The quantum harmonic oscillator is the quantum-mechanical analog of the classical harmonic oscillator. Because an arbitrary potential can be approximated as a harmonic potential at the vicinity of a stable equilibrium point, it is one of the most important model systems in quantum mechanics. Furthermore, it is one of the few quantum-mechanical systems for which an exact, analytical solution is known.[1] [2] [3]
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The Hamiltonian of the particle is:
where is the position operator, and is the momentum operator, given by
The first term in the Hamiltonian represents the kinetic energy of the particle, and the second term represents the potential energy in which it resides. In order to find the energy levels and the corresponding energy eigenstates, we must solve the time-independent Schrödinger equation,
We can solve the differential equation in the coordinate basis, using a spectral method. It turns out that there is a family of solutions. In the position basis they are
The functions Hn are the Hermite polynomials:
The corresponding energy levels are
This energy spectrum is noteworthy for three reasons. First, the energies are quantized, meaning that only discrete energy values (half-integer multiples of ) are possible; this is a general feature of quantum-mechanical systems when a particle is confined. Second, these discrete energy levels are equally spaced, unlike in the Bohr model of the atom, or the particle in a box. Third, the lowest achievable energy (the energy of the state, called the ground state) is not equal to the minimum of the potential well, but above it; this is called zero-point energy. Because of the zero-point energy, the position and momentum of the oscillator in the ground state are not fixed (as they would be in a classical oscillator), but have a small range of variance, in accordance with the Heisenberg uncertainty principle. The zero-point energy also has important implications in quantum field theory and quantum gravity.
Note that the ground state probability density is concentrated at the origin. This means the particle spends most of its time at the bottom of the potential well, as we would expect for a state with little energy. As the energy increases, the probability density becomes concentrated at the classical "turning points", where the state's energy coincides with the potential energy. This is consistent with the classical harmonic oscillator, in which the particle spends most of its time (and is therefore most likely to be found) at the turning points, where it is the slowest. The correspondence principle is thus satisfied.
The spectral method solution, though straightforward, is rather tedious. The "ladder operator" method, due to Paul Dirac, allows us to extract the energy eigenvalues without directly solving the differential equation. Furthermore, it is readily generalizable to more complicated problems, notably in quantum field theory. Following this approach, we define the operators a and its adjoint a†
The operator a is not Hermitian since it and its adjoint a† are not equal.
We can also define a number operator N which has the following property:
The following commutators can be easily obtained by substituting the canonical commutation relation,
And the Hamilton operator can be expressed as
so the eigenstate of N is also the eigenstate of energy.
The commutation properties yields
and similarly,
This means that a acts on to produce, up to a multiplicative constant, , and a† acts on to produce . For this reason, a is called a "lowering operator", and a† a "raising operator". The two operators together are called ladder operators. In quantum field theory, a and a† are alternatively called "annihilation" and "creation" operators because they destroy and create particles, which correspond to our quanta of energy.
Given any energy eigenstate, we can act on it with the lowering operator, a, to produce another eigenstate with -less energy. By repeated application of the lowering operator, it seems that we can produce energy eigenstates down to E = −∞. However, since
the smallest eigen number is 0, and
In this case, subsequent applications of the lowering operator will just produce zero kets, instead of additional energy eigenstates. Furthermore, we have shown above that
Finally, by acting on with the raising operator and multiplying by suitable normalization factors, we can produce an infinite set of energy eigenstates , such that
which matches the energy spectrum which we gave in the preceding section.
Arbitrary eigenstate can be expressed in terms of
The quantum harmonic oscillator possesses natural scales for length and energy, which can be used to simplify the problem. These can be found by nondimensionalization. The result is that if we measure energy in units of and distance in units of , then the Schrödinger equation becomes:
and the energy eigenfunctions and eigenvalues become
where are the Hermite polynomials.
To avoid confusion, we will not adopt these natural units in this article. However, they frequently come in handy when performing calculations.
The vibrations of a diatomic molecule are an example of a two-body version of the quantum harmonic oscillator. In this case, the angular frequency is given by
where is the reduced mass and is determined by the mass of the two atoms. [4]
The one-dimensional harmonic oscillator is readily generalizable to N dimensions, where N = 1, 2, 3, ... . In one dimension, the position of the particle was specified by a single coordinate, x. In N dimensions, this is replaced by N position coordinates, which we label x1, ..., xN. Corresponding to each position coordinate is a momentum; we label these p1, ..., pN. The canonical commutation relations between these operators are
The Hamiltonian for this system is
As the form of this Hamiltonian makes clear, the N-dimensional harmonic oscillator is exactly analogous to N independent one-dimensional harmonic oscillators with the same mass and spring constant. In this case, the quantities x1, ..., xN would refer to the positions of each of the N particles. This is a convenient property of the potential, which allows the potential energy to be separated into terms depending on one coordinate each.
This observation makes the solution straightforward. For a particular set of quantum numbers {n} the energy eigenfunctions for the N-dimensional oscillator are expressed in terms of the 1-dimensional eigenfunctions as:
In the ladder operator method, we define N sets of ladder operators,
By a procedure analogous to the one-dimensional case, we can then show that each of the ai and a†i operators lower and raise the energy by ℏω respectively. The Hamiltonian is
This Hamiltonian is invariant under the dynamic symmetry group U(N) (the unitary group in N dimensions), defined by
where is an element in the defining matrix representation of U(N).
The energy levels of the system are
As in the one-dimensional case, the energy is quantized. The ground state energy is N times the one-dimensional energy, as we would expect using the analogy to N independent one-dimensional oscillators. There is one further difference: in the one-dimensional case, each energy level corresponds to a unique quantum state. In N-dimensions, except for the ground state, the energy levels are degenerate, meaning there are several states with the same energy.
The degeneracy can be calculated relatively easily. As an example, consider the 3-dimensional case: Define n = n1 + n2 + n3. All states with the same n will have the same energy. For a given n, we choose a particular n1. Then n2 + n3 = n − n1. There are n − n1 + 1 possible groups {n2, n3}. n2 can take on the values 0 to n − n1, and for each n2 the value of n3 is fixed. The degree of degeneracy therefore is:
Formula for general N and n [gn being the dimension of the symmetric irreducible nth power representation of the unitary group U(N)]:
The special case N = 3, given above, follows directly from this general equation. This is however, only true for distinguishable particle, or one particle in N dimensions (as dimensions are distinguishable). For the case of N bosons in a one dimension harmonic trap, the degeneracy scales as the number of ways to partition an integer using integers less than or equal to N.
This arises due to the constraint of putting N quanta into a state ket where and , which are the same constraints as in integer partition.
The Schrödinger equation of a spherically-symmetric three-dimensional harmonic oscillator can be solved explicitly by separation of variables, see this article for the present case. This procedure is analogous to the separation performed in the hydrogen-like atom problem, but with the spherically symmetric potential
where is the mass of the problem. (Because m will be used below for the magnetic quantum number, mass is indicated by , instead of m, as earlier in this article.)
The solution reads
where
The energy eigenvalue is
The energy is usually described by the single quantum number
Because k is a non-negative integer, for every even n we have and for every odd n we have . The magnetic quantum number m is an integer satisfying , so for every n and l there are 2l+1 different quantum states, labeled by m. Thus, the degeneracy at level n is
where the sum starts from 0 or 1, according to whether n is even or odd. This result is in accordance with the dimension formula above.
In this problem, we consider N equal masses which are connected to their neighbors by springs, in the limit of large N. The masses form a linear chain in one dimension, or a regular lattice in two or three dimensions.
As in the previous section, we denote the positions of the masses by x1, x2, ..., as measured from their equilibrium positions (i.e. xk = 0 if particle k is at its equilibrium position.) In two or more dimensions, the xs are vector quantities. The Hamiltonian of the total system is
The potential energy is summed over "nearest-neighbor" pairs, so there is one term for each spring.
Remarkably, there exists a coordinate transformation[5] to turn this problem into a set of independent harmonic oscillators, each of which corresponds to a particular collective distortion of the lattice. These distortions display some particle-like properties, and are called phonons. Phonons occur in all (even amorphous) solids and are extremely important for understanding many of the phenomena studied in solid state physics.