Locked Laser Systems for Quantum Positioning, Navigation and Timing

Introduction

Reliable Positioning, Navigation and Timing (PNT) underpins critical civil and defence infrastructure, from maritime navigation and aviation to telecommunications and autonomous systems. While Global Navigation Satellite Systems (GNSS) provide global coverage and high accuracy, their vulnerability to jamming, spoofing and denial has become an operational concern rather than a theoretical risk. As a result, significant public and private investment is being directed towards complementary and alternative PNT technologies that can operate independently of satellite signals.

Quantum PNT has emerged as one of the most promising responses to this challenge. In 2025 alone, global public and private investment in quantum technologies exceeded USD 17 billion, with quantum sensing identified as a priority area for near term deployment. In particular, quantum PNT represents a rapidly growing segment, with the global market projected to grow from approximately USD 200 million in 2026 to over USD 1 billion by 2036. More than a dozen specialist companies, alongside major defence and aerospace integrators, are actively developing quantum inertial navigation systems, supported by large scale government programs in the US, UK, Europe and Australia. [1-3]

By exploiting the intrinsic stability of atomic systems, quantum sensors offer long term accuracy and drift performance that exceed those of classical technologies. Government programs and industrial consortia worldwide are now transitioning quantum PNT from laboratory demonstrations to field trials and early deployment, particularly for applications requiring autonomous operation in GNSS denied environments.

Among the enabling technologies for quantum PNT, laser systems play a central and often underestimated role. The Radiantis Locked Laser System has been developed specifically to meet the spectral purity, long term stability and robustness requirements of cold atom quantum sensors, supporting the transition of quantum PNT from controlled laboratory environments to real world operation

Quantum PNT and the Role of Laser Frequency Stability

Quantum PNT is a subset of complementary PNT in which quantum clocks and quantum sensors are used to provide time holdover, inertial navigation, or position fixing when GNSS signals are unavailable. The most mature quantum PNT technologies today are atomic clocks for precision timing and cold‑atom inertial sensors based on atom interferometry for acceleration and rotation measurements. These technologies are increasingly being demonstrated in operationally relevant environments, highlighting both their performance potential and their system‑level requirements. [4]

In cold‑atom inertial sensors, laser light is used to cool, trap and coherently manipulate atomic ensembles. Laser pulses act as beam splitters and mirrors for atomic matter waves, forming an interferometer whose output phase encodes inertial quantities. In this architecture, the laser defines the phase and frequency reference against which atomic motion is measured. Consequently, laser frequency noise, phase noise and long‑term drift directly map onto interferometric phase noise, bias instability and scale‑factor errors in the inertial solution. Similar considerations apply to atomic clocks used for time holdover, where the stability of the interrogation laser strongly influences achievable timing performance over relevant integration times.

Although quantum sensors derive their intrinsic accuracy from atomic transitions defined by fundamental constants, fully realizing this advantage requires laser systems whose spectral purity and long‑term stability are comparable to the atomic references themselves. Free‑running or passively stabilized lasers typically exhibit frequency excursions that far exceed atomic linewidths over operational time scales, limiting sensor performance. Atomic‑referenced locked laser systems address this limitation by continuously anchoring the laser frequency to an atomic transition, enabling quantum sensors to operate closer to their fundamental limits while maintaining long‑term stability and autonomy.

Locked Laser System and Its Performance for Quantum PNT Applications

The Radiantis Locked Laser System is a turn-key solution designed to meet the stringent requirements of rubidium‑based quantum PNT sensors. The system automatically locks a narrow‑linewidth laser to a saturated absorption feature of the ⁸⁷Rb D2 transition, providing an absolute frequency reference that is independent of environmental conditions and component ageing.

The architecture is based on a low‑noise and narrow linewidth 1560 nm seed laser, fiber amplification and closed‑loop frequency doubling to 780 nm with Covesion’s proven frequency conversion technology. Saturated absorption spectroscopy (SAS) is implemented with balanced detection to suppress Doppler background and enhance the frequency discriminator slope. Importantly, frequency modulation for lock generation is applied via an electro‑optic modulator in a separate optical branch, preserving the intrinsic linewidth of the seed laser while enabling fast and robust error‑signal generation.

Once locked, the system delivers short‑term linewidths below 10 kHz RMS and long‑term frequency stability <250 kHz (RMS) over periods extending from 12 to 60 hours. Fractional frequency stability, expressed as Allan deviation, reaches the 10⁻¹¹ level over integration times from seconds to thousands of seconds. At 780 nm, the system provides hundreds of milliwatts of spectrally pure optical power with intensity noise below 1% RMS, supporting efficient atom cooling, trapping and interrogation.

A key capability for quantum PNT architectures is the ability to apply user‑defined frequency offsets of several gigahertz from the atomic reference while maintaining lock. This allows cooling, repumping and interrogation frequencies to be derived from a single absolute reference, simplifying system architecture and improving long‑term reproducibility.

Quantitative Comparison of Laser Architectures for Rubidium‑Based Quantum Inertial Sensing

Parameter	Free‑Running Diode Laser (Typical)	Commercial 780 nm Laser [5]	Radiantis Locked Laser System
Frequency reference	None	Factory‑set wavelength, no atomic lock	87Rb saturated absorption lock
Short‑term linewidth	<10–100 kHz	<400 kHz	<10 kHz RMS
Long‑term frequency stability	>100 MHz (hours–days)	10–100 MHz (hours)	<250 kHz (RMS) (12–60 h)
Allan deviation	Not specified	Not specified	<8 × 10⁻¹¹ (1 s), <1.5 × 10⁻¹¹ (1000 s)
Frequency reproducibility	Manual re‑tuning	Factory dependent	Automatic re‑lock to atomic line
Frequency offset capability	Thermal/current tuning only	Limited	±kHz to multi‑GHz while locked
Output power @ 780 nm	20–100 mW (typical)	>30 mW	up to 1 W
Power stability	1–5% RMS	~1–2% RMS	<1% RMS (12 h)
Environmental robustness	Lab environment typically	Benchtop laboratory use	Validated in harsh, mobile environments
Suitability for deployable Q‑PNT	Low	Moderate	High