Green quantum sensor CSU realizes first sunlight-driven quantum magnetometer

The team of academician Du Jiangfeng of the University of Science and Technology of China has demonstrated a quantum sensor that can be powered by sunlight and ambient magnetic fields, helping to reduce the energy cost of this energy-consuming technology. on October 17, the paper was officially published in the journal Physical Review X Energy [1].

Newly designed quantum sensor made from diamond defects can operate without external power supply.

01Conventional quantum sensors are an energy-consuming technology

When building new quantum sensors, most researchers focus on making the most accurate devices possible, which often requires the use of advanced, energy-consuming technologies. All for remote areas of the Earth, in space or for the Internet of Things not connected to a power source, this high energy consumption can pose a problem when designing sensors. To reduce the dependence of quantum sensors on external energy sources, the team of academician Jiangfeng Du at the University of Science and Technology of China has now demonstrated a quantum sensor that can directly use renewable energy sources to obtain the energy it needs to operate [2]. The new device could expand the use of quantum sensors and help to reduce the energy cost of quantum sensors in existing applications.

Nowadays, quantum technologies are found mainly in research laboratories, which have almost unlimited access to energy. Typical devices operate at low temperatures and require powerful lasers, microwave frequency amplifiers and waveform generators. Such equipment can consume several kilowatts and operate 24 hours a day. One way to reduce these energy costs is to build sensors using systems that do not require cryogenic cooling, such as diamond defects known as nitrogen vacancy (NV) centers. However, such sensors still require powerful lasers that can easily consume 100-1000 W, as well as requiring about 100 W of microwave power. Although researchers are working on miniaturized sensors, a process that typically reduces power consumption; current versions of these smaller sensors still require power from the grid [2].

02Sunlight-driven quantum magnetometer

Taking a different approach, Jiangfeng Du's team has developed a quantum sensor that draws energy from a renewable energy source (in this case, solar energy). The team's sensor is made from a collection of NV centers in diamond, a proven solid-state quantum sensing platform that can operate over a wide range of temperatures (0-600 K), pressures (up to 40 GPa), and magnetic fields (0-12 T).

Nitrogen vacancy centers are defects that are typically created by injecting nitrogen ions into the diamond lattice. These centers confine charge carriers (e.g., electrons or holes), resulting in a localized electronic state. Users can read out the spin of this state by exciting the defect with a laser. NV centers then emit radiation by fluorescence whose intensity correlates with the spin of the system. Researchers typically use green lasers for this excitation because this color of light produces the strongest fluorescence in the system (the emitted radiation is red).

For quantum applications, NV centers are ideal because they operate at room temperature and therefore do not require cooling equipment. They do, however, require a laser to excite the NV center. They also require a magnetic field generator and a microwave frequency amplifier: the fluorescence frequencies of the NV center can be split in two by applying a bias magnetic field, and the two resulting emission peaks can be obtained by sweeping the microwave amplifier across these frequencies. The exact location of these peaks encodes information about any changes in the ambient magnetic field relative to the bias, as well as changes in device temperature or strain.

The experimental team's equipment eliminated the laser and amplifier. Instead of using a laser to excite the NV center, the researchers used sunlight, filtering it with an optical band-pass filter so that only green wavelengths were incident on the NV center. They also used a so-called flux concentrator made of iron to amplify the Earth's magnetic field to about 100-300 G. At these magnetic field strengths, the energy structure of the NV center allows all-optical detection of changes in the ambient magnetic field simply by monitoring the brightness of the device's fluorescence. This capability allows the team to run the sensor without a separate magnetic field generator or a separate external microwave frequency amplifier.

Green light shines on a diamond-based sensor in a quantum device that can be used to measure magnetic fields. In this prototype, a lens (top) collects sunlight, which is filtered to leave only green wavelengths of light. This green light provides an environmentally friendly alternative to the light produced by the high energy-consuming lasers on which conventional quantum devices rely.

Schematic diagram of the magnetometry method driven by sunlight.

Demonstration of sunlight-driven magnetometer.

The team's device requires only 0.1 W to operate - the power needed to run a low-energy photodetector to take spin readings. The researchers showed that they could obtain reasonable sensitivity to detect terrestrial variations in the Earth's magnetic field, such as the presence of nearby power lines or trains. This sensitivity is less than 1 nT/sqrt(Hz), comparable to that achieved by diamond with natural carbon isotope concentrations: a level suitable for detecting changes in biomagnetic fields in the heart or skeletal muscle. In the future, the experimental team could achieve such a level of sensitivity by increasing the sunlight energy entering the device or by tailoring the isotopic content and NV center concentration of diamond [3].

This demonstration is a first step towards using renewable energy sources directly to power quantum technology without connecting to an external power source; also, the experimental data show that their device is more energy efficient than similar grid-connected devices.

Reference link：

[1]https://journals.aps.org/prxenergy/abstract/10.1103/PRXEnergy.1.033002

[2]https://onlinelibrary.wiley.com/doi/10.1002/qute.202000111

[3]https://physics.aps.org/articles/v15/158#c1