The goal of iqClock

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Optical clocks are amazingly stable frequency standards, which would be off by only 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 iqClock consortium, assembling leading experts from academia, strong industry partners, and relevant end users. We will seize on recent developments in clock concepts and technology to start-up a clock development pipeline along the TRL scale. 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 first product prototype will be a field-ready strontium optical clock, which we will benchmark in real use cases, such as network synchronization (TRL 6). This clock will be based on a modular concept, already with the next-generation clocks in mind, which our academic partners will realize (TRL 3-4). By their operation principle, these optical clocks are more robust than the current ones and have come into reach by recent breakthroughs, some of which achieved by our partners. We will leverage the foundational work by the consortia 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

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. Superradiant clocks can help to overcome both difficulties.

Superradiant clocks

A superradiant clock is a laser that operates directly on an ultranarrow optical transition. Most commercial and scientific lasers are good-cavity lasers , where the cavity mode is much narrower than the gain profile of the lasing medium. In such a laser the coherence is stored in the cavity field, and the oscillation frequency ωL of the laser is primarily determined by the cavity length. Thermal and mechanical fluctuations of the cavity length cause fluctuations of the laser frequency. By contrast, in bad-cavity or superradiant lasers the cavity mode is much broader than the gain profile and the coherence is stored in the polarization of the gain medium. As a result, the frequency of such a laser is largely insensitive to fluctuations of the cavity length. The difference between standard- and superradiant lasers is illustrated in the figure below:

This robustness of the radiation frequency with respect to the cavity fluctuations promotes the idea of a high-performance active optical frequency standard using an atomic clock transition as a gain medium. Above the lasing threshold the atomic ensemble itself produces a highly stable oscillation which is also locked to the atomic line. The very same basic principle is already successfully exploited in hydrogen masers, forming the most successful microwave active frequency references as used in the Galileo satellites.

The stability of such a laser can also be observed in terms of its phase diffusion. A more stable laser will exhibit a more robust phase. This can be nicely illustrated by looking at the phase space of the laser cavity field amplitude. In any quantum mechanical system, detection will disturb the system and thereby induce noise. A measurement of the phase (homodyne detection) thus causes a random walk of the amplitude in phase space. With a little help from some numerical tools, we can see that the phase of a superradiant laser is kept stable (to some extent) due to the collective coherence properties of the gain medium (see animation on the right). The better this collective coherence is, the more stable the phase.