Physics – “green” quantum sensor
• physics 15, 158
Researchers have demonstrated a quantum sensor that can be powered using sunlight and an ambient magnetic field, an achievement that could help reduce the energy costs of this energy-hungry technology.
No longer the realm of science fiction, quantum sensors are now used in applications ranging from timing and gravitational wave detection to nano-magnetometry . When developing new quantum sensors, most researchers focus on creating devices that are as precise as possible, which usually requires the use of advanced – energy-hungry – technologies. This high power consumption can be problematic for sensors designed for use in remote locations on Earth, in space, or in off-grid IoT sensors. To reduce the reliance of quantum sensors on external energy sources, Yunbin Zhu of the University of Science and Technology of China and colleagues are now demonstrating a quantum sensor that directly uses renewable energy sources to get the energy it needs to operate. . The new device could expand the use of quantum sensors, as well as help significantly reduce the energy costs of quantum sensors in existing applications.
Today, quantum technologies are mostly found in research laboratories, which have virtually unlimited access to energy. A typical device operates at cryogenic temperatures and requires powerful lasers, microwave frequency amplifiers and waveform generators. Such a device can consume thousands of watts and works 24 hours a day. One way to reduce these energy costs is to make sensors from systems that do not require cryogenic cooling, such as diamond defects known as nitrogen vacancy (NV) centers. However, such sensors still require a powerful laser, which can easily consume 100-1000 W, and a microwave power supply that requires about 100 W. Researchers are also working to miniaturize the sensor, a process that typically reduces power consumption. But current versions of these smaller sensors still draw power from the grid .
Zhu and colleagues take a different approach by developing a quantum sensor that generates its own energy from a renewable energy source, in this case solar energy (Fig. 1). The team’s sensor is made from an ensemble of NV centers in diamond, a well-established 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 typically created by implantation of nitrogen ions into the diamond lattice. Centers confine charge carriers—such as electrons or holes—creating a localized electronic state. Users can read the spin of this state by exciting the defect with a laser. The NV center then emits radiation, via fluorescence, whose intensity is correlated with the spin of the system. Researchers typically use a green laser for this excitation, because that color of light produces the strongest fluorescence in the system (the emitted radiation is red).
For use in quantum applications, NV centers are ideal because they operate at room temperature, so no cooling apparatus is required. However, they require a laser to excite the NV center. They also require a magnetic field generator and a microwave frequency amplifier: the fluorescence frequency of the NV center can be split into two parts by applying a bias magnetic field, and the two resulting emission peaks can be accessed by passing the microwave amplifier through these frequencies. The exact positions of these peaks encode information about any changes in the ambient magnetic field relative to the bias, as well as changes in device temperature or voltage.
Zhu and his colleagues’ device removes both the laser and the amplifier. Instead of using laser light to excite the NV center, the researchers use sunlight, filtering it with an optical transmission filter so that only green wavelengths enter the NV center. They also use 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 centers allows full optical detection of changes in the ambient magnetic field just by monitoring the device’s fluorescence intensity. This capability allows the team to operate the sensor without a separate magnetic field generator or a separate external microwave frequency amplifier.
The team’s device requires only 0.1 W to operate — that power is needed to drive a low-power photo detector to read the spin. The researchers show that they can achieve reasonable sensitivity for detecting changes in the Earth’s magnetic field caused, for example, by the presence of nearby power lines or trains. This sensitivity — less than 1 nT/sqrt(Hz) — is equal to that achieved for diamond having natural concentrations of carbon isotopes — diamond typically contains two isotopes, C12 go13. Greater sensitivity has been achieved with isotopically pure, laboratory-grown diamond, and the best is around 1 pT/sqrt(Hz)—a level suitable for detecting changes in biological magnetic fields in the heart or in skeletal muscle. I imagine they could achieve that level of sensitivity by increasing the energy of sunlight entering the device or by adjusting the isotopic content of the diamond and the concentration of the NV center.
This demonstration is the first step towards directly powering quantum technologies with renewable energy, eliminating the need to connect them to an external power source. In doing so, Zhu and colleagues demonstrate that their device is much more energy efficient than similar grid-connected devices.