title: Improvement of an Atomic Clock using Squeezed Vacuum.
abstract: Since the pioneering work of Ramsey, atom interferometers are employed for precision metrology, in particular to measure time and to realize the second. In a classical interferometer, an ensemble of atoms is prepared in one of the two input states, whereas the second one is left empty. In this case, the vacuum noise restricts the precision of the interferometer to the standard quantum limit (SQL). Here, we propose and experimentally demonstrate a novel clock configuration that surpasses the SQL by squeezing the vacuum in the empty input state. We create a squeezed vacuum state containing an average of 0.75 atoms to improve the clock sensitivity of 10000 atoms by 2.05_{-0.37}^{+0.34}  dB. The SQL poses a significant limitation for today's microwave fountain clocks, which serve as the main time reference. We evaluate the major technical limitations and challenges for devising a next generation of fountain clocks based on atomic squeezed vacuum.
authors.affiliation: Institut für Quantenoptik, Leibniz Universität Hannover, Welfengarten 1, D-30167 Hannover, Germany.
authors.forename: I
authors.initials: I
authors.lastname: Kruse
authors.affiliation: Institut für Quantenoptik, Leibniz Universität Hannover, Welfengarten 1, D-30167 Hannover, Germany.
authors.forename: K
authors.initials: K
authors.lastname: Lange
authors.affiliation: Institut für Quantenoptik, Leibniz Universität Hannover, Welfengarten 1, D-30167 Hannover, Germany.
authors.forename: J
authors.initials: J
authors.lastname: Peise
authors.affiliation: Institut für Quantenoptik, Leibniz Universität Hannover, Welfengarten 1, D-30167 Hannover, Germany.
authors.forename: B
authors.initials: B
authors.lastname: Lücke
authors.affiliation: QSTAR, INO-CNR and LENS, Largo Enrico Fermi 2, I-50125 Firenze, Italy.
authors.forename: L
authors.initials: L
authors.lastname: Pezzè
authors.affiliation: Institut for Fysik og Astronomi, Aarhus Universitet, Ny Munkegade 120, DK-8000 Århus C, Denmark.
authors.forename: J
authors.initials: J
authors.lastname: Arlt
authors.affiliation: Institut für Quantenoptik, Leibniz Universität Hannover, Welfengarten 1, D-30167 Hannover, Germany.
authors.forename: W
authors.initials: W
authors.lastname: Ertmer
authors.affiliation: Physikalisch-Technische Bundesanstalt, Bundesallee 100, D-38116 Braunschweig, Germany.
authors.forename: C
authors.initials: C
authors.lastname: Lisdat
authors.affiliation: Institut für Theoretische Physik, Leibniz Universität Hannover, Appelstraße 2, D-30167 Hannover, Germany.
authors.forename: L
authors.initials: L
authors.lastname: Santos
authors.affiliation: QSTAR, INO-CNR and LENS, Largo Enrico Fermi 2, I-50125 Firenze, Italy.
authors.forename: A
authors.initials: A
authors.lastname: Smerzi
authors.affiliation: Institut für Quantenoptik, Leibniz Universität Hannover, Welfengarten 1, D-30167 Hannover, Germany.
authors.forename: C
authors.initials: C
authors.lastname: Klempt