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Researchers at the U. S. Department of Energy’s Ames Laboratory, the University of California, Santa Barbara, and Microsoft Station Q have made significant advancements in understanding a fundamental problem of quantum mechanics – one that is blocking efforts to develop practical quantum computers with processing speeds far superior to conventional computers. Their respective theoretical and experimental studies investigate how microscopic objects lose their quantum-mechanical properties through interactions with the environment. The results of the researchers’ investigations were announced at the American Physical Society meeting held March 10-14 in New Orleans and also reported in Science Express, the advance online publication of the journal Science.

“Quantum-mechanical particles can interact with their environments: visible light, or photons; molecules of the air; crystal vibrations; and many other things,” said Viatcheslav Dobrovitski, an Ames Laboratory theoretical physicist. “All these uncontrollable interactions randomly ‘kick’ the system, destroying quantum phases, or the ability of particles to preserve coherence between different quantum states.”

Quantum coherence is essential to developing quantum computers in which information would be stored and processed on quantum mechanical states of quantum bits, called qubits. So the self-destructive nature of quantum-mechanical states interacting with the environment is a huge problem.

To find out more about how quantum coherence breaks down and to study the dynamics of this decoherence process, the Ames Lab, UCSB and Microsoft Station Q team studied certain spin systems, called nitrogen-vacancy, N-V, impurity centers, in diamond. (Spin is the intrinsic angular momentum of an elementary particle, such as an electron.) N-V impurity centers in diamond are interesting because of the ability to control and manipulate the quantum state of a single center, allowing scientists to study the loss of coherence at a single-particle scale.

The Ames Lab, UCSB and Microsoft Station Q researchers were able to manipulate the N-V centers interacting with an environment of nitrogen spins in a piece of diamond. Amazingly, the physicists were able to tune and adjust the environmental interference extremely well, accessing surprisingly different regimes of decoherence in a single system. The scientists showed that the degree of interaction between the qubit and the interfering environment could be regulated by applying a moderate magnetic field. By using analytical theory and advanced computer simulations, they gained a clear qualitative picture of the decoherence process in different regimes, and also provided an excellent quantitative description of the quantum spin dynamics. The experiments were performed at room temperature rather than the extremely low temperatures often required for most atomic scale investigations.

Dobrovitski noted that quantum coherence of N-V centers in diamond is being studied by leading scientific groups worldwide. “The combined efforts of these groups could help in opening the way to developing a series of interacting qubits – steps to a quantum computer – where each N-V center would act as a qubit,” he said.

“In addition to quantum computers, quantum coherence plays an important role for future less exotic, but not less spectacular, applications,” said Dobrovitski. “For instance, quantum spins can be employed to develop coherent spintronic devices, which would work much faster than traditional microelectronic elements and dissipate much less energy. Quantum coherence between many spins can be employed to perform measurements with ultrahigh precision for metrology applications or to drastically increase the sensitivity of modern nuclear magnetic resonance, NMR, or electron spin resonance, ESR, experiments.

“However, in order to implement these appealing proposals, a very good understanding of quantum coherence and its destruction by the environment is needed,” Dobrovitski emphasized. In particular, from the application point of view, it is important to understand the loss of coherence of quantum systems in solid-state environments, which form the basis of modern technology.”

Note for Quantum Mechanics
In physics, quantum mechanics is the study of the relationship between quanta and elementary particles. Among other relationships the valence shell electrons and photons are quantized. Quantum mechanics is a fundamental branch of physics with wide applications in both experimental and theoretical physics. Quantum theory generalizes all classical theories, including mechanics, electromagnetism (except general relativity), and provides accurate descriptions for many previously unexplained phenomena such as black body radiation and stable electron orbits.The effects of quantum mechanics are typically not observable on macroscopic scales, but become evident at the atomic and subatomic level.

The word “quantum” came from the Latin word which means "what quantity". In quantum mechanics, it refers to a discrete unit that quantum theory assigns to certain physical quantities, such as the energy of an atom at rest. The discovery that waves have discrete energy packets (called quanta) that behave in a manner similar to particles led to the branch of physics that deals with atomic and subatomic systems which we today call quantum mechanics. It is the underlying mathematical framework of many fields of physics and chemistry, including condensed matter physics, solid-state physics, atomic physics, molecular physics, computational chemistry, quantum chemistry, particle physics, and nuclear physics. The foundations of quantum mechanics were established during the first half of the twentieth century by Werner Heisenberg, Max Planck, Louis de Broglie, Albert Einstein, Niels Bohr, Erwin Schrödinger, Max Born, John von Neumann, Paul Dirac, Wolfgang Pauli and others. Some fundamental aspects of the theory are still actively studied.

It is currently necessary to use quantum mechanics to understand the behavior of systems at atomic length scales and smaller. For example, if Newtonian mechanics governed the workings of an atom, electrons would rapidly travel towards and collide with the nucleus. However, in the natural world the electrons normally remain in an unknown orbital path around the nucleus, defying classical electromagnetism.

Quantum mechanics was initially developed to provide a better explanation of the atom, especially the spectra of light emitted by different atomic species. The quantum theory of the atom developed as an explanation for the electron's staying in its orbital, which could not be explained by Newton's laws of motion and by Maxwell's laws of classical electromagnetism.

In the formalism of quantum mechanics, the state of a system at a given time is described by a complex wave function (sometimes referred to as orbitals in the case of atomic electrons), and more generally, elements of a complex vector space. This abstract mathematical object allows for the calculation of probabilities of outcomes of concrete experiments. For example, it allows one to compute the probability of finding an electron in a particular region around the nucleus at a particular time. Contrary to classical mechanics, one can never make simultaneous predictions of conjugate variables, such as position and momentum, with arbitrary accuracy. For instance, electrons may be considered to be located somewhere within a region of space, but with their exact positions being unknown. Contours of constant probability, often referred to as “clouds” may be drawn around the nucleus of an atom to conceptualize where the electron might be located with the most probability. It should be stressed that the electron itself is not spread out over such cloud regions. It is either in a particular region of space, or it is not. Heisenberg's uncertainty principle quantifies the inability to precisely locate the particle.

The other exemplar that led to quantum mechanics was the study of electromagnetic waves such as light. When it was found in 1900 by Max Planck that the energy of waves could be described as consisting of small packets or quanta, Albert Einstein exploited this idea to show that an electromagnetic wave such as light could be described by a particle called the photon with a discrete energy dependent on its frequency. This led to a theory of unity between subatomic particles and electromagnetic waves called wave–particle duality in which particles and waves were neither one nor the other, but had certain properties of both. While quantum mechanics describes the world of the very small, it also is needed to explain certain “macroscopic quantum systems” such as superconductors and superfluids.

Broadly speaking, quantum mechanics incorporates four classes of phenomena that classical physics cannot account for: (i) the quantization (discretization) of certain physical quantities, (ii) wave-particle duality, (iii) the uncertainty principle, and (iv) quantum entanglement. Each of these phenomena is described in detail in subsequent sections.

Note for Coherence
Coherence is the property of wave-like states that enables them to exhibit interference. It is also the parameter that quantifies the quality of the interference (also known as the degree of coherence). It was originally introduced in connection with Young’s double-slit experiment in optics but is now used in any field that involves waves, such as acoustics, electrical engineering, neuroscience, and quantum physics. In interference, at least two wave-like entities are combined and, depending on the relative phase between them, they can add constructively or subtract destructively. The degree of coherence is equal to the interference visibility, a measure of how perfectly the waves can cancel due to destructive interference. The property of coherence is the basis for commercial applications such as holography, the Sagnac gyroscope, radio antenna arrays, optical coherence tomography and telescope interferometers (astronomical optical interferometers and radio telescopes).

The coherence of two waves follows from how well correlated the waves are as quantified by the cross-correlation function. The cross-correlation quantifies the ability to predict the value of the second wave by knowing the value of the first. As an example, consider two waves perfectly correlated for all times. At any time, if the first wave changes, the second will change in the same way. If combined they can exhibit complete constructive interference at all times. It follows that they are perfectly coherent. As will be discussed below, the second wave need not be a separate entity. It could be the first wave at a different time or position. In this case, sometimes called self-coherence, the measure of correlation is the autocorrelation function.

Note for Nuclear Magnetic Resonance
Nuclear magnetic resonance (NMR) is a physical phenomenon based upon the quantum mechanical magnetic properties of an atom's nucleus. NMR also commonly refers to a family of scientific methods that exploit nuclear magnetic resonance to study molecules.
All nuclei that contain odd numbers of protons or neutrons have an intrinsic magnetic moment and angular momentum. The most commonly measured nuclei are hydrogen-1 (the most receptive isotope at natural abundance) and carbon-13, although nuclei from isotopes of many other elements (e.g. 15N, 14N 19F, 31P, 17O, 29Si, 10B, 11B, 23Na, 35Cl, 195Pt) can also be observed.
NMR resonant frequencies for a particular substance are directly proportional to the strength of the applied magnetic field, in accordance with the equation for the Larmor precession frequency.

NMR studies magnetic nuclei by aligning them with an applied constant magnetic field and perturbing this alignment using an alternating magnetic field, those fields being orthogonal. The resulting response to the perturbing magnetic field is the phenomenon that is exploited in NMR spectroscopy and magnetic resonance imaging, which use very powerful applied magnetic fields in order to achieve high resolution spectra, details of which are described by the chemical shift and the Zeeman effect.
Nuclear magnetic resonance was first described and measured in molecular beams by Isidor Rabi in 1938. Eight years later, in 1946, Felix Bloch and Edward Mills Purcell refined the technique for use on liquids and solids, for which they shared the Nobel Prize in physics in 1952.
Purcell had worked on the development and application of RADAR during World War II at Massachusetts Institute of Technology's Radiation Laboratory. His work during that project on the production and detection of radiofrequency energy, and on the absorption of such energy by matter, preceded his discovery of NMR.

They noticed that magnetic nuclei, like 1H and 31P, could absorb RF energy when placed in a magnetic field of a strength specific to the identity of the nuclei. When this absorption occurs, the nucleus is described as being in resonance. Interestingly, for analytical scientists, different atoms within a molecule resonate at different frequencies at a given field strength. The observation of the resonance frequencies of a molecule allows a user to discover structural information about the molecule.
The development of nuclear magnetic resonance as a technique of analytical chemistry and biochemistry parallels the development of electromagnetic technology and its introduction into civilian use.
NMR spectroscopy is one of the principal techniques used to obtain physical, chemical, electronic and structural information about molecules due to the chemical shift and Zeeman effect on the resonant frequencies of the nuclei. It is a powerful technique that can provide detailed information on the topology, dynamics and three-dimensional structure of molecules in solution and the solid state. Also, nuclear magnetic resonance is one of the techniques that has been used to build elementary quantum computers.

Note for Electron Spin Resonance
Electrons, and other particles, have an intrinsic angular momentum, known as spin. This creates a magnetic dipole moment. When the electron is placed in a magnetic field, the intrinsic magentic dipole can align in one of two ways, parallel or anti-parallel to the field. The anti-parallel state is of lower energy. However, applying radiation of a certain frequency to the electron can raise it to the higher energy state, in which its magnetic dipole is parallel to the applied magnetic field. It will then fall back to the lower energy state, emitting a photon. If radiation continues to be applied, then the electron will "resonate" between the two energy states. This is known as electron spin resonance, and is used to identify compounds, which each have a unique spectrum of radiation absorption. This occurrence is used in both NMR and MRI.

The DOE Office of Science, Basic Energy Sciences Office and the Air Force Office of Scientific Research funded this research on the fundamental physics of a single quantum spin.

Ames Laboratory is operated for the Department of Energy by Iowa State University. The Lab conducts research into various areas of national concern, including the synthesis and study of new materials, energy resources, high-speed computer design, and environmental cleanup and restoration.

Figure 1 depicting coherently driven spin oscillations of a nitrogen-vacancy (N-V) center show the excellent level of agreement achieved between experiment, analytical theory and computer simulation in the research on the fundamental physics of a single quantum spin by Ames Laboratory, the University of California, Santa Barbara, and Microsoft Station Q.