Quantum-Referenced Long-Term Stability for PPM-Level High-Voltage Power Supplies
In the most demanding scientific and industrial applications, the stability of a high-voltage power supply is not just a desirable feature; it is the fundamental enabler of the entire measurement or process. Experiments in fundamental physics, such as those searching for the electric dipole moment of the electron or measuring the mass of the neutrino, require voltage stabilities at the parts-per-million level or even parts-per-billion over days or weeks of data collection. Similarly, the calibration of high-resolution particle spectrometers and the operation of precision ion beams for materials analysis demand a voltage reference that is, for all practical purposes, immutable. For decades, the best we could achieve relied on temperature-stabilized Zener diode references and precision resistive dividers, all housed in temperature-controlled enclosures. These systems, while excellent, are still subject to long-term drift due to component aging and mechanical stress. The next frontier, and one that I have been privileged to witness emerging, is the use of quantum phenomena to provide an absolute, drift-free reference for high-voltage generation. This is the realm of quantum-referenced high-voltage power supplies, where the volt is tied directly to fundamental constants of nature.
The most promising quantum standard for high-voltage applications is the Josephson effect. When a superconducting junction is irradiated with microwave radiation, its current-voltage characteristic develops steps at voltages precisely equal to n times the frequency times a fundamental constant, h/2e. This provides a voltage that is known with incredible accuracy, determined only by the frequency of the microwave radiation, which can be referenced to a atomic clock. However, the voltage generated by a single Josephson junction is only a few millivolts. To reach the kilovolt levels required for many applications, we must use a series array of tens of thousands of junctions. These Josephson voltage standards are now commercially available and are used in national metrology institutes to maintain the primary standard for the volt. The challenge, and the focus of much current research, is to integrate such a standard into a practical, programmable high-voltage power supply that can be used outside of a primary metrology laboratory.
The concept of a quantum-referenced power supply is to use a Josephson array as the ultimate reference for a feedback loop that controls a conventional, high-power high-voltage supply. The output of the conventional supply is divided down using a precision resistive divider, and the divided voltage is compared to the voltage from the Josephson array. Any difference is amplified and used to correct the conventional supply's output, locking it to the quantum reference. The accuracy of this system is then limited only by the accuracy of the resistive divider and the noise in the comparison circuit. The resistive divider itself becomes the critical component. It must have an exceptionally low temperature coefficient, and its ratio must be stable over time. It must also be able to withstand the full high voltage without breakdown or leakage. The construction of such a divider is a high-voltage engineering challenge in its own right, requiring careful selection of materials, shielding, and thermal management. The comparison circuit, which must detect differences of a few nanovolts in the presence of the divided high voltage, is equally challenging. It requires ultra-low-noise amplifiers and careful guarding to prevent interference.
The stability achievable with such a system is unprecedented. Once locked to the quantum reference, the output voltage is, in principle, as stable as the atomic clock driving the Josephson array. Drift becomes a non-issue. This opens up new possibilities for long-duration experiments and for the calibration of other instruments with an accuracy that was previously only available at national metrology institutes. However, the system is complex and expensive. The Josephson array must be cooled to liquid helium temperatures, requiring a cryostat. The microwave drive and the low-level detection electronics add further complexity. The practical application of these systems is therefore currently limited to the most demanding use cases, such as in fundamental physics experiments and in primary calibration laboratories. But as technology advances, and as high-temperature superconductors and more compact cryocoolers become available, we may see these quantum-referenced supplies become more widely accessible. In my half-century of work, I have seen the evolution of voltage measurement from the simple galvanometer to the digital multimeter, and now to the quantum standard. The integration of this standard into a practical high-voltage source is the culmination of this journey, offering a level of precision and stability that was once the stuff of science fiction. It represents the ultimate marriage of quantum physics and high-voltage engineering, providing a rock-solid foundation for the most exacting measurements in science and industry.
