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U. Sterr

Publications and source records attributed to U. Sterr.

32 records · Page 2Linked to original sources
arXiv2011

Remote frequency measurement of the 1S0-3P1 transition in laser cooled Mg-24

We perform Ramsey-Bordé spectroscopy on laser-cooled magnesium atoms in free fall to measure the 1S0 \rightarrow 3P1 intercombination transition frequency. The measured value of 655 659 923 839 730 (48) Hz is consistent with our former atomic beam measurement (Friebe et al 2008 Phys. Rev. A 78 033830). We improve upon the fractional accuracy of the previous measurement by more than an order of magnitude to 7e-14. The magnesium frequency standard was referenced to a fountain clock of the Physikalisch-Technische Bundesanstalt (PTB) via a phase-stabilized telecom fiber link and its stability was characterized for interrogation times up to 8000 s. The high temperature of the atomic ensemble leads to a systematic shift due to the motion of atoms across the spectroscopy beams. In our regime, this leads to a counterintuitive reduction of residual Doppler shift with increasing resolution. Our theoretical model of the atom-light interaction is in agreement with the observed effect and allows us to quantify its contribution in the uncertainty budget.

physics.atom-ph↗
arXiv2011

The 87-Sr optical frequency standard at PTB

With 87-Sr atoms confined in a one dimensional optical lattice, the frequency of the optical clock transition 5s^2 ^1S_0 - 5s5p ^3P_0 has been determined to be 429 228 004 229 872.9(5) Hz. The transition frequency was measured with the help of a fs-frequency comb against one of PTB's H-masers whose frequency was measured simultaneously by the PTB Cs fountain clock CSF1. The Sr optical frequency standard contributes with a fractional uncertainty of 1.5 10^-16 to the total uncertainty. The agreement of the measured transition frequency with previous measurements at other institutes supports the status of this transition as secondary representation of the second with the currently smallest uncertainty.

physics.atom-ph↗
arXiv2010

Hyper-Ramsey Spectroscopy of Optical Clock Transitions

We present non-standard optical Ramsey schemes that use pulses individually tailored in duration, phase, and frequency to cancel spurious frequency shifts related to the excitation itself. In particular, the field shifts and their uncertainties of Ramsey fringes can be radically suppressed (by 2-4 orders of magnitude) in comparison with the usual Ramsey method (using two equal pulses) as well as with single-pulse Rabi spectroscopy. Atom interferometers and optical clocks based on two-photon transitions, heavily forbidden transitions, or magnetically induced spectroscopy could significantly benefit from this method. In the latter case these frequency shifts can be suppressed considerably below a fractional level of 10^{-17}. Moreover, our approach opens the door for the high-precision optical clocks based on direct frequency comb spectroscopy.

physics.atom-ph↗
arXiv2009

Compensation of Field-Induced Frequency Shifts in Ramsey Spectroscopy of Optical Clock Transitions

We have extended Ramsey spectroscopy by stepping the probe frequency during the two Ramsey excitation pulses to compensate frequency shifts induced by the excitation itself. This makes precision Ramsey spectroscopy applicable even for transitions that have Stark and Zeeman shifts comparable to the spectroscopic resolution. The method enables a new way to evaluate and compensate key frequency shifts, which benefits in particular, optical clocks based on magnetic field-induced, spectroscopy, two-photon transitions, or heavily forbidden transitions.

physics.optics↗
arXiv2009

Collisional losses, decoherence, and frequency shifts in optical lattice clocks with bosons

We have quantified collisional losses, decoherence and the collision shift in a one-dimensional optical lattice clock with bosonic 88Sr. The lattice clock is referenced to the highly forbidden transition 1S0 - 3P0 at 698 nm, which becomes weakly allowed due to state mixing in a homogeneous magnetic field. We were able to quantify three decoherence coefficients, which are due to dephasing collisions, inelastic collisions between atoms in the upper and lower clock state, and atoms in the upper clock state only. Based on the measured coefficients, we determine the operation parameters at which a 1D-lattice clock with 88Sr shows no degradation due to collisions on the relative accuracy level of 10-16.

physics.atom-ph↗
arXiv2009

Optical frequency transfer via 146 km fiber link with 10^{-19} relative accuracy

We demonstrate the long-distance transmission of an ultra-stable optical frequency derived directly from a state-of-the-art optical frequency standard. Using an active stabilization system we deliver the frequency via a 146 km long underground fiber link with a fractional instability of 3*10^{-15} at 1 s, which is close to the theoretical limit for our transfer experiment. The relative uncertainty for the transfer is below 1*10^{-19} after 30 000 seconds. Tests with a very short fiber show that noise in our stabilization system contributes fluctuations which are two orders of magnitude lower, namely 3*10^{-17} at 1 s, reaching 10^{-20} after 4000 s.

physics.optics↗
arXiv2008

Interrogation laser for a strontium lattice clock

We report on the setup and characterization of a 698 nm master-slave diode laser system to probe the 1S0-3P0 clock transition of strontium atoms confined in a one-dimensional optical lattice. A linewidth in the order of around 100 Hz of the laser system has been measured with respect to an ultrastable 657 nm diode laser with 1 Hz linewidth using a femtosecond fiber comb as transfer oscillator. The laser has been used to measure the magnetically induced 1S0-3P0 clock transition in 88Sr where a linewidth of 165 Hz has been observed. The transfer oscillator method provides a virtual beat signal between the two diode lasers that has been used to phase lock the 698 nm laser to the 1 Hz linewidth laser at 657 nm, transferring its stability to the 698 nm laser system.

physics.optics↗
arXiv2008

The stability of an optical clock laser transferred to the interrogation oscillator for a Cs fountain

We stabilise a microwave oscillator at 9.6 GHz to an optical clock laser at 344 THz by using a fibre-based femtosecond laser frequency comb as a transfer oscillator. With a second frequency comb we measure independently the instability of the microwave source with respect to another optical clock laser frequency at 456 THz. The total fractional frequency instability of this optic-to-microwave and microwave-to-optic conversion resulted in an Allan deviation sigma_y, of sigma_y=1.2E-14 at 1 s averaging time (band width 50 kHz). The residual phase noise density is -97 dBc/Hz at 10 Hz offset from the 9.6 GHz carrier. Replacing the existing quartz-based interrogation oscillator of the PTB caesium fountain CSF1 with this optically stabilised microwave source will reduce the instability contribution due to the Dick effect from the 1E-13-level at 1s averaging time to an insignificant level at the current status of CSF1. Therefore this new microwave source can be an alternative to cryogenic sapphire-loaded cavity oscillators in order to overcome the limitations of state-of-the-art quartz oscillators.

physics.atom-ph↗
arXiv2006

Optical Clocks in Space

The performance of optical clocks has strongly progressed in recent years, and accuracies and instabilities of 1 part in 10^18 are expected in the near future. The operation of optical clocks in space provides new scientific and technological opportunities. In particular, an earth-orbiting satellite containing an ensemble of optical clocks would allow a precision measurement of the gravitational redshift, navigation with improved precision, mapping of the earth's gravitational potential by relativistic geodesy, and comparisons between ground clocks.

gr-qc↗
arXiv2004

The optical calcium frequency standards of PTB and NIST

We describe the current status of the Ca optical frequency standards with laser-cooled neutral atoms realized in two different laboratories for the purpose of developing a possible future optical atomic clock. Frequency measurements performed at the Physikalisch-Technische Bundesanstalt (PTB) and the National Institute of Standards and Technology (NIST) make the frequency of the clock transition of 40Ca one of the best known optical frequencies (relative uncertainty 1.2e-14) and the measurements of this frequency in both laboratories agree to well within their respective uncertainties. Prospects for improvement by orders of magnitude in the relative uncertainty of the standard look feasible.

physics.atom-ph↗
arXiv2002

Photoassociation spectroscopy of cold calcium atoms

Photoassociation spectroscopy experiments on 40Ca atoms close to the dissociation limit 4s4s 1S0 - 4s4p 1P1 are presented. The vibronic spectrum was measured for detunings of the photoassociation laser ranging from 0.6 GHz to 68 GHz with respect to the atomic resonance. In contrast to previous measurements the rotational splitting of the vibrational lines was fully resolved. Full quantum mechanical numerical simulations of the photoassociation spectrum were performed which allowed us to put constraints on the possible range of the calcium scattering length to between 50 a_0 and 300 a_0.

physics.atom-ph↗
arXiv2002

An Optical Clock with Ultracold Neutral Atoms

We demonstrate how to realize an optical clock with neutral atoms that is competitive to the currently best single ion optical clocks in accuracy and superior in stability. Using ultracold atoms in a Ca optical frequency standard we show how to reduce the relative uncertainty to below 1E-15. We observed atom interferences for stabilization of the laser to the clock transition with a visibility of 0.36, which is 70 % of the ultimate limit achievable with atoms at rest. A novel scheme was applied to detect these atom interferences with the prospect to reach the quantum projection noise limit at an exceptional low instability of 4E-17 in 1 s.

physics.atom-ph↗
arXiv2001

Doppler cooling and trapping on forbidden transitions

Ultracold atoms at temperatures close to the recoil limit have been achieved by extending Doppler cooling to forbidden transitions. A cloud of ^40Ca atoms has been cooled and trapped to a temperature as low as 6 μK by operating a magneto-optical trap on the spin-forbidden intercombination transition. Quenching the long-lived excited state with an additional laser enhanced the scattering rate by a factor of 15, while a high selectivity in velocity was preserved. With this method more than 10% of pre-cooled atoms from a standard magneto-optical trap have been transferred to the ultracold trap. Monte-Carlo simulations of the cooling process are in good agreement with the experiments.

physics.atom-ph↗