POLARIS: high POwer, phase-locked LAseRs for atom InterferometerS

Quantum technology relies on the behaviour of quantum superposition states. It is generally desirable to work with longlived superpositions as these give the greatest benefit. For example, when measuring forces, accelerations, magnetic and electric fields, or just the passage of time, long-lived superpositions give the highest sensitivity and that is the key advantage of quantum sensing. For computation, long coherence time increases the available computing power. The most convenient way to manipulate the internal quantum states of an atom (or ion or molecule) is using laser light that couples to the charge distortion of the atom. Two stable, low-lying atomic states can be superposed using a pair of laser beams whose frequencies differ by the microwave frequency linking the two states. This is called a Raman transition. The microwave beat note between the two laser frequencies is transferred to the quantum superposition of atomic states, which then oscillates at the microwave frequency, with the phase impressed by the lasers. To make the most of quantum interference, one needs lasers of stable intensity -- preferably high intensity -- and exceptionally low phase noise in the beat note. For many such applications, no suitable laser system is commercially available. That is the problem we address here. The SolsTis produced by M-Squared Lasers is a very stable, high-power laser, which provides the ideal starting point
for developing a suitable product. It is broadly-tuneable across the near infrared range (700 nm - 1,000 nm), so it can address a wide range of relevant atoms, particularly the two workhorse atoms rubidium and caesium.

The team at Imperial College is developing accelerometers for inertial navigation using atomic quantum coherence in rubidium atoms. This application is challenging as the lasers driving the Raman transition must combine high power, exceptionally low phase noise, low drift, and great agility of power, frequency and phase. We propose to work closely with M-Squared to develop a packaged laser system that optimises the performance of these accelerometers. This involves research to define the optimum specifications and development to deliver those specifications in a commercial package. The goal of the present proposal is to define and then produce a suitable laser system, and to validate it by demonstrating high performance, first in a 1-axis accelerometer and then in a 3-axis prototype.

The system will be developed and validated in the context of a new method for navigating without recourse to the satellite network, which has military and transport applications. However, it will be much more widely useful because of the general importance of Raman transitions in quantum technology. Other probable applications include geological surveying, mining, ultra-precise time stamping, medical imaging, and quantum information processing.

In addition to the academic impact, described above under Academic Beneficiaries, this project will have impact in the developing area of quantum technology, as described fully in Appendix A, the business case.

This proposal focuses on optimising the performance of an accelerometer based on atom interferometry, with a view to enhancing the navigation of military craft, especially submarines, in the absence of communication with the global navigation satellite system. This is a capability that the MoD is seeking, and achieving that will be our first major impact outside academia.

Beyond that, the preparation of stable superposition states in atoms, ions and molecules is a generic approach to almost all applications in quantum technology. These include the measurement of electric and magnetic fields, the sensing of gravitational forces for imaging underground or inside containers, the mapping of gravity gradients, applications in secure quantum communication, and the processing of quantum information. This project will produce a laser system capable of driving Raman transitions with exceptionally good phase stability and high power in a range of atomic systems. The wavelength will be tuneable from 700 nm to 1,000 nm. This product will occupy a unique position among high-end commercial laser systems. Initially the main consumers will be academic groups, national laboratories, and military researchers. As quantum technology continues to develop over the coming 5-10 years, commercial applications in navigation, mining, and secure communications are expected to grow. All these groups will benefit from the availability of a professionally engineered, commercial laser system specifically designed to do the job.