Alexander Friedrich

I am a physicist. My work sits where precision measurement meets fundamental physics. On the one side: light-pulse atom interferometers, optical atomic clocks, frequency metrology and quantum sensing – instruments that measure time, frequency and acceleration to a precision little else can match. On the other side: the question of what such instruments can tell us about gravity, in particular through tests of the equivalence principle, relativistic effects in matter-wave interferometers and quantum field theory in curved and non-inertial spacetimes.

In short, I like very good clocks and rulers, and I like to ask what they can measure that nothing else can.

In the following you will find my publications, a few projects, short technical notes and, on occasion, a blog post.

selected publications

Opt. Express

Molecular iodine at 508 nm: hyperfine spectroscopy and operating regimes for optical frequency references

M. Weller, K. Rostan, A. Friedrich, and 5 more authors

Optics Express, Aug 2026

DOI Bib HTML

@article{Weller2026,
  title = {{Molecular iodine at 508 nm: hyperfine spectroscopy and operating regimes for optical frequency references}},
  author = {Weller, M. and Rostan, K. and Friedrich, A. and Wüst, J. and Oswald, M. and Kuschewski, F. and Schuldt, T. and Braxmaier, C.},
  journal = {Optics Express},
  publisher = {Optica Publishing Group},
  volume = {34},
  number = {17},
  pages = {31567},
  month = aug,
  year = {2026},
  doi = {10.1364/oe.605121},
}

APPA

The Wave Functional of the Vacuum in a Resonator

A. Friedrich, D. Moll, M. Freyberger, and 2 more authors

Acta Phys. Pol. A, Jun 2023

DOI arXiv Bib HTML

@article{Friedrich2023,
  title = {{The Wave Functional of the Vacuum in a Resonator}},
  author = {Friedrich, A. and Moll, D. and Freyberger, M. and Plimak, L. and Schleich, W. P.},
  journal = {Acta Phys. Pol. A},
  volume = {143},
  number = {6},
  pages = {S52--S77},
  issn = {1898-794X},
  month = jun,
  year = {2023},
  doi = {10.12693/APhysPolA.143.S52},
}

PRX Quantum

Gravitational Redshift Tests with Atomic Clocks and Atom Interferometers

F. Di Pumpo, C. Ufrecht, A. Friedrich, and 3 more authors

PRX Quantum, Nov 2021

DOI arXiv Bib HTML

@article{DiPumpo2021,
  title = {{Gravitational Redshift Tests with Atomic Clocks and Atom Interferometers}},
  author = {Di Pumpo, F. and Ufrecht, C. and Friedrich, A. and Giese, E. and Schleich, W. P. and Unruh, W. G.},
  journal = {PRX Quantum},
  publisher = {American Physical Society},
  volume = {2},
  issue = {4},
  pages = {040333},
  numpages = {23},
  month = nov,
  year = {2021},
  doi = {10.1103/PRXQuantum.2.040333},
}

PRR

Atom-interferometric test of the universality of gravitational redshift and free fall

C. Ufrecht, F. Di Pumpo, A. Friedrich, and 6 more authors

Phys. Rev. Res., Nov 2020

DOI arXiv Bib HTML

@article{Ufrecht2020,
  title = {{Atom-interferometric test of the universality of gravitational redshift and free fall}},
  author = {Ufrecht, C. and Di Pumpo, F. and Friedrich, A. and Roura, A. and Schubert, C. and Schlippert, D. and Rasel, E. M. and Schleich, W. P. and Giese, E.},
  journal = {Phys. Rev. Res.},
  publisher = {American Physical Society},
  volume = {2},
  issue = {4},
  pages = {043240},
  numpages = {9},
  month = nov,
  year = {2020},
  doi = {10.1103/PhysRevResearch.2.043240},
}

Sci. Adv.

Interference of clocks: A quantum twin paradox

S. Loriani, A. Friedrich, C. Ufrecht, and 15 more authors

Sci. Adv., Oct 2019

DOI arXiv Bib HTML

@article{Loriani2019,
  title = {{Interference of clocks: A quantum twin paradox}},
  author = {Loriani, S. and Friedrich, A. and Ufrecht, C. and Di Pumpo, F. and Kleinert, S. and Abend, S. and Gaaloul, N. and Meiners, C. and Schubert, C. and Tell, D. and Wodey, {\'{E}}. and Zych, M. and Ertmer, W. and Roura, A. and Schlippert, D. and Schleich, W. P. and Rasel, E. M. and Giese, E.},
  journal = {Sci. Adv.},
  volume = {5},
  number = {10},
  pages = {eaax8966},
  issn = {2375-2548},
  month = oct,
  year = {2019},
  doi = {10.1126/sciadv.aax8966},
}