Quantum sensing breakthrough Realization of the smallest size optical lattice atomic clock

In collaboration with and funded by the UK Defence Science and Technology Laboratory (Dstl), the UK Centre for Quantum Sensors and Timing Technologies, led by the University of Birmingham, has designed a new technology for quantum sensors: one that not only reduces the size of the clock, but also makes it robust enough to be taken out of the lab and used in the "real world". This is the smallest package of an optical lattice atomic clock to date. The results were published in the journal Quantum Science and Technology.

The optical lattice atomic clock in this work is the smallest package to date

01Quantum clocks: widely used, but not portable

Quantum/atomic clocks are widely considered essential for increasingly accurate global online communication, navigation or global stock trading systems: areas where a fraction of a second can make a huge economic difference. Atomic clocks with optical clock frequencies can be 10,000 times more accurate than microwave clocks, thus opening up the possibility of redefining the standard (SI) unit of measurement.

Even more advanced optical clocks could one day have a major impact in everyday life and basic science. By providing a longer time to resynchronize than other types of clocks, they could provide greater resilience to national timing infrastructure and unlock future positioning and navigation applications for self-driving cars. The unparalleled accuracy of these clocks could also help us build models of physics that go beyond the norm to understand some of the most mysterious aspects of the universe (including dark matter and dark energy). Such clocks will also help solve fundamental physics questions, such as whether fundamental constants are really "constants" or whether they change with time.

Once we have a system that can be used outside the lab, we can use them for: for example, on a terrestrial navigation network where all these clocks are connected and communicate with each other via fiber optics. Such a network would reduce our reliance on GPS systems, which can sometimes fail.

In this regard, Dr. Yogeshwar Kale, principal investigator of sensors and timers at the UK Centre for Quantum Technologies, stated [2].

The stability and accuracy of optical clocks make them critical for many future information networks and communications. These movable optical clocks will not only help to improve geodetic measurements (geodetic measurements, which measure fundamental properties such as the shape of the Earth and changes in gravity), but also enable the monitoring and identification of geodynamic signals such as early stage earthquakes and volcanoes.

While such quantum clocks are rapidly evolving, the main obstacle to their deployment is their size: current clocks are about 1,500 liters and need to be packed in vans or car trailers; and their environmental sensitivity also limits their transport between locations.

02Core technology: Compact Atomic Packaging (CAP)

The Birmingham team of Sensors and Timers at the UK Centre for Quantum Technologies has proposed a solution to the challenge of carrying clocks by encapsulating alkaline-earth atoms (e.g., strontium (Sr) and ytterbium (Yb), etc.) in a "box" weighing less than 75 kg and measuring about 120 liters.

The clock works by using a laser to generate and measure quantum oscillations in the atoms. These oscillations can be measured with a high degree of accuracy; and in terms of frequency, time can also be measured. The main challenge in this process is to minimize external influences on the measurement, such as mechanical vibrations, electromagnetic interference. For this purpose, the measurements have to be performed in a vacuum and the external interference needs to be minimized.

(left) The basic building blocks of a typical optical clock. The oscillator (cavity-locked narrow linewidth clock laser) is tuned to the desired clock frequency by interrogating the "reference" (reference, ultracold trapped atom/ion set) and using an appropriate feedback mechanism. The calibrated optical frequency comb counts and is down-converted to the corresponding radio frequency (RF) signal for various practical applications. For such a clock, it is critical to have an accurate and stable atomic/ionic frequency reference, so the atomic package is central. (Right) Correlation energy level diagram and energy level jumps in a strontium atom.

Considering that mobility is a key criterion for field-deployable quantum sensors, the Compact Atomic Package (CAP) was designed and developed so that it can be easily transported from one location to another and quickly adjusted to an operational state. Its robust design is inherently modular: it includes the necessary sub-modules, and compact ultra-high vacuum (UHV) components. The core of the new design is an ultra-high vacuum chamber - it is smaller than any vacuum chamber used in the field of quantum timing. The chamber can be used to trap atoms and then cool them to temperatures very close to "absolute zero" so that they reach a state where they can be used for precision quantum sensors.

(left) 3D rendering of the UHV assembly; (right) CAP for ultracold strontium atoms and a simplified CAP module.

Finally, the team demonstrated that nearly 160,000 ultracold atoms could be captured inside the cavity in less than a second. In addition, they demonstrated the ability to transport the system over 200 km and then set it up to be ready for measurements within a 90-minute time frame. At the same time, the system was able to withstand a temperature rise of 8 degrees above room temperature during the journey.

Dr. Kale adds：

We have been able to demonstrate a robust and resilient system that can be quickly transported and set up by one trained technician. This brings us closer to the use of these high-precision quantum instruments in challenging environments outside the laboratory setting.

The experimental timing in the CAP (the lattice is always kept constant). the CAP can load up to 1.7 × 107 atoms into the first cooling stage; about 1.1 × 107 atoms are loaded into the blue magneto-optical trap (MOT) at a temperature T ≈ 3 mK; nearly 1.6 × 105 atoms are successfully loaded into the 1D lattice at a temperature of ≈ 110 μK in the single-frequency red MOT; by lowering the lattice temperature Further reducing the lattice temperature to about 22 μK, the CAP is still able to trap ≈ 2.9 × 104 atoms.

Pre-transportation preparation phase: (a) CAP protected by antistatic packaging placed in a transport box (the data logger and its mobile power supply are mounted on top of the CAP); (b) box with CAP located in the back of the van for short-distance transport scenarios. Transport phases: (c) round trip activity in urban traffic with short distances covering about 65.2 km and a maximum speed of 100 km/h; (d) representative route for medium distance intercity transport with a distance covering about 222 km and a maximum speed of 104 km/h.

03Optical clock: a key driving technology for the future of national defense

In this experiment, the team designed, developed and demonstrated a powerful movable atom package operation that is proficient in producing 88Sr ultracold atoms. Also, this is the smallest size of an optical lattice clock to date: with a repetition time <1s, the system is able to produce ~1.6×105 ultracold atoms in a one-dimensional optical lattice.

In addition, the team demonstrated the transport and restart process to prove that the packaging technology is suitable for field-deployable applications and that, in the future, the CAP can be easily modified, keeping the overall functionality and operation intact while using it for the fermionic 87Sr atom isotope.

In response to this achievement, Dstl stated.

Dstl sees optical clock technology as a key enabler of future DoD capabilities. Such clocks have the potential to shape the future by increasing the resilience of the nation's infrastructure and changing the way communications and sensor networks are designed. With Dstl's support, the University of Birmingham has made significant progress in miniaturizing many subsystems of optical lattice clocks and has overcome many significant engineering challenges in the process. We look forward to seeing them make further progress in this exciting and rapidly evolving field.

In the future, such stand-alone packages based on ultracold strontium atoms could form the basis for optical lattice clocks as well as large momentum-atom interferometers in an out-of-lab environment, and would be a key step toward achieving high-bandwidth quantum navigation systems.

Reference link：

[1]https://iopscience.iop.org/article/10.1088/2058-9565/ac7b40

[2]https://www.birmingham.ac.uk/news/2022/next-generation-atomic-clocks-are-a-step-closer-to-real-world-applications