What Is Rubidium?

Rubidium is a soft, silvery-white alkali metal with atomic number 37 and the chemical symbol Rb. It belongs to Group 1 of the periodic table, placing it in the same family as lithium, sodium, and cesium. In engineering and physics, rubidium is valued less for its bulk material properties than for the quantum-mechanical behavior of its electrons, which makes it the basis of atomic frequency standards and a preferred species for ultracold atom experiments.

Rubidium was discovered in 1861 by German chemists Robert Bunsen and Gustav Kirchhoff using the technique of flame spectroscopy, which revealed the element's characteristic deep-red emission lines that gave it its name. Two naturally occurring isotopes exist: rubidium-85, which is stable, and rubidium-87, which is weakly radioactive with a half-life of approximately 49 billion years and is the isotope central to most precision measurement applications.

Rubidium Atomic Frequency Standards

The most significant engineering application of rubidium is in atomic frequency standards. A rubidium frequency standard operates by locking a crystal oscillator to the hyperfine transition of the rubidium-87 ground state, which occurs at 6,834,682,610.904 Hz. This hyperfine transition is exceptionally stable, enabling oscillators that outperform quartz by many orders of magnitude in long-term frequency stability. Rubidium clocks are the least expensive and most compact atomic frequency standards commercially available, making them the preferred choice for applications that require atomic-level stability without the cost and complexity of cesium primary standards. Research published by the IEEE on rubidium atomic frequency standards has demonstrated short-term stability reaching the 10^-14 per root-Hz level, approaching the performance of primary standards.

Laser Cooling and Bose-Einstein Condensation

Rubidium-87 is one of the most widely used atomic species in ultracold physics. Its electronic structure is amenable to laser cooling using standard diode lasers near 780 nm, allowing researchers to cool atomic samples to temperatures in the micro- and nanokelvin range. In 1995, Eric Cornell and Carl Wieman at JILA used laser-cooled rubidium-87 to produce the first Bose-Einstein condensate (BEC), a state of matter in which a dilute gas of atoms collapses into the lowest quantum energy state and exhibits collective quantum behavior. That achievement earned Cornell, Wieman, and Wolfgang Ketterle the 2001 Nobel Prize in Physics. Rubidium BECs remain a central tool in experimental quantum physics, and the NIST atomic physics program continues to use rubidium in precision measurement research.

Sensing and Other Applications

Beyond timekeeping and cold-atom physics, rubidium finds use in several sensing and specialty applications. Rubidium vapor cells serve as the active medium in optically pumped magnetometers, which can detect magnetic field variations at the femtotesla level and are employed in biomedical magnetometry and geophysical surveys. In nuclear medicine, the radioisotope rubidium-82 is used as a positron-emission tomography (PET) tracer for myocardial perfusion imaging, exploiting its potassium-like uptake by cardiac muscle tissue. Rubidium is also used in photocells and as a getter material in vacuum tubes, where its high chemical reactivity serves to remove residual gas molecules. A historical survey of frequency standard development, available as a reference from W. Riley's archive on rubidium frequency standards, traces the technology from early laboratory demonstrations in the 1950s to modern miniaturized modules.

Applications

Rubidium has applications across a range of technical fields, including:

• Satellite navigation and telecommunications, as the timekeeping element in GPS satellite clocks and cellular base stations
• Quantum computing and atom-based qubit platforms requiring ultracold neutral atoms
• Biomedical imaging via rubidium-82 PET tracers for cardiac diagnostics
• Precision magnetometry in geophysical and neuroscientific measurements
• Photonics research, including laser-cooled atom interferometry