Nobel Prize in Physics 2012: Trapping and Measuring Single Quantum Systems

This note covers the Nobel Prize in Physics 2012: who won it, how Serge Haroche and David J. Wineland learned to trap and measure single photons and single ions without destroying their fragile quantum states, how the discovery unfolded, why it matters for quantum computers and ultra-precise clocks, and quick facts for exams.

What was the Nobel Prize in Physics 2012 awarded for?

The Royal Swedish Academy of Sciences gave the award "for ground-breaking experimental methods that enable measuring and manipulation of individual quantum systems". This is the official citation, quoted exactly as issued.

In plain words, the two laureates each found a way to trap a single tiny quantum object, such as one atom or one particle of light, and to watch it closely without ruining the very quantum behaviour they wanted to study.

Before their work, physicists could only imagine such experiments as "thought experiments" because any real attempt to look at a lone quantum particle tended to destroy its delicate state by letting it interact with its surroundings.

Haroche and Wineland built laboratory tools clever enough to get around that problem.

The full official name of this award is the Nobel Prize in Physics, given each year by the Royal Swedish Academy of Sciences. The 2012 prize was announced on 9 October 2012 and carried a prize amount of 8,000,000 Swedish kronor, split equally between the two winners.

Who are the laureates?

Serge Haroche

Serge Haroche was born on 11 September 1944 in Casablanca, Morocco. At the time of the award he was affiliated with the Collège de France and the École Normale Supérieure, both in Paris, France, and he received one half of the prize.

According to the nobelprize.org facts page, he moved to France at age 12 and later worked at École Normale Supérieure, the CNRS, École Polytechnique and finally Collège de France, with periods at Stanford, Harvard and Yale in the United States.

Haroche's contribution was to trap single particles of light, called photons, bouncing between two mirrors, and then to send specially prepared atoms through that trap one at a time to read out what the photon was doing, all without destroying the photon.

David J. Wineland

David J. Wineland was born on 24 February 1944 in Milwaukee, Wisconsin, USA. At the time of the award he was affiliated with the National Institute of Standards and Technology (NIST) in Boulder, Colorado, and the University of Colorado, Boulder, and he also received one half of the prize.

The facts page notes that he studied at the University of California, Berkeley, earned his PhD at Harvard University under Norman Ramsey, then worked with Hans Dehmelt's team at the University of Washington in Seattle before joining NIST in 1975.

Wineland's contribution was the opposite approach: he trapped electrically charged atoms, called ions, using electric fields, and then used carefully tuned laser light (photons) to cool, control and measure those trapped ions.

What problem were they trying to solve?

Quantum mechanics describes how the smallest building blocks of nature, atoms and photons, behave. One of its strangest predictions is superposition: a single quantum particle can exist in two different states at the same time, something like a marble that is somehow both "here" and "there" until it is measured.

The trouble, as the popular information page explains, is that lone particles are hard to keep apart from everything around them.

The instant a lone atom or photon interacts with the outside world, even just a little, it loses its special quantum properties.

So for decades after quantum theory was developed in the 1920s and 1930s, researchers could only describe such effects through imagined "thought experiments" rather than see them directly in a real laboratory.

The Austrian physicist Erwin Schrödinger, himself a 1933 physics laureate, captured this puzzle with his famous thought experiment of a cat sealed in a box with a vial of poison triggered by random radioactive decay.

According to quantum rules, the cat should be in a superposition of being both dead and alive until someone looks inside, at which point the superposition "collapses" into one definite outcome.

Schrödinger used this to highlight how strange it is that such superpositions, common at the atomic scale, seem never to show up in the everyday, macroscopic world we see.

Both quantum optics researchers, Haroche and Wineland, worked in the field that studies exactly this boundary: how light and matter interact, and how a fragile quantum superposition turns into a definite classical outcome when it is observed.

Their task was to build equipment precise enough to watch this transition happen, step by step, instead of only guessing about it.

How did Wineland trap and control single ions?

In Wineland's laboratory in Boulder, Colorado, electrically charged atoms, or ions, are held inside a trap by surrounding electric fields. The experiment is run in a vacuum at extremely low temperatures so that the ion is isolated from stray heat and radiation.

A central tool is the laser. Laser light can push an ion's thermal (heat-driven) motion down until the ion sits in its lowest possible energy state inside the trap, a technique described in the scientific background as sideband cooling.

Once the ion is this cold and still, its quantum behaviour becomes easy to study.

A carefully tuned laser pulse can then put the ion into a superposition state: instead of simply sitting in its lowest energy level, the ion is nudged only halfway towards a higher energy level, so it ends up with an equal chance of being found in either level when measured.

1. Trap the charged ion using surrounding electric fields inside an ultra-high vacuum chamber.
2. Cool the ion with laser light until its vibrational motion drops to the lowest energy state of the trap (sideband cooling).
3. Apply a precisely tuned laser pulse to nudge the ion's internal state only halfway, creating a superposition of two energy levels.
4. Use further laser pulses to transfer this superposition from the ion's internal (electronic) state into its vibrational motion in the trap.
5. If a second ion shares the same trap, pass the superposition on to it as well, since the ions share vibrational motion.

Draw a small chamber with electric field lines curving in from surrounding electrodes to hold one charged ion at the centre.

Add an arrow from a laser pointing at the ion, and label the ion's two possible energy levels with a small "superposition" bracket connecting them.

The scientific background notes that Wineland's group was the first anywhere to perform a quantum logic operation on two quantum bits, an early building block for a future quantum computer.

How did Haroche trap and control single photons?

Haroche took the opposite route: instead of trapping matter and probing it with light, he trapped light itself and probed it with atoms.

In his Paris laboratory, a microwave photon travels repeatedly between two mirrors forming a cavity about 2.7 centimetres apart, cooled to a temperature close to absolute zero.

These mirrors, made of a superconducting material, are reflective enough that a single photon can remain trapped for close to a tenth of a second before it disappears, a record lifetime that means the photon effectively travels about 40,000 kilometres, roughly one trip around the Earth, while bouncing between the mirrors.

To read out what is happening inside the cavity without destroying the photon, Haroche's team sends specially prepared Rydberg atoms (named after the physicist Johannes Rydberg) through the cavity one at a time.

A Rydberg atom is unusually large, with a radius of about 125 nanometres, roughly a thousand times bigger than an ordinary atom.

1. Prepare a Rydberg atom in a chosen quantum state before it enters the cavity.
2. Send the atom through the cavity at a carefully controlled speed so it interacts with the trapped microwave photon for a precise, known time.
3. Let the interaction shift the phase of the atom's quantum state, a shift that depends on whether a photon is present in the cavity or not.
4. Measure the atom's phase shift after it exits the cavity to find out whether a photon was present, leaving the photon itself undisturbed.
5. Repeat with further atoms, using different timings and phases, to count several photons or to follow an evolving quantum state step by step.

Draw two curved, facing mirrors a short distance apart with a photon bouncing between them as a zig-zag line.

Draw a single atom entering from one side, crossing the gap between the mirrors, and exiting the other side with an arrow showing its phase has shifted.

Haroche and his team also used this method to create "cat-like" states inside the cavity and, as the committee described, to watch the superposition "collapse" into a single definite outcome, giving a direct laboratory picture of Schrödinger's cat paradox.

Why do both methods belong to the same field?

The scientific background document points out that both experiments rest on the same underlying physics problem, described by what physicists call the Jaynes-Cummings Hamiltonian: a two-level quantum system coupled to a quantized oscillator.

For Wineland this two-level system is the ion's internal state coupled to its vibrational motion; for Haroche it is a Rydberg atom's two levels coupled to the microwave field in the cavity.

Both groups work within quantum optics, the field studying how light and matter interact at the most fundamental level, which the committee said has progressed considerably since the mid-1980s.

How did the discovery unfold?

The scientific background traces a long chain of experiments across several decades that led up to the 2012 prize, beginning with the invention of the ion trap itself and running through to direct demonstrations of controlling single quantum systems.

Why does it matter?

The committee highlighted two main practical directions opened up by this work. The first is the quantum computer. In an ordinary computer, each bit of information is either 0 or 1.

In a quantum computer, a quantum bit or qubit can be 0 and 1 at the same time, so that, as the popular information page notes, a device with just 300 qubits could in principle hold more states simultaneously than there are atoms in the universe.

Wineland's group carried out the world's first two-qubit quantum operation, an early practical step towards such machines, though building a large, working quantum computer remains, in the committee's words, "an enormous practical challenge" because qubits must be shielded from the environment yet still made to communicate their results.

The second direction is precision timekeeping. Wineland's team used trapped ions to build optical clocks that run on visible light rather than the microwave transitions used in ordinary caesium atomic clocks, reaching a precision better than one part in 10¹⁷.

The popular information page gives a vivid comparison: such a clock, run since the Big Bang about 14 billion years ago, would be off by only around five seconds today.

This precision is already useful for testing Einstein's prediction that time runs slightly differently depending on speed and gravity, for example the small timing corrections applied to GPS satellite clocks because gravity is weaker several hundred kilometres above the Earth.

More broadly, both laureates allowed physicists, for the first time, to watch a quantum superposition collapse into a classical outcome in real time inside the laboratory, turning what had been a thought experiment about Schrödinger's cat into an observable process.

How does this connect to what you study?

This prize links directly to the basic ideas of atomic structure and quantum mechanics covered in physics and chemistry courses, such as energy levels, photons and the wave-particle nature of matter.

The idea of superposition, where a particle can be in two states at once, extends the simpler picture of electrons jumping between fixed energy levels that students first meet when studying atomic spectra and the Bohr model of the atom.

The use of lasers to cool and control atoms also connects to practical ideas about light, frequency and energy transfer that appear in the study of electromagnetic radiation, since sideband cooling works by choosing a laser frequency that removes a precise amount of vibrational energy each time a photon is absorbed.

The Schrödinger's cat thought experiment, which both laureates turned into real laboratory demonstrations, is often used in school-level introductions to quantum mechanics to show why strange quantum effects, so clear for single atoms and photons, seem to vanish for everyday objects like a cat or a spinning top.

Students who go on to study relativity will also meet a direct use of this prize's work: Wineland's optical clocks were precise enough to measure the small changes in the flow of time caused by differences in speed and in gravity, the same effects that must be corrected for in GPS satellite systems because gravity is slightly weaker several hundred kilometres above the Earth's surface.

Finally, the qubit, the basic unit of a quantum computer that can hold the values 0 and 1 at the same time, gives a concrete example of superposition that connects to any introductory discussion of binary numbers and computer logic.

Quick facts for exams

The Nobel Prize in Physics 2012 was awarded jointly to Serge Haroche and David J. Wineland "for ground-breaking experimental methods that enable measuring and manipulation of individual quantum systems".

The announcement was made on 9 October 2012 by the Royal Swedish Academy of Sciences, which awards the Nobel Prize in Physics each year.

Haroche, born in Casablanca, Morocco, worked at the Collège de France and École Normale Supérieure in Paris, trapping single photons between mirrors and probing them with Rydberg atoms.

Wineland, born in Milwaukee, USA, worked at NIST and the University of Colorado, Boulder, trapping single charged ions with electric fields and controlling them with laser light. Each received one half of the 8,000,000 Swedish kronor prize.

Their work opened early paths towards quantum computers and ultra-precise optical clocks.

Note: Source. The prize facts in this note are from the Nobel Prize's official site, nobelprize.org.

Glossary

• Quantum mechanics — the branch of physics describing the behaviour of very small particles such as atoms and photons.
• Superposition — a quantum state in which a particle exists in two or more distinct states at the same time until measured.
• Ion — an atom that has gained or lost electrons and so carries an electric charge.
• Photon — a single particle (or packet) of light or other electromagnetic radiation.
• Ion trap — a device that holds a charged atom in place using electric fields, usually inside a vacuum.
• Rydberg atom — an atom excited to a very high energy level, making it unusually large, used by Haroche to probe trapped photons.
• Cavity — in this context, the space between two highly reflective mirrors where a photon is trapped and bounces back and forth.
• Sideband cooling — a laser technique that removes vibrational energy from a trapped ion until it reaches its lowest energy state.
• Qubit — the basic unit of information in a quantum computer, which can represent 0 and 1 simultaneously.
• Quantum non-demolition measurement — a way of measuring a quantum property, such as photon number, without destroying the particle being measured.
• Decoherence — the process by which a quantum superposition breaks down into an ordinary classical state through interaction with its environment.
• Optical clock — an extremely precise clock based on a light (optical) transition in a trapped ion, rather than the microwave transition used in caesium clocks.
• Schrödinger's cat — a thought experiment by Erwin Schrödinger imagining a cat in a superposition of being both alive and dead, used to illustrate puzzles of quantum measurement.

Common errors and misconceptions

• Misconception: Haroche and Wineland worked together on one shared experiment. Correct: They worked independently, in separate laboratories, using opposite approaches: one trapping ions, the other trapping photons.
• Misconception: The prize was for building a working quantum computer. Correct: The citation and committee statements describe methods that opened the first steps towards a future quantum computer, not a completed machine.
• Misconception: Wineland trapped photons and Haroche trapped ions. Correct: It is the other way round: Wineland traps charged ions and probes them with light; Haroche traps photons and probes them with atoms.
• Misconception: Superposition means a particle is physically in two separate places at once in the everyday sense. Correct: It means the particle's quantum state carries a defined probability of being found in either of two states when measured.
• Misconception: Schrödinger's cat was an actual experiment performed in a laboratory. Correct: It was a thought experiment proposed by Schrödinger in 1935 to highlight a paradox, and the 2012 laureates later created analogous "cat-like" quantum states in real experiments.
• Misconception: Optical clocks use caesium. Correct: Standard atomic clocks use a caesium microwave transition; Wineland's optical clocks use a different, visible-light transition in a trapped ion, which is far more precise.
• Misconception: The prize money and citation were different for each laureate. Correct: Both Haroche and Wineland received the identical citation and an equal one-half share of the 8,000,000 Swedish kronor prize.

Exam-style questions with model answers

Q1. In which year was the Nobel Prize in Physics 2012 announced? [1 mark]

1. It was announced on 9 October 2012 by the Royal Swedish Academy of Sciences.

Q2. Name the two laureates of the Nobel Prize in Physics 2012 and state their countries of birth. [2 marks]

1. The laureates were Serge Haroche, born in Casablanca, Morocco, and David J. Wineland, born in Milwaukee, Wisconsin, USA.

Q3. State the official citation for the Nobel Prize in Physics 2012. [2 marks]

1. The citation reads "for ground-breaking experimental methods that enable measuring and manipulation of individual quantum systems".

Q4. Explain the difference between Wineland's and Haroche's experimental approaches. [4 marks]

1. David Wineland trapped electrically charged atoms, called ions, inside electric fields and used laser light (photons) to cool, control and measure them.
2. Serge Haroche took the opposite approach: he trapped microwave photons bouncing between two highly reflective superconducting mirrors and used specially prepared Rydberg atoms, sent through the cavity one at a time, to measure and control the trapped photon without destroying it.
3. Both methods belong to the same field of quantum optics, which studies how light interacts with matter.
4. Both rely on a similar underlying physics problem, a two-level system coupled to a quantized oscillator (the Jaynes-Cummings Hamiltonian).

Q5. What is sideband cooling and why was it important? [3 marks]

1. Sideband cooling is a laser technique that removes vibrational energy from a trapped ion step by step until it reaches the lowest possible energy state of the trap.
2. It was important because it let Wineland's group control and study the ion's quantum states with precision, since a very cold, still ion shows quantum effects far more clearly than a thermally moving one.
3. It became the foundation for later techniques such as the two-qubit quantum gate and optical clocks based on trapped ions.

Q6. Describe how Haroche measured the presence of a photon without destroying it. [5 marks]

1. Haroche's group trapped microwave photons bouncing between two superconducting mirrors cooled to nearly absolute zero, where a photon could survive for almost a tenth of a second.
2. To find out whether a photon was present, they sent a specially prepared Rydberg atom, an atom excited to a very large size, through the cavity at a controlled speed.
3. As the atom passed through the cavity, its quantum state picked up a phase shift that depended on whether a photon was present or not, while the photon itself was left essentially undisturbed.
4. By measuring this phase shift once the atom exited the cavity, Haroche's team could tell whether zero or one photon was present, and by repeating the process with further atoms at different phases they could count more photons or track an evolving state.
5. This approach is called a quantum non-demolition measurement, because it reveals information about the photon without destroying it.

Q7. Discuss two applications that the committee said might follow from this prize-winning work. [6 marks]

1. The first application is the quantum computer: an ordinary bit is either 0 or 1, but a qubit can represent 0 and 1 simultaneously.
2. A system of many qubits can in principle hold an enormous number of states at once, for example 300 qubits could outnumber the atoms in the universe.
3. Wineland's group achieved the world's first two-qubit quantum operation, an early step towards such a machine, though the committee noted that building a large, working device remains an enormous practical challenge.
4. The second application is highly precise optical clocks built from trapped ions, using visible-light transitions rather than the microwave transitions of standard caesium clocks, reaching a precision better than one part in 10¹⁷.
5. Such clocks are precise enough to detect tiny relativistic effects, such as the slowing of time at small changes in speed or height, effects already relevant to correcting GPS satellite signals.

Q8. Who proposed the Schrödinger's cat thought experiment, and what question was it meant to illustrate? [3 marks]

1. The thought experiment was proposed by Erwin Schrödinger, the Austrian physicist and 1933 Nobel physics laureate, in 1935.
2. It imagined a cat sealed in a box with a poison triggered by random radioactive decay, placing the cat in a superposition of being both alive and dead until observed.
3. This illustrated the puzzle of how quantum superpositions, common at the atomic scale, relate to the everyday classical world where such effects are never seen.

Q9. What share of the prize did each laureate receive, and what was the total prize amount? [2 marks]

1. Serge Haroche and David J. Wineland each received one half of the prize.
2. The total prize amount was 8,000,000 Swedish kronor.

Key takeaways

• Serge Haroche and David J. Wineland shared the Nobel Prize in Physics 2012 for methods to measure and control single quantum systems.
• Wineland trapped single charged ions with electric fields and controlled them using laser light.
• Haroche trapped single photons between superconducting mirrors and probed them using Rydberg atoms.
• Both approaches made it possible to observe quantum superpositions directly, instead of only imagining them in thought experiments.
• Wineland's group performed the world's first two-qubit quantum operation, an early step towards quantum computers.
• Wineland's trapped-ion optical clocks reach precisions better than one part in 10¹⁷, far beyond caesium clocks.
• Haroche's team created laboratory versions of Schrödinger's cat-like states and watched them collapse from superposition to a classical outcome.
• The prize amount of 8,000,000 Swedish kronor was shared equally between the two laureates.

Test yourself

Where was Serge Haroche born?

Serge Haroche was born in Casablanca, Morocco, on 11 September 1944.

Where was David J. Wineland working at the time of the award?

David J. Wineland was affiliated with the National Institute of Standards and Technology and the University of Colorado, Boulder, USA.

What does a qubit do that an ordinary computer bit cannot?

A qubit can represent 0 and 1 at the same time through superposition, unlike an ordinary bit, which is always only 0 or 1.

What is a Rydberg atom used for in Haroche's experiments?

A Rydberg atom, excited to a very large size, is sent through the photon cavity to measure the trapped photon's presence without destroying it.

Why does an optical clock based on trapped ions beat a caesium clock in precision?

It uses a visible-light transition with a much higher frequency than caesium's microwave transition, giving precision better than one part in 10¹⁷.

What real-world puzzle did Schrödinger's cat illustrate?

It illustrated how a quantum superposition, such as a cat being both alive and dead, seems to collapse into one definite state once observed.

How long could a single photon survive in Haroche's cavity?

A single photon could survive almost a tenth of a second, travelling roughly 40,000 kilometres while bouncing between the mirrors.