114 These days, inertial sensing is the basis of all navigation systems that don’t rely on an external signal or some measurement of the environment outside the vehicle, while the inertial sensors themselves are mechanical or micro-electromechanical accelerometers or gyroscopes (writes Peter Donaldson). The former measures changes in linear velocity, while the latter measure changes in angular velocity. The problem with even the best inertial navigation systems (INSs) in service at the moment though is drift, which is the accumulation of small errors in measurement that gradually but inevitably make the navigation solution less and less accurate over time. Drift is why INSs are usually back-ups used to maintain navigation performance through brief periods when a GNSS signal is not available, for example. In future, however, inertial navigation based on quantum sensing techniques could reduce drift significantly. One example of such a quantum sensor is a highly sensitive accelerometer based on atom interferometry involving rubidium atoms cooled to a temperature close to absolute zero, which is being developed by the Centre for Cold Matter at Imperial College, London, in cooperation with M Squared. When the atoms are that cold they form a condensate, a state in which their wave functions – a mathematical description of the quantum state of an isolated quantum system – are very sensitive to even tiny disturbances such as accelerations, and the changes in the atoms’ wave functions can be measured using laser interferometry. In quantum theory, particles including atoms can be thought of as waves and described mathematically as probabilistic wave functions. It is well-known that waves can be made to interfere constructively or destructively by being combined in or out of phase. Atom interferometers make use of this by using lasers to ‘prepare’ the quantum states of the ultra-cold rubidium atoms. These are internal states, such as the electron configuration or spin polarisation, and the motion states of position and momentum, which are described by a wave function. The instrument then splits the wave function to make it propagate along two separate paths before recombining them. If the system has been disturbed by an acceleration of the vehicle, there will be a phase difference between them, and measuring this using a fluorescence detector allows the change in the vehicle’s position to be measured extremely accurately. According to the Imperial College/M Squared team, the conversion from phase shift to acceleration depends only on the wavelength of the laser light and the timing of the light pulses, both of which are very accurate and stable. The team adds that the instrument measures acceleration much more reliably than the best conventional accelerometers, and that the one in their laboratory is sensitive enough to measure changes of a few billionths of a g. The system has been tested at sea in cooperation with the Royal Navy aboard the research vessel XV Patrick Blackett, with the equipment housed in an ISO container secured to the deck at the stern. Clearly therefore, there is some way to go before the quantum navigation can start to replace existing INS in operational vehicles, but it is transitioning from the science to technology. In quantum theory atoms can be thought of as waves and can therefore be made to interfere by being combined in or out of phase with each other August/September 2023 | Uncrewed Systems Technology PS | Quantum navigation Now, here’s a thing
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