100 Years of Quantum Research

From its inception 100 years ago, the field of quantum mechanics has produced some of the most unexpected insights into the natural world, unlocking a vast universe of technologies that enhance our daily lives in ways we may not even realize. 

Below, we highlight some of NIST's most influential and important papers that helped advance the frontiers of quantum science research. 

Make sure to check back throughout the year as we highlight additional papers from NIST authors.

Foundational Tests

Quantum mechanics is the science of the very small and the very cold. The world can behave in very nonintuitive ways at these extreme scales, and these behaviors can be exploited to develop fundamentally new tools and technologies. NIST research in basic quantum science has laid a foundation on which we continue to build as more discoveries are made, in turn leading to more technologies and lasting impacts on our quality of life. Below is a sampling of the work NIST researchers have done to test the underlying principles, or foundations, of quantum physics. 

Experimental Test of Parity Conservation in Beta Decay (1957): Researchers from NIST (then the National Bureau of Standards, or NBS) proved that one of the fundamental forces in physics — the “weak interaction” that governs radioactive decay of atoms — violates what was considered a universal law of nature, known as parity conservation. This upended the prevailing theoretical understanding of how particles interact, leading to a new watershed of discoveries.

How Quantum State Mixing Gives Rise to Autoionization (1961): NBS scientist Ugo Fano introduced an important theory describing what happens when atoms and molecules in different quantum states mix. The quantum states combine or “interfere” in such a way that the atoms and molecules can spontaneously emit an electron, a process known as “autoionization.” This definitive theory helped to describe underlying processes in nuclear, condensed matter, atomic, molecular and optical science. Fano’s work also helped scientists better control the production of laser light. Autoionization has proved to be an important process in solar plasma physics and atmospheric science. 

New Autoionizing Atomic Energy Levels in He, Ne and Ar (1963): Shortly after Ugo Fano published his theory paper, NBS researchers Robert Madden and Keith Codling performed an experiment demonstrating autoionization from quantum interference using a unique tool — a synchrotron light source, in which light is emitted by charged particles following a curved trajectory. Synchrotrons are extremely valuable for studying the properties of everything from the light used to make cutting-edge semiconductor chips to gecko feet. NIST continues to operate a synchrotron today, known as SURF-III, at its Gaithersburg campus, and has developed and operates a beamline at the National Synchrotron Light Source II at the Brookhaven National Laboratory in Upton, New York.

Generation of Squeezed States by Parametric Down Conversion (1986): Any quantum state has a certain amount of noise or randomness associated with it. A quantum state is said to be “squeezed” if a component of that noise is lower than that usually dictated by quantum physics. Nobel Prize winner and NIST Senior Fellow Emeritus Jan Hall and colleagues then at the University of Texas at Austin were among the first to demonstrate squeezed light in this groundbreaking optics experiment.

Quantum Zeno Effect (1990): The quantum Zeno effect is a fascinating phenomenon in which researchers make frequent measurements of an atom (or other object obeying the principles of quantum physics) to slow down the rate at which it jumps between quantum states. NIST scientists, including Nobel Prize winner David Wineland, observed the quantum Zeno effect for an ion (charged atom) moving between two energy states by trapping the ions with electric and magnetic fields and making rapid measurements. This is the quantum version of “a watched pot never boils,” but in this case observation doesn’t just seem to slow down the process, it actually does slow down the process. (Make sure to check back, as ion trapping will be featured in a future installment!) 

Sideband Cooling of Micromechanical Motion to the Quantum Ground State (2011): The field of optomechanics is dedicated to the study and use of the quantum states of mechanical resonators — tiny devices that vibrate like microscopic tuning forks. NIST scientist John Teufel and colleagues were the first to cool a mechanical resonator to its lowest quantum state of motion by using light. They made an exquisitely precise measurement of the tiny distance that the mechanical resonator moved as it vibrated, which was the smallest measurement of an object’s displacement to that date. Read the NIST news story about this paper.

Significant-Loophole-Free Test of Bell’s Theorem With Entangled Photons (2015): Bell’s theorem provides a fascinating glimpse into one of the most intriguing aspects of the quantum world — that a quantum object’s physical properties may not exist independently of measurement. Many experiments have sought to confirm Bell’s theorem, but none succeeded without containing significant loopholes that prevent an airtight proof of the theorem. NIST researchers Thomas Gerrits, Adriana Lita, Lynden Krister Shalm and Sae Woo Nam were part of a team that used entangled photons in one of the first tests of Bell’s theorem free of significant loopholes.

Strong Loophole-Free Test of Local Realism (2015): Simultaneously, Gerrits, Lita, Shalm and Nam joined other NIST scientists plus collaborators from elsewhere in the U.S. and abroad to do a second loophole-free test of Bell’s theorem at NIST. This was the culmination of five decades of experimental effort, worth it because proving Bell’s theorem shows that quantum mechanics permits instantaneous connections between objects in far-apart locations, crucial for quantum cryptography and quantum communication. Read the NIST news story about this paper.

Experimental Shot-by-Shot Estimation of Quantum Measurement Confidence (2022): To use quantum measurements for practical tasks, it is necessary to understand and attain the fundamental limits on how accurate the measurements can be. A team of scientists from NIST and the University of Maryland was the first ever to experimentally obtain estimates of the accuracy of an individual “single-shot” measurement of a quantum system.

Clocks

Time is the most measured quantity of all. Among many other applications, timekeeping plays a critical role in synchronizing computers and data networks, time-stamping business transactions, ensuring that the power grid operates efficiently, and enabling us to precisely know our location: As clocks get better, GPS gets better. None of these impacts would be possible without the precision that comes from basing clocks on intrinsic, immutable quantum properties of atoms. Ultraprecise clocks are vital to basic science as well, offering the possibility of unifying general relativity and quantum mechanics once they reach sufficient precision. NIST scientists have always been at the forefront of research in atomic clocks and are moving us ever closer to this monumental goal.

The Atomic Clock (1949): Harold Lyons of the National Bureau of Standards (now known as NIST) led a team that built the first-ever atomic clock. It ushered in a new paradigm where time and frequency, and indeed measurements of all kinds, would be based upon immutable quantum properties. The length of an Earth day is an intuitive benchmark to define time, but it is difficult to make precision measurements based on our rotating planet. And as the solar system ages, the rotation speed of the Earth changes, making it a fundamentally poor standard for time. In the original atomic clock, time was standardized using the internal quantum properties of atoms in an ammonia molecule. NIST continues to lead the world in developing ever-better atomic clocks.

A Microfabricated Atomic Clock (2004): One of the tenets of modern technology is that the smaller you can make a device, the better. NIST researchers were the first to demonstrate that it’s possible to shrink an atomic clock small enough that it can be embedded in hand-held devices and still be capable of delivering the accuracy and precision promised by atomic clocks that fill an entire laboratory. The success of this project inspired NIST to launch the NIST on a Chip program, which seeks to miniaturize other NIST technologies.

Optical Atomic Clocks (2015): Atoms have intrinsic oscillations that can be used as frequency standards for clocks, and some of these oscillations make better clocks than others. Just like smaller gradations on a ruler enable more precise distance measurements, faster oscillations in an atomic clock enable more precise time measurements. This review paper describes how NIST scientists and their colleagues overcame difficult technical hurdles to access faster oscillations — those in the optical as opposed to microwave frequency range — to make a new generation of highly stable and precise atomic clocks: optical atomic clocks.

Resolving the Gravitational Redshift Across a Millimeter-Scale Atomic Sample (2022): Einstein predicted that time passes more slowly in regions where gravity is stronger, an effect known as the gravitational redshift. Ultraprecise clocks are necessary to test this effect: The better the clock, the smaller the length scale that can be tested. NIST has long been a leader in pushing the limits of precision in clocks, and indeed the work referenced here has observed gravitational redshifts at the millimeter scale. As we push even further to shorter and shorter scales, we may eventually enable the unification of general relativity and quantum mechanics.

Nuclear Clocks (2024): As clocks get better, the technologies that depend on them get better, and so it is essential that we continually push toward new and better ways of determining time and frequency. While there are still ways to make incremental improvements in conventional atomic clock designs, the next major step forward will be in using quantum properties of the nucleus for timekeeping. In this recent paper, NIST scientist Jun Ye describes the world’s first nuclear clock.

Laser Cooling

Cooling an atomic gas — that is, slowing down the random thermal motion of atoms in the gas phase — was one of the most important developments of 20th-century physics, enabling a multitude of new discoveries, techniques and technologies. With contributions leading to Nobel Prizes, NIST researchers were among the very first to develop the ability to cool atoms with pressure from light and hold the cold atoms in a trap. NIST continues to lead in this arena today as techniques from laser cooling are exploited for a variety of applications in measurement science and quantum information, while expanding our knowledge of other quantum systems.

Radiation-Pressure Cooling of Bound Resonant Absorbers (1978): In 1978, David Wineland and colleagues at NIST (then the National Bureau of Standards or NBS) were the first to demonstrate cooling of particles, specifically magnesium ions, by shining light on them. They cooled a hot gas of ions (electrically charged atoms) from a temperature of about 700 kelvin to less than 40 kelvin, well into the cryogenic temperature range, using only laser light.

Laser Cooling of Atoms (1979): David Wineland from NIST and Harvard colleague Wayne Itano, who later joined NIST, presented a theoretical framework for cooling atomic gases. By laying out what is essentially a recipe for laser cooling of atoms and determining the lowest temperatures to which the gas could theoretically be cooled, they enabled research groups around the world to begin working in earnest on this nascent idea. Wineland’s groundbreaking contributions to the field of laser cooling led to experiments in which he quantum mechanically manipulated single or several ions, work that would be recognized with a Nobel Prize in 2012, shared with Serge Haroche of the College de France.

Laser Deceleration of an Atomic Beam (1982): The cooling of neutral atoms presents challenges beyond cooling of ions, because there isn’t an electric charge to “grab onto” to confine them. To devise a method for decelerating a beam of neutral sodium atoms, William Phillips and Harold Metcalf at NIST exploited the fact that atomic energy levels shift in the presence of magnetic fields (known as the Zeeman shift). A spatially varying magnetic field called a “Zeeman slower” allowed them to reduce the atoms’ speed from about 1,000 meters per second to about 10 meters per second, enabling their confinement.

Stopping Atoms With Laser Light (1985): Building upon their success in cooling or slowing neutral atoms, Phillips and colleagues went a step further to demonstrate that an ensemble of atoms can be brought effectively to rest and confined to one spot or trapped. Their innovation was to allow cold atoms to drift out of the Zeeman slower described in the paper above, stop them with an additional pulse of light, then capture them in a magnetic trap. These and other experiments, such as the discovery of sub-Doppler cooling, led to NIST’s first Nobel Prize, awarded to Bill Phillips in 1997 with Steven Chu and Claude Cohen-Tannoudji.

Observation of Atoms Laser Cooled Below the Doppler Limit (1988): Atoms in a gas randomly emit and absorb photons, which add heat to the gas and impose a theoretical lower limit to the achievable temperature, calculated by Wineland and Itano as described above. Early experiments in laser cooling reported temperatures at or above this so-called Doppler limit, which for sodium atoms is 240 µK (millionths of a degree above absolute zero). Phillips and colleagues serendipitously discovered that it was possible to achieve temperatures well below the Doppler limit (because of mechanisms more complicated than those considered in the first theoretical treatments). This sub-Doppler cooling made possible new classes of experiments in ultracold matter.

Generating Solitons by Phase Engineering of a Bose-Einstein Condensate (2000): Early work in laser cooling and trapping led to the creation of Bose-Einstein condensates or BECs (check back later for our section on “New States of Matter” to learn more about BECs). Because they comprise many atoms in the same quantum state, BECs are great tools for “quantum phase engineering,” a method to carefully control and manipulate quantum systems. The atoms in BECs act in very interesting ways; they can flow as frictionless “superfluids” or propagate as “solitary waves” or solitons that preserve their shape as they move through space, unlike ordinary matter waves, which spread out in space as they propagate. The first quantum phase engineering experiment was carried out by Phillips and colleagues, who demonstrated the generation and propagation of solitary waves or solitons in a BEC.