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Put simply, a quantum sensor is a quantum device that responds to stimulus. For a sensor to be considered a quantum sensor it has to be able to do one of the following:
- Measure a physical quantity using quantum coherence.
- Use entanglement to improve traditional measurements taken with classical sensors.
Scientists dealing with quantum sensors generally apply 4 criteria to classify whether or not a quantum sensor works:
- The system being used has to be able to resolve energy levels
- A sensor has to be initialized and be able to provide a measurable answer
- A user must be able to manipulate the sensor
- The sensor must be able to interact with a physical quality and be able to respond to that quality
With the uncertainty involved in quantum mechanics, such a sensor, in theory, would be so far beyond classical sensors as to render them obsolete. A good analogy would be to compare Galileo’s telescope with the James Webb Telescope due to go online in 2021. In order to understand what a quantum sensor does though, one has to first understand what a quantum sensor is. To do this it is best to use the example of radiation that drives photosynthesis in nature. This particular type of radiation is called photosynthetically active radiation (PAR) and is defined as the solar radiation across a range of 400-700 nanometers. PAR is often expressed as photosynthetic photon flux density (PPFD) and sensors that can measure this density are called quantum sensors because of the quantized nature of radiation.
It is difficult to simplify quantum mechanics due to the almost constant uncertainty involved. Quantum sensors rely on the unpredictable behavior of sub-atomic particles. Newtonian physics ceases to apply at the quantum level which is why we often struggle to understand quantum behavior. In the PAR example, a quantum refers to the minimum quantity of radiation i.e. 1 photon involved in physical interactions. Traditional sensors can detect photons but they cannot measure their interactions as well as a quantum sensor due to the unpredictability factor. But if we think of quantum particles (like photons), as waves instead of individual particles, our understanding is improved. A wave is easier to measure.
Now that we have a better understanding of quantum sensors, let us look at some of the sensors currently available. Apogee Instruments offer 2 main types; an original Quantum Sensor and a Full Spectrum Quantum Sensor. The original sensor uses a blue lens to detect photons using different pigments in the filter to improve the measurable spectrum response. The full spectrum sensor utilizes an improved detector allowing incredibly accurate measurements under different light sources including LED.
Other Types of QS
In March 2019, a team led by Professor Kai Bongs of the University of Birmingham reported on their development of quantum gravity sensors. Dubbed the ‘Gravity Pioneer’ the new sensor would represent a huge leap forward for everyday construction and engineering projects. The Pioneer QS uses lasers to cool rubidium atoms to just above absolute zero (-273 degrees Celsius). The atoms are propelled upward in a vacuum and then measured by the sensor as gravity pulls them back. Because the atoms have been cooled to absolute zero, they move slower and are therefore easier to detect and measure as they fall back down.
Why is this so Exciting and How Does it Help in Engineering and Construction Work?
Traditional surveying sensors on building sites are too sensitive to the density of the surrounding buildings, wind or ground variations. They cannot provide an accurate survey of the area because these interactions limit their resolution. For example, they are unable to detect buried mineshafts, sinkholes and other factors that would affect construction. The Gravity Pioneer is able to detect these factors, meaning engineering teams could use it to survey a site and be confident that the QS would give them an accurate lay of ground above and below.
The Gravity Pioneer represents an even more exciting technological advance beyond engineering and construction. The advances made in quantum sensors mean that it might be possible to detect the minuscule electrical signals in the body that current technology is unable to accurately measure. Were we able to measure these signals using a QS, we might be able to understand more about how the brain works and perhaps even detect and prevent neurological diseases before they cause irreparable damage. Perhaps the best analogy to use is that of the clock. Our basic measurements of time are in seconds, minutes and hours (measurements recorded by the aforementioned Galileo when he took his pulse). Quantum sensors could allow us to see all the different values between each second, perhaps giving us a new understanding of time and how we measure it.
The methods being used by Professor Kai and others open the door to enhanced quantum sensing. Because the rubidium atoms have been cooled, it could allow us to see the quantum fluctuations imprinted onto individual photons, allowing us to read them for the first time. This could allow us to predict quantum behavior in a way we have never seen before and lead us into a better understanding of the universe.