The Joint Quantum Institute would like to again congratulate the 2012 Nobel Prize in physics recipients, David Wineland and Serge Haroche. The Nobel Prize committee cites Wineland and Haroche “for ground-breaking experimental methods that enable measuring and manipulation of individual quantum systems.” The awards were bestowed during the ceremony in Stockholm on December 10, 2012.
So why does quantum matter, matter? The short answer is that it allows us to not only understand some of the most mystical workings of the microscopic universe, but even harness quantum physics for constructing powerful computers. These computers would likely be limited in their tasks, but the tasks themselves are critical things like cryptography and database searching. While leaps have been made in the technology, a quantum computer is still a ways off. However, there is currently no fundamental reason why scientists will not eventually succeed in this (ad)venture.
Because no one knows for sure how a full-scale quantum computer will look in the future, physicists and engineers are taking an all-out approach. The work of Haroche and Wineland may seem quite specific to their systems, but quantum physics can actually be realized in many platforms. These pioneering methods have been extended, with qubits (short for quantum bit) being proposed and/or built in superconductors, photonic networks, neutral atoms/optical lattices, quantum dots, topological matter, and more.
Since the announcement of the prize in October, many articles have nicely explained the physics* (see, for example, this Scientific American article which has been re-posted). Below is a brief summary of the research. Along the way, it is worth noting that quite a bit of the underlying physics and technology has also received Nobel prizes.
It's cold out there: David Wineland’s research
NIST has a long history with cold atoms and ions. In the 1980s Wineland pioneered techniques to cool atomic ions with lasers. NIST/JQI scientist William Phillips received the 1997 nobel prize, along with Claude Cohen-Tannoudji (École Normale Supérieure) and Steven Chu (Stanford, now US Secretary of Energy), for laser cooling and trapping neutral atoms. In 1995 Carl Wieman (U. Colorado, now U. British Columbia), Eric Cornell (NIST), and Wolfgang Ketterle (independently at MIT) first created a Bose- Einstein condensate, which is the coldest state of matter ever made (this earned them the 2001 prize).
The mid-nineties was also when Wineland’s team used a beryllium ion to house a quantum state and entangle the ion’s internal state with its physical position in the ion trap. (Hans Dehmelt and Wolfgang Paul shared one half of the 1989 Nobel prize for ion traps; Norman Ramsey received the other half for developing the standard technique for probing for quantum systems, which is central to both Wineland’s and Haroche’s methods.)
In this platform, a single electrically charged particle is laser cooled until it is effectively stationary. That by itself is neat because at room temperature and atmospheric conditions atoms fly about at hundreds of meters per second and there is virtually no chance of grabbing one out of the air, let alone manipulating it. But at low temperatures, the quantum nature of the ion's motion emerges with only certain allowed states, described by quanta of vibration called phonons (analogous to the quanta of light--a photon).
At this point the atom is so cold that it has only a few phonons of vibration. In order for this experiment to work, the team also needs control over the amount of phonons in the system (motional temperature) and so they use lasers to remove those leftover phonons. This method was developed by Wineland’s group and is called “sideband cooling.” (The types of laser transitions employed here generally fall under the term Raman spectroscopy--after Sir Chandrasekhara Venkata Rāman, who received the Nobel prize in 1930.)
Now with the ion effectively stationary, the scientists can use lasers to precisely select the atom’s internal energy, and also map quantum states between the internal levels of the ion and its motion. This allows the operation of quantum logic gates, which when extended to multiple ions, constitutes a primitive form of quantum computation. Wineland's group showed this mapping step in 1995, and a few years later "entangled" two ions. Several groups have followed with bigger systems and variations on this scheme.
Quantum logic with ions is also used to create the world’s most accurate clock, located in Wineland’s lab. His work with cold atoms and ions has paved the way for researchers to scale up quantum devices in these systems. JQI/UMD professor Chris Monroe worked with Wineland on the early quantum information experiments. Currently, Monroe, building upon these earlier techniques, can manipulate up to 16 ions, simulating quantum magnetism. Rainer Blatt’s group has shown entanglement of up to 14 ions. Many groups around the world focus on other issues related to designing a scalable quantum computer--such as creating quantum networks with hybrid quantum platforms, increasing the number of qubits, and improving logic gate quality.
Seeing without looking: Serge Haroche’s research
One of the unusual and powerful aspects of quantum mechanics involves superpositions, in which systems reside simultaneously in two states. The rub comes when measuring these superpositions-- they collapse. Quantum systems are delicate and are destroyed upon interacting with the outside world, whether through measurement, or just by random chance.
Haroche, who works at the Ecole Normale Superiore and the College de France, has spent years extracting information from quantum systems in a way that doesn’t totally destroy the object being studied--in this case he wants to determine the number of photons in a cavity without looking directly at the light. One of his research goals is to understand the boundary between the classical (non-quantum) and quantum descriptions of nature. While other systems could, in principle, be used to study these questions, Haroche has certainly refined his cavity system to be an ideal testing ground for photons. In his system, photons can be studied, but not using the conventional (destructive) means of a camera or photomultiplier tube.
Haroche uses an approach where the qubit is again made from two atomic levels. Instead of an ion, these levels are in a Rydberg atom, which is a highly excited species. The atom’s electron is promoted to such a high energy (here the 50th and 51st principle quantum numbers) that it is nearly detached all- together. They insert atoms into a cavity that traps electromagnetic radiation whose frequency is slightly different from the separation between the qubit energy levels. The cavity construction is a precision art all by itself-- made of two finely polished superconducting mirrors separated by a few centimeters, and held at a fraction of a degree Kelvin above absolute zero. (Many Nobel prizes have been given related to superconductivity. These are listed below). The configuration is so stable, with such shiny mirrors, that a photon can bounce back and forth about 40,000 kilometers before leaking out.
When the Rydberg atom enters the cavity, it interacts strongly with the microwave field. This alters the atomic energy levels in a way that depends on the number of photons that are trapped in the cavity. (This research falls under the sub-field of cavity quantum electrodynamics (CQED); the 1965 Nobel prize was awarded to Richard Feynman, Sin-Itiro Tomonaga, and Julian Schwinger for QED). When the atom is in the 50th (51st) state, the cavity is holding one (zero) photon. The atom’s internal states can be entangled with the cavity field. Because the atoms can be prepared and analyzed before and after interacting with the cavity field, the scientists can tell if they encountered a photon along the way.
In later experiments, Haroche’s group adjusted the parameters so that they could detect the photon number-dependent light shifts for up to seven photons in the cavity. Using this modification, they examined how a quantum state can become non-quantum (classical). This is of philosophical importance because large objects, such as humans, are not described by quantum physics. Many scientists have long sought to unify the descriptions of the very small and the very large.
All of the research from both Haroche’s and Wineland’s groups, and in most of quantum information science requires the use of the laser, which, also earned a Nobel prize (1964--Charles Hard Townes, the other half jointly to Nicolay Gennadiyevich Basov and Aleksandr Mikhailovich Prokhorov).
Nobel prize-winning research that is related to or used in quantum information research (in Chronological order; some years, the prize was split and only half is related to current research):
- 1913 Heike Kamerling Onnes - Liquid Helium
- 1918 Max Planck - Energy Quanta
- 1921 Albert Einstein - Photoelectric Effect and Theoretical Contributions to Physics
- 1922 Niels Bohr - Atomic Structure
- 1930 Sir Chandrasekhara Venkata Raman - Raman Transitions
- 1932 Werner Heisenberg - "Creation of Quantum Mechanics" (quote from Nobel citation)
- 1933 Paul Dirac and Erwin Schrodinger - New Theories in Atomic Physics
- 1944 Isidor Isaac Rabi - Magnetic Resonance Methodology (Used in MRI)
- 1954 Max Born - Fundamental Research in Quantum Mechanics, Statistical Interpretation of the Wavefunction
- 1955 Willis Eugene Lamb - Fine Structure of the Hydrogen Spectrum; Polykarp Kusch - Precisions Measurement of Magnetic Moment of the Electron
- 1962 Lev Landau - Theoretical Work in Condensed Matter, Liquid Helium
- 1964 Charles Hard Townes, the other half jointly to Nicolay Gennadiyevich Basov and Aleksandr Mikhailovch Prokhorov - The Laser
- 1965 Richard Feynman, Sin-Itiro Tomonaga, and Julian Schwinger - Quantum Electrodynamics
- 1966 Alfred Kaslter - Optical Methods for Studying Hertzian Resonances
- 1972 John Bardeen, Leon Neil Cooper, and John Rober Schrieffer - Developed BCS Theory
- 1973 Leo Esaki and Ivar Giaever - Tunneling in Semiconductors and Superconductors; Brian David Josephson - Theoretical Work on Supercurrents through a Tunnel Barrier
- 1981 Nicolaas Bloembergen and Arthur Leonard Schawlow - Development of Laser Spectroscopy; Kai M. Siegbahn - High-Resolution Electron Spectroscopy
- 1985 Klaus von Klitzing - Quantized Hall Effect
- 1987 J. Georg Bednorz and K. Alexander Müller - Superconductivity in Ceramic Materials
- 1989 Norman F. Ramsey - Development of Oscillatory Methods to Probe Quantum Systems; Hans G. Dehmelt and Wolfgang Paul - Ion Traps
- 1996 David M. Lee, Douglas D. Osheroff, and Robert C. Richardson - Superfluidity in Helium-3
- 1997 Steven Chu, Claude Cohen-Tannoudji, and William D. Phillips - Laser Cooling and Trapping of Atoms
- 1998 Robert B. Laughlin, Horst L. Störmer, and Daniel C. Tsui - New Quantum Fluid with Fractionally Charged Excitations
- 2001 Carl Wieman, Eric Cornell, and Wolfgang Ketterle - Bose-Einstein Condensation
- 2003 Alexei A. Abrikosov, Vitaly L. Ginzburg, Another J. Legget - Superfluidity and Superconductivity
- 2005 Roy J. Glauber, John L. Hall, Theodor W. Hänsch - Glauber States, Precision Laser Spectroscopy, and Optical Frequency Combs
- 2009 Charles Kuen Kao, Willard S. Boyle, George E. Smith - Optical Fibers and CCD Imagers
* For nice illustrations of the physics, we suggest Nobelprize.org, which has summaries that include graphics.