PERSPECTIVES
Quantum sensors
QUANTUM SENSORS AND THEIR LIMITS
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Mathieu JUAN * Mathieu Juan, Institut quantique et Département de Physique, Université de Sherbrooke, Sherbrooke J1K 2R1 Québec, Canada * Mathieu. Juan @ USherbrooke. ca
https:// doi. org / 10.1051 / photon / 202513468
A fundamental aspect of quantum systems lies in their extreme sensitivity to external perturbations. This characteristic, which generally represents a considerable challenge for physicists seeking to isolate these systems in order to observe their quantum properties, can actually be transformed into an advantage. Indeed, this intrinsic sensitivity makes quantum systems ideal candidates for high-precision metrology. Quantum sensors exploit this susceptibility to measure physical quantities with precision difficult to match with classical approaches.
To illustrate how quantum system and their interaction with the environment can be tuned for a given application, it is interesting to consider the example of the single-Cooper-pair box. Composed of a small superconducting electrode connected to a reservoir via a Josephson junction, the Cooper-pair-box constitutes an artificial two-level system that can be used as a qubit: the charge qubit. By controlling the exact configuration, particularly the ratio of Josephson energy to charging energy, this type of superconducting circuit can be made sensitive or unsensitive to charges, a characteristic directly observable in the transition energy spectrum as a function of the effective charge offset( see Fig. 1). While quantum information processing demands systems that are highly isolated from environmental noise to preserve quantum coherence, quantum sensors deliberately exploit their interaction with the environment.
In this context, it is interesting to precisely identify the important properties required for a quantum sensor. Quantum sensors constitute a specific class of measurement devices that exploit the properties of quantum mechanics to detect and quantify physical quantities. Inspired by DiVincenzo ' s criteria for quantum computers, Degen et al. [ 1 ] proposed that a quantum sensor is distinguished by four essential characteristics:
•( i) The system must possess discrete energy levels, a fundamental characteristic of quantum systems.
•( ii) It must be possible to initialize the system in a well-defined quantum state, generally the ground state or a specific superposition state.
•( iii) The system must be able to be manipulated coherently, thus allowing controlled quantum operations.
•( iv) The quantum system must interact with the physical quantity of interest V( t), and this interaction must modify the transition energy of the system, generally in a linear or quadratic manner.
These criteria define a conceptual framework that encompasses a wide variety of physical systems- from cold atoms to nitrogen-vacancy color centers in diamond, to Josephson junctions and superconducting qubits. It also becomes apparent that
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