-photon laser excitation, starting from the S$_{1/2}\rightarrow$ D$_{5/2}$ quadrupole transition at 729~nm followed by the D$_{5/2}\rightarrow$ P$_{3/2}$ dipole transition at 854~nm, from where the ions decay back to the S$_{1/2}$ ground state via emission of a photon near 393~nm. We realize spin-selective excitation from the Zeeman-split ground states S$_{1/2}$, m$= \pm 1/2$, of the $\text{Ca}^+$ ions and observe in the far field spin-dependent interference patterns displaying the spin textures of the ion crystals. We investigate their dynamics by measuring the temporal evolution of the spatial Fourier frequencies of the observed patterns.

physics.atom-ph↗

arXiv2025

Surpassing the wave-particle duality relation via feed-forward of phase information

Complementarity constitutes a central aspect of quantum theory. It manifests itself, for example, in a two-way interferometer, where the simultaneous observation of an interference pattern and the acquisition of which-way information are limited by an inequality known as the duality relation. Here, we investigate which-way information in a double-slit interferometer and show that it can be correlated to the phase of the quantum object at the detection screen, leading to a phase-dependent which-way knowledge. In specific cases, this knowledge can locally exceed the limit set by the duality relation. Based on this observation, we propose a feed-forward protocol that aims at maximizing the which-way information locally for each phase after the particle has been recorded on the screen. This allows us to surpass the duality relation limit even globally. We present analytical results as a proof of principle of our protocol as well as numerical outcomes quantifying the amount of maximally achievable which-way knowledge.

quant-ph↗

arXiv2025

On-sky demonstration of an ultra-fast intensity interferometry instrument utilizing hybrid single photon counting detectors

Intensity interferometry is a reemerging astronomical technique for performing high angular resolution studies at visible wavelengths, benefiting immensely from the recent improvements in (single) photon detection instrumentation. We developed an ultra-fast, single photon counting and highly stable intensity interferometry instrument for 1 m class optical telescopes. The instrument records on sky the expected stellar photon rates and reaches the temporal coherence times as measured in the laboratory. In addition, all components, especially the photon detection hardware, of the instrument are easily upgradeable with custom hardware currently being developed. The collimated telescope output is spectrally filtered via an ultra narrow band pass of 2 nm at a central wavelength of 405 nm. We use hybrid photon detectors (HPDs) for single photon detection and a constant fraction discriminator (CFD) for signal conditioning. A time to digital converter (TDC) is used for time stamping. The combination of HPDs and CFDs is optimized for large active area and high timing resolution. We successfully measured photon bunching of three bright A-type stars - Vega, Altair and Deneb at the 1.04 m Omicron telescope of C2PU at the Calern Observatory in the south of France. In all cases the observed coherence time fits well to both the pre-calculated expectations as well as the values measured in preceding laboratory tests. We obtained the previously estimated photon count rates at the telescope and achieved highly stable coupling of the star light to the detectors. Utilizing a new class of large area single photon detectors based on multichannel plate amplification, high resolution spatial intensity interferometry experiments are within reach at 1 m diameter class telescopes within one night of observation time for bright stars.

astro-ph.IM↗

arXiv2025

Multi-Path and Multi-Particle Tests of Complex vs. Hyper-Complex Quantum Theory

The axioms of quantum mechanics provide limited information regarding the structure of the Hilbert space, such as the underlying number system. The latter is generally regarded as complex, but generalizations of complex numbers, so-called hyper-complex numbers, cannot be ruled out in theory. Therefore, specialized experiments to test for hyper-complex quantum mechanics are needed. To date, experimental tests are limited to single-particle interference exploiting a closed phase relation in a three-path interferometer called the Peres test. The latter distinguishes complex quantum mechanics from quaternionic quantum mechanics. Here, we propose a general matrix formalism putting the Peres test on a solid mathematical ground. On this basis, we introduce multi-path and multi-particle interference tests, which provide a direct probe for any dimension of the number system of quantum mechanics.

quant-ph↗

arXiv2025

Hanbury Brown and Twiss interference of electrons in free space from independent needle tip sources

We investigate two-electron interference in free space using two laser-triggered needle tips as independent electron sources, a fermionic realisation of the landmark Hanbury Brown and Twiss interferometer. We calculate the two-electron interference pattern in a quantum path formalism taking into account the fermionic nature and the spin configuration of the electrons. We also estimate the Coulomb repulsion in the setup in a semiclassical approach. We find that antibunching resulting from Pauli's exclusion principle and repulsion stemming from the Coulomb interaction can be clearly distinguished.

quant-ph↗

arXiv2024

TEMPUS, a Timepix4-based system for event-based X-rays detection

A readout system based on the Timepix4 ASIC, is being developed for photon science. The TEMPUS detector can be operated in two distinct modes: a photon counting mode, which allows for conventional full-frame readout at rates up to 40 kfps; and an event-driven time-stamping mode, which allows excellent time resolution in the nanosecond regime in measurements with moderate X-ray flux. In this paper, we introduce the initial prototype, a single-chip system, and present the first results obtained at PETRA III and ESRF.

physics.ins-det↗

arXiv2023

Studying the phase diagram of the three-flavor Schwinger model in the presence of a chemical potential with measurement- and gate-based quantum computing

We propose an ansatz quantum circuit for the variational quantum eigensolver (VQE), suitable for exploring the phase structure of the multi-flavor Schwinger model in the presence of a chemical potential. Our ansatz is capable of incorporating relevant model symmetries via constrains on the parameters, and can be implemented on circuit-based as well as measurement-based quantum devices. We show via classical simulation of the VQE that our ansatz is able to capture the phase structure of the model, and can approximate the ground state to a high level of accuracy. Moreover, we perform proof-of-principle simulations on superconducting, gate-based quantum hardware. Our results show that our approach is suitable for current gate-based quantum devices, and can be readily implemented on measurement-based quantum devices once available.

hep-lat↗

arXiv2023

Photon counting intensity interferometry in the blue at a 0.5 m telescope

Intensity interferometry is a re-emerging interferometry tool that alleviates some of the challenges of amplitude interferometry at the cost of reduced sensitivity. We demonstrate the feasibility of intensity interferometry with fast single photon counting detectors at small telescopes by utilising a telescope of diameter of merely $0.5$\,m. The entire measurement setup, including collimation, optical filtering, and two single photon detectors, is attached directly to the telescope without the use of optical fibres, facilitated by the large area of our single photon detectors. For digitisation and timing, we utilise a Time-To-Amplitude-Converter. Observing $α$ Lyrae (Vega) for a total exposure time of $32.4$\,h over the course of six nights, an auto-correlation signal with a contrast of $(9.5 \pm 2.7) \times 10^-3$ and a coherence time of $(0.34 \pm 0.12)$ ps at a SNR of 2.8 is measured. The result fits well to preceding laboratory tests as well as expectations calculated from the optical and electronic characteristics of our measurement setup. This measurement, to our knowledge, constitutes the first time that a bunching signal with starlight was measured in the B band with single photon counting detectors. Simultaneously, this is to date the stellar intensity interferometry measurement utilising the smallest telescope. Our successful measurement shows that intensity interferometry can be adopted not only at large scale facilities, but also at readily available and inexpensive smaller telescopes.

astro-ph.IM↗

arXiv2022

Exploring the phase structure of the multi-flavor Schwinger model with quantum computing

We propose a variational quantum eigensolver suitable for exploring the phase structure of the multi-flavor Schwinger model in the presence of a chemical potential. The parametric ansatz circuit we design is capable of incorporating the symmetries of the model, present in certain parameter regimes, which allows for reducing the number of variational parameters substantially. Moreover, the ansatz circuit can be implementated on both measurement-based and circuit-based quantum hardware. We numerically demonstrate that our ansatz circuit is able to capture the phase structure of the model and allows for faithfully approximating the ground state. Our results show that our approach is suitable for current intermediate-scale quantum hardware and can be readily implemented on existing quantum devices.

quant-ph↗

arXiv2022

Engineering of spontaneous emission in free space via conditional measurements

We study the collective spontaneous emission of three identical two-level atoms initially prepared in the excited states by measuring Glauber's third-order photon correlation function. Assuming two atoms at sub-wavelength distance from each other such that they are subject to the dipole-dipole interaction while the third one is located several wavelengths away, we observe super- and subradiant decay alike, depending on the direction of observation. Unlike the case where no remote atom is introduced or no conditional measurements are performed, the spontaneous emission behavior of the conditioned three-atom system differs strongly from the single-atom and the canonical two-atom configuration. The conditional measurements associated with the three-photon correlation function in combination with the dipole-dipole interaction between the adjacent atoms lead to quantum interference among the different decay channels allowing to engineer the spontaneous emission in space and time.

quant-ph↗

arXiv2022

Collective photon emission patterns from two atoms in free space

Modification of spontaneous decay in space and time is a central topic of quantum physics. It has been predominantly investigated in the context of cavity quantum electrodynamics (QED), gaining new interest recently in the domain of nano-optics. Beyond cavity-QED, spontaneous emission may be modified also in free space due to correlations among the photon emitters, a phenomenon known as super- and sub-radiance. Correlations may stem either from direct interactions between the particles, from long-range exchange of photons, or by measuring single photons in a common mode. Yet, the genuine spatial spontaneous emission pattern of an atomic ensemble in an entangled quantum state has not been observed so far, due to the lack of ultra-fast cameras with high spatial resolution suited for recording single photons from single atoms. Preparing two trapped ions in free space in entangled Dicke states via photon detection, we study the resulting collective spontaneous emission patterns. Depending on the symmetry of the Dicke states, associated with the direction of detection of the first state-determining photon, we observe fundamentally different emission patterns for the subsequently scattered photon, including super- and sub-radiance. Our results demonstrate that the detection of a single photon can profoundly modify the collective emission of an atomic array, here represented by its most elementary building block of two atoms in free space.

quant-ph↗

arXiv2022

Comparing Different Approaches for Stellar Intensity Interferometry

Stellar intensity interferometers correlate photons within their coherence time and could overcome the baseline limitations of existing amplitude interferometers. Intensity interferometers do not rely on phase coherence of the optical elements and thus function without high grade optics and light combining delay lines. However, the coherence time of starlight observed with realistic optical filter bandwidths (> 0.1 nm) is usually much smaller than the time resolution of the detection system (> 10 ps), resulting in a greatly reduced correlation signal. Reaching high signal to noise in a reasonably short measurement time can be achieved in different ways: either by increasing the time resolution, which increases the correlation signal height, or by increasing the photon rate, which decreases statistical uncertainties of the measurement. We present laboratory measurements employing both approaches and directly compare them in terms of signal to noise ratio. A high time-resolution interferometry setup designed for small to intermediate size optical telescopes and thus lower photon rates (diameters < some meters) is compared to a setup capable of measuring high photon rates, which is planned to be installed at Cherenkov telescopes with dish diameters of > 10 m. We use a Xenon lamp as a common light source simulating starlight. Both setups measure the expected correlation signal and work at the expected shot-noise limit of statistical uncertainties for measurement times between 10 min and 23 h. We discuss the quantitative differences in the measurement results and give an overview of suitable operation regimes for each of the interferometer concepts.

astro-ph.IM↗