What is a quantum sensor?

The principal concept of any sensor is based on the interaction between the sensor and a system of interest which can change the state of the sensor. Readout sensor’s state change provides information about physical quantities that characterize the system. A classic example is a thermistor which is nowadays widely used to monitor the temperature of things i.e. battery pack while charging. When the thermistor interacts to the battery pack via thermal contact, heat from the pack transfer to the thermistor and change its electrical resistance. Measuring this resistance provides a mean to measure the temperature of the battery pack. However, the thermistor is not a quantum sensor because its state does not possess quantum coherence.

In quantum-enhanced sensors or so-called quantum sensors, the sensor utilizes quantum coherence, such as superposition and entanglement, of quantum objects i.e. atoms, molecules, photons or even artificial atoms to enhance sensitivity, precision, and accuracy. Since quantum superposition is naturally very sensitive to the environment, it can, therefore, be used to make high sensitivity sensors. Quantum entanglement provides means for designing high precision sensor beyond the standard quantum limit (SQL) while at the same time maintain the sensitivity. Moreover, some atoms and molecules possess special quantum states which are sensitive to a physical quantity in a unique way while insensitive to others. Combining this with technological advances in quantum controls which allows one to manipulate these quantum states, it can be used to design an accurate quantum sensor for that quantity.

The Figure 1 shows a diagram providing an overview of quantum sensors and the associated measuring physical quantities. All of these devices are undergoing research around the world and many prototypes have been developed to real-world applications.

The impact of quantum sensing technologies is expected to be broad and considerable. From ultra-high-precision spectroscopy and microscopy, positioning systems, clocks, gravitational, electrical and magnetic field sensors, to optical resolution beyond the wavelength limit.

Electric field sensing

Why electric field sensing?
Electric field measurement happens almost everywhere around us in this modern world, in particular, measuring electric field component of electromagnetic wave. We use them for communication i. e. Wi-Fi, Radar, Bluetooth, radio FM, Cellular. It involves the electric spectrum whose frequency is anywhere from about kilohertz which are a kilometer long wavelengths to hundreds of gigahertz with a wavelength of about a millimeter. We commonly use lower frequencies but there is also a push to go to higher frequencies so that data transfer can be higher.

One of many motivation why we need high data transfer rate is that there is signal drop out on cellular (3G/4G LTE) when you go to the BITEC stadium for a concert or exhibitions and you cannot call or use your internet. That’s really because the bandwidth is too low and everyone is trying to use. Therefore going to higher and higher frequencies is something that the federal government and mobile phone companies are trying to do.
An important key to do this is to be able to both emit and receive high-frequency radiation. Antenna technology has been around for a while since its invention. It converts electromagnetic radiation to current through a metal structure. The first horn antenna was implemented in 1938, World War II with radar in the 1940s, and until the first iPhone in 2007. In 2020 it is predicted that with internet of things (IoT) we are all synced up and there is going to be over 50 billion devices that are all connected to internet and talking to one another. What is amazing is that antenna technology has held for this long so we rely on antenna technology.
However, there are limitations that these antennas have and some of the problems are listed below,

  • Narrow bandwidth (for a certain structure of an antenna)
  • Require calibration (sensitive to material and shape)
  • Susceptible to electromagnetic wave interference

Although there are a lot of effort and advanced techniques in engineering design of antenna, physicists are still looking forward to utilize quantum objects to create better electric field sensor which beside from probin 1.2 Rydberg Atom as a quantum antenna
Rydberg atom refers to any atom which is in highly excited quantum state. This kind of special state is very sensitive to external electric field.