Cold atom technology is becoming a cornerstone of advanced systems for positioning, navigation, and timing (PNT), as well as next-generation communication networks. By cooling atoms to microkelvin temperatures, their motion is slowed to the point where quantum effects dominate, enabling exceptionally precise measurements. This underpins quantum sensors, atomic clocks, and other devices that achieve unprecedented levels of accuracy and stability.
Quantum technology (2.0) can deliver significant performance benefits over existing technologies. One such example is atomic clocks due to their ability to keep time with extremely high precision when compared to the quartz-based and mechanical clocks that are used today. Quartz oscillators are found in almost all digital electronics, including phones, computers, watches and measurement instruments. Their operation depends on counting vibrations of the crystal to establish time, but temperature changes and aging can cause drifting and timing inaccuracies. On the other hand, atomic clocks use atoms of a given element, which always have the same vibration. For example, Caesium-133 can generate a vibration of 9,192,631,770 times per second; this behaviour is then used to define the “second” in the International System of Units (SI).
The reason this is important is due to the stability, or drift, aspect of the clock. When comparing quartz oscillators to atomic clocks, there is a vast difference in stability. Over 24 hours, a quartz oscillator drifts on the order of seconds, whereas an atomic clock will have drifted by nano (10-9) seconds – 0.000000001 seconds. This accuracy is critical in applications such as navigation (GPS), where a denial-of-service attack would have a significant impact to the economy. A UK government report estimates a daily loss of £1.42 billion to the UK economy in the case of a GPS attack. The high stability of atomic clocks mitigates these risks by extending what is called hold-over time, this term refers to the duration that the system can use accurate timing to continue to track position without access to the satellites. Other application areas requiring precise timing include telecommunications, where high data rate signals need to arrive in sync, science and research for gravity-related experiments and in finance, where high-frequency trading and precise timestamping are required.
Within this white paper, we discuss the process of trapping atoms within a trap, which we call a magneto-optical trap (MOT) and using the FLAME laser to stimulate the atoms in the correct way to achieve a precise electrical signal that can be used within a timing system.
The main issues with uptake of the new technology are those of cost, weight and size. To address this, Alter Technology UK has developed the Frequency-stabilised LAser ModulE (FLAME): a compact, robust laser module operating at 780 nm and 795 nm, targeting the D2 and D1 transitions of rubidium-87. These transitions are essential for laser cooling, trapping, and repumping in quantum sensors, as well as for the clock transition in atomic frequency references. Packaging techniques adapted from the telecommunications industry—including gold-coated internal surfaces, flux-free soldering, low-outgassing adhesives, and laser-welded fibre outputs—ensure stability, cleanliness, and optical performance.
FLAME integrates a rubidium vapour reference cell for absolute frequency stabilisation via double-pass saturated absorption spectroscopy (SAS). The internal optical layout, shown in Fig. 1, diverts a small portion (5%) of the output to generate a Doppler-free transmission spectrum using a heated rubidium reference cell. The DBR output is collimated, and a polarising beam splitter (PBS) and quarter-wave plate (QWP) enable a retroreflected probe configuration for the SAS signal.
SAS offers sharp, narrow spectral features, ideal for generating a frequency lock without requiring external modulators such as AOMs or EOMs. Laser frequency tuning is achieved by modulating the drive current of the internal DBR laser diode. As the laser sweeps across the D1 transition, an optical transmission signal is recorded (Fig. 2b), which is then demodulated to produce an electronic error signal (Fig. 2c). This signal forms the basis of a feedback loop that locks the laser frequency to a selected feature—typically a crossover resonance—on the rubidium spectrum.
The desired lock point should now be chosen (e.g., a side-of-fringe zero-crossing), the laser current is centred, and the control loop of the laser driver or lock-in amplifier is engaged. The FLAME output is now stabilised and resonant with the chosen transition.
Once frequency locked, the main output of the FLAME module—delivered via a polariation-maintaining (PM) fibre—can be routed to the trapping optics. A common compact MOT design uses a single beam and a reflective pyramidal mirror structure to generate the necessary six-beam geometry for cooling and trapping. An example setup is shown in Fig. 3.
Figure 1: Optical layout of a FLAME module. Servo is added externally through control electronics. 5% of the output is diverted from the main beam to create part of the frequency lock. QWP: quarter-wave plate, M1: mirror, BL: ball lens, PBS: polarizing beam splitter, PM fibre: polarization-maintaining fibre.
Figure 2: (a) The D2 line for 87Rb. (b) Saturated absorption spectroscopy for the trapping transitions only, for 87Rb and 85Rb. (c) The Doppler-free features for the trapping transitions in 87Rb. (d) and (e) show the de-modulated error signals for (b) and (c). Images: Elvin, R. (2020). Phase-sensitive optical spectroscopy of a laser-cooled, microwave atomic clock
Download the FLAME-780: Frequency Stabilised Laser datasheet for more information
Gain detailed insights into ALTER’s FLAME-780 datasheet. This compact, frequency-stabilized laser module with an integrated vapor cell is crafted using advanced high-reliability telecoms manufacturing and space-qualified processes. Download now for comprehensive details on this cutting-edge technology.
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Figure 3: A simplified diagram of a pyramid MOT set-up. The anti-Helmholtz coils and chamber that contain the pyramid MOT are shown in cross-section. The FLAME 780nm output delivers the light for cooling and traps atoms directly into the chamber.
To operate the MOT:
- The frequency-stabilised 780 nm output should be red-detuned from the cooling transition (typically the F = 2 to F’ = 3 transition in 87Rb D2).
- The beam should be circularly polarised (via a QWP) before entering the pyramid.
- A repump frequency (addressing the F = 1 to F’ = 2 transition) may be derived from the same laser using direct modulation or an additional sideband.
- A magnetic quadrupole field must be applied around the trapping region, typically generated by a pair of anti-Helmholtz coils.
The FLAME module provides sufficient optical power for the entire MOT setup and, combined with its robust frequency lock, supports cold atom loading without the need for external cavity stabilisation or modulation hardware. Simplification of the development of deployable laser-cooled atom systems by integrating narrow-linewidth laser output with a robust absolute frequency lock to rubidium transitions. By leveraging compact packaging techniques and saturated absorption locking, FLAME eliminates the need for bulky optics and modulators, making it well-suited for compact cold-atom experiments such as magneto-optical traps.
This application note outlines the use of FLAME to lock to rubidium transitions and generate a functional MOT using a pyramid configuration. With careful alignment and modest external optics, FLAME enables rapid MOT deployment outside the laboratory, opening the door to fieldable cold-atom sensors.
Figure 4: Left: A fibre-coupled FLAME operating at 795 nm to target the D1 transition in 87Rb. Right: A CAD rendering showing internal components.