What is the Planck constant? The tiny number behind World Quantum Day and the kilogram

The latest “Google Doodle” on the homepage of Google celebrates advances in quantum physics to spark wider public curiosity about the field. The April 14, 2026 illustration highlights the Bloch sphere, a visual model used to explain how qubits, the building blocks of quantum computing, function.

Google’s Doodle webpage explains the distinction between classical and quantum computing. “While a classical computing bit is strictly 1 or 0, a qubit can exist in a blend of both, called a superposition,” Google explained. “By harnessing superposition, along with other quantum properties, quantum computers can solve specific, complex problems that are out of reach for today’s supercomputers to solve.” But why was April 14 chosen as World Quantum day and what's its relationship to the measuring unit, the kilogram?

Why is April 14 celebrated as World Quantum Day?

The Doodle coincides with World Quantum Day, observed globally each year on April 14. The date is not arbitrary; it references 4.14, the rounded first digits of the Planck constant: 4.1356677×10⁻¹⁵ eV⋅s = 0.0000000000000041356677 electron volt seconds, a product of energy and time that is the fundamental constant governing quantum physics.

What is the Planck constant?

The Planck constant defines how energy behaves at the smallest measurable scale and appears throughout quantum mechanics. In simpler terms, this constant is used for describing the behavior of particles and waves at an atomic scale. It is valued at h = 6.6260693(11) x 10⁻³⁴ J s., where uncertain values in the decimal place are contained in brackets. Planck’s constant has the units of action (energy x time, which can be shown to be the same as momentum x length).

It was first introduced by Max Planck in his explanation of blackbody radiation. Later, Albert Einstein showed it represents the proportionality between energy (E) and frequency (f) of photons: E = hf

Although highly technical, quantum mechanics influences everyday technology. Systems such as GPS navigation, MRI scanners, semiconductors, lasers, and smartphones rely on principles derived from quantum physics, making the Planck constant foundational to modern life.

How the Planck constant redefined the kilogram

The impact of the Planck constant extends beyond theory to measurement standards. For decades, the kilogram was defined by the International Prototype Kilogram, a platinum–iridium cylinder stored in Paris. It remained the only SI base unit tied to a physical artifact.

That changed on World Metrology Day in 2019, which commemorates the 1875 Metre Convention. The kilogram was redefined using precise measurements of the Planck constant. Since the meter and second are already based on constants of nature, this shift allowed the kilogram to be defined universally.

“The value of a kilogram can be obtained without relying on comparison with a physical reference block," explained CERN.