Thomas Legero

Thomas Legero
Physikalisch-Technische Bundesanstalt | PTB · Department 4.3 Quantum Optics and Unit of Length

About

61
Publications
13,023
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3,986
Citations
Citations since 2017
12 Research Items
2425 Citations
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Introduction
Thomas Legero currently works at the Department 4.3 Quantum Optics and Unit of Length, Physikalisch-Technische Bundesanstalt. Thomas does research in Atomic, Molecular and Optical Physics and Optics. Their current project is 'Cryogenic Silicon Resonators'.

Publications

Publications (61)
Article
Full-text available
Ultrastable lasers are essential tools in optical frequency metrology enabling unprecedented measurement precision that impacts on fields such as atomic timekeeping, tests of fundamental physics, and geodesy. To characterise an ultrastable laser it needs to be compared with a laser of similar performance, but a suitable system may not be available...
Preprint
Full-text available
Precise wavelength calibration is a critical issue for high-resolution spectroscopic observations. The ideal calibration source should be able to provide a very stable and dense grid of evenly distributed spectral lines of constant intensity. A new method which satisfies all mentioned conditions has been developed by our group. The approach is to a...
Preprint
Full-text available
We demonstrate a time scale based on a phase stable optical carrier that accumulates an estimated time error of $48\pm94$ ps over 34 days of operation. This all-optical time scale is formed with a cryogenic silicon cavity exhibiting improved long-term stability and an accurate $^{87}$Sr lattice clock. We show that this new time scale architecture o...
Article
Full-text available
We report on a laser locked to a silicon cavity operating continuously at 4 K with 1×10−16 instability and a median linewidth of 17 mHz at 1542 nm. This is a tenfold improvement in short-term instability, and a 104 improvement in linewidth, over previous sub-10-K systems. Operating at low temperatures reduces the thermal noise floor and, thus, is a...
Article
Full-text available
We report on a laser locked to a silicon cavity operating continuously at 4 K with $1 \times 10^{-16}$ instability and a median linewidth of 17 mHz at 1542 nm. This is a ten-fold improvement in short-term instability, and a $10^4$ improvement in linewidth, over previous sub-10 K systems. Operating at low temperatures reduces the thermal noise floor...
Article
Full-text available
We report on two ultrastable lasers each stabilized to independent silicon Fabry-Pérot cavities operated at 124 K. The fractional frequency instability of each laser is completely determined by the fundamental thermal Brownian noise of the mirror coatings with a flicker noise floor of 4×10−17 for integration times between 0.8 s and a few tens of se...
Article
Full-text available
We report on two ultrastable lasers each stabilized to independent silicon Fabry-P\'erot cavities operated at 124 K. The fractional frequency instability of each laser is completely determined by the fundamental thermal Brownian noise of the mirror coatings with a flicker noise floor of $4 \times 10^{-17}$ for integration times between 0.8 s and a...
Conference Paper
The tremendous progress with optical atomic clocks asks for super-stable lasers to interrogate the best clock transitions with only a few mHz line width. Previously, we developed a cryogenic silicon Fabry-Perot resonator that demonstrated short-term instability below 1 χ 10 −16 . We have set up two independent updated systems with the goal of reach...
Article
Full-text available
We have set up an improved vertically mounted silicon cavity operating at the zero-crossing temperature of the coefficient of thermal expansion (CTE) near 123 K with estimated thermal noise limited instability of 4 x 10-17 in the modified Allan deviation. Owing to the anisotropic elasticity of single-crystal silicon, the vertical acceleration sensi...
Article
Full-text available
Leveraging the unrivaled performance of optical clocks in applications in fundamental physics beyond the standard model, in geo-sciences, and in astronomy requires comparing the frequency of distant optical clocks truthfully. Meeting this requirement, we report on the first comparison and agreement of fully independent optical clocks separated by 7...
Article
We present a laser system based on a 48 cm long optical glass resonator. The large size requires a sophisticated thermal control and optimized mounting design. A self balancing mounting was essential to reliably reach sensitivities to acceleration of below $\Delta \nu / \nu$ < 2E-10 /g in all directions. Furthermore, fiber noise cancellations from...
Article
Full-text available
We present a compact and robust transportable ultra-stable laser system with minimum fractional frequency instability of $1\times10^{-15}$ at integration times between 1 to 10 s. The system was conceived as a prototype of a subsystem of a microwave-optical local oscillator to be used on the satellite mission STE-QUEST (Space-Time Explorer and QUant...
Article
Full-text available
Cryogenic single-crystal optical cavities have the potential to provide high dimensional stability. We have investigated the long-term performance of an ultrastable laser system that is stabilized to a single-crystal silicon cavity operated at 124 K. Utilizing a frequency comb, the laser is compared to a hydrogen maser that is referenced to a prima...
Article
Full-text available
Cryogenic single-crystal optical cavities have the potential to provide highest dimensional stability. We have investigated the long-term performance of an ultra-stable laser system which is stabilized to a single-crystal silicon cavity operated at 124 K. Utilizing a frequency comb, the laser is compared to a hydrogen maser that is referenced to a...
Conference Paper
Full-text available
Thermal noise fundamentally limits the frequency stability of optical resonators used in optical clocks or to generate ultrapure microwaves by optical frequency division. We will give an overview of the relevant noise mechanisms, their calculation and discuss different current developments to reduce the noise. In one approach a cryogenic silicon re...
Article
Full-text available
Active control and cancellation of residual amplitude modulation (RAM) in phase modulation of an optical carrier is one of the key technologies for achieving the ultimate stability of a laser locked to an ultrastable optical cavity. Furthermore, such techniques are versatile tools in various frequency modulation-based spectroscopy applications. In...
Article
Full-text available
We have measured the frequency of the extremely narrow 1S-2S two-photon transition in atomic hydrogen using a remote cesium fountain clock with the help of a 920 km stabilized optical fiber. With an improved detection method we obtain f1S-2S=2466 061 413 187 018 (11) Hz with a relative uncertainty of 4.5×10-15, confirming our previous measurement o...
Article
Thermal noise in optical cavities imposes a severe limitation in the stability of the most advanced frequency standards at a level of a few 10^(-16) (s/t)^(1/2) for long averaging times t. In this paper we describe two schemes for reducing the effect of thermal noise in a reference cavity. In the first approach, we investigate the potential and lim...
Article
We report on the detection of single, slowly moving Rubidium atoms using laser-induced fluorescence. The atoms move at 3 m/s while they are detected with a time resolution of 60 [mu]s. The detection scheme employs a near-resonant laser beam that drives a cycling atomic transition, and a highly efficient mirror setup to focus a large fraction of the...
Article
Full-text available
We investigate the photon statistics of a single-photon source that operates under non-stationary conditions. The photons are emitted by shining a periodic sequence of laser pulses on single atoms falling randomly through a high-finesse optical cavity. Strong antibunching is found in the intensity correlation of the emitted light, demonstrating tha...
Article
Full-text available
Optical frequency transfer via a 920 km fiber link has been investigated. Active noise compensation enables the transfer of a stable optical frequency with a stability of 4x10e-14 at 1 s reaching 4x10e-18 after 10^4 s and an accuracy of 3.6e-19.
Article
Full-text available
Synchronize Watches Time standards based on the energy-level transitions of atoms and ions provide the most accurate and precise methods of time keeping. Measurements made in one laboratory and in another must be done with clocks that have been synchronized and calibrated to ensure that the same measurement is being made. Such clocks, however, are...
Conference Paper
We describe a system that combines a cavity-stabilized laser, a hydrogen maser and a cesium fountain clock. It provides an optical reference frequency that over the course of half a day drifts with less than 100 μHz/s. Furthermore it allows for a convenient measurement of an absolute frequency in the optical domain.
Article
Distributing a stable, absolute optical reference frequency via fiber network would serve research and development in academia and industry. Lasers stabilized to high-finesse Fabry–Pérot cavities can achieve fractional frequency instabilities of less than 10−15 for periods up to several seconds. Their instabilities increase for longer averaging tim...
Conference Paper
We reference high-precision spectroscopy on atomic hydrogen measured with an uncertainty of 4×10-15 to a remote Cs-fountain clock using a 920 km actively noise-compensated fiber link.
Article
Full-text available
State-of-the-art optical oscillators based on lasers frequency stabilized to high finesse optical cavities are limited by thermal noise that causes fluctuations of the cavity length. Thermal noise represents a fundamental limit to the stability of an optical interferometer and plays a key role in modern optical metrology. We demonstrate a novel des...
Article
Thermal noise of optical reference cavities sets a fundamental limit to the frequency instability of ultra-stable lasers. Using Levin's formulation of the fluctuation-dissipation theorem we correct the analytical estimate for the spacer contribution given by Numata et al.. For detailed analysis finite-element calculations of the thermal noise focus...
Conference Paper
We present the setup of a cryogenic single-crystal silicon optical cavity with a potential thermal noise floor below 10-16. The system comprises a vibration-free cryostat based on gaseous nitrogen as coolant.
Article
We present the setup and test of a transportable clock laser at 698 nm for a strontium lattice clock. A master-slave diode laser system is stabilized to a rigidly mounted optical reference cavity. The setup was transported by truck over 400 km from Braunschweig to D\"usseldorf, where the cavity-stabilized laser was compared to a stationary clock la...
Article
Full-text available
We demonstrate a fully optical, long-distance remote comparison of independent ultrastable optical frequencies reaching a short term stability that is superior to any reported remote comparison of optical frequencies. We use two ultrastable lasers, which are separated by a geographical distance of more than 50 km, and compare them via a 73 km long...
Article
Full-text available
We presented the first prototype of transportable laser-cooled strontium source. Novel design solutions have been discussed with a final volume, mass and power consumption budget for the atomic-package of 210 liters, 120 kg and 110 W, respectively. Most of the home made components have been assembled and tested. The system is currently under charac...
Article
We investigate the thermal expansion of low thermal noise Fabry-Perot cavities made of Low Thermal Expansion (LTE) glass spacers and Fused Silica (FS) mirrors. The different thermal expansion of mirror and spacer deforms the mirror. This deformation strongly contributes to the cavity's effective Coefficient of Thermal Expansion (CTE), decreasing th...
Article
Full-text available
We have explored the performance of 2 "dark fibers" of a commercial telecommunication fiber link for a remote comparison of optical clocks. These fibers establish a network in Germany that will eventually link optical frequency standards at PTB with those at the Institute of Quantum Optics (IQ) at the Leibniz University of Hanover, and the Max Plan...
Article
Full-text available
We have explored the performance of two “dark fibers” of a commercial telecommunication fiber link for a remote comparison of optical clocks. The two fibers, linking the Leibniz University of Hanover (LUH) with the Physikalisch-Technische Bundesanstalt (PTB) in Braunschweig, are connected in Hanover to form a total fiber length of 146 km. At PTB th...
Article
Full-text available
Optical clocks largely rely on interrogation lasers with sub-Hz linewidth and low short term instability. The laser stability is mostly determined by the properties of the cavities that are used as short term references. With suitable mounting the influence of vibrations is strongly suppressed and the short term stability is limited by thermal fluc...
Article
Full-text available
We have characterized the 24Mg optical frequency standard at the Institute of Quantum Optics (IQ), Hanover, using a clock laser at the Physikalisch-Technische Bundesanstalt (PTB), Braunschweig, via a noise compensated 73 km fiber link and present preliminary results for the stability of the Mg standard. The stability of the clock laser (lambda = 65...
Article
Full-text available
We report on the setup and characterization of a 698-nm master-slave diode laser system to probe the <sup>1</sup>S<sub>0</sub>-<sup>3</sup>P<sub>0</sub> clock transition of strontium atoms confined in a 1-D optical lattice. A linewidth on the order of about 100 Hz of the laser system has been measured with respect to an ultrastable 657-nm diode las...
Article
We report the observation and interpretation of collision-induced perturbations in a 88Sr lattice clock. Losses are observed in the collision channels 1S0+3P0 and 3P0+3P0. Furthermore, we observe broadening and shift of the clock transition by collisions.
Conference Paper
We report on the setup and characterization of a 698 nm master-slave diode laser system to probe the <sup>1</sup>S<sub>0</sub>-<sup>3</sup>P<sub>0</sub> clock transition of strontium atoms confined in a 1D optical lattice. The instability and the line-width of the laser system have been measured with respect to an ultrastable 657 nm diode laser wit...
Conference Paper
Full-text available
This paper reports results of cooling and trapping of <sup>88</sup>Sr for future use in an optical lattice clock. Strontium atoms are cooled to ultra-cold temperatures using a two-stage cooling process. In the first stage, atoms are captured from a Zeeman-slowed atomic beam and cooled to 2 mK in a magneto optical trap operated on the <sup>1</sup>S<...
Article
Full-text available
Due to their orders-of-magnitude higher frequencies, optical frequency standards are beginning to outper-form the best microwave standards with respect to their stability and accuracy and, hence, offer very promising prospects for novel space-based applications. We report on recent results for optical standards in PTB based on neutral atoms and dis...
Article
Adiabatic passage techniques allow the generation of single photons which are very long compared to the typical detector time resolution. Therefore the detection time of a photon can be measured within the duration of the single-photon wavepacket. As a consequence, two-photon interference can be investigated in a time-resolved manner, i.e., the coi...
Article
The interference of two single photons impinging on a beam splitter is measured in a time-resolved manner. Using long photons of different frequencies emitted from an atom-cavity system, a quantum beat with a visibility close to 100% is observed in the correlation between the photodetections at the output ports of the beam splitter. The time depend...
Article
The interference of two independent single-photon pulses impinging on a beam splitter is analysed in a generalised time-resolved manner. Different aspects of the phenomenon are elaborated using different representations of the single-photon wave packets, like the decomposition into single-frequency field modes or spatio-temporal modes matching the...
Article
Deterministic single-photon source is used to investigate the feasibility of processing quantum information with linear optics. This scheme requires indistinguishable single-photon wave packets, which give rise to second-order interference phenomena.
Article
The paper reports on the experimental demonstration of elementary quantum-network links and nodes with narrowband and transform-limited single-photon pulses that cannot be distinguished from one another. The photon source is realized in form of a strongly coupled atom-cavity system and it employs a controlled energy exchange between the atom and th...
Article
We review some recent work performed with single moving atoms strongly coupled to high-finesse optical cavities, emphasizing cavity-mediated light forces and vacuum-stimulated generation of single photons.
Article
Full-text available
Vacuum-stimulated Raman scattering in strongly coupled atom-cavity systems allows one to generate free-running single photon pulses on demand. Most properties of the emitted photons are well defined, provided spontaneous emission processes do not contribute. Therefore, electronic excitation of the atom must not occur, which is assured for a system...
Article
Full-text available
We report on the first observation of stimulated Raman scattering from a Lambda-type three-level atom, where the stimulation is realized by the vacuum field of a high-finesse optical cavity. The scheme produces one intracavity photon by means of an adiabatic passage technique based on a counterintuitive interaction sequence between pump laser and c...
Article
Summary form only given. Many attempts to realize elementary quantum-logic gates as well as the feasible schemes for quantum cryptography and quantum teleportation are based on the availability of a single photon in a well defined mode of the radiation field. However, most employed schemes used for photon generation rely on spontaneous processes or...
Article
Summary form only given. In quantum information processing, a qubit is realised as a superposition of atomic or ionic eigenstates, or as a superposition of the states of a photon. A coupling between these different types of qubits is necessary to perform quantum operations or quantum transmissions, e.g. in a quantum network where single atoms or io...
Article
The magneto-optic properties of multilayered MnBi/Al films have been investigated and compared to pure MnBi films. The Al interlayers act as diffusion barrier during the MnBi formation process leading to a reduced MnBi particle size of approximately 50nm. The smaller particle size changes the dominant magnetization reversal process from domain wall...
Article
In this thesis, the quantum interference of two single photons with individually tunable wavelength is studied. The photons are generated one-after-the-other using an adiabatic passage in an atom-cavity system. They travel through two different optical fibers and impinge simultaneously on a beam splitter. The photon duration is much longer than the...

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