Rb cell clock

Many scientific and technological applications need very precise time and frequency reference signals. Very often, only atomic clocks can guarantee the required high level of accuracy and stability. In the current scenario of atomic frequency standards, vapor-cell clocks are particularly suited to be employed in activities that demand for good frequency stability performances joined to compactness, reliability and low power consumption. Recently, due to better performing laser sources and to innovative techniques to prepare and detect the atoms, several cell-based prototypes exhibiting unprecedented frequency stability have been developed.

At INRiM we developed a maser based on the coherent population trapping (CPT) phenomenon and more recently a Rb clock based on the pulsed optical pumping (POP) in a Ramsey scheme.

Specifically, the POP clock concept relies on three operation phases. First, a strong laser pulse generates an imbalance between the two ground-state levels through optical pumping. Then, the microwave interrogation of the reference transition takes place fully in the dark with a temporal Ramsey technique. Finally, the atoms that have made the clock transition are detected.

Schematic of the POP clock setup.

Analogously to the CPT standards, the detection of the hyperfine reference transition can occur in the microwave domain, through the free-induction decay signal (POP maser), or in the optical domain, through the transmitted optical signal. The optical detection of the clock transition exhibits some advantages compared to the maser approach. Due to the higher energy carried by optical photons, a better signal-to-noise ratio is expected. This results in an improving of the short-term frequency stability. In addition, some requirements on the cavity are relaxed. Since the cavity plays a role only during the interrogation and not in the detection phase, it is not necessary to have a very high cavity quality factor which instead fundamental in the maser approach. The measured frequency stability of the POP clock with optical detection is 1E-13/t^-1/2 where t is the measurement time and the white frequency region extends up to 10’000 s, reaching the level of 1E-15.

Since 2012, the EMRP MClocks project aimed at developing and producing three types of vapor-cell microwave clocks which match the performance and compactness needed for upcoming demanding industrial and technical applications. Specifically, the goal of the project was:

1) to realize a vapor-cell clock based on the pulsed optical pumping (POP) principle targeted on industrial applications in terms of size, power consumption and reliability.

2) to develop a vapor-cell clock based on cold atoms (Rubiclock) with performance comparable to that of pulsed optical pumping in the short term but with better long term performance including an accuracy within an order of magnitude of that of primary standards. The JRP will identify the compromises required in order to obtain the expected performance while still targeting industrial applications.

3) to study alternatives principles such as coherent population trapping (CPT) or electromagnetically induced transparency to investigate the possibility of realizing a clock optimized in terms of compactness.

The consortium included INRiM (Coordinator of the project), UFC, OBSPARIS and TUBITAK (funded partners), UniNe (REG), Muquans (unfunded industrial partner), Selex ES and Spectratime (unfunded industrial collaborators).

More recently, INRIM has been involved in a technology transfer activity in favor of Leonardo Spa with the aim of creating an engineered prototype of this clock suitable for space applications such as GNSS. The activity so far has concerned the creation and subsequent characterization of the physical clock system only. The results obtained in terms of frequency stability are particularly good. The clock physics package has been tested at INRIM laboratories and exhbited a short term stability of 2x10-13 at 1s; when the environmental parameters are under control the white noise region extends up to 10000 s. Moreover, a frequency drift as low as 1x10-14/day has been measured.

Due to these performances and to compactness of the clock, ESA will fund the subsequent phase concerning the engineering of the optics and clock electronics (call GSTP Element 2 Make-Industrialization of the Rubidium Pulsed Optically Pumped clock).

Leonardo POP physics package (courtesy Leonardo)

INRIM test setup used to characterize the Leonardo POP clock.

Reducing size and weight is of crucial importance for any device intended for space applications. It is not only a matter of cost: reducing the size also implies a better ability to control the temperature of all the system. Even though the size of the POP clock physics package is already relatively compact (2.5 L), at INRIM the possibility of further reducing the physics package volume has been investigated. Specifically, the microwave cavity has been loaded with a material of high dielectric constant (alumina). In this way, the inner cavity volume has been reduced by a factor of 10 compared to the un-loaded cylindrical design. At the same time, the cell volume is reduced of a factor of 8. In terms of frequency stability performance, a short term of this so called mini-POP approach has been measured, reporting an Allan deviation in the mid 10-13 t-1/2, fully compliant with current GNSS requirements. More studies are on going to characterize the performances in the medium-long term.

The loaded cavity of the mini POP clock.

Contacts: s.micalizio[at]inrim.it, c.calosso[at]inrim.it, f.levi[at]inrim.it