Duration: 24 months

Ultracold strontium (Sr) is a prime candidate for the redefinition of the second [1] and is proposed as a free-flying test mass in infrasound gravitational wave antennas [2]. So far most optical atomic clocks and atom interferometers work in pulsed mode, suffering from measurement dead time during ultracold Sr preparation [3]. Continuous measurements and feedback would enable optical clocks to reach the desired precision much faster (proportional to averaging time instead of the square root of it), overcome the Dick effect, and provide a higher measurement bandwidth. This relaxes the stability requirements for the local oscillator, an ultrastable optical resonator, which otherwise can become more complicated than the ultracold atom package.

We will build a compact, continuous ultracold strontium source and demonstrate its benefits for a zero-deadtime optical clock, following four steps.

1) Development of a Sr desorption source, which overcomes the need for a bulky and power-hungry Sr oven and Zeeman slower. Sr atoms will be desorbed from samples of Sr oxide, nitride or metal by laser ablation and directly loaded into a magneto-optical trap.

2) Development of a compact version (<0.5m) of our ultracold Sr source [4, picture], either based on 1) or, as risk mitigation, a compact oven and Zeeman slower.

3) The frequency of optical clocks is shifted by black-body radiation, making it necessary to temperature stabilize the environment of the interrogated atoms. We will develop a compact black-body shield that is temperature stabilized by Peltier coolers.

4) Demonstration of 1-3) in a zero-deadtime clock, in this project with one interrogation zone. Ultimately four zones will be alternately interrogated with zero dead time. One pair will be interrogated quickly (<1s) in order to prestabilize the local oscillator. This enables seconds long interrogation of the other pair, leading to high precision.