Why Simple Tasks Like Charging Rely on Mobile Phone Measurements

If you’re anything like me, your smartphone is almost always connected to a charger. What we often overlook is that the capacity to safely conduct intricate quantum measurements in cutting-edge physics hinges on safety standards.

To grasp this, consider what occurs when you connect the charger to a standard socket. The electricity flowing from the outlet exceeds 100 volts, yet the charger is engineered to reduce it to around a dozen volts as it reaches the phone. Without this voltage reduction, the device would be damaged.

Essentially, the precise voltage matters in a specific way. But how can one truly know the value of a single volt? Moreover, when reporting voltages, can we fully trust the manufacturers of phone chargers?

This may appear to be merely a scientific query; however, in the U.S., the volt has a legal definition established in 1904, governed by the National Institute of Standards and Technology (NIST). Various countries maintain their own national measurement units for the same purpose, such as the UK’s National Physics Institute.

For volts, NIST’s definition has relied on quantum devices for over three decades. In this process, the metrologist begins with a series of superconducting junctions—like crosswalks in narrow superconducting regions separated by insulation—and exposes them to microwaves of extremely specific frequencies. This stimulates a purely quantum phenomenon that creates voltage differences across junctions. The number of volts contributing to this difference is directly linked to two of the universe’s fundamental constants. This allows scientists to define a volt based on what we understand as foundational to our physical reality.

Specifically, the two constants involved are Planck’s constants that connect the charge of an electron—a fundamental quantum particle—to the energy of a photon (a quantum particle of light) and its frequency. Remarkably, the connection between charging a mobile phone and the most basic elements of the quantum realm is quite brief.

However, volts are not solely entrenched in the quantum realm. In fact, in 2018, metrologists globally unanimously voted to redefine several entries in the International System of Units (SI Units) with close ties to microscopic details.

Some unit changes were quite radical. For instance, kilograms are now defined in terms of a combination of Planck’s constant, the speed of light, and the frequency at which electrons in a specific type of atomic clock “click,” derived from platinum alloy polished only by the hide of endangered European goats. If you’ve recently stood on a scale at your doctor’s office, you’re witnessing how quantum physics influences the numbers displayed there.

The shift towards quantum-based definitions of units underscores the remarkable scientific advancements achieved in the past decades concerning our understanding, control, and exploration of the microscopic world. For example, I spoke in January with Alexander Epri at the University of Colorado Boulder, a key player in developing some of the most accurate clocks globally. “Frequency measurements have reached unprecedented levels of precision,” he noted. The frequencies from these clocks are linked to the electron transitions between energy levels within atoms, governed by quantum physics.

This extraordinary control over quantum systems places humans at the “top tier” of quantum measurements, yielding benefits beyond merely defining time. For example, atomic-based clocks may play vital roles in next-generation early warning systems for earthquakes and volcanic activities.

Moreover, the move towards quantum methodology could democratize access to the world’s premier metrics. Before the 2018 SI unit redefinition, manufacturers, researchers, and technicians needing to validate the accuracy of their devices often had to seek certification at local Metrology Institutes, where certified experts operated. The current standard for certification essentially requires sophisticated labs. “As we’ve mentioned previously, the aim is to put ourselves out of business,” Richard Davis from the International Bureau of Weights and Measures stated, which oversees SI systems. “The entire system has become more adaptable and significantly less Euro-centric.”

“We possess ample equipment, so individuals come to us. However, this redefinition is one of our focal points since people aren’t sending their instruments to us; we’re teaching them how to measure independently,” Jason Underwood explained to me in August. “Currently, this framework operates under the new SI. Our aim is to develop instruments that can establish traceability to the basic constants of the universe.”

He and his team recently introduced a prototype of a quantum device capable of measuring three distinct electrical units simultaneously, including volts. By offering this three-in-one functionality, such devices could make it much simpler and more cost-effective to compare electronic devices against relevant standards, assuming they remain portable.

As we have evolved our understanding of units, what might the future hold? For electrical units like those designed by Underwood and his team, the Quantum Standard has yet to achieve international acceptance akin to the second or kilogram, with further experiments necessary to reach that milestone. Similar innovations are emerging in other parts of the world, including the EU-based Quahmet Consortium.

The concept of the second, too, is fluid, reflecting researchers’ ongoing endeavors to refine atomic-based clocks and redefine our understanding of time measurement. In April, I reported on some cutting-edge timepieces created by an international team on a mission to compare models from Japan, Germany, and other nations. This research is ongoing, and I look forward to sharing more about quantum clocks in the future.

Despite metrologists’ pursuit of stability in definitions, measurement work is inherently variable, tied closely to national funding strategies and international relations. This was evident in 1875, as representatives of the first international measurement treaty confronted political tensions between France and Germany following the Franco-Prussian War. This remains relevant today—as I reported on NIST’s work in August, discussions included the institutional infrastructure’s challenges, highlighted by a proposed 43% budget cut by the Trump administration earlier this year. Though Congress ultimately dismissed this proposal, it underscores the complexities of disentangling Metrology Institute operations from national politics.