Quantum Sensing – Introduction

This article was written by B. d’Aligny and reviewed by G. Morlat.

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Quantum sensing is the field which consists in measuring physical quantities using quantum phenomena. It is already in use in several areas such as non-invasive medical imagery for its outstanding precision. But quantum sensing technologies is also bringing new kinds of applications, such as locating oneself on the Earth without the need of satellites, as well as monitoring volcanic activities below our feet.

These technologies’ record-breaking precision comes from the high sensitivity of quantum objects. The range of approaches is extremely diverse, due to the many existing physical quantities we can measure, and to the numerous objects which can be used as qubits. For example, if a cold atom is restricted to 2 possible trajectories, it can be described as a qubit. Because an atom’s trajectory changes significantly under the effect of gravity, such a qubit can be used to detect massive objects through walls (indeed, the precision of this method allow us to detect little variation of the gravitational field induced by objects). In a totally different approach, particles have a property that can be compared to a tiny magnet called “spin”. This spin moves differently depending on the surrounding magnetic field and can be used to observe brain activity through the skull.

In the following we introduce several quantum sensing technologies, and a few of its use cases. These examples will give you an overview of the key principles of quantum sensing. Curious readers can then read more by clicking on our suggestions at the end of the article, or by reading the references.

Table of contents

Medical imagery : Optically pumped magnetometers

Measuring gravity using cold atom clouds

Gravitometry : general method

NV centers for all kinds of sensing

Entanglement-enhanced quantum sensing

source : ETH Zurich

Medical imagery : Optically pumped magnetometers

Quantum sensing presents unparalleled results when it comes to medical imagery. It is particularly the case of magnetoencephalography, which is the mapping of the brain using its magnetic activity. These brain mappings are precious tools for detecting brain diseases such as dementia, Alzheimer’s or Parkinson’s. In fact, they can be obtained with particularly high details by using quantum sensors known as “Optically pumped magnetometers”.

In this technology, the Qubit which is used for measurement is the spin orientation of atoms. These spins can point downwards, upwards, or in any superposition of these orientations. The atoms are trapped in a glass box called a “cell”, and are illuminated by a laser: they are “pumped” by the laser. We then measure the light coming out of the cell. The atoms will absorb a different amount of light depending on the atoms’ spin orientation. In the presence of a magnetic field, the atoms’ spin orientation is changed, thus changing how much the cell absorbs light. We can therefore deduce the magnetic field’s intensity from how much light comes out of the cell.

Because our brain activity emits small magnetic fields, we can map it by measuring the magnetic field around our skull. This mapping is called magnetoencephalography. It is currently mainly achieved using MRIs (IRM in French), but “Optically Pumped Magnetometers” are promising as they are more compact and more precise. This is particularly the case because on the contrary to traditional methods, they do not require voluminous cooling equipment, and the detectors can be put in direct contact with the skull.

This technology shows us a first key component of quantum sensing: it relies on measuring the behaviour of qubits which interact with their environment. However, “Optically Pumped Magnetometers” are particular among quantum sensing techniques because they do not rely on a single qubit, but rather a crowd of qubits. Indeed, in each cell there are several atoms, which each correspond to one qubit.


ONERA’s Girafe1 gravimeter (2009). Used for gravity mapping on boats and planes
In this device, the atoms fall over 6cm [4]

Measuring gravity using cold atom clouds

Gravitometers based on cold atoms are quantum sensors which detect tiny fluctuations of gravity. From these fluctuations, scientists can deduce the underground structure of the Earth. This can be useful for monitoring volcanic activity, as magma displacement generates a change in the gravitational field. It can also be used for mapping the gravity strength around the globe. These gravity maps can then be used for locating oneself without the need of satellite (like the GPS) [5][6]. It is important to notice that these use cases were not doable with previous gravitometers, which were not sensitive enough. 

Gravitometry : general method

The general method for this technology, called “cold atom interferometry” consists in dropping an atom, and making its possible trajectories interfere during the downfall [7]. The atom is put in a superposition of two states, which correspond to two different directions of propagation. In this method, interference of the atom’s trajectories — which is a direct consequence of quantum superposition — is exploited to increase the precision of the measurement. The steps of the measurements are the following:

Initialize: We use lasers to precisely localize the atom, and cool the system down. We then apply a first laser pulse to the atom, which puts it in a superposition of two different initial speeds. These speeds correspond to the two states of the qubit.

Dropping atoms: The atom will fall in a superposition of two paths. At half the falling time, we apply another laser pulse, which inverts the qubit states. That way, at the end of the falling time, both “paths” will have joined again. However, both paths will have been slightly distorted by the gravitational pull on the atom.

Measurement and interpretation: At the end of the dropping, we send a last laser pulse, which will read the interference of the two paths of the atom. Distortions of the two atom’s trajectories can now be determined from the interference pattern. From there we can deduce a highly precise value of gravity.

Usually, several atoms are dropped together for more precision. Also, the process can be redone regularly in order to obtain the evolution of gravity through time, for example for measuring underground lava movement. If the gravimeter is moving on a boat or a plane for example, repeating the protocol allows us to build gravity maps.

This second approach to quantum sensing shows us that superposition is a useful phenomenon for increasing the precision of measurements. In fact, this method is now so precise that some armies are trying to use it to detect submarines!

NV centers for all kinds of sensing

NV centers in diamond (abbreviation for Nitrogen-Vacancy centers) are one of the most promising hardware setups in quantum sensing. NV centers are particular defects of diamond which interact strongly with light. Technologies based on NV centers operate on the defect’s spin, similarly to “Optically Pumped Magnetometer” (mettre un lien qui renvoie plus haut) systems. It is very promising for a large variety of applications, such as magnetometry, and temperature measurement.

An NV center defect is made of 2 atoms only in the diamond, and it is possible to isolate a single one of these defects as a probe. This allows us to obtain unbeatable spatial resolutions. This is a potential gamechanger in biology, as we can now measure temperatures inside living cells [10]. Another advantage of NV centers is its solid diamond structure, which allows it to measure phenomena under high-pressures [11], while other sensors would be crushed.

NV centers are expected to become a cornerstone of quantum sensing, and the few examples we introduced are just the tip of the iceberg.

A nanodiamond (very small diamond) containing a single NV center is placed at the tip of a stick, and moved around in order to build a 2D image of the magnetic field.

Entanglement-enhanced quantum sensing

The previously presented methods do not exploit the entanglement phenomenon, yet it could strongly improve the precision of quantum sensing. This has already been proven in laboratories, but practical difficulties makes it difficult to industrialise and use it in real-life situations. In fact, entangling quantum objects is one of the most difficult operations in quantum technologies, because of the limitations of decoherence (see “Introduction to Quantum physics” for an explanation of decoherence). The two main approaches to entanglement-enhanced quantum sensing are “Distributed quantum sensing”, and the “storage and retrieval” method.

“Distributed quantum sensing” consists in entangling a set of qubits, and then measure a given quantity with each qubit. In certain cases, the entanglement will reduce unwanted measurement perturbations.

Entanglement-enhanced microscope – ©University of Queensland

Entanglement can also be leveraged through the “storage and retrieval” method. In this method, a highly sensitive “probe” qubit, and a very stable “auxiliary” qubit work together. At the start of the measurement, they are entangled. Then, the “probe” interacts with the physical quantity we want to measure, while the “auxiliary” is protected from its environment to avoid any unwanted perturbation. The “information” about the physical quantity is thus stored in the probe’s state. We then swap the states of the entangled qubits (it would have not been possible without the entanglement). Finally, we can measure the state of the “auxiliary” qubit, which is less sensitive to unwanted perturbations. This method has already provided better-than-classical measurement result, in an experiment which used two photon qubits to measure a distance [13].

Conclusion

As a conclusion, quantum sensing exists in many forms, and promises to impact a large variety of areas, from medicine to biology and natural risk assessment. The single idea uniting quantum technologies is that they exploit quantum properties of light and matter for measuring stuff with high precisions. In particular, superposition and entanglement are very useful to increase the precision of sensing, yet the latter is still very difficult to master at a commercial scale.

References

[1] Tierney, Tim M., et al. “Optically pumped magnetometers: From quantum origins to multi-channel magnetoencephalography.” NeuroImage 199 (2019): 598-608.

[2] Hill, R. M., et al. “A tool for functional brain imaging with lifespan compliance. Nat. Commun. 10.” (2019).

[3] Wikipedia page « Magnetic resonance imaging »

[4] Presentation « Gravimétrie absolue marinetrie absolue marine à l’aide d’atomes froids : GIRAFE 2l’aide d’atomes froids : GIRAFE 2 », 16 novembre 2016, ENSTA-Brest, Colloque CNFG2.

[5] Stray, B., Lamb, A., Kaushik, A. et al. Quantum sensing for gravity cartography. Nature 602, 590–594 (2022). https://doi.org/10.1038/s41586-021-04315-3

[6] 23655 Phillips, Alexander M., et al. “Position fixing with cold atom gravity gradiometers.” AVS Quantum Science 4.2 (2022).

[7] Cold atom gravitometers: Peters, A., Chung, K. & Chu, S. Measurement of gravitational acceleration by dropping atoms. Nature 400, 849–852 (1999). https://doi.org/10.1038/

[8] Fig 688 de Oez UQT page 799. https://www.itu.int/en/ITU-T/Workshops-and-Seminars/2019060507/Documents/Shuai_Chen_Presentation.pdf

[9] UQT Oezratty page 801 (Real source???)

[10] Kucsko, Georg, et al. “Nanometre-scale thermometry in a living cell.” Nature 500.7460 (2013): 54-58.

[11] Lesik, Margarita, et al. “Magnetic measurements on micrometer-sized samples under high pressure using designed NV centers.” Science 366.6471 (2019): 1359-1362.

[12] Celano, Umberto, et al. “Probing magnetic defects in ultra-scaled nanowires with optically detected spin resonance in nitrogen-vacancy center in diamond.” Nano letters 21.24 (2021): 10409-10415.

[13] Assouly, R., Dassonneville, R., Peronnin, T. et al. Quantum advantage in microwave quantum radar. Nat. Phys. 19, 1418–1422 (2023). https://doi.org/10.1038/s41567-023-02113-4

More readings

“Quantum sensors will start a revolution — if we deploy them right”, Nature, 2023

Understanding Quantum Technologies 2023 (par Olivier Ezratty)

Développement de capteurs quantiques pour des mesures de magnétisme à haute pression, « Chocs avancés n°14 » page 30 (in french),

General overview of quantum sensing: Degen, C. L., Reinhard, F., & Cappellaro, P. (2017). Quantum sensing. Reviews of modern physics, 89(3), 035002.

Rondin, Loïc, et al. “Magnetometry with nitrogen-vacancy defects in diamond.” Reports on progress in physics 77.5 (2014): 056503.

Distributed quantum sensing: Zhuang, Q., Zhang, Z., & Shapiro, J. H. (2018).

Shaji, A., & Caves, C. M. (2007). Qubit metrology and decoherence. Physical Review A, 76(3), 032111.