Introduction to quantum sensing
You probably don’t realize it, but you very likely already employ quantum technology on a regular basis. Get in your car, switch on Waze or Google Maps, and you are already harnessing quantum effects. A GPS receiver works by measuring the tiny time delays in signals from multiple satellites separated in space. Doing this requires very stable and very accurate time measurement: enter the atomic clock. Such clocks, which reside inside every GPS satellite, often use quantum superposition. They employ atoms of Cesium or Rubidium to achieve an extremely stable “tick,” one accessible only within the atoms themselves. The primary standard for time, operated using this kind of physics, is so stable that it will lose just one second in 100 million years. That kind of stability powers not just GPS but other systems as well, including the synchronization protocols that govern Internet operations.
A clock that loses just a second in 100 million years (or more) may sound like more than we need, but this early application of quantum technology represents the start of something much bigger - building a new generation of quantum-enhanced sensors.
Quantum sensors turn the inherent weakness of quantum technology - its instability against the environment - into a strength. It takes a huge amount of work to isolate a quantum system in a way that allows it to be used faithfully as a clock. In general these devices are REALLY sensitive to everything around them; the most sensitive experiments to date have shown that such clocks can measure the effect of lifting the clock by a bit more than one foot (gravity changes as you move away from the center of the Earth). But quantum sensors deliver more than just sensitivity - quantum sensors also give the benefit of stability over long times. Conventional sensor instruments slowly change over time, meaning that averaging longer to reduce measurement noise becomes impossible. But because quantum sensors use immutable quantities - like the structure of atoms - their measurements tend to be very stable over long times.
Let’s explore one exciting kind of quantum sensor based on the same core technology as used in atomic clocks - cold trapped atoms. Cold atoms can be exploited for ultra-sensitive interferometric measurements using the wavelike nature of matter. Instead of building interferometers with light reflected off of (matter-based) mirrors (as widely used in telecom optical modulators), one can build atom interferometers using matter “reflected”off of pulses of light. Such atom interferometers have the benefit that the atoms themselves have mass, making them sensitive to both gravity and general acceleration. Accordingly, there is an emerging area of work on quantum-enabled “PNT” or positioning, navigation, and timing. Here, atomic accelerometers may enable dead reckoning navigation in environments such as space or GPS-denied battlefields.
More broadly, leveraging these capabilities and advantages, atomic devices are routinely used for both magnetometry and gravimetry. They could thus be deployed by military personnel to detect underground, hardened structures, submarines, or hidden weapons systems. Imagine a detector which can measure via changes in gravity whether a mountain is being hollowed out in a hostile nation with a furtive weapons program. In civilian applications these devices form the basis of new ways to monitor the climate - from underground aquifer levels through to ice-sheet thickness. Totally new forms of Earth observation for the space sector are now emerging, enabled by new small-form quantum sensors. Those capabilities flow into new data streams for long-term weather forecasting and insurance against weather events in agriculture. And of course the mining industry has long relied on advanced instrumentation for improved aerial survey and productivity enhancement.
Of course trapped atoms aren’t the only technology relevant to quantum sensing. There’s been a huge amount of research showing how solid-state devices like imperfections in diamond can be used as sensitive magnetometers. These have the advantage that they can be used in biological environments - even in vivo. They may not be as sensitive as atomic devices, but by virtue of their tiny size they can access new applications that are not possible with trapped atoms.
Overall, quantum sensing provides a route to gain massive technological advantages well before quantum computing comes online. And if you aren’t sure how much impact quantum sensors may have, just take a step back and think about how atomic clocks and GPS have already shaped your daily life.
Q-CTRL’S WORK IN QUANTUM SENSING
Q-CTRL is active across all applications of quantum technology, producing the fundamental enabling capabilities in quantum control to help our customers realize the true potential of quantum tech. But with quantum sensors we go one step further, taking a “software-first” approach to building and designing our own hardware powered by quantum control. Placing the advantages of quantum control front and center enables huge reductions in system size and improvements in noise rejection, ultimately unlocking totally new applications.
We’re excited to be building a new generation of atomic navigation systems for space exploration and terrestrial applications. And we’re thrilled to have assembled one of the most impressive teams of quantum sensing experts in the world.
Partially adapted from The Leap into Quantum Technology: A Primer for National Security Professionals
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