arXiv subjects

Christian Kurtsiefer

Publications and source records attributed to Christian Kurtsiefer.

At least 55 records · Page 3Linked to original sources

arXiv2016

Characterization of very narrow spectral lines with temporal intensity interferometry

Context: Some stellar objects exhibit very narrow spectral lines in the visible range additional to their blackbody radiation. Natural lasing has been suggested as a mechanism to explain narrow lines in Wolf-Rayet stars. However, the spectral resolution of conventional astronomical spectrographs is still about two orders of magnitude too low to test this hypothesis. Aims: We want to resolve the linewidth of narrow spectral emissions in starlight. Methods: A combination of spectral filtering with single-photon-level temporal correlation measurements breaks the resolution limit of wavelength-dispersing spectrographs by moving the linewidth measurement into the time domain. Results: We demonstrate in a laboratory experiment that temporal intensity interferometry can determine a 20 MHz wide linewidth of Doppler-broadened laser light, and identify a coherent laser light contribution in a blackbody radiation background.

astro-ph.IM↗

arXiv2016

Time-resolved Scattering of a Single Photon by a Single Atom

Scattering of light by matter has been studied extensively in the past. Yet, the most fundamental process, the scattering of a single photon by a single atom, is largely unexplored [1-3]. One prominent prediction of quantum optics is the deterministic absorption of a traveling photon by a single atom, provided the photon waveform matches spatially and temporally the time-reversed version of a spontaneously emitted photon [4-12]. Here, we experimentally address this prediction and investigate the influence of the temporal profile of the photon on the scattering dynamics using a single trapped atom and heralded single photons. In a time-resolved measurement of the atomic excitation we find a 56(11)% increase of the peak excitation by photons with an exponentially rising profile compared to a decaying one. This result demonstrates that tailoring the envelope of single photons enables precise control of the photon-atom interaction.

quant-ph↗

arXiv2016

Random numbers from vacuum fluctuations

We implement a quantum random number generator based on a balanced homodyne measurement of vacuum fluctuations of the electromagnetic field. The digitized signal is directly processed with a fast randomness extraction scheme based on a linear feedback shift register. The random bit stream is continuously read in a computer at a rate of about 480 Mbit/s and passes an extended test suite for random numbers.

quant-ph↗

arXiv2015

Optical Intensity Interferometry through Atmospheric Turbulence

Conventional ground-based astronomical observations suffer from image distortion due to atmospheric turbulence. This can be minimized by choosing suitable geographic locations or adaptive optical techniques, and avoided altogether by using orbital platforms outside the atmosphere. One of the promises of optical intensity interferometry is its independence from atmospherically induced phase fluctuations. By performing narrowband spectral filtering on sunlight and conducting temporal intensity interferometry using actively quenched avalanche photon detectors (APDs), the Solar $g^{(2)}(τ)$ signature was directly measured. We observe an averaged photon bunching signal of $g^{(2)}(τ) = 1.693 \pm 0.003$ from the Sun, consistently throughout the day despite fluctuating weather conditions, cloud cover and elevation angle. This demonstrates the robustness of the intensity interferometry technique against atmospheric turbulence and opto-mechanical instabilities, and the feasibility to implement measurement schemes with both large baselines and long integration times.

astro-ph.IM↗

arXiv2015

Approaching Tsirelson's bound in a photon pair experiment

Quantum theory introduces a cut between the observer and the observed system, but does not provide a definition of what is an observer. Based on an informational definition of observer, Grinbaum has recently predicted an upper bound on bipartite correlations in the Clauser-Horne-Shimony-Holt (CHSH) Bell scenario equal to 2.82537, which is slightly smaller than the Tsirelson bound of standard quantum theory, but is consistent with all the available experimental results. Not being able to exceed Grinbaum's limit would support that quantum theory is only an effective description of a more fundamental theory and would have a deep impact in physics and quantum information processing. Here we present a test of the CHSH Bell inequality on photon pairs in maximally entangled states of polarization in which a value 2.82759+-0.00051 is observed, violating Grinbaum's bound by 4.3 standard deviations and providing the smallest distance with respect to Tsirelson's bound ever reported, namely, 0.00084+-0.00051. This sets a new lower experimental bound for Tsirelson's bound, strengthening the value of principles that predict Tsirelson's bound as possible explanations of all natural limits of correlations, and has important consequences for cryptographic security, randomness certification, characterization of physical properties in device-independent scenarios, and certification of quantum computation.

quant-ph↗

arXiv2015

Polarization entanglement and quantum beats of photon pairs from four-wave mixing in a cold Rb-87 ensemble

We characterize correlations in polarization and time of photon pairs generated from a cold cloud of Rb-87 atoms via a four-wave mixing process in a cascade level scheme. Quantum state tomography reveals entangled polarization states of high purity for each of the decay paths through two different intermediate hyperfine levels. When allowing both decay paths, we observe quantum beats in time-resolved correlation measurements.

quant-ph↗

arXiv2015

Probing quantum-classical boundary with compression software

We experimentally demonstrate that it is impossible to simulate quantum bipartite correlations with a deterministic universal Turing machine. Our approach is based on the Normalized Information Distance (NID) that allows the comparison of two pieces of data without detailed knowledge about their origin. Using NID, we derive an inequality for output of two local deterministic universal Turing machines with correlated inputs. This inequality is violated by correlations generated by a maximally entangled polarization state of two photons. The violation is shown using a freely available lossless compression program. The presented technique may allow to complement the common statistical interpretation of quantum physics by an algorithmic one.

quant-ph↗

arXiv2015

Hong-Ou-Mandel interference between triggered and heralded single photons from separate atomic systems

We present Hong-Ou-Mandel interference of single photons generated via two different physical processes by two independent atomic systems: scattering by a single atom, and parametric generation via four-wave mixing in a cloud of cold atoms. Without any spectral filtering, we observe a visibility of V=62$\pm$4%. After correcting for accidental coincidences, we obtain V=93$\pm$6%. The observed interference demonstrates the compatibility of the two sources, forming the basis for an efficient quantum interface between different physical systems.

quant-ph↗

arXiv2014

Reversing the temporal envelope of a heralded single photon using a cavity

We demonstrate a way to prepare single photons with a temporal envelope that resembles the time reversal of photons from the spontaneous decay process. We use the photon pairs generated from a time-ordered atomic cascade decay: the detection of the first photon of the cascade is used as a herald for the ground-state transition resonant second photon. We show how the interaction of the heralding photon with an asymmetric Fabry-Perot cavity reverses the temporal shape of its twin photon from a decaying to a rising exponential envelope. This single photon is expected to be ideal for interacting with two level systems.

quant-ph↗

arXiv2014

Experimental implementation of bit commitment in the noisy-storage model

Fundamental primitives such as bit commitment and oblivious transfer serve as building blocks for many other two-party protocols. Hence, the secure implementation of such primitives are important in modern cryptography. In this work, we present a bit commitment protocol which is secure as long as the attacker's quantum memory device is imperfect. The latter assumption is known as the noisy-storage model. We experimentally executed this protocol by performing measurements on polarization-entangled photon pairs. Our work includes a full security analysis, accounting for all experimental error rates and finite size effects. This demonstrates the feasibility of two-party protocols in this model using real-world quantum devices. Finally, we provide a general analysis of our bit commitment protocol for a range of experimental parameters.

quant-ph↗

arXiv2014

Diffraction-limited Fabry-Perot Cavity in the Near Concentric Regime

Nearly concentric optical cavities can be used to prepare optical fields with a very small mode volume. We implement an anaclastic design of a such a cavity that significantly simplifies mode matching to the fundamental cavity mode. The cavity is shown to have diffraction-limited performance for a mode volume of $\approx10^4λ^3$. This is in sharp contrast with the behavior of cavities with plano-concave mirrors, where aberrations significantly increase the losses in the fundamental mode. We estimate the related cavity QED parameters and show that the proposed cavity design allows for strong coupling without a need for high finesse or small physical cavity volume.

quant-ph↗

arXiv2014

Measuring Temporal Photon Bunching in Blackbody Radiation

Light from thermal black body radiators such as stars exhibits photon bunching behaviour at sufficiently short timescales. However, with available detector bandwidths, this bunching signal is difficult to be directly used for intensity interferometry with sufficient statistics in astronomy. Here we present an experimental technique to increase the photon bunching signal in blackbody radiation via spectral filtering of the light source. Our measurements reveal strong temporal photon bunching in light from blackbody radiation, including the Sun. Such filtering techniques may revive the interest in intensity interferometry as a tool in astronomy.

quant-ph↗

arXiv2014

Counterintuitive temporal shape of single photons

We prepare heralded single photons from a photon pair source based on non-degenerate four-wave mixing in a cold atomic ensemble via a cascade decay scheme. Their statistics shows strong antibunching with g(2)(0) < 0.03, indicating a near single photon character. In an optical homodyne experiment, we directly measure the temporal envelope of these photons and find, depending on the heralding scheme, an exponentially decaying or rising profile. The rising envelope will be useful for efficient interaction between single photons and microscopic systems like single atoms and molecules. At the same time, their observation illustrates the breakdown of a realistic interpretation of the heralding process in terms of defining an initial condition of a physical system.

quant-ph↗

arXiv2013

A universal setup for active control of a single-photon detector

The influence of bright light on a single-photon detector has been described in a number of recent publications. The impact on quantum key distribution (QKD) is important, and several hacking experiments have been tailored to fully control single-photon detectors. Special attention has been given to avoid introducing further errors into a QKD system. We describe the design and technical details of an apparatus which allows to attack a quantum-cryptographic connection. This device is capable of controlling free-space and fiber-based systems and of minimizing unwanted clicks in the system. With different control diagrams, we are able to achieve a different level of control. The control was initially targeted to the systems using BB84 protocol, with polarization encoding and basis switching using beamsplitters, but could be extended to other types of systems. We further outline how to characterize the quality of active control of single-photon detectors.

quant-ph↗

arXiv2013

Narrow Band Source of Transform-Limited Photon Pairs via Four-Wave Mixing in a Cold Atomic Ensemble

We observe narrowband pairs of time-correlated photons of wavelengths 776\,nm and 795\,nm from non-degenerate four-wave mixing in a laser-cooled atomic ensemble of $^{87}${Rb} using a cascade decay scheme. Coupling the photon pairs into single mode fibers, we observe an instantaneous rate of 7700 pairs per second with silicon avalanche photodetectors, and an optical bandwidth below 30\,MHz. Detection events exhibit a strong correlation in time ($g^{(2)}(τ=0)\approx5800$), and a high coupling efficiency indicated by a pair-to-single ratio of 23%. The violation of the Cauchy-Schwarz inequality by a factor of $8.4\times10^6$ indicates a strong non-classical correlation between the generated fields, while a Hanbury--Brown--Twiss experiment in the individual photons reveals their thermal nature. The narrow bandwidth and brightness of our source makes it ideal for interacting with atomic ensembles in quantum communication protocols.

quant-ph↗

arXiv2013

Excitation of a single atom with exponentially rising light pulses

We investigate the interaction between a single atom and optical pulses in a coherent state with a controlled temporal envelope. In a comparison between a rising exponential and a square envelope, we show that the rising exponential envelope leads to a higher excitation probability for fixed low average photon numbers, in accordance to a time-reversed Weisskopf-Wigner model. We characterize the atomic transition dynamics for a wide range of the average photon numbers, and are able to saturate the optical transition of a single atom with ~50 photons in a pulse by a strong focusing technique. For photon numbers of ~1000 in a 15ns long pulse, we clearly observe Rabi oscillations.

quant-ph↗

arXiv2012

The black paper of quantum cryptography: real implementation problems

The laws of physics play a crucial role in the security of quantum key distribution (QKD). This fact has often been misunderstood as if the security of QKD would be based only on the laws of physics. As the experts know well, things are more subtle. We review the progresses in practical QKD focusing on (I) the elements of trust that are common to classical and quantum implementations of key distribution; and (II) some threats to security that have been highlighted recently, none of which is unredeemable (i.e., in principle QKD can be made secure). This leads us to guess that the field, similar to non-quantum modern cryptography, is going to split in two directions: those who pursue practical devices may have to moderate their security claims; those who pursue ultimate security may have to suspend their claims of usefulness.

quant-ph↗

arXiv2012

Preparation of an Exponentially Rising Optical Pulse for Efficient Excitation of Single Atoms in Free Space

We report on a simple method to prepare optical pulses with exponentially rising envelope on the time scale of a few ns. The scheme is based on the exponential transfer function of a fast transistor, which generates an exponentially rising envelope that is transferred first on a radio frequency carrier, and then on a coherent cw laser beam with an electro-optical phase modulator (EOM). The temporally shaped sideband is then extracted with an optical resonator and can be used to efficiently excite a single Rb-87 atom.

quant-ph↗