Optical clocks are amazingly stable frequency standards, which would be off by around one second over the age of the universe. Bringing those clocks from the laboratory into a robust and compact form will have a large impact on telecommunication (e.g. network synchronization, traffic bandwidth, GPS free navigation), geology (e.g. underground exploration, monitoring of water tables or ice sheets), astronomy (e.g. low-frequency gravitational wave detection, radio telescope synchronization), and other fields. Likewise, techniques developed for robust clocks will improve laboratory clocks, potentially leading to physics beyond the standard model.
To make this a reality, we have founded the Quantum Flagship AQuRA consortium, assembling leading industry partners with experts from academia, national measurement labs and relevant end users. Our consortium represents a nucleus for a European optical clock ecosystem, which will continuously deliver competitive products and foster the development of clock applications. Our prototype will be a field-tested strontium optical lattice clock, which we will benchmark in some of the most demanding real use cases, in fields such as geodesy and radio astronomy. The development and manufacture of this clock will be led by industry with a path to a fully commercial product. We will leverage the foundational work by the consortia iqClock, QuantERA Q-Clocks and JRP f17 USOQS, which have joined partners with us, and translate their work into a higher TRL. To increase our impact and to broaden our industry base, we will reach out to all stakeholders, train the next generation of quantum engineers, educate and listen to end users, and enrich the exchange of scientific ideas.
Optical atomic clocks are the most precise scientific instruments available to humanity. Their accuracy and stability reach eighteen significant digits. A standard optical atomic clock consists of two state-of-the-art components: an ultra-stable high-Q optical cavity which transfers stability of the length into stability of the frequency, and an atomic sample which transfers accuracy of the energy of the ultra-precise atomic clock transition into accuracy of the frequency. These two frequencies are compared with the help of a frequency shifter (FS) and a feedback is added to the laser frequency.
Optical lattice clocks
The blue fluorescence from a magneto-optical trap of ~1 billion strontium atoms
In an optical lattice atomic clock, atoms are tightly confined in a Lamb-Dicke regime in an optical lattice formed by a standing wave inside an optical cavity. The Lamb-Dicke regime effectively suppresses all motion effects, preventing any Doppler shifts of the measured transition. The lattice is operated at the so-called magic wavelength at which the light shifts of ground and excited energy levels compensate each other with high accuracy.
The cavity instability, which is transferred to the probe laser as a phase noise, is the limiting factor for the interaction time between atoms and the laser. Even with the best state-of-the-art optical cavities the stability of the whole optical atomic clock is limited by the Dick effect, the down-conversion of cavity frequency noise because of the need to periodically prepare a new sample of atoms. Furthermore the need for an ultra-stable reference cavity is a complication for a commercial optical lattice clock.
A standard optical atomic clock consists of two state-of-the-art components: an ultra-stable high-Q optical resonator, which transfers stability of its length into stability of an optical frequency, and an atomic sample, which transfers accuracy of the energy of the ultra-narrow atomic clock transition into accuracy of an optical frequency. On short timescales the laser is frequency stabilized to the cavity, and both together are called the reference oscillator. It is used to spectroscopically interrogate the atoms, which provide long-term stability and accuracy (this uses a frequency shifter (FS) and a feedback loop). The short- and long-term frequency-stabilized laser frequency is the clock output. In an optical lattice clock, atoms are tightly confined in the Lamb-Dicke regime in an optical lattice formed by a standing wave inside an optical cavity. The Lamb-Dicke regime effectively suppresses all motion effects, preventing any Doppler shifts of the measured transition. The lattice is operated at the so-called magic wavelength at which the light shifts of ground and excited energy levels compensate each other with high accuracy.
A trap for strontium atoms in an optical clock
Frequency control and stabilization
Working principle of the frequency stabilization system: An optical frequency comb operating around 1550 nm central wavelength is referenced to an ultra-stable CW-laser, thereby inheriting its ultra-high spectral purity. By the required extending the spectrum of the frequency comb using nonlinear optical techniques, reference signals for the CW lasers used to pump, cool, trap and interrogate the strontium atoms are provided.