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Two-state Quantum System
Quantum system that can be measured as one of two values; sought for "quantum bits" in quantum computing
An electrically neutral silver atoms beam through Stern-Gerlach experiment's inhomogeneous magnetic field splits into two, each of which corresponds to one possible spin value of the outermost electron of the silver atom.
Two-state systems are the simplest quantum systems that can exist, since the dynamics of a one-state system is trivial (i.e. there is no other state the system can exist in). The mathematical framework required for the analysis of two-state systems is that of linear differential equations and linear algebra of two-dimensional spaces. As a result, the dynamics of a two-state system can be solved analytically without any approximation. The generic behavior of the system is that the wavefunction's amplitude oscillates between the two states.
A very well known example of a two-state system is the spin of a spin-1/2 particle such as an electron, whose spin can have values +?/2 or −?/2, where ? is the reduced Planck constant.
The two-state system cannot be used as a description of absorption or decay, because such processes require coupling to a continuum. Such processes would involve exponential decay of the amplitudes, but the solutions of the two-state system are oscillatory.
Analytical solutions for stationary state energies and time-dependence
or which is a 2 2 matrix Eigenvalues and eigenvectors problem. Because of the hermiticity of the eigenvalues are real, or rather vice versa it is the requirement that the energies are real that implies the hermiticity of . The eigenvectors represent the stationary states, i.e. those for whom the absolute magnitude of the squares of the probability amplitudes do not change with time.
Eigenvalues of the Hamiltonian
The most general form of a 2 2 Hermitian matrix such as the Hamiltonian of a two-state system is given by
where and are real numbers with units of energy. The allowed energy levels of the system, namely the eigenvalues of the Hamiltonian matrix, can be found in the usual way.
Alternatively, this matrix can be decomposed as,
Here, and are real numbers. The matrix is the 2 2 identity matrix and the matrices are the Pauli matrices. This decomposition simplifies the analysis of the system, especially in the time-independent case where the values of and are constants.
The Hamiltonian can be written even more compactly as:
The vector is given by and is given by . This representation simplifies the analysis of the time evolution of the system and is easier to use with other specialized representations such as the Bloch sphere.
If the two-state system's time-independent Hamiltonian is defined as above, then its eigenvalues are given by . Evidently is the average energy of the two levels, and the norm of is the splitting between them. The corresponding eigenvectors are denoted and .
We now assume that the probability amplitudes are time-dependent, though the basis states are not. The Time-dependent Schrödinger equation states , and proceeding as before (substituting for and premultiplying by again produces a pair of coupled linear equations, but this time they are first order partial differential equations: . If is time independent there are several approaches, to find the time dependence of , such as normal modes. The result is that
where is the statevector at .
Here the exponential of a matrix may be found from the series expansion. The matrix is called the time evolution matrix (which comprises the matrix elements of the corresponding time evolution operator ). It is easily proved that is unitary, meaning that . It can be shown that
When one changes the basis to the eigenvectors of the Hamiltonian, in other words, if the basis states are chosen to be the eigenvectors then and and so the Hamiltonian is diagonal, i.e. and is of the form,
Now the unitary time evolution operator is easily seen to be given by:
The factor only contributes to the overall phase of the operator and can usually be ignored to yield a new time evolution operator that is physically indistinguishable from the original operator. Moreover, any perturbation to the system (which will be of the same form as the Hamiltonian) can be added to the system in the eigenbasis of the unperturbed Hamiltonian and analysed in the same way as above. Therefore, for any perturbation the new eigenvectors of the perturbed system can be solved for exactly, as mentioned in the introduction.
Rabi formula for a static perturbation
Suppose that the system starts in one of the basis states at , say so that , and we are interested in the probability of occupation of each of the basis states as a function of time when is the time-independent Hamiltonian.
The probability of occupation of state is . In the case of the starting state, , and from above, . Hence
Obviously due to the initial condition. The frequency is called the generalised Rabi frequency, is called the Rabi frequency, and is called the detuning. At zero detuning, , i.e. there is Rabi flopping from guaranteed occupation of state 1, to guaranteed occupation of state 2, and back to state 1 etc with frequency . As the detuning is increased away from zero, the frequency of the flopping increases (to ) and the amplitude decreases to .
Consider the case of a spin-1/2 particle in a magnetic field . The interaction Hamiltonian for this system is
where is the magnitude of the particle's magnetic moment and is the vector of Pauli matrices. Solving the time dependent Schrödinger equation yields
where and . Physically, this corresponds to the Bloch vector precessing around with angular frequency . Without loss of generality, assume the field is uniform points in , so that the time evolution operator is given as
It can be seen that such a time evolution operator acting on a general spin state of a spin-1/2 particle will lead to the precession about the axis defined by the applied magnetic field (this is the quantum mechanical equivalent of Larmor precession)
The above method can be applied to the analysis of any generic two-state system that is interacting with some field (equivalent to the magnetic field in the previous case) if the interaction is given by an appropriate coupling term that is analogous to the magnetic moment. The precession of the state vector (which need not be a physical spinning as in the previous case) can be viewed as the precession of the state vector on the Bloch sphere.
The representation on the Bloch sphere for a state vector will simply be the vector of expectation values . As an example, consider a state vector that is a normalized superposition of and , that is, a vector that can be represented in the basis as
The components of on the Bloch sphere will simply be . This is a unit vector that begins pointing along and precesses around in a left-handed manner. In general, by a rotation around , any state vector can be represented as with real coefficients and . Such a state vector corresponds to a Bloch vector in the xz-plane making an angle with the z-axis. This vector will proceed to precess around . In theory, by allowing the system to interact with the field of a particular direction and strength for precise durations, it is possible to obtain any orientation of the Bloch vector, which is equivalent to obtaining any complex superposition. This is the basis for numerous technologies including quantum computing and MRI.
Evolution in a time-dependent field: Nuclear magnetic resonance
Nuclear magnetic resonance (NMR) is an important example in the dynamics of two-state systems because it is involves the exact solution to a time dependent Hamiltonian. The NMR phenomenon is achieved by placing a nucleus in a strong, static field B0 (the "holding field") and then applying a weak, transverse field B1 that oscillates at some radiofrequency ?r. Explicitly, consider a spin-1/2 particle in a holding field and a transverse rf field B1 rotating in the xy-plane in a right-handed fashion around B0:
As in the free precession case, the Hamiltonian is , and the evolution of a state vector is found by solving the time-dependent Schrödinger equation . After some manipulation (given in the collapsed section below), it can be shown that the Schrödinger equation becomes
where and .
As per the previous section, the solution to this equation has the Bloch vector precessing around with a frequency that is twice the magnitude of the vector. If is sufficiently strong, some proportion of the spins will be pointing directly down prior to the introduction of the rotating field. If the angular frequency of the rotating magnetic field is chosen such that , in the rotating frame the state vector will precess around with frequency , and will thus flip from down to up releasing energy in the form of detectable photons. This is the fundamental basis for NMR, and in practice is accomplished by scanning until the resonant frequency is found at which point the sample will emit light. Similar calculations are done in atomic physics, and in the case that the field is not rotating, but oscillating with a complex amplitude, use is made of the rotating wave approximation in deriving such results.
Relation to Bloch equations
The optical Bloch equations for a collection of spin-1/2 particles can be derived from the time dependent Schrödinger equation for a two level system. Starting with the previously stated Hamiltonian , it can be written in summation notation after some rearrangement as
Multiplying by a Pauli matrix and the conjugate transpose of the wavefunction, and subsequently expanding the product of two Pauli matrices yields
Adding this equation to its own conjugate transpose yields a left hand side of the form
where the fact that has been used. In vector form these three equations can be expressed in terms of a cross product
Classically, this equation describes the dynamics of a spin in a magnetic field. An ideal magnet consists of a collection of identical spins behaving independently, and thus the total magnetization is proportional to the Bloch vector. All that is left to obtain the final form of the optical Bloch equations is the inclusion of the phenomenological relaxation terms.
Two-state systems are the simplest non-trivial quantum systems that occur in nature, but the above-mentioned methods of analysis are not just valid for simple two-state systems. Any general multi-state quantum system can be treated as a two-state system as long as the observable one is interested in has two eigenvalues. For example, a spin-1/2 particle may in reality have additional translational or even rotational degrees of freedom, but those degrees of freedom are irrelevant to the preceding analysis. Mathematically, the neglected degrees of freedom correspond to the degeneracy of the spin eigenvalues.
Another case where the effective two-state formalism is valid is when the system under consideration has two levels that are effectively decoupled from the system. This is the case in the analysis of the spontaneous or stimulated emission of light by atoms and that of charge qubits. In this case it should be kept in mind that the perturbations (interactions with an external field) are in the right range and do not cause transitions to states other than the ones of interest.
Significance and other examples
Pedagogically, the two-state formalism is among the simplest of mathematical techniques used for the analysis of quantum systems. It can be used to illustrate fundamental quantum mechanical phenomena such as the interference exhibited by particles of the polarization states of the photon, but also more complex phenomena such as neutrino oscillation or the neutral K-meson oscillation.
Two-state formalism can be used to describe simple mixing of states, which leads to phenomena such as resonance stabilization and other level crossing related symmetries. Such phenomena have a wide variety of application in chemistry. Phenomena with tremendous industrial applications such as the maser and laser can be explained using the two-state formalism.
The two-state formalism also forms the basis of quantum computing. Qubits, which are the building blocks of a quantum computer, are nothing but two-state systems. Any quantum computational operation is a unitary operation that rotates the state vector on the Bloch sphere.
An excellent treatment of the two-state formalism and its application to almost all the applications mentioned in this article is presented in the third volume of The Feynman Lectures on Physics.
The following set of lecture notes covers the necessary mathematics and also treats a few examples in some detail: