arXiv subjects

Christian Kurtsiefer

Publications and source records attributed to Christian Kurtsiefer.

At least 19 recordsLinked to original sources

arXiv2026

Distinguishing thermal and pseudothermal light by testing the Siegert relation

Thermal light, including blackbody radiation and spontaneous emission, exhibits photon bunching. Thermal light sources, however, typically yield low spectral densities, limiting their practical utility. Pseudothermal light sources with higher brightness and longer coherence time are often employed instead. While pseudothermal light also exhibits photon bunching, this property may not suffice to fully replicate the behavior of genuine thermal light. Here we demonstrate a method to directly test the Siegert relation for two sources of photon-bunched light, laser light scattered from a rotating ground glass and spontaneously emitted light from a gas discharge lamp, probing a fundamental criterion expected of thermal light.

physics.optics↗

arXiv2026

Clock Synchronization with Weakly Correlated Photons

Clock synchronization is necessary for communication and distributed computing tasks. Previous schemes based on photon timing correlations use pulsed light or photon pairs for their strong timing correlations. In this work, we demonstrate successful synchronization of crystal clocks using weakly time-correlated photons of 180 ns coherence time from a bunched light source. A synchronization timing jitter of 10 ns is achieved over symmetric -102 dB optical channel loss between two parties, over a span of 25 hours. We also present a model that gives better estimates to the coherence peak finding success probabilities under low signal.

quant-ph↗

arXiv2026

Near-lossless method for generating thermal photon-bunched light

Thermal light sources exhibiting photon bunching have been suggested for sensing applications that exploit timing correlations of stationary light, including range finding, clock synchronization, and non-line-of-sight imaging. However, these proposals have remained unrealized in practice because available sources of photon bunching either possess coherence times too short to be timing resolved by photodetectors, or produce brightness levels too low to tolerate realistic return losses. In this work, we demonstrate a low-loss method for generating photon bunching with a conversion efficiency nearly 9 orders of magnitude higher than that achieved by many other bunching processes.

quant-ph↗

arXiv2026

Narrowband four-photon states from spontaneous four-wave mixing

We observe time-correlated four photons within a correlation window of 20ns from spontaneous four-wave mixing via a double-Lambda scheme in a cold cloud of Rb-87 atoms. In contrast to high-power pulsed pumping of chi^(2) nonlinear processes in crystals, our scheme generates correlated four-photon states by direct continuous-wave pumping at nominal powers. We verify the presence of genuinely correlated four-photon states over accidentals by higher-order intensity cross-correlation measurements and accidental subtraction. We infer a time-correlated four-photon generation rate of 2.5(4)x10^6 counts per second close to saturation. The photons produced are near-resonant with atomic transitions, and have a bandwidth in the order of MHz, making them readily compatible with quantum networking applications involving atoms.

quant-ph↗

arXiv2025

Inline Quantum Measurements with SNSPDs Coupled to Photonic Bound States

We report the realization of inline quantum measurements with waveguide-integrated superconducting nanowire single-photon detectors (SNSPDs). To suppress parasitic scattering at detector terminations, while ensuring compatibility with standard photonic substrates and cryogenic operation, we developed a photonic bound states in the continuum (BIC) platform based on etchless polymer waveguides. We show BIC-coupled inline detectors with on-chip efficiency exceeding 80%, recovery time of less than 2 ns, and intrinsic jitter of less than 70 ps. As a proof of principle, we implement Hanbury Brown and Twiss interferometry and photon number resolution with two collinear detectors within a footprint of $60 \cdot 6$ $μm^2$. The concept of inline quantum measurements could be further developed to support more complex circuit functionalities, such as higher-order correlation measurements, quantum state tomography, and multi-photon subtraction, within a compact architecture.

quant-ph↗

arXiv2023

Fano Resonance in Excitation Spectroscopy and Cooling of an Optically Trapped Single Atom

Electromagnetically induced transparency (EIT) can be used to cool an atom in a harmonic potential close to the ground state by addressing several vibrational modes simultaneously. Previous experimental efforts focus on trapped ions and neutral atoms in a standing wave trap. In this work, we demonstrate EIT cooling of an optically trapped single neutral atom, where the trap frequencies are an order of magnitude smaller than in an ion trap and a standing wave trap. We resolve the Fano resonance feature in fluorescence excitation spectra and the corresponding cooling profile in temperature measurements. A final temperature of around 6 $μ$K is achieved with EIT cooling, a factor of two lower than the previous value obtained using olarization gradient cooling.

quant-ph↗

arXiv2023

Low Noise Near-Concentric Optical Cavity Design

Near-concentric cavities are excellent tools for enhancing atom--light interaction as they combine a small mode volume with a large optical access for atom manipulation. However, they are sensitive to longitudinal and transverse misalignment. To address this sensitivity, we present a compact near-concentric optical cavity system with a residual cavity length variation $δL_{C, rms}$=36(9) pm. A key part of this system is a cage-like tensegrity mirror support structure that allows to correct for longitudinal and transverse misalignment. The system is stable enough to allow the use of mirrors with higher cavity finesse to enhance the atom--light coupling strength in cavity-QED applications.

quant-ph↗

arXiv2023

Direct measurement of coherent light proportion from a practical laser source

We present a technique to estimate the proportion of coherent emission in the light emitted by a practical laser source without spectral filtering. The technique is based on measuring interferometric photon correlations between the output ports of an asymmetric Mach-Zehnder interferometer. With this, we characterize the fraction of coherent emission in the light emitted by a laser diode when transiting through the lasing threshold.

quant-ph↗

arXiv2023

Practical Quantum Sensing with Thermal Light

Many quantum sensing suggestions rely on temporal correlations found in photon pairs generated by parametric down-conversion. In this work, we show that the temporal correlations in light with a thermal photon statistics can be equally useful for such applications. Using a sub-threshold laser diode as an ultrabright source of thermal light, we demonstrate optical range finding to a distance of up to 1.8 km.

quant-ph↗

arXiv2022

Observation of the Mollow Triplet from an optically confined single atom

Resonance fluorescence from atomic systems consists of a single spectral peak that evolves into a Mollow triplet for a strong excitation field. Photons from different peaks of the triplet show distinct photon correlation that make the fluorescence a useful light source for quantum information purpose. We characterize the fluorescence of a single optically trapped $^{87}$Rb atom that is excited resonantly at different power levels. Second-order correlation measurements reveal the single photon nature of the fluorescence concurrently with Rabi oscillations of a strongly excited atom. The asymmetry in correlations between photons from two sidebands of the fluorescence spectrum when the atom is exposed to an off-resonant field further indicates that there is a preferred time-ordering of the emitted photons from different sidebands.

quant-ph↗

arXiv2022

Countering detector manipulation attacks in quantum communication through detector self-testing

In practical quantum key distribution systems, imperfect physical devices open security loopholes that challenge the core promise of this technology. Apart from various side channels, a vulnerability of single-photon detectors to blinding attacks has been one of the biggest concerns, and has been addressed both by technical means as well as advanced protocols. In this work, we present a countermeasure against such attacks based on self-testing of detectors to confirm their intended operation without relying on specific aspects of their inner working, and to reveal any manipulation attempts. We experimentally demonstrate this countermeasure with a typical InGaAs avalanche photodetector, but the scheme can be easily implemented with any single photon detector.

quant-ph↗

arXiv2022

Distributing Polarization Entangled Photon Pairs with High Rate over Long Distance through Standard Telecommunication Fiber

Entanglement distribution over long distances is essential for many quantum communication schemes like quantum teleportation, some variants of quantum key distribution, or implementations of a quantum internet. Distributing entanglement through standard telecommunication fiber is particularly important for quantum key distribution protocols with low vulnerability over metropolitan distances. However, entanglement distribution over long distance through optical fiber so far could only be accomplished with moderate photon pair rates. In this work, we present entanglement distribution over 50km of standard telecommunication fiber with pair rate more than 10,000 s$^{-1}$ using a bright non-degenerate photon pair source. Signal and idler wavelengths of this source are optimized for low dispersion in optical fiber and high efficiency for single-photon avalanche diode detectors, respectively. The resulting modest hardware requirement and high rate of detected entangled photon pairs could significantly enhance practical entanglement-based quantum key distribution in existing metropolitan fiber networks.

quant-ph↗

arXiv2021

Fibre polarization state compensation in entanglement-based quantum key distribution

Quantum Key Distribution (QKD) using polarisation encoding can be hard to implement over deployed telecom fibres because the routing geometry and the birefringence of the fibre link can alter the polarisation states of the propagating photons. These alterations cause a basis mismatch, leading to an increased Quantum Bit Error Rate (QBER). In this work we demonstrate a technique for dynamically compensating fibre-induced state alteration in a QKD system over deployed fibre. This compensation scheme includes a feedback loop that minimizes the QBER using a stochastic optimization algorithm.

quant-ph↗

arXiv2020

Temperature insensitive type II quasi-phasematched spontaneous parametric downconversion

The temperature dependence of the refractive indices of potassium titanyl phosphate (KTP) are shown to enable quasi-phasematched type II spontaneous parametric downconversion (SPDC) with low temperature sensitivity. Calculations show the effect to be maximised for emission of photons at around 1165nm, as well as producing potentially useful regions for wavelengths throughout the telecommunications bands. We demonstrate the effect experimentally, observing temperature-insensitive degenerate emission at 1326nm, within the telecommunications O band. This result has practical applications in the development of entangled photon sources for resource-constrained environments, and we demonstrate a simple polarization entangled source as a proof of concept.

quant-ph↗

arXiv2020

Coupling Light to Higher Order Transverse Modes of a Near-Concentric Optical Cavity

Optical cavities in the near-concentric regime have near-degenerate transverse modes; the tight focusing transverse modes in this regime enable strong coupling with atoms. These features provide an interesting platform to explore multi-mode interaction between atoms and light. Here, we use a spatial light modulator (SLM) to shape the phase of an incoming light beam to match several Laguerre-Gaussian (LG) modes of a near-concentric optical cavity. We demonstrate coupling efficiency close to the theoretical prediction for single LG modes and well-defined combinations of them, limited mainly by imperfections in the cavity alignment.

physics.optics↗

arXiv2020

Absolute clock synchronization with a single time-correlated photon pair source over 10km

We demonstrate a point-to-point clock synchronization protocol based on bidirectionally propagating photons generated in a single spontaneous parametric down-conversion (SPDC) source. Tight timing correlations between photon pairs are used to determine the single and round-trip times measured by two separate clocks, providing sufficient information for distance-independent absolute synchronization secure against symmetric delay attacks. We show that the coincidence signature useful for determining the round-trip time of a synchronization channel, established using a 10\,km telecommunications fiber, can be derived from photons reflected off the end face of the fiber without additional optics. Our technique allows the synchronization of multiple clocks with a single reference clock co-located with the source, without requiring additional pair sources, in a client-server configuration suitable for synchronizing a network of clocks.

quant-ph↗

arXiv2020

A wide-range wavelength-tunable photon-pair source for characterizing single-photon detectors

The temporal response of single-photon detectors is usually obtained by measuring their impulse response to short-pulsed laser sources. In this work, we present an alternative approach using time-correlated photon pairs generated in spontaneous parametric down-conversion (SPDC). By measuring the cross-correlation between the detection times recorded with an unknown and a reference photodetector, the temporal response function of the unknown detector can be extracted. Changing the critical phase-matching conditions of the SPDC process provides a wavelength-tunable source of photon pairs. We demonstrate a continuous wavelength-tunability from 526 nm to 661 nm for one photon of the pair, and 1050 nm to 1760 nm for the other photon. The source allows, in principle, to access an even wider wavelength range by simply changing the pump laser of the SPDC-based source. As an initial demonstration, we characterize single photon avalance detectors sensitive to the two distinct wavelength bands, one based on Silicon, the other based on Indim Gallium Arsenide.

quant-ph↗

arXiv2020

Stable Polarization Entanglement based Quantum Key Distribution over Metropolitan Fibre Network

We demonstrate a quantum key distribution implementation over deployed dark telecom fibers with polarisation-entangled photons generated at the O-band. One of the photons in the pairs are propagated through 10km of deployed fiber while the others are detected locally. Polarisation drifts experienced by the photons propagating through the fibers are compensated with liquid crystal variable retarders. This ensures continuous and stable QKD operation with an average QBER of 6.4% and a final key rate of 109 bits/s.

quant-ph↗