APS Journals

The probability that an excited two-level system (e.g., an excited atom) will emit a photon decreases exponentially with time. However, this emission rate is much faster if a second atom is placed nearby—even if the second atom is in its ground state. Robert Dicke made this surprising theoretical discovery, which occurs because the quantum states of nearby atoms are correlated, in 1954. Dicke also showed that emission events in a large group of particles are not independent, leading to a significant increase in the radiative power of the system, a behavior he named superradiance. Since Dicke’s exploratory study, the phenomena of superradiance has been observed in many physical systems such as optically pumped hydrogen fluoride gas, quantum dots, and superconducting qubits. The effect has recently been used to make a superradiant laser, where the correlated atomic emission boosted photon emission by a factor of 10,000.

Losses can occur in devices, such as semiconductor lasers, when light of unwanted frequencies is spontaneously emitted. In the late 1980s, Eli Yablonovitch and Sajeev John were separately working on ways to avoid these losses. Yablonovitch observed that losses didn’t occur in mediums that prevented the unwanted frequencies from propagating. He proposed that such a medium could be made by carving voids into an optically transparent material, where the material and voids had different refractive indices. Later termed “photonic crystals” by Yabonovitch and John, these structures reduced losses because the periodic voids caused waves with certain frequencies to destructively interfere. Photonic crystals are now widely used in optical fibers, light-emitting diodes, high-energy particles accelerators, and medical devices.

Since the 19th century, light was known to be a wave of electric and magnetic fields. In the early 20th century, the physics revolution known as quantum mechanics included the alternative description of light as a collection of particles called photons. But Roy Glauber of Harvard University realized that with the invention of the laser and the detectors capable of sensing a single photon, a more complete quantum theory of light was needed. His theory, published in 1963, explained that photons are not entirely independent objects and that detecting a photon from a light beam affects the probability of detecting another photon. His theory marked the birth of the field of quantum optics, which led to a wide range of advances, such as a type of optics-based cryptography that has already been used for some bank transactions. Glauber shared the 2005 Nobel Prize in physics for his work. [Reprinted from Physics].

Squeezed light is a special state of light where the noise caused by quantum effects has been reduced, or “squeezed out.” Predicted by Horace Yuen, squeezed light can be advantageous for certain precision measurements, or for optical communications that require a low signal-to-noise ratio. In the late 1980s, three experimental groups used three different media—an optical cavity, an optical fiber, and a parametric crystal—to demonstrate squeezing of light. The parametric crystal method of Ling-An Wu and colleagues has gone on to be the most widely adopted of the three. This method uses an incoming photon to excite a resonant transition in a nonlinear crystal, which then decays into two quantum-correlated photons. This correlation reduces the uncertainty in one parameter, e.g., electric-field amplitude, leading to outgoing photons with reduced noise.

In 1964, John Bell derived a set of inequalities that allowed arguments surrounding the foundations of quantum mechanics to be experimentally settled. Quantum mechanics predicts strong correlations between measurements performed on particles that have interacted, even if they are physically separated at the time of measurement. Einstein, Podolsky, and Rosen believed these predictions meant quantum mechanics was incomplete, and they argued that additional “hidden variables” were needed to make the theory compatible with the classical ideas of causality and locality. Bell showed that if “hidden variables” existed, certain inequalities had to hold for polarization measurements on particles that had interacted, but that these inequalities would be violated in a purely quantum system. In 1981 and 1982, Alain Aspect and colleagues tested Bell’s inequalities using pairs of photons emitted by a calcium atom undergoing two-step decay. They found that all of Bell’s inequalities were violated, and the idea of “hidden variables” was discredited. Bell’s reading of quantum mechanics, and its subsequent test, has been hugely influential, serving today as a basis for research into quantum technologies, quantum communications, and quantum metrology.

Stable, easily controllable sources of entangled photons are needed to encode quantum information and relay it elsewhere. Stray photons, or noise, could cause unwanted errors. Early attempts at creating an entangled photon source used atoms that emitted entangled light in a two-step decay called a cascade, but the process was too inefficient to be useful. In 1995, Paul Kwiat and colleagues overcame these efficiency issues using a nonlinear crystal to prepare entangled photons. In their method, photons from an incident laser excited a resonant transition in the crystal, which then decayed into two correlated photons. This light could then be collected into a single beam that was an order of magnitude brighter than that produced by other methods. The scheme developed by Kwiat et al. has today evolved to become the most reliable way to produce entangled photons.

Careful control of atom excitations can make an opaque material transparent to certain wavelengths of light. Klaus Boller and co-workers first demonstrated this phenomenon in the early 1990s using strontium vapor. Strontium has two ground states that can both be excited to the same excited state. By carefully tuning the frequencies of two incident lasers, Boller et al. were able to ensure that the probabilities of the different excitation pathways destructively interfered, cancelling out any excitation. The strontium vapor, which was opaque to the separate lasers, was now transparent to both. Electromagnetically induced transparency has since been achieved in atomic gases, diamond, and superconducting qubits. As well as making materials transparent, this effect has been used to slow and stop light, to measure the velocity of cold atoms, to induce lasing, and for high-precision magnetometry.

Quantum technologies require that information encoded in a photon be stored without damaging its quantum properties. Michael Fleischauer and Mikhail Lukin predicted the controlled propagation of photons in a material with electromagnetic- induced transparency, showing that photons could be stopped, and stored, and their quantum state preserved. Lukin and colleagues then went on to demonstrate this in a hot rubidium vapor, where they decelerated a light pulse, trapped it for up to 200 microseconds, and then released it. In the last fifteen years, using these methods, the storage and retrieval of light has been achieved in a variety of systems ranging from gases to defects in solids. The longest storage time of a photon to date is greater than 39 minutes.

In 1989, William Moerner and Lothar Kador were the first to optically detect a single molecule. Prior to this, optical imaging techniques had been limited by the wavelength of visible light—anything smaller than about 500 nanometers couldn’t be imaged and instead appeared blurry. By measuring the absorption spectrum of a crystal containing a few pentacene molecules, Moerner and Kador were able to overcome this limit to detect much smaller structures. The team carefully tuned the frequency of their laser so that, at most, one of the molecules would absorb (and then emit) light. Double modulation techniques were then used to remove background noise, allowing light from a single molecule to be detected. Single-molecule imaging was further progressed by the discovery, again by Moerner, of fluorescent molecules that could be turned on and off using different wavelengths of light. Today, these fluorescent molecules can be used to stain and label heart cells, neurons, and other complex structures, allowing high-resolution images to be obtained. In 2014, Moerner shared the Nobel Prize in Chemistry for his work on single-molecule imaging.

The 1962 invention of the diode laser allowed lasers to go from expensive, specialized devices to cheap and ubiquitous components of consumer appliances. A diode is a chunk of semiconductor material that allows electric current to pass in only one direction, and in 1962 researchers reported that diodes made from gallium arsenide emit large amounts of infrared light. The emission comes from the collision and merging of the two types of charge carriers found in semiconductors—negative electrons and positive holes (vacancies that correspond to missing electrons). To make a resonant “box” for this light, equivalent to the box used in the maser, Robert Hall and his colleagues at the General Electric Research Laboratory in Schenectady, New York, made a small crystal of gallium arsenide and polished two opposite faces. This diode produced an infrared beam, but later improvements led to the standard red light we see in use today. [Reprinted from Physics].

Precise measurements of the frequencies of light waves emitted by atoms have helped physicists make great progress in understanding the quantum world and the nature of atoms. But a truly high-precision measurement of the frequency of light has traditionally been a huge project, available at only a handful of labs. The problem is that there’s a large gap between the precisely-known frequency of an atomic clock (10⁹ cycles per second, or Hz) and that of the visible light emitted by atoms (around 10¹⁵ Hz).

In 2000, a team in Colorado reported a much simpler technique. John Hall of the National Institute of Standards and Technology (NIST) and the University of Colorado in Boulder and his colleagues used a laser that produces a short pulse a hundred million times per second in a highly controllable way, which is synchronized with an atomic clock. After being sent through an optical fiber, the beam contains the equivalent of millions of laser beams with precisely-known frequencies that are equally spaced, like teeth on a comb. This comb is like a ruler, and the team showed they could use it to precisely measure another laser’s frequency by finding the closest “tooth” on the comb. Hall won a portion of the 2005 Nobel Prize in physics for this work.

A lens focuses light from an object to create an image. But diffraction limits the size of the smallest feature that can be formed with a lens to roughly half the wavelength of light. In 2000, John Pendry proposed a way to circumvent this limit using a material with a negative refractive index to perfectly focus light. As light travels between two media, some parts of the wave are lost in rapidly decaying evanescent waves. Pendry showed that in a negative-refractive-index material these evanescent waves grow rather than decay. In a lens formed of such a material, all parts of the light are collected, making it possible to image objects much smaller than light’s wavelength. Formed from an array of split-ring resonators, David Smith and colleagues demonstrated the first negative-refractive-index material six months prior to Pendry’s proposal. Despite this, a perfect lens has not yet materialized, largely because the material in the lens needs to absorb less light than is currently achievable.

A ball dropped from the top of a building accelerates because of Earth’s gravitational pull. How much it speeds up and how fast the ball travels depends on how you view the ball (if you were accelerating at the same speed as the ball it would appear to be stationary). This holds for all moving things except light, which travels at the same speed in all reference frames. But Einstein’s theory of general relativity predicted that gravity’s pull would shift a light beam’s frequency slightly higher as it got closer to Earth’s surface. In 1959, Robert Pound and Glen Rebka discovered a technique with the precision needed to measure the small frequency shift, which was expected to be only a few parts in 10¹⁵ for a photon falling from the top of a building. Using a gamma-ray source positioned on the roof of a building at Harvard University, they verified Einstein’s predictions to within ten percent. By 1964, this agreement was to within one percent.

In the 1970s and 80s, physicists learned to use lasers to slow down and trap atoms in order to study them in more detail. Three important developments in atom cooling techniques led to a shared Nobel prize in 1997. First Steven Chu, then at Bell Labs in New Jersey, and his colleagues, developed “optical molasses.” In this technique, laser beams hit a cloud of atoms from six different directions. The lasers are tuned to frequencies of light that the atoms can absorb only when they are moving toward a beam. Absorbing the light slows an atom down, and the process can rapidly cool a cloud of sodium atoms to less then a thousandth of a degree above absolute zero. Three years later, two other teams came up with additional tricks to “thicken” the molasses and cool it down to two millionths of a degree above absolute zero. These techniques for slowing and cooling atoms led to dramatic improvements in extremely precise clocks based on atoms and to the creation of a new ultracold state of matter called a Bose-Einstein condensate (see Physics article The Coolest Atoms).

Light can slow down and trap atoms, allowing them to be individually manipulated and probed. These cooled atoms are also useful for understanding light. In 1996, researchers led by David Wineland used light to change the mechanical state of beryllium ions that had been captured and cooled in a laser trap. Varying the frequency of two additional lasers incident on the ion, they were able to induce different vibrational states in the ion, showing that its mechanical motion could be controlled by light. Simultaneously, the group of Serge Haroche was able to probe microwave-length photons trapped in a cavity with rubidium atoms. By measuring the atom after it had passed through the cavity, the group was able to determine the precise number of photons in the cavity, showing that the light was quantized. David Wineland and Serge Haroche won the 2012 Nobel Prize in Physics for this and other work, which led to new tools for studying the quantum nature of light.

Light’s electric field can polarize the atoms in a material. For low light intensity this polarization is linearly proportional to the electric field. But at high intensities (electric fields ~ 10⁸ volts per meter) the linear dependency doesn’t hold, and new phenomena are observed, such as a change in the light’s frequency as it travels through the material. In 1961, Peter Franken and colleagues used a high-intensity ruby laser to induce nonlinear behavior in a quartz crystal. They showed that light emitted from the crystal had double the frequency of the incident beam. An in-depth quantum-mechanical treatment of the phenomena was later carried out by John Armstrong and colleagues, providing the theoretical foundations for nonlinear optics. A number of applications in the medical and biological sciences rely on nonlinear behavior, including endoscopy and tissue imaging.