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The second: Optical atomic clocks

Optical atomic clocks are the next generation of atomic clocks. Currently, they are still at the development stage. In the case of today's atomic clocks, the clock frequency lies in the microwave range, and cesium atoms are usually used as a reference. In the case of optical clocks, the clock frequency is more than 10 000 times higher (100 THz - 1000 THz) and lies, thus, in the optical spectral range. Due to this fact, their accuracy – which can be achieved after clearly shorter averaging times – is approx. a 100 times higher. There are different variants, with different reference atoms or reference ions, for which different technologies have been used and which, therefore, also have different advantages and disadvantages. However, no special type has so far established itself. But the race to find the best clock of the future has already started. At PTB, several possible variants are being investigated.

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A “light highway” for highprecision frequencies

The German part of the link uses commercially rented optical fibres and facilities of the German National Research and Education Network (DFN). The French part of the link uses the Network for Education and Research, RENATER, which is operated by the GIP RENATER. Approximately midway, signals from LNE-SYRTE and PTB meet at the IT Centre of the University of Strasbourg, so that the clocks of the two institutes can be compared there. (Fig.: PTB)

In the past few years, optical atomic clocks have made spectacular progress. They have become 100 times more precise than the best cesium clocks. So far, their precision has been available only locally, since frequency transfer via satellite cannot provide sufficient resolution. This has now changed thanks to a novel 1,400 km optical fiber link between Braunschweig and Paris. This link allows...

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1400 km of optical fibre connect optical clocks in France and Germany

Glasfaser-Uhrenvergleich

In the past few years, optical atomic clocks have made spectacular progress, becoming 100 times more precise than the best caesium clocks. So far, their precision has been available only locally, since frequency transfer via satellite cannot provide sufficient resolution. This has recently changed thanks to a new direct optical connection between France and Germany, established by joint work of...

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World record for two optical clocks

Schematic representation: Measuring the influence of thermal ambient radiation on the frequency of the trapped ion in the ytterbium clock. The “clock laser” (blue beam) excites the trapped ion (yellow) with a special pulse sequence. The resonance frequency of the ion is shifted by infrared radiation (here by an infrared laser, red beam). This can be measured by means of the clock laser.

With two clocks that are currently the best worldwide in terms of accuracy and stability, respectively, PTB is well-prepared for future tasks in fields such as fundamental physics where such clocks are necessary to detect possible changes in fundamental constants.

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Spectroscopy of single trapped molecules

(a) Conceptual set-up of the experiment with MgH<sup>+</sup>(orange) and Mg<sup>+</sup> (green) in a linear ion trap. The ionic crystal is cooled to the ground state via Mg+. An oscillating dipole force changes the motion state, depending on the rotation state of MgH<sup>+</sup>. This excitation is read out via Mg<sup>+</sup>. (b) Typical detection signal in which a quantum jump into the (J = 1) rotation state (transition from the red to the blue area) of the molecule and out of it (from blue to red) can be seen.

The QUEST Institute at PTB has, for the first time, succeeded in proving the quantum state of trapped and indirectly laser-cooled molecular ions without destroying the molecule itself or its internal state. In this way, quantum jumps which were induced by thermal environmental radiation could be observed directly in a single molecule, and a new form of spectroscopy could be demonstrated. The new...

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Involvet Working Groups

Optische Strontium-Atomuhren

In einer optischen Strontium-Atomuhr wird ein Übergang zwischen zwei Energieniveaus im Strontium-Atom als Referenz verwendet. Dazu werden Strontium-Atome im Interferenzmuster zweier Laserstrahlen festgehalten. Entsprechend dem Streifenmuster der Interferenz sind sie dadurch Gitter-artig angeordnet. Deshalb werden optische Atomuhren dieser Art auch als Strontium-Gitteruhren bezeichnet. Mit der höheren Genauigkeit werden auch neue Anwendungsfelder zugänglich. So wird auch eine transportable Strontium-Atomuhr entwickelt, die z.B. für geodätische Untersuchungen eingesetzt werden kann.

Opens internal link in current windowArbeitsgruppe 4.32: Optische Gitteruhren

Frequency Dissemination with Fibres

Optical clock comparisons over large distances require the transmission of the signal of optical clocks beyond the confines of the laboratory of origin. The network of optical telecom fibres offers unique opportunities for such frequency dissemination. Using links based on telecom fibres, frequency dissemination with an accuracy outperforming traditional means has been demonstrated in recent years. These links enable comparing the world's best optical clocks without any noticeable impact of the link between them. The working group "Frequency Dissemination with Fibres" not only strives to continue improving the performance of the links but also pursues application of the technology for example to relativistic geodesy.

Opens internal link in current windowWorking Group 4.34: Frequency Dissemination with Fibres

Optische Ytterbium-Ionenuhren

Eine optische Ytterbium-Ionenuhr basiert auf einem einzelnen Ion, das im elektrischen Feld einer Paulfalle gefangen ist. Als Referenz dient ein Übergang zwischen zwei Energieniveaus des Ytterbium-Ions. Damit die Übergangsfrequenz nicht durch das elektrische Feld der Falle gestört wird, ist diese gerade so konzipiert, dass im Zentrum der Falle das elektrische Feld gleich null ist. Genau dort befindet sich das Ion.

Opens internal link in current windowArbeitsgruppe 4.43: Optische Uhren mit gespeicherten Ionen

Optische Aluminium-Ionenuhren

Bei optischen Aluminium-Ionenuhren wird ein Übergang zwischen zwei Energieniveaus eines Aluminium-Ions als Referenz verwendet. Das Aluminium Ionen wird dazu im elektrischen Feld einer linearen Paulfalle gehalten. Zum Auslesen der Übergangsfrequenz wird ein zweites Ion verwendet, das in der gleichen Falle gefangen ist. Mittels sogenannter Quantenlogik-Spektroskopie kann die Übergangsfrequenz hochgenau ausgelesen werden.

QUEST-Institut: Quantum Logic Spectroscopy

Optische Multi-Ionenuhren

Während bei Atomuhren basierend auf neutralen Atomen stets über viele Atome gemittelt wird, steht in Ionenuhren üblicherweise lediglich ein einzelnes Ion zur Verfügung. Deshalb ist eine längere Mittelungszeit erforderlich. Demgegenüber haben sie allerdings den Vorteil, dass sie weniger anfällig für Störungen von außen sind. In der optischen Multi-Ionenuhr sollen beide Vorteile kombiniert werden, indem mehrere Ionen in der gleichen linearen Paul-Falle gehalten werden. Als Referenz dient ein Übergang zwischen zwei Energieniveaus des Indium-Ions.

QUEST-Institut: Multi-Ion Clocks