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Glossary: Unit 7

Bose-Einstein condensate
A Bose-Einstein condensate, or BEC, is a special phase of matter in which the quantum mechanical wavefunctions of a collection of particles line up and overlap in a manner that allows the particles to act as a single quantum object. The electrons in a superconductor form a BEC; superfluid helium is an example of a liquid BEC. BECs can also be created from dilute gases of ultracold atoms and molecules.
Doppler shift (Doppler effect)
The Doppler shift is a shift in the wavelength of light or sound that depends on the relative motion of the source and the observer. A familiar example of a Doppler shift is the apparent change in pitch of an ambulance siren as it passes a stationary observer. When the ambulance is moving toward the observer, the observer hears a higher pitch because the wavelength of the sound waves is shortened. As the ambulance moves away from the observer, the wavelength is lengthened and the observer hears a lower pitch. Likewise, the wavelength of light emitted by an object moving toward an observer is shortened, and the observer will see a shift to blue. If the light-emitting object is moving away from the observer, the light will have a longer wavelength and the observer will see a shift to red. By observing this shift to red or blue, astronomers can determine the velocity of distant stars and galaxies relative to the Earth. Atoms moving relative to a laser also experience a Doppler shift, which must be taken into account in atomic physics experiments that make use of laser cooling and trapping.
In quantum mechanics, entanglement occurs when the quantum states of two particles that may be spatially separated are linked together. A measurement of one of the entangled particles implies the result of the same measurement made on the other entangled particle.
evaporative cooling
Evaporative cooling is a process used in atomic physics experiments to cool atoms down to a few billionths of a degree above absolute zero. The way it works is similar to how a cup of hot coffee cools through evaporation. Atoms are pre-cooled, usually with some kind of laser cooling, and trapped in a manner that imparts no additional energy to the atoms. The warmest atoms are removed from the trap, and the remaining atoms reach a new, lower equilibrium temperature. This process is typically repeated many times, creating small clouds of very cold atoms.
ground state
The ground state of a physical system is the lowest energy state it can occupy. For example, a hydrogen atom is in its ground state when its electron occupies the lowest available energy level.
index of refraction
A material's index of refraction is defined as the speed that light travels in the material divided by the speed of light in a vacuum. Therefore, the index of refraction of the vacuum is equal to one. Light slows down as it enters a material due to the interaction between the oscillating electric and magnetic fields of the light wave and the constituent parts of the material. The index of refraction of air is 1.003, and the index of refraction of water is 1.33.
Interference is an effect that occurs when two or more waves overlap. In general, the individual waves do not affect one another, and the total wave amplitude at any point in space is simply the sum of the amplitudes of the individual waves at that point. In some places, the two waves may add together, and in other places they may cancel each other out, creating an interference pattern that may look quite different than either of the original waves. Quantum mechanical wavefunctions can interfere, creating interference patterns that can only be observed in their corresponding probability distributions.
A light-year is the distance that light, which moves at a constant speed, travels in one year. One light-year is equivalent to 9.46 x 1015 meters, or 5,878 billion miles.
optical molasses
Optical molasses is formed when laser beams for Doppler cooling are directed along each spatial axis so that atoms are laser cooled in every direction. Atoms can reach microkelvin temperatures in optical molasses. However, the molasses is not a trap, so the atoms can still, for example, fall under the influence of gravity.
In physics, the term phase has two distinct meanings. The first is a property of waves. If we think of a wave as having peaks and valleys with a zero-crossing between them, the phase of the wave is defined as the distance between the first zero-crossing and the point in space defined as the origin. Two waves with the same frequency are "in phase" if they have the same phase and therefore line up everywhere. Waves with the same frequency but different phases are "out of phase." The term phase also refers to states of matter. For example, water can exist in liquid, solid, and gas phases. In each phase, the water molecules interact differently, and the aggregate of many molecules has distinct physical properties. Condensed matter systems can have interesting and exotic phases, such as superfluid, superconducting, and quantum critical phases. Quantum fields such as the Higgs field can also exist in different phases.
A qubit is the quantum counterpart to the bit for classical computing. A bit, which is short for binary digit, is the smallest unit of binary information and can assume two values: zero and one. A qubit, or quantum bit, is in a quantum superposition of the values zero and one. Because the qubit is in a superposition and has no definite value until it is measured directly, quantum computers can operate exponentially faster than classical computers.
Snell's Law
Snell's law describes how the path of a light ray changes when it moves into a material with a different index of refraction. According to Snell's law, if a light ray traveling through a medium with index of refraction n1 hits the boundary of a material with index n2 at angle , the light path is bent and enters the new material at an angle given by the relation n1sin = n2 sin.
In quantum mechanics, it is possible for a particle or system of particles to be in a superposition state in which the outcome of a measurement is unknown until the measurement is actually made. For example, neutrinos can exist in a superposition of electron, muon, and tau flavors (Units 1 and 2). The outcome of a measurement of the neutrino's flavor will yield a definite result—electron, muon, or tau—but it is impossible to predict the outcome of an individual measurement. Quantum mechanics tells us only the probability of each outcome. Before the measurement is made, the neutrino's flavor is indeterminate, and the neutrino can be thought of as being all three flavors at once.
valence electron
A valence electron is an electron in the outermost shell of an atom in the Lewis model, or in the orbital with the highest value of the principal quantum number, n, in the quantum mechanical description of an atom. The valence electrons determine most of the chemical and physical properties of the atom. It is the valence electrons that participate in ionic and covalent chemical bonds, and that make the primary contributions to an atom's magnetic moment.

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