Quantum sensors for controlling prosthetics
Researchers are pursuing photonic integration to miniaturise quantum sensors that can detect neural signals to control artificial limbs, offering a reliable and non-invasive alternative to electrodes implanted in the body
A multidisciplinary consortium called Human Machine Interface Based on Quantum Sensors (QHMI) based in Stuttgart is developing a new approach for the control of prosthetic limbs. Currently, implanting electrodes to detect the body’s signals is the most common technique, but this is invasive, and electrodes can deteriorate or move position over time. The QHMI team are pursuing an alternative approach of using quantum sensors to detect the incredibly small and fast nerve signals. The ultrasensitive quantum magnetometers will be carried outside the body measuring the neural signals through the skin. At this stage, the scientists are using Spectrum Instrumentation’s ultrafast digitisers (M5i.3357) and arbitrary waveform generators (M4x.6631) to characterise the signals and to finally design the required ASICs and PICs.
Jens Anders, a professor at the University of Stuttgart, who is in charge of the project ‘Cluster4Future QSens’ and a leading scientist of the QHMI consortium, explained: “This is one of the first real-world applications for quantum sensor probes as there is no other way to non-invasively detect such tiny magnetic changes that are in the order of 10 to 100 picoTeslas for muscles: that is six orders of magnitude smaller than the Earth’s magnetic field. Our tests show that our sensors are sensitive enough that they can detect neural signals to muscles through the skin. Even a small amount of remaining, say, lower arm muscle can in principle be used for this. We are working on even greater sensitivity for the femtoTesla magnetic changes we need to measure to detect signals within the brain without breaking the skin.”
At the heart of this technology is an optically detected magnetic resonance (ODMR) device made of a tiny slice of diamond. The diamond is doped with so-called nitrogen-vacancy centres (NV centres), which have a net electron spin and, therefore, behave like tiny bar magnets. When green laser light is shone on them, they produce a red fluorescence signal. By applying a suitable microwave magnetic field, this fluorescence signal is very sensitive to external magnetic fields, which can be used to measure neural signals with utmost precision.
The microwave magnetic fields required to control the NV centre spins are generated using suitable coils driven by a microwave transmitter. The baseband signals for this transmitter are generated using an arbitrary waveform generator (AWG) to provide the required phase and amplitude modulation of the carrier signal that make the excitation signal more robust against experimental nonidealities. The resulting fluorescence signals, which carry the information of the neural magnetic fields, are then captured by a photodiode, amplified, filtered and digitised for advanced signal processing.
According to Spectrum, the team chose its products for several reasons, including their extremely high dynamic range and good noise performance – which is vital for such tiny signals – and their high speed and ability to capture the fast signals associated with advanced pulsed excitation schemes, which can require bandwidth beyond 100 MHz.
The quantum sensor probes are currently matchbox-sized and, in the future, will be around one cubic centimetre and go to a control box that is roughly the size of a large matchbox that houses the processing electronics and the battery. The aim is to use microelectronic and photonic integration to shrink the control box further and extend the battery life to give a day of use before recharging. It is hoped that prosthetics will start becoming available in three to four years.
