Katharina Predehl

A 920-Kilometer Optical Fiber Link for Frequency Metrology at the 19th Decimal Place

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 not particularly mobile and are housed in national metrology labs. Predehl et al. (p. 441; see the Perspective by Warrington) linked two optical clocks separated by over 900 kilometers using optical fiber to show that the clocks can be synchronized, with the clocks showing a frequency stability better than 3.7 × 10−19. Such long-distance synchronization should allow for tests of fundamental physics, such as general relativity and quantum electrodynamics. A long-distance fiber network is used to synchronize two optical clocks with high precision. Optical clocks show unprecedented accuracy, surpassing that of previously available clock systems by more than one order of magnitude. Precise intercomparisons will enable a variety of experiments, including tests of fundamental quantum physics and cosmology and applications in geodesy and navigation. Well-established, satellite-based techniques for microwave dissemination are not adequate to compare optical clocks. Here, we present phase-stabilized distribution of an optical frequency over 920 kilometers of telecommunication fiber. We used two antiparallel fiber links to determine their fractional frequency instability (modified Allan deviation) to 5 × 10−15 in a 1-second integration time, reaching 10−18 in less than 1000 seconds. For long integration times τ, the deviation from the expected frequency value has been constrained to within 4 × 10−19. The link may serve as part of a Europe-wide optical frequency dissemination network.

Improved Measurement of the Hydrogen 1S - 2S Transition Frequency

We have measured the 1S-2S transition frequency in atomic hydrogen via two-photon spectroscopy on a 5.8 K atomic beam. We obtain f(1S-2S) = 2,466,061,413,187,035 (10)  Hz for the hyperfine centroid, in agreement with, but 3.3 times better than the previous result [M. Fischer et al., Phys. Rev. Lett. 92, 230802 (2004)]. The improvement to a fractional frequency uncertainty of 4.2 × 10(-15) arises mainly from an improved stability of the spectroscopy laser, and a better determination of the main systematic uncertainties, namely, the second order Doppler and ac and dc Stark shifts. The probe laser frequency was phase coherently linked to the mobile cesium fountain clock FOM via a frequency comb.

Optical frequency transfer via 146 km fiber link with 10 -19 relative accuracy.

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 3.8 × 10^-14 at 1 s reaching 4 × 10^-18 after 10⁴ s and an accuracy of 3.6 × 10^-19.

Precision Spectroscopy of Atomic Hydrogen

Precise determinations of transition frequencies of simple atomic systems are required for a number of fundamental applications such as tests of quantum electrodynamics (QED), the determination of fundamental constants and nuclear charge radii. The sharpest transition in atomic hydrogen occurs between the metastable 2S state and the 1S ground state. Its transition frequency has now been measured with almost 15 digits accuracy using an optical frequency comb and a cesium atomic clock as a reference [1]. A recent measurement of the 2S ? 2P3/2 transition frequency in muonic hydrogen is in significant contradiction to the hydrogen data if QED calculations are assumed to be correct [2, 3]. We hope to contribute to this so-called proton size puzzle by providing additional experimental input from hydrogen spectroscopy.

Long-distance remote comparison of ultrastable optical frequencies with 10 −15 instability in fractions of a second

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 phase-stabilized fiber in a commercial telecommunication network. The remote characterization spans more than one optical octave and reaches a fractional frequency instability between the independent ultrastable laser systems of 3 x 10 (-15) in 0.1 s. The achieved performance at 100 ms represents an improvement by one order of magnitude to any previously reported remote comparison of optical frequencies and enables future remote dissemination of the stability of 100 mHz linewidth lasers within seconds.

Telecommunication fiber link for the remote characterization of a magnesium optical frequency standard

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 (λ = 657 nm) is transferred with a femtosecond frequency comb to a telecommunication laser at λ = 1542 nm. The signal is then transmitted from PTB through the fiber link to IQ. A second comb at IQ (the remote end) is used to compare the transmitted laser frequency with that of the Mg clock laser λ = 914 nm. The frequency ratio of the clock lasers νMg/νCa shows a relative instability < 10-14 at 1 s. The upper limit for the contribution of the fiber link to the frequency instability is measured by connecting another optical fiber following the same 73 km route at Hanover computer center. The comparison performed at PTB between the local and the transmitted signal after a round trip length of 146 km showed a relative uncertainty below 1 x 10-19 and a short term instability σy(τ)= 3.3 x 10-15 / (τ/s).

Optical frequency transfer via 1840 km fiber link with superior stability

We investigated the transfer of a 194 THz optical frequency along a 1840 km fiber link. The fractional frequency instability expressed as the modified Allan deviation is 2.7 × 10^-15 at 1 s with 4 × 10^-19 after 100 s. The transferred frequency shows no systematic offset within an uncertainty of 3 × 10^-19. Analyzing the underlying noise structure reveals a τ^-2 response in the modified Allan deviation, which is confirmed by calculations of the frequency instability from frequency counter data as well as from phase noise data.

Optical Frequency Transfer via 1840 km Fiber Link with superior Stability

We transferred an optical frequency along a 1840km fiber link and achieved an instability of 3x10^-15 at 1s with 4x10^-19 after 100s. The transferred frequency shows no systematic offset within an uncertainty of 3x10^-19. Detailed analysis revealed a t^-2 response in the modified Allan deviation.

Optical frequency transfer over a single-span 1840 km fiber link

Optical frequency transfer via a 1840 km fiber link has been investigated. Twenty fiber amplifiers and two fiber Brillouin amplifiers are needed to compensate for 420 dB of loss. Active noise compensation reduces the instability to 2.7 × 10−15 at 1 s with 4 × 10−19 after 100 s and an accuracy of a few parts in 1019.

Frequency dissemination at the 19th decimal place

Optical frequency transmission via phase-stabilized telecommunication fiber is currently the most precise technique available for disseminating frequencies. We summarize some key advances, results and applications for fiber networks spanning up to 1000 km.