TRJ QUANTUM LEAP: THE FIRST QUANTUM LEAPS EVER OBSERVED: HOW SCIENTISTS CAPTURED ONE OF PHYSICS’ MOST ELUSIVE PHENOMENA

In 1986, three independent research teams achieved a historic breakthrough in experimental quantum physics, observing individual atomic transitions that had been predicted for decades. Their discoveries transformed quantum jumps from a theoretical description of matter into directly detectable events, opening an extraordinary new chapter in the scientific investigation of individual atoms

For much of the twentieth century, one of the most fundamental processes in quantum mechanics remained beyond the reach of direct experimental observation at the level of an individual atom. Scientists had developed mathematical descriptions of atomic energy states, measured the radiation associated with transitions between them, and established that matter behaved according to principles fundamentally different from those governing the visible world. Yet the ability to isolate a single atomic particle and monitor its internal quantum transitions presented a technological challenge that demanded exceptional advances in precision, electromagnetic confinement, and optical detection.

That challenge reached a historic turning point in 1986, when researchers working at the University of Washington, the National Bureau of Standards in Colorado, and the University of Hamburg independently reported observations of individual quantum jumps. Their experiments involved ions held within electromagnetic traps, illuminated by carefully controlled radiation, and monitored through changes in the light those particles emitted. The researchers were not witnessing an electron travel through space along a visible trajectory. They were detecting changes in the internal quantum states of individual ions through measurable interruptions in their fluorescence, providing evidence that the transitions predicted by quantum theory could be identified within isolated atomic systems.

The achievement represented a major advance in experimental physics because it brought individual quantum-state behavior into an observable laboratory setting. Earlier measurements had already demonstrated the accuracy of atomic energy quantization, but the 1986 experiments allowed scientists to distinguish the changing states of individual trapped particles through their optical signals. The resulting measurements exposed sequences of activity and inactivity that reflected quantum transitions occurring within the ions, providing a new opportunity to investigate the statistical and dynamical properties of processes previously studied largely through large populations of atoms.

The work established a foundation that extended far beyond the immediate observation of quantum jumps. Once researchers could isolate a single atomic ion and distinguish its internal states, they possessed an experimental starting point for advanced techniques involving state preparation, controlled excitation, precision spectroscopy, and quantum measurement. The same general capabilities would eventually become essential to atomic frequency standards, trapped-ion quantum computing, and investigations into how quantum systems respond to repeated observation and carefully engineered interactions.

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The Scientific Challenge That Preceded the Breakthrough

The possibility of quantum jumps emerged from the early development of quantum theory, particularly Niels Bohr’s 1913 atomic model. Bohr proposed that electrons within atoms could occupy distinct permitted energy states and that radiation was emitted or absorbed when an atom transitioned between those states. His model helped explain the characteristic spectral lines of hydrogen and introduced a framework in which energy changes at the atomic level could occur through discrete transitions rather than unrestricted continuous variations.

The later development of quantum mechanics during the 1920s replaced the limitations of Bohr’s early orbital picture with mathematical descriptions based on quantum states, wavefunctions, and measurable probabilities. Atomic energy levels remained central to the new framework, but the behavior of particles could no longer be adequately represented as miniature objects traveling along fixed classical paths. Instead, the theory described how quantum states evolve and how interactions with radiation, measurement devices, and surrounding environments influence the probabilities of different outcomes.

By the middle of the twentieth century, spectroscopy had established extensive experimental support for quantized atomic energy levels. Scientists could identify particular wavelengths of emitted and absorbed light, determine the characteristic frequencies associated with atomic transitions, and calculate how various interactions affected those measurements. These achievements confirmed essential predictions of quantum mechanics, but the experiments generally involved large numbers of atoms or other particles whose combined behavior produced the measured signals.

Observing an individual quantum jump required overcoming several substantial obstacles. A single atom produces extremely weak optical signals compared with an ensemble containing enormous numbers of particles, making its fluorescence difficult to distinguish from background noise. The atom also had to remain confined long enough for researchers to measure changes in its internal state repeatedly, while unwanted collisions and external disturbances had to be reduced sufficiently to preserve the conditions required for observation.

The development of electromagnetic ion traps and laser-based cooling techniques transformed what experimental physicists could accomplish. Electrically charged atoms could be confined through carefully arranged electromagnetic fields, while laser interactions could reduce their motion and permit detailed examination of their optical transitions. Combined with sensitive photon-detection equipment, these advances made it possible to investigate the behavior of an individual ion over extended periods, creating the experimental conditions needed to identify quantum jumps as separate, measurable events.

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The University of Washington Experiment That Changed Quantum Observation

On June 30, 1986, Physical Review Letters published a paper by Warren Nagourney, Jon Sandberg, and Hans Dehmelt of the University of Washington titled “Shelved Optical Electron Amplifier: Observation of Quantum Jumps.” Their experiment demonstrated the direct detection of changes between internal quantum states of an individual laser-cooled barium ion held within a radio-frequency trap. The researchers used a technique known as electron shelving, which allowed a weak atomic transition to produce a much more conspicuous change in the ion’s fluorescence.

The experimental design depended upon the existence of an atomic state in which the barium ion could remain for a comparatively long period before returning to its ordinary fluorescence cycle. Under normal experimental conditions, laser excitation caused the ion to scatter photons repeatedly, producing a detectable optical signal. When an additional excitation process transferred the ion into a long-lived metastable state, the ordinary fluorescence cycle was interrupted because that state did not participate in the same laser-driven transitions.

This interruption was the crucial element of the experiment. Rather than attempting to detect the extremely weak radiation associated with the metastable transition directly, the researchers observed the disappearance of a much stronger fluorescence signal. The barium ion continued to remain physically confined inside the electromagnetic trap, but its internal electronic configuration no longer permitted the same pattern of photon scattering. When the metastable state eventually decayed, the ion returned to the fluorescence cycle, and the optical signal reappeared.

The researchers thereby converted an internal atomic transition into an experimentally accessible change between bright and dark intervals. The effect was especially valuable because the difference between the two conditions could be measured through photon counts collected over time. A single ion could produce enough scattered light during its bright periods for the temporary disappearance of fluorescence to become readily detectable, even though the underlying transition itself involved a comparatively weak process.

The experiment provided a practical demonstration of how the quantum state of one trapped ion could be monitored repeatedly without requiring the direct collection of the photon associated with every individual transition. This principle became one of the most influential experimental techniques in atomic physics, demonstrating that carefully designed optical measurements could reveal information about quantum states through their indirect effects on observable radiation.

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How One Barium Ion Produced the Evidence

The University of Washington experiment employed a laser-cooled barium ion, Ba⁺, confined within a radio-frequency trap under high-vacuum conditions. The researchers used two stabilized laser beams operating near wavelengths of 493 nanometers and 650 nanometers to maintain the ion’s cooling and fluorescence processes. An additional filtered barium hollow-cathode lamp supplied radiation near 455.4 nanometers, allowing the ion to be excited through another atomic level and subsequently enter the metastable state whose occupancy researchers intended to detect.

The metastable state was particularly important because its lifetime was long compared with the rapid optical transitions responsible for ordinary fluorescence. Once the ion entered that state, the stronger fluorescence signal became suppressed. The researchers could then monitor how long the ion remained dark before the signal returned, providing experimental information about the lifetime of the metastable level and the statistical behavior of individual quantum transitions.

In the published research, the fluorescence measurements displayed alternating intervals of strong signal and reduced signal. The recorded trace, presented in Figure 2 of the original paper, showed the ion’s fluorescence falling to the background level and subsequently returning. These interruptions were identified with transitions into the metastable state, allowing the research team to associate visible changes in the measurement record with the internal quantum-state behavior of one ion.

The paper also presented a statistical analysis of the dark intervals, drawing upon 203 observed periods during which the ion occupied the metastable state. The measured distribution was consistent with an exponential lifetime model, and the researchers reported a fitted lifetime of approximately 30 seconds, with an uncertainty of about three seconds. The results supplied more than an illustration of fluorescence switching; they provided quantitative evidence that the observed intervals corresponded to the expected properties of the metastable atomic state.

These details are significant because they distinguish a controlled quantum experiment from an unexplained interruption in an optical signal. The researchers knew which atomic transitions their equipment was driving, understood the energy-level structure required for electron shelving, and could compare their measurements with theoretical expectations for the probability of leaving the metastable state. The agreement between the observed fluorescence behavior and predicted lifetime statistics strengthened the interpretation that the experiment was detecting individual quantum jumps.

The result demonstrated the value of combining several established experimental methods into a system capable of resolving the internal-state changes of a single particle. Electromagnetic trapping provided confinement, laser cooling improved experimental stability, optical excitation produced the relevant transitions, and fluorescence detection converted the internal quantum behavior into data that could be recorded and analyzed.

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The Breakthrough Was Not Confined to One Laboratory

Although the University of Washington paper appeared in June 1986, the historical record includes two additional independent research groups that reported closely related observations later that year. Their work is essential to understanding how the scientific community established the experimental reality of individual quantum jumps through different apparatus designs and atomic systems.

At the University of Hamburg in Germany, Thilo Sauter, Werner Neuhauser, Rainer Blatt, and Peter E. Toschek conducted experiments involving laser-excited barium ions. Their findings were published on October 6, 1986, in Physical Review Letters under the title “Observation of Quantum Jumps.” The team monitored the resonance fluorescence of systems containing between one and three trapped barium ions, observing interruptions associated with transitions into a metastable state.

The Hamburg experiment demonstrated that quantum-jump behavior could be resolved in systems involving a small, identifiable number of confined ions. Fluorescence changes provided information about when an ion entered a nonfluorescing state, establishing another experimental demonstration of the connection between quantum transitions and measurable changes in optical emission. The study also investigated the processes responsible for producing the observed shelving events, including Raman-Stokes scattering associated with specific atomic energy levels.

On the same date, another paper appeared in Physical Review Letters from researchers at the National Bureau of Standards in Boulder, Colorado. John C. Bergquist, Randall G. Hulet, Wayne M. Itano, and David J. Wineland published “Observation of Quantum Jumps in a Single Atom,” describing measurements involving an individual laser-cooled mercury ion, Hg⁺. Their experiment identified transitions into a metastable state through the sudden disappearance of fluorescence from a laser-excited resonance line, followed by the return of the optical signal when the ion transitioned back to its ground state.

The National Bureau of Standards experiment was particularly important because it used a different atomic species and investigated a radiatively driven electric-quadrupole transition. The researchers also examined statistical properties of the observed transitions, including photon antibunching associated with the metastable-state emission. Their findings provided another demonstration that the fluorescence-interruption method could reveal the behavior of individual atomic systems under controlled laboratory conditions.

Taken together, these investigations established that the observation of quantum jumps was not dependent upon one particular laboratory, one experimental apparatus, or one atomic species. Three independent teams had developed methods capable of detecting discrete internal-state changes through the optical behavior of confined ions, making 1986 a defining year in the experimental investigation of quantum measurement.

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What the Bright and Dark Intervals Actually Revealed

The fluorescence records obtained during these experiments offered a way to distinguish quantum-state transitions from ordinary variations in light intensity. An ion participating in a rapid laser-driven fluorescence cycle repeatedly absorbs and emits photons, producing a stream of detectable radiation. When the ion enters a metastable state outside that cycle, the stream is interrupted, producing a dark interval whose duration depends upon the relevant transition processes.

The visible switching between these conditions must be interpreted carefully. The measured fluorescence signal is a record of photon detection over time, not a photograph of an electron moving between physical locations. Its significance lies in the relationship between the optical response and the atom’s internal quantum state. Through the design of the experiment, researchers could determine that extended intervals without ordinary fluorescence corresponded to occupation of the metastable level.

This method also helps explain why the process could be monitored despite the weakness of the transition under investigation. The relevant state change did not need to produce a bright photon that the apparatus could immediately detect. Instead, the ion’s inability to continue participating in the normal fluorescence cycle created a measurable absence of many photons that otherwise would have appeared. This indirect amplification of state information made weak transitions accessible through ordinary photon-counting measurements.

The recorded timing of the bright and dark periods also provided information about the random character of individual quantum events. Even under stable experimental conditions, the precise moment when an ion entered a metastable state could not be predetermined from the apparatus settings alone. What physicists could calculate were the rates and probability distributions governing transitions, allowing the collective statistics of many recorded events to be compared with theoretical predictions.

This relationship between individual unpredictability and statistically regular behavior remains central to quantum mechanics. A specific transition may occur at an unpredictable time, while the probability distribution of many comparable transitions follows well-defined mathematical laws. The 1986 experiments demonstrated this relationship within an isolated atomic system, supplying data that could be examined through both the timing of individual events and the overall distribution of their durations.

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Why Electron Shelving Became Such a Powerful Scientific Tool

Electron shelving solved a difficult measurement problem by exploiting the relationship between an internal quantum state and an externally detectable optical response. In a suitable atomic system, a transition into a long-lived metastable state suppresses the ordinary fluorescence process. By observing whether the fluorescence is present or absent, scientists can determine which state the system occupies with a level of confidence governed by the measurement conditions.

The practical advantage extends beyond the detection of a rare transition. A single quantum-state change can influence the scattering of a large number of photons during a measurement interval, making it possible to distinguish states that would otherwise be difficult to identify. This amplification of information allows scientists to investigate individual ions without depending upon the direct detection of every weak photon associated with the transition being studied.

The technique became important to precision spectroscopy because researchers could prepare an ion in a known state, apply radiation intended to drive a particular transition, and then determine whether the transition occurred by examining its fluorescence. Repeating the procedure while varying the frequency of the applied radiation allows the characteristics of narrow atomic transitions to be investigated with exceptional sensitivity.

The same general approach is relevant to modern trapped-ion quantum information experiments, where two internal states of an ion can represent the logical values used to encode a quantum bit. Researchers can manipulate those states through controlled electromagnetic interactions and determine the outcome through state-dependent fluorescence. The ability to discriminate reliably between quantum states is necessary for extracting information from a quantum register and checking whether an intended operation produced the expected result.

These applications demonstrate why electron shelving became a lasting contribution to experimental physics. The 1986 observations were not solely historic demonstrations of a theoretical phenomenon. They also established an effective method for converting microscopic quantum-state information into signals accessible to conventional measurement equipment, helping bridge the gap between the behavior of individual particles and the instruments used to study them.

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Quantum Observation and the Measurement Problem

The direct observation of individual quantum jumps also renewed attention to questions concerning what a quantum measurement reveals. In classical physics, a measurement is frequently understood as determining a physical property that exists independently of the instrument used to observe it. Quantum mechanics presents a more complex situation because the state of a system and the outcome of a measurement must be interpreted through the mathematical rules governing probabilities, observables, and interactions.

The 1986 fluorescence experiments did not settle every philosophical question associated with quantum measurement. They did establish that individual atomic systems could produce measurement records containing distinguishable state changes under controlled conditions. These records could be analyzed using quantum theory, allowing scientists to compare observed transition statistics with predictions derived from the known atomic energy structure and the processes driving the system.

Continuous monitoring introduces an additional layer of complexity because measurement is itself a physical interaction. Detecting the fluorescence of an ion requires coupling it to electromagnetic radiation, and that interaction can influence its behavior. The experimental challenge is therefore not simply to collect the greatest possible amount of light, but to design a measurement process that provides reliable information about the desired state while accounting for the effects of the monitoring interaction.

In electron-shelving experiments, the information obtained from fluorescence depends upon a carefully constructed relationship between the bright states, the metastable state, and the applied radiation. Scientists use this relationship to interpret the photon-detection record without claiming that the apparatus provides unrestricted access to every property of the ion. Quantum measurements reveal particular observables under specific experimental conditions rather than supplying a complete classical description of a particle’s behavior.

The importance of these experiments lies partly in demonstrating that quantum measurement could be investigated through repeated observations of individual systems instead of relying exclusively upon large ensembles. This capability created opportunities to explore how measurements evolve over time, how quantum-state information can be extracted, and how measurement outcomes can eventually be incorporated into active feedback systems.

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The Transition From Observation to Quantum Control

The ability to detect quantum jumps established an important experimental foundation, but observation represents only one part of understanding a quantum system. Scientists also need methods for preparing particular states, initiating controlled transitions, maintaining coherence, and determining how accurately the system responds to applied electromagnetic fields.

Trapped-ion research provided an environment in which these capabilities could be developed together. An ion can remain confined through electromagnetic forces while lasers or other radiation sources interact with selected internal transitions. Researchers can use those interactions to cool the particle, prepare a chosen state, apply controlled operations, and measure the resulting quantum behavior. Each capability reinforces the others, making trapped ions valuable experimental systems for precision measurements and investigations into quantum information processing.

The success of the 1986 experiments also illustrated the importance of experimental isolation. Collisions with residual gas particles, fluctuations in electromagnetic fields, and uncontrolled interactions with the surrounding environment can disturb a trapped ion and interfere with the measurement of its internal states. Maintaining suitable vacuum conditions and stable control fields therefore became essential to improving both the accuracy and reproducibility of the experiments.

As laboratory techniques advanced, researchers gained greater control over transitions that could previously be investigated only through their spontaneous occurrence or statistical effects. Carefully timed electromagnetic pulses made it possible to manipulate quantum-state populations and coherent superpositions, while improvements in detection allowed measurements to become more precise. These capabilities supported the development of advanced atomic clocks, quantum logic operations, and experimental methods for controlling the interactions of individual quantum systems.

The scientific progression was substantial. Researchers had moved from demonstrating that distinct atomic energy levels existed to observing individual transitions, and the experimental techniques emerging from that work would help make quantum-state manipulation a practical laboratory discipline. Moving beyond detection toward deliberate control of quantum-state transitions became one of the central challenges driving subsequent research.

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The Scientists and Institutions Behind the Historic Achievement

The three independent investigations of 1986 reflected years of progress in atomic trapping, optical spectroscopy, and the development of methods for studying isolated charged particles. Their success was not the result of one instrument or one scientific insight, but the convergence of theoretical understanding, experimental engineering, and improvements in the sensitivity of optical measurement.

Hans Dehmelt had spent years advancing techniques for confining and studying individual charged particles. His research into trapped-particle measurement contributed to the experimental methods that eventually made the direct observation of individual quantum jumps possible. In 1989, Dehmelt shared the Nobel Prize in Physics with Wolfgang Paul for their development of ion-trap techniques, while Norman Ramsey received the other half of the prize for his work on separated oscillatory fields. The Nobel recognition concerned broader advances in precision spectroscopy and particle trapping rather than the quantum-jump observation alone.

At the National Bureau of Standards, the research team involving Bergquist, Hulet, Itano, and Wineland contributed to the development of high-precision methods for studying individual atomic ions. Their 1986 experiment formed part of a research direction that would continue into trapped-ion spectroscopy and precision frequency standards. Wineland’s later contributions to the manipulation of individual quantum systems were recognized with the 2012 Nobel Prize in Physics, shared with Serge Haroche.

The University of Hamburg researchers contributed another independent demonstration through their investigation of trapped barium ions and the fluorescence changes associated with metastable-state transitions. Their results reinforced the broader experimental understanding that individual quantum jumps could be detected through carefully controlled interactions between radiation and atomic systems.

These achievements illustrate the international nature of fundamental physics research. Laboratories operating with different atomic species, equipment configurations, and measurement arrangements reached compatible conclusions about the behavior of individual quantum systems. Such independent results are particularly important when a theoretical concept is being examined through experimental methods that introduce new capabilities or measurement challenges.

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What the Experiments Established—and What They Did Not

The 1986 observations demonstrated that individual quantum-state transitions could be detected through measurable changes in the fluorescence of trapped ions. They also supplied experimental information about the timing and statistical properties of those transitions, allowing researchers to compare their findings with established quantum mechanical descriptions of spontaneous emission, excitation, and metastable-state lifetimes.

The experiments did not demonstrate that electrons travel instantaneously through ordinary space or that particles can be transported across arbitrary distances through quantum jumps. The observed phenomenon involved changes between internal atomic states, with the measurement apparatus identifying those changes through their effect on photon scattering. The ion itself remained confined within the experimental trap throughout the observations.

The results also did not establish that scientists could predict the exact time of every quantum jump. The observed transitions displayed stochastic timing, even though their probability distributions and average rates could be examined quantitatively. This combination of unpredictable individual events and measurable statistical regularity supported the quantum mechanical description of the system.

Another limitation concerned the precise evolution occurring between the identified states. The fluorescence records clearly distinguished bright and dark periods, but they did not provide a complete time-resolved reconstruction of every stage of the underlying quantum-state evolution. Later experiments using different measurement techniques and specially engineered quantum systems would investigate those dynamics in greater detail.

These boundaries do not weaken the historical importance of the work. They identify what the original experiments actually measured and explain why their findings became the starting point for further investigations into quantum-state monitoring and control. Scientific progress depends upon accurately establishing the capabilities of experimental instruments and evaluating discoveries according to the evidence obtained.

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Why the 1986 Breakthrough Still Matters

Four decades after the first published observations, the underlying experimental principles remain relevant to advanced quantum technologies. Researchers studying trapped ions continue to depend upon techniques capable of preparing, manipulating, and measuring individual quantum states, while precision spectroscopy relies upon the ability to resolve transitions associated with extremely narrow energy differences.

Atomic clocks provide a particularly significant example of how quantum-state measurements can contribute to technologies with applications far beyond the laboratory. Their performance depends upon the stability and reproducibility of atomic transition frequencies, allowing precisely controlled interactions between atoms and electromagnetic radiation to serve as references for measuring time. Improvements in trapping, cooling, and state detection have contributed to the development of frequency standards capable of extraordinary precision.

Quantum computing represents another major field in which individual quantum-state measurement is fundamental. A trapped-ion quantum processor must distinguish the logical states of its qubits, apply controlled operations, and obtain measurement outcomes without introducing uncontrolled errors that compromise the computation. State-dependent fluorescence remains an important method of extracting information from such systems, demonstrating the lasting value of experimental techniques that connect internal atomic behavior with detectable optical signals.

The broader lesson of the 1986 experiments concerns the relationship between scientific theory and technological capability. Quantum mechanics had already supplied successful mathematical descriptions of atomic transitions, but the ability to observe individual events required instruments capable of operating at a previously inaccessible level of sensitivity. Once those instruments became available, researchers could ask new experimental questions, refine their understanding of quantum-state behavior, and develop methods for influencing the physical processes they were studying.

The achievement therefore occupies a distinctive place in the history of quantum physics. It helped transform the study of isolated quantum systems from a predominantly indirect investigation of energy levels and radiation into an experimental discipline capable of examining the changing states of individual particles. That transformation continues to influence the design of instruments and technologies whose operation depends upon precise control over matter at its smallest accessible scales.

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TRJ VERDICT

The first direct observations of individual quantum jumps in 1986 represented a decisive advancement in experimental physics because they demonstrated that a fundamental quantum process could be identified through the measurable behavior of a single confined atomic particle. Scientists had already established extensive evidence for quantized energy states, but the ability to detect individual transitions marked a new level of experimental access to the microscopic world. What had long been expressed through mathematical theory and collective atomic measurements could now be investigated through the fluorescence of individual ions.

The achievements of the University of Washington, the National Bureau of Standards, and the University of Hamburg deserve recognition as complementary contributions to a historic development. Their experiments demonstrated that distinct atomic systems could reveal quantum-state transitions through controlled optical measurements, while the resulting data supported established predictions concerning metastable states and transition statistics. These findings provided a foundation for techniques that would become essential as researchers sought greater precision in the preparation, manipulation, and measurement of individual quantum systems.

The significance extends beyond the history of atomic physics. Modern quantum technologies depend upon the ability to identify and control physical states that cannot be described adequately through ordinary classical models. The development of reliable methods for measuring those states remains essential to precision spectroscopy, atomic clocks, quantum information processing, and experimental investigations into the limits of quantum control. The 1986 breakthroughs helped establish that such measurements could be performed at the level of individual particles, giving researchers new opportunities to connect theoretical predictions with directly recorded experimental behavior.

The discovery also demonstrates why scientific progress must be evaluated through verifiable achievements rather than speculative interpretations. The researchers did not establish a mechanism for instantaneous transportation, unrestricted movement through space, or the manipulation of time. They accomplished something more clearly defined and scientifically consequential: they developed methods capable of identifying individual quantum transitions within isolated atomic systems, providing evidence that could be examined, measured, and independently reproduced.

The first observed quantum jumps did not reveal a shortcut through the universe. They revealed a new level of access to the physical laws governing matter, establishing an experimental foundation upon which decades of quantum research would continue to build.

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National Institute of Standards and Technology (NIST), Trapped Ions and Laser Cooling II, NIST Technical Note 1324, October 1988. Edited by David J. Wineland, Wayne M. Itano, James C. Bergquist, and John J. Bollinger. Published by the U.S. Department of Commerce. Includes original research on quantum jumps, trapped ions, laser cooling, and atomic spectroscopy. (Free Download)

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Warren Nagourney, Jon Sandberg, and Hans Dehmelt, “Shelved Optical Electron Amplifier: Observation of Quantum Jumps.” Physical Review Letters, Volume 56, Number 26, pages 2797–2799, June 30, 1986. Department of Physics, University of Washington. Published by the American Physical Society. © 1986 American Physical Society. (Free Download)

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