Quantum sensors and high-precision measurements: what are they, and how do they work?

03.05.2021 – 12.50 – One of the most promising lines of development of quantum technologies is provided by the so-called quantum sensing, or quantum sensors, devices that using the laws of quantum mechanics can perform high-precision measurements.
In general, being able to make more and more precise measurements has had a double value for the scientific method and technological applications: on the one hand to test scientific theories with increasing accuracy – both to verify them and to find deviations from known laws – and on the other hand to provide useful tools for our daily life. A typical example is given by the research of clocks that can measure time with increasing accuracy, but there are many others: at the beginning of air navigation, in the first part of the last century, the navigation time on long distances could be significantly decreased when airplanes were equipped with more accurate and reliable gyroscopes (i.e. rotation meters).

Typical measurements that have an impact on both the study of scientific theories and technological applications are precisely those of time and rotation, but also, for example, of magnetic fields, acceleration, and speed.
The second quantum revolution we are witnessing provides us with a series of physical systems in which we are able not only to control the device in which the system itself is positioned with high precision, but also to induce and exploit states with strong quantum correlations. The principle of quantum sensing is in short based on:

  •  putting the device in contact with the external environment in which the quantity we want to measure acts (e.g. rotation, or masses we want to detect such as archaeological finds or oil fields);
  • measure the state of the system;
  • based on the result and how the system “reacted” to the perturbation we want to measure, extract the measure we want to find.

The crucial point is that in a whole series of applications it is observed that the greater the amount of quantum correlation that the state of the system has, the greater the accuracy with which we can measure the quantity that interests us. For this reason, quantum sensors in many applications are believed to be able – and in many cases already succeed in doing so – to make much more precise and accurate measurements than their “classical” counterparts, i.e., non-quantum, which cannot exploit quantum correlations.

Figure 1

A typical example of the operation of a quantum sensor is schematized in figure 1, which refers to a “matter-wave Sagnac interferometer”. This is a ring configuration in which two atomic gases at low temperatures are placed in a region of the ring, (first picture on the left); at this point the two gases are rotated separately in the two directions, clockwise and counterclockwise, (central picture); when they recombine at the interference points their state is measured, (right picture) more precisely their “interference fringe” is measured. Suppose we do this when the ring is not rotating, and then when it is in a rotation that we want to measure. By comparing the two interference fringes we can estimate how much the value of the rotation that makes the ring rotate is (for example: the rotation of the Earth). If the initial state is strongly quantum, then the accuracy with which we can measure this rotation is greater than if the initial state was classical. Clearly, this strategy works if we can prepare the system in a controllable way in a “full” quantum state, which is becoming increasingly achievable in the last two decades of the second quantum revolution. In addition, since the procedure of determining the value sought is based on the analysis of the measurements made (in this case, the interference fringes with and without rotation), it is interesting to note that there is increasing use of artificial intelligence techniques to efficiently perform the data analysis.

Figure 2

What has just been illustrated is just one example of measurement, but similar procedures can be used to measure a variety of quantities. Several physical systems are being used to build new classes of quantum sensors: spin qubits, trapped ions, low-temperature atomic gases, NV centers (which are defects embedded in diamonds), to name a few.

A practical example of application of quantum sensors is provided by the so-called gravity gradiometry, namely the study and measurement of variations of the Earth’s gravitational field. In addition to monitoring the territory, studying these variations can have very specific uses. Figure 2 shows a reproduction of a vast territory in Gabon, where geological studies indicate the possibility of finding deposits of natural gas (the yellow areas of the Figure). By placing a high-precision gradiometer in an aircraft and flying over the area, the anomaly study detects where to look for the resource, which is marked with a cross. Using similar techniques, the detection of artifacts in archaeological areas is also now being actively studied.
Both scientific and technological applications are numerous, and at the same time quantum sensors are also an important economic opportunity: many companies that produce and sell quantum sensors have been founded in recent years, and others well established in the market are moving in this direction. In many cases, these companies employ or are led by young people who have got graduate or doctoral degrees on topics related to quantum technologies and quantum sensors. These examples, it must be said, are still quite rare in Italy, and the strong hope, as well as a direction to strive towards, is certainly that the country does not fall behind in this strategic field.

by Andrea Trombettoni 

[Andrea Trombettoni: is currently a researcher at the University of Trieste, after having previously worked at SISSA and the Istituto Officina dei Materiali of the CNR in Trieste. He works on low-temperature atomic gases and quantum devices and is currently Vice-Chair of the COST Action “Quantum Technologies with Ultra-Cold Atoms” and Trieste coordinator of the MIT – FVG (Friuli Venezia Giulia) Seed Fund project “Non-Equilibrium Thermodynamics of Dissipative Quantum Systems“].