Derivation of the optical Bloch equations

The master equation is an equation of motion for a density matrix describing an open quantum system, much like the Schrodinger equation describes the evolution of a closed quantum system. This section provides a derivation of the master equation for a spontaneously emitting atom, driven by a classical field, which is known as the Optical Bloch Equation.

▼ Category:8.422

► Category:Quantum Light

► Category:Optical Bloch Equations

► Category:Bose-Einstein condensates

► Category:Ion traps and quantum information

► Category:Light forces

► Category:Photon-atom interactions

  Introduction to Atomic Physics

  Quantum light: states and dynamics

  Optical Bloch equations

  Photon-atom interactions

  Frontiers in Atomic Physics

  Ion traps and quantum information

  Quantum gases

Classical model of atom and field

A good starting point, to appreciate the problem of open quantum systems, is the classical model for a two-level atom coupled to a black-body electromagnetic field, the Einstein rate equations

Failed to parse (MathML with SVG or PNG fallback (recommended for modern browsers and accessibility tools): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle \begin{array}{rcl} \frac{dN_g}{dt} &=& A_{eg} N_e + u(\omega_{eg}) \left[ { B_{eg}N_e - B_{ge} N_g } \right] \\ \frac{dN_e}{dt} &=& - A_{eg} N_e + u(\omega_{eg}) \left[ { B_{ge}N_g - B_{eg} N_e } \right] \,, \end{array}}

where $A_{eg}$ is the spontaneous emission rate, $B_{ge}$ and $B_{eg}$ are stimulated emission rates, and $u(\omega _{eg})$ is the field energy density at atomic frequency $\omega _{eg}$, for levels denoted by $|e{\rangle }$ and $|g{\rangle }$.

Is there a straightforward quantum analogue of this? We might be tempted to simply add a damping term to the Schrödinger equation, like

Failed to parse (MathML with SVG or PNG fallback (recommended for modern browsers and accessibility tools): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle i\hbar \partial_t |\psi \rangle = H |\psi \rangle - i\Gamma |\psi{\rangle} \,, }

but this is not physically allowed by quantum mechanics! How, then, can we construct a fully quantum-mechanical description of open system dynamics? The key concept is that we must properly account for noise:

From classical thermodynamics, we know that any time there is energy transfer from a system to the environment, there is entropy exchanged back from the environment to the system. This is a simple illustration of the very basic fluctuation--dissipation principle: there can be no relaxation without con-commitment noise! To study quantum open system, we must model the appropriate quantum noise contribution which goes along with relaxation.

Density matrices and closed system dynamics

The main tool we shall use to model open quantum systems is the density matrix representation for quantum states, so it is helpful to begin with a review of density matrices and how they evolve under Hamiltonian dynamics.

Review of density matrices: definition, properties, and unravlings

Recall that a density matrix Failed to parse (MathML with SVG or PNG fallback (recommended for modern browsers and accessibility tools): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle \rho} for a pure state Failed to parse (MathML with SVG or PNG fallback (recommended for modern browsers and accessibility tools): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle |\psi{\rangle}} is the matrix Failed to parse (MathML with SVG or PNG fallback (recommended for modern browsers and accessibility tools): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle |\psi{\rangle}{\langle}\psi|} . Thus, for example

Failed to parse (MathML with SVG or PNG fallback (recommended for modern browsers and accessibility tools): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle \begin{array}{rcl} |0 \rangle &\rightarrow& \left[ \begin{array}{cc}{1}&{0}\\{0}&{0}\end{array}\right] \\ &&~ \\ |1 \rangle &\rightarrow& \left[ \begin{array}{cc}{0}&{0}\\{0}&{1}\end{array}\right] \\ &&~ \\ \frac{|0{\rangle}+|1{\rangle}}{\sqrt{2}} &\rightarrow& \frac{1}{2} \left[ \begin{array}{cc}{1}&{1}\\{1}&{1}\end{array}\right] \,. \end{array}}

Density matrices may also represent statistical mixtures of pure states; this state

Failed to parse (MathML with SVG or PNG fallback (recommended for modern browsers and accessibility tools): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle \frac{1}{2} \left[ \begin{array}{cc}{1}&{0}\\{0}&{1}\end{array}\right] }

can be interpreted as a 50/50 mixture of Failed to parse (MathML with SVG or PNG fallback (recommended for modern browsers and accessibility tools): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle |0{\rangle}} and Failed to parse (MathML with SVG or PNG fallback (recommended for modern browsers and accessibility tools): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle |1{\rangle}} . However, one must be careful, because there are infinitely many ways to unravel a density matrix into statistical mixtures of pure states.

For example,

$\rho ={\frac {1}{4}}\left[{\begin{array}{cc}{3}&{0}\\{0}&{1}\end{array}}\right]$

is

Failed to parse (MathML with SVG or PNG fallback (recommended for modern browsers and accessibility tools): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle \rho = \frac{3}{4} |0 \rangle \langle 0| + \frac{1}{4} |1 \rangle \langle 1| \,, }

but it is also

Failed to parse (MathML with SVG or PNG fallback (recommended for modern browsers and accessibility tools): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle \rho = \frac{1}{2} |a \rangle \langle a| + \frac{1}{2} |b \rangle \langle b| \,, }

where

Failed to parse (MathML with SVG or PNG fallback (recommended for modern browsers and accessibility tools): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle \begin{array}{rcl} |a \rangle &=& \frac{\sqrt{3}|0{\rangle}+|1{\rangle}}{2} \\ |b \rangle &=& \frac{\sqrt{3}|0{\rangle}-|1{\rangle}}{2} \,. \end{array}}

In general, a density matrix $\rho$ may always be written as a statistical mixture of pure states,

Failed to parse (MathML with SVG or PNG fallback (recommended for modern browsers and accessibility tools): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle \rho = \sum_k p_k |\psi_k{\rangle} \,, }

where Failed to parse (MathML with SVG or PNG fallback (recommended for modern browsers and accessibility tools): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle p_k} are probabilities, such that Failed to parse (MathML with SVG or PNG fallback (recommended for modern browsers and accessibility tools): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle \sum_k p_k = 1} .

A matrix $\rho$ is a valid density matrix if and only if the eigenvalues of Failed to parse (MathML with SVG or PNG fallback (recommended for modern browsers and accessibility tools): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle \rho} are non-negative, and sum to one, such that Failed to parse (MathML with SVG or PNG fallback (recommended for modern browsers and accessibility tools): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle {\rm Tr}(\rho)=1} . Failed to parse (MathML with SVG or PNG fallback (recommended for modern browsers and accessibility tools): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle \rho} represents a pure state if and only if Failed to parse (MathML with SVG or PNG fallback (recommended for modern browsers and accessibility tools): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle {\rm Tr}(\rho^2)=1} .

Density matrices: closed system evolution

How does a density matrix evolve in a closed system? From the Schrodinger equation

$i\hbar \partial _{t}|\psi \rangle =H|\psi {\rangle }\,,$

it follows that a pure state density matrix Failed to parse (MathML with SVG or PNG fallback (recommended for modern browsers and accessibility tools): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle \rho=|\psi{\rangle}{\langle}\psi|} evolves as

Failed to parse (MathML with SVG or PNG fallback (recommended for modern browsers and accessibility tools): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle \begin{array}{rcl} \dot{\rho} &=& |\dot{\psi}{\rangle}{\langle}\psi| + |\psi{\rangle}{\langle}\dot{\psi}| \\ &=& -\frac{i}{\hbar}[H,\rho] \,. \end{array}}

For example, if Failed to parse (MathML with SVG or PNG fallback (recommended for modern browsers and accessibility tools): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle \rho} is a two-level atom evolving under the classical field Jaynes-Cummings Hamiltonian

Failed to parse (MathML with SVG or PNG fallback (recommended for modern browsers and accessibility tools): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle H = \frac{\hbar\omega_0}{2}(|e \rangle \langle e|-|g \rangle \langle g|) + \Omega (|g \rangle \langle e|+|e \rangle \langle g|) \,, }

which we may express using Pauli matrices as

$H={\frac {\hbar \omega _{0}}{2}}Z+\Omega X\,,$

then for

Failed to parse (MathML with SVG or PNG fallback (recommended for modern browsers and accessibility tools): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle \rho = \left[ \begin{array}{cc}{\rho_{gg}}&{\rho_{ge}}\\{\rho_{eg}}&{\rho_{ee}}\end{array}\right] \,, }

the equation of motion for Failed to parse (MathML with SVG or PNG fallback (recommended for modern browsers and accessibility tools): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle \rho} is

Failed to parse (MathML with SVG or PNG fallback (recommended for modern browsers and accessibility tools): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle \dot{\rho} = \left[ \begin{array}{cc}{i\Omega(\rho_{eg}-\rho_{ge})}&{-i\omega_0\rho_{eg}+i\Omega(\rho_{ee}-\rho_{gg})}\\{i\omega_0\rho_{eg}-i\Omega(\rho_{ee}-\rho_{gg})}&{-i\Omega(\rho_{eg}-\rho_{ge})}\end{array}\right] \,. }

We can recognize this as a rotation of the Bloch sphere about the axis defined by

Failed to parse (MathML with SVG or PNG fallback (recommended for modern browsers and accessibility tools): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle \vec{n} = \frac{\hbar\omega_0}{2} \hat{z} + \Omega \hat{x} \,, }

by using the Bloch sphere representation for a density matrix,

Failed to parse (MathML with SVG or PNG fallback (recommended for modern browsers and accessibility tools): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle \rho = \frac{I+\vec{r}\cdot{\sigma}}{2} \,. }

Density matrices and open system dynamics: approach

We can build a mathematical model for quantum open system dynamics, based on four basic ideas:

Density matrix evolution	Instead of pure states (eg Failed to parse (MathML with SVG or PNG fallback (recommended for modern browsers and accessibility tools): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle |\psi{\rangle}} ), we describe the system state using a density matrix Failed to parse (MathML with SVG or PNG fallback (recommended for modern browsers and accessibility tools): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle \rho} . The equation of motion for $\rho$ is Failed to parse (MathML with SVG or PNG fallback (recommended for modern browsers and accessibility tools): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle \dot{\rho} = { \mathcal L}[ \rho ] \,, } where Failed to parse (MathML with SVG or PNG fallback (recommended for modern browsers and accessibility tools): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle { \mathcal L}} is known as the Liouvillian operator (or a "superoperator"). For example, for Hamiltonian evolution, we have: Failed to parse (MathML with SVG or PNG fallback (recommended for modern browsers and accessibility tools): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle i\hbar \partial_t |\psi \rangle = H |\psi{\rangle} } which, for Failed to parse (MathML with SVG or PNG fallback (recommended for modern browsers and accessibility tools): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle \rho = |\psi{\rangle}{\langle}\psi|} gives Failed to parse (MathML with SVG or PNG fallback (recommended for modern browsers and accessibility tools): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle \begin{array}{rcl} \partial_t \rho &=& |\dot\psi{\rangle}{\langle}\psi| + |\psi{\rangle}{\langle}\dot\psi| \\ &=& -\frac{i}{\hbar} H |\psi{\rangle}{\langle}\psi| + |\psi{\rangle}{\langle}\psi| \frac{i}{\hbar} H \\ &=& -\frac{i}{\hbar} [H,\rho] \,. \end{array}} This is unitary evolution, but in general, the differential equation for Failed to parse (MathML with SVG or PNG fallback (recommended for modern browsers and accessibility tools): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle \rho} can describe non-unitary evolution. Such differential equations are known as "master equations." They are nontrivial to construct, because they must restrict Failed to parse (MathML with SVG or PNG fallback (recommended for modern browsers and accessibility tools): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle \rho} to be a legitimate density matrix at all times.
Partial trace	We are interested in the state of the system alone, and want to disregard the state of the environment. If Failed to parse (MathML with SVG or PNG fallback (recommended for modern browsers and accessibility tools): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle \rho_{\rm total}} is the state of the whole system + environment, then the state of the system alone is Failed to parse (MathML with SVG or PNG fallback (recommended for modern browsers and accessibility tools): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle \begin{array}{rcl} \rho_{sys} &=& {\rm Tr}_{env} \left[ { \rho_{\rm total} } \right] \\ &=& \sum_{\rm env} {\langle}{\rm env}| \rho_{\rm total} |{\rm env}{\rangle} \,. \end{array}}
Assumptions about the environment	The environment is also known as a "bath" or a "reservoir" (cf API). We model it as being an ensemble of oscillators, of a variety of frequencies, which are weakly coupled to the system. It has several important properties: Large and unchanging -- Born approximation Short correlation time Failed to parse (MathML with SVG or PNG fallback (recommended for modern browsers and accessibility tools): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle \tau_c} -- Markov approximation
Two (very different) timescales	There are two important timescales in this model: $T_{relax}$: A slow evolution of the system Failed to parse (MathML with SVG or PNG fallback (recommended for modern browsers and accessibility tools): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle \tau_c} : The fast fluctuations of the environment We will build equations of motion which have a timescale Failed to parse (MathML with SVG or PNG fallback (recommended for modern browsers and accessibility tools): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle \Delta t} , chosen such that Failed to parse (MathML with SVG or PNG fallback (recommended for modern browsers and accessibility tools): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle \tau_c \ll \Delta t \ll T_{relax}} .

Our goal is to construct a model dynamical equation of motion for Failed to parse (MathML with SVG or PNG fallback (recommended for modern browsers and accessibility tools): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle \rho_{sys}} of the form

${\frac {\Delta \rho _{sys}}{\Delta t}}=M\rho _{sys}\,,$

where Failed to parse (MathML with SVG or PNG fallback (recommended for modern browsers and accessibility tools): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle M} is time independent. This is known as a "coarse grained" evolution equation. It is desirable to obtain Failed to parse (MathML with SVG or PNG fallback (recommended for modern browsers and accessibility tools): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle M} for a variety of scenarios, including interactions where the system + environment are atom + light, light + light, and atom + motion, for example. Below, we construct a master equation for the atom + vacuum using two different approaches.

Beamsplitter model of the master equation

The physical intuition behind the master equation we desire to construct can be captured with a simple example, which builds on the beamsplitter we studied in previous lectures.

Single photon through beamsplitter: pure state description

Consider a single photon state Failed to parse (MathML with SVG or PNG fallback (recommended for modern browsers and accessibility tools): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle |\psi \rangle = \alpha |0 \rangle + \beta |1{\rangle}} (for simplicity, let the coefficients be real-valued) entering a beamsplitter of angle Failed to parse (MathML with SVG or PNG fallback (recommended for modern browsers and accessibility tools): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle \theta} :

A vacuum state Failed to parse (MathML with SVG or PNG fallback (recommended for modern browsers and accessibility tools): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle |0{\rangle}} is input to the other port, whose output we discard. Let us consider the photon as being our system, and the other (initially vacuum) mode as being our environment. What is the quantum state of the undiscarded output? Naively, we might argue that a single photon is discarded into the environment with probability Failed to parse (MathML with SVG or PNG fallback (recommended for modern browsers and accessibility tools): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle p_1 = \beta ^2 \sin^2 \theta} , so that we might expect the output to be Failed to parse (MathML with SVG or PNG fallback (recommended for modern browsers and accessibility tools): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle |0{\rangle}} with probability $p_{1}$, and Failed to parse (MathML with SVG or PNG fallback (recommended for modern browsers and accessibility tools): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle |\psi{\rangle}} with probability Failed to parse (MathML with SVG or PNG fallback (recommended for modern browsers and accessibility tools): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle 1-p_1} .

However, that (semi-)classical argument is incorrect. The output state Failed to parse (MathML with SVG or PNG fallback (recommended for modern browsers and accessibility tools): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle |\phi{\rangle}} of the system + environment is

${\begin{array}{rcl}|\phi \rangle &=&e^{i\theta (a^{\dagger }b+b^{\dagger }a)}\left[{|\psi \rangle \otimes |0\rangle }\right]\\&=&\alpha |00\rangle +\beta \left[{\cos \theta |10\rangle +\sin \theta |01\rangle }\right]\\&=&\left[{\alpha |0\rangle +\beta \cos \theta |1\rangle }\right]\otimes |0{\rangle }+\left[{\beta \sin \theta |0\rangle }\right]\otimes |1{\rangle }\,.\end{array}}$

Thus, the correct result is that the output is Failed to parse (MathML with SVG or PNG fallback (recommended for modern browsers and accessibility tools): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle |\psi_0 \rangle = ( \alpha |0 \rangle + \beta \cos\theta |1 \rangle )/\sqrt{ \alpha ^2 + \beta ^2 \cos^2\theta}} , with probability Failed to parse (MathML with SVG or PNG fallback (recommended for modern browsers and accessibility tools): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle 1-p_1} , and $|\psi _{1}\rangle =|0{\rangle }$ with probability Failed to parse (MathML with SVG or PNG fallback (recommended for modern browsers and accessibility tools): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle p_1} .

Single photon through beamsplitter: density matrix description

These states can conveniently be written as density matrices. The input state is

Failed to parse (MathML with SVG or PNG fallback (recommended for modern browsers and accessibility tools): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle \rho_{in} = |\psi{\rangle}{\langle}\psi| = \left[ \begin{array}{cc}{ \alpha ^2}&{ \alpha \beta }\\{ \alpha \beta }&{ \beta ^2}\end{array}\right] \,, }

(3.3.1)

and the output state is

Failed to parse (MathML with SVG or PNG fallback (recommended for modern browsers and accessibility tools): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle \begin{array}{rcl} \rho_{out} = p_1 |\psi_1{\rangle}{\langle}\psi_1| + p_0 |\psi_0{\rangle}{\langle}\psi_0| = \left[ \begin{array}{cc}{ \alpha ^2 + \beta ^2 \sin^2\theta}&{ \alpha \beta \cos\theta }\\{ \alpha \beta \cos\theta }&{ \beta ^2\cos^2\theta}\end{array}\right] \,. \end{array}}

Note that Failed to parse (MathML with SVG or PNG fallback (recommended for modern browsers and accessibility tools): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle \rho_{out}} cannot be written as Failed to parse (MathML with SVG or PNG fallback (recommended for modern browsers and accessibility tools): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle |\chi{\rangle}{\langle}\chi|} for any pure state $|\chi {\rangle }$, because it is not pure (it is a statistical mixture). The change in Failed to parse (MathML with SVG or PNG fallback (recommended for modern browsers and accessibility tools): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle \rho} is

Failed to parse (MathML with SVG or PNG fallback (recommended for modern browsers and accessibility tools): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle \Delta \rho = \rho_{out} - \rho_{in} = \left[ \begin{array}{cc}{-\beta^2 (\cos 2\theta-1)/2}&{ \alpha \beta ( \cos\theta -1)}\\{ \alpha \beta ( \cos\theta -1)}&{ \beta ^2(\cos 2\theta-1)/2}\end{array}\right] \,. }

Single photon through many beamsplitters: coarse-grained evolution

Now imagine that we send the single photon state through many beamsplitters in a sequence, each with some small tap angle Failed to parse (MathML with SVG or PNG fallback (recommended for modern browsers and accessibility tools): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle \theta = \sqrt{\Gamma \Delta t/2}} :

We make two assumptions: the environment modes always begin in the vacuum Failed to parse (MathML with SVG or PNG fallback (recommended for modern browsers and accessibility tools): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle |0{\rangle}} (this is the Born approximation), and the environment is completely different and uncorrelated between scattering events (this is the Markov approximation).

This allows us to write a coarse-grained differential equation for the photon state

${\frac {\Delta \rho }{\Delta t}}\approx \left[{\begin{array}{cc}{{\dot {\rho }}_{00}}&{{\dot {\rho }}_{01}}\\{{\dot {\rho }}_{10}}&{{\dot {\rho }}_{11}}\end{array}}\right]=-\Gamma \left[{\begin{array}{cc}{-\beta ^{2}}&{\alpha \beta /2}\\{\alpha \beta /2}&{\beta ^{2}}\end{array}}\right]\,.$

Expressed as differential equations for each of the independent matrix elements, we get (using Eq.(3.3.1) for Failed to parse (MathML with SVG or PNG fallback (recommended for modern browsers and accessibility tools): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle \rho(t=0)} )

Failed to parse (MathML with SVG or PNG fallback (recommended for modern browsers and accessibility tools): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle \begin{array}{rcl} \frac{d}{dt} \rho_{00} &=& +\Gamma \rho_{11} \\ \frac{d}{dt} \rho_{11} &=& -\Gamma \rho_{11} \end{array}}

for the diagonal elements.

These describe the evolutions of the probabilities of finding the photon in the Failed to parse (MathML with SVG or PNG fallback (recommended for modern browsers and accessibility tools): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle |0{\rangle}} and Failed to parse (MathML with SVG or PNG fallback (recommended for modern browsers and accessibility tools): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle |1{\rangle}} states, and are analogous to the Einstein rate equations. And for the off-diagonal elements we get

Failed to parse (MathML with SVG or PNG fallback (recommended for modern browsers and accessibility tools): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle \begin{array}{rcl} \frac{d}{dt} \rho_{01} &=& -\frac{\Gamma}{2} \rho_{01} \\ \frac{d}{dt} \rho_{10} &=& -\frac{\Gamma}{2} \rho_{10} \end{array}}

which show the decay of the quantum coherence of the state.

Note that the input state we have used in this model is a pure state, but essentially the same result holds if the input is a general mixed state. Specifically, if the input were an arbitrary density matrix

Failed to parse (MathML with SVG or PNG fallback (recommended for modern browsers and accessibility tools): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle \rho = \left[ \begin{array}{cc}a&b\\c&d\end{array}\right] }

then the coarse-grained differential equation for this input would be

Failed to parse (MathML with SVG or PNG fallback (recommended for modern browsers and accessibility tools): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle \dot{\rho} = -\frac{\Gamma}{2} \left[ \begin{array}{cc}{ 2 a}&{b}\\{c}&{-2a}\end{array}\right] \,, }

(3.3.2)

Comparison to the general master equation

The form of these differential equations, which are master equations, is very general, and almost exactly the same result is obtained for a two-level atom interacting with the vacuum. In particular, we shall see that in the master equation for a two-level atom undergoing spontaneous emission, the coherences (the off-diagonal terms) decay twice as fast as the populations (the diagonal terms). That essential physics is captured perfectly by the beamsplitter model, namely Eq.(3.3.2).

There are differences in the atomic master equation, however. In particular,the coherences evolve as

${\begin{array}{rcl}{\frac {d}{dt}}\rho _{01}&=&-\left({i\Delta +{\frac {\Gamma }{2}}}\right)\rho _{01}\,,\end{array}}$

where Failed to parse (MathML with SVG or PNG fallback (recommended for modern browsers and accessibility tools): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle \Delta} is a frequency shift of the system known as the "Lamb shift," which is due to virtual excitations to higher atomic levels.

In the atomic master equation, Failed to parse (MathML with SVG or PNG fallback (recommended for modern browsers and accessibility tools): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle \Gamma} is the spontaneous emission rate, given by Fermi's golden rule

Failed to parse (MathML with SVG or PNG fallback (recommended for modern browsers and accessibility tools): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle \Gamma = \frac{2\pi}{\hbar} \sum_{\kappa \epsilon} | \langle g;\kappa\epsilon| V | e; 0{\rangle}|^2 \delta(\hbar\omega - \hbar\omega_{ge}) \,, }

as derived elsewhere.

Full derivation -- walk-through

We now turn to a full derivation of the general master equation.

Master equation in integral form for arbitrary interaction

Following the notation used in API, Chapter 4, let Failed to parse (MathML with SVG or PNG fallback (recommended for modern browsers and accessibility tools): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle A} denote the system, and $R$ the environment (known as the reservoir in API). The full Hamiltonian is

Failed to parse (MathML with SVG or PNG fallback (recommended for modern browsers and accessibility tools): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle H = H_A + H_R + V \,, }

where Failed to parse (MathML with SVG or PNG fallback (recommended for modern browsers and accessibility tools): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle V} is the system-reservoir interaction potential. In the interaction picture defined by $H_{A}$ and Failed to parse (MathML with SVG or PNG fallback (recommended for modern browsers and accessibility tools): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle H_R} , the equation of motion for the full system + reservoir density matrix is

Failed to parse (MathML with SVG or PNG fallback (recommended for modern browsers and accessibility tools): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle i\hbar \dot{\rho} = [V,\rho] \,. }

(3.5.1)

Integrating this once gives

Failed to parse (MathML with SVG or PNG fallback (recommended for modern browsers and accessibility tools): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle i\hbar \rho(t) = \int_0^t [ V(t'),\rho(t') ]\, dt' \,. }

Substituting this back into Eq.(3.5.1) gives

${\dot {\rho }}=-{\frac {1}{\hbar ^{2}}}\int _{0}^{t}[V(t),[V(t'),\rho (t')]]\,dt'\,.$

If we assume that the system and reservoir are initially uncorrelated, and make the approximation that the reservoir stays unchanged (the Born approximation), then

Failed to parse (MathML with SVG or PNG fallback (recommended for modern browsers and accessibility tools): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle \rho(t) \approx \rho_A(t) \otimes \rho_R(0) = \rho_A(t) \otimes \rho_R }

This gives us our starting point for a general master equation:

Failed to parse (MathML with SVG or PNG fallback (recommended for modern browsers and accessibility tools): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle \dot{\rho}_{A} = -\frac{1}{\hbar^2} {\rm Tr}_R \int_0^t [V(t),[V(t'),\rho_A(t') \otimes \rho_R ]]\,dt' \,. }

(3.5.2)

Master equation for a two-level atom interacting with the vacuum

An example is helpful in seeing how this equation works. Generally, we will take system + reservoir interactions of the form Failed to parse (MathML with SVG or PNG fallback (recommended for modern browsers and accessibility tools): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle V = -A R} , where $A$ acts only on the system, and Failed to parse (MathML with SVG or PNG fallback (recommended for modern browsers and accessibility tools): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle R} acts only on the reservoir. Specifically, let the system be a two-level atom, and the environment be a single electromagnetic mode initially in the vacuum state Failed to parse (MathML with SVG or PNG fallback (recommended for modern browsers and accessibility tools): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle |0{\rangle}} . The atom interacts with the usual dipole interaction,

Failed to parse (MathML with SVG or PNG fallback (recommended for modern browsers and accessibility tools): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle V = -\frac{\Gamma}{2} ( b^\dagger |g \rangle \langle e| + b |e \rangle \langle g| ) \,, }

which is conveniently written using Pauli raising and lowering operators

$\sigma _{\pm }={\frac {\sigma _{x}\pm i\sigma _{y}}{2}}$

where

Failed to parse (MathML with SVG or PNG fallback (recommended for modern browsers and accessibility tools): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle \sigma_- = |g \rangle \langle e| ~~~~~~{\rm and}~~~~~~ \sigma_+ = |e \rangle \langle g| \,. }

Insert this now into Eq.(3.5.2), but disregard the integral over time (this lets us see what the essential dynamics are, at the expense of not obtaining the correct specific rates). The relevant commutators are

Failed to parse (MathML with SVG or PNG fallback (recommended for modern browsers and accessibility tools): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle \left[ {V, \rho_A\otimes |0 \rangle \langle 0| } \right] = ( \sigma_- \rho_A) \otimes |1 \rangle \langle 0| - (\rho_A \sigma_+ ) \otimes |0 \rangle \langle 1| }

and

Failed to parse (MathML with SVG or PNG fallback (recommended for modern browsers and accessibility tools): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle \left[ { V, \left[ { V,\rho_A\otimes |0 \rangle \langle 0| } \right] } \right] = ( \sigma_+ \sigma_- \rho_A)\otimes |0 \rangle \langle 0| - 2 ( \sigma_- \rho_A \sigma_+ )\otimes |1 \rangle \langle 1| + (\rho_A \sigma_+ \sigma_- ) |0 \rangle \langle 0| + {\rm other} \,, }

where the "other" terms are not diagonal in the electromagnetic mode states, and thus disappear in the partial trace over the reservoir. We find, finally:

${\dot {\rho }}_{A}=-{\frac {\Gamma }{2}}\left[{\sigma _{+}\sigma _{-}\rho _{A}-2\sigma _{-}\rho _{A}\sigma _{+}+\rho _{A}\sigma _{+}\sigma _{-}}\right]\,.$

This is the master equation for a two-level atom dipole coupled to a single electromagnetic mode initially in the vacuum state. It is written in a form known as the "Lindblad" form, which is very common. In atomic physics, you will often see master equations like this.

The Lindblad form has the special property that it ensures Failed to parse (MathML with SVG or PNG fallback (recommended for modern browsers and accessibility tools): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle \rho} is a legitimate density matrix at all times; not only does Failed to parse (MathML with SVG or PNG fallback (recommended for modern browsers and accessibility tools): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle {\rm Tr}(\rho)=1} always, but also, its eigenvalues remain non-negative. And more importantly, the map from Failed to parse (MathML with SVG or PNG fallback (recommended for modern browsers and accessibility tools): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle \rho(t)} to $\rho (t')$ is completely positive, meaning that if the map operates on just part of a larger system, the state of the larger system remains described by a valid, positive density matrix. Using the definitions for Failed to parse (MathML with SVG or PNG fallback (recommended for modern browsers and accessibility tools): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle \sigma_{\pm}} , if we express Failed to parse (MathML with SVG or PNG fallback (recommended for modern browsers and accessibility tools): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle \rho_A} as

Failed to parse (MathML with SVG or PNG fallback (recommended for modern browsers and accessibility tools): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle \rho_A = \left[ \begin{array}{cc}{a}&{b}\\{c}&{d}\end{array}\right] \,, }

then we find

${\dot {\rho }}_{A}=-{\frac {\Gamma }{2}}\left[{\begin{array}{cc}{2a}&{b}\\{c}&{-2a}\end{array}}\right]\,,$

which is identical to the master equation we constructed for the beamsplitter example, Eq.(3.3.2), up to a relabeling of Failed to parse (MathML with SVG or PNG fallback (recommended for modern browsers and accessibility tools): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle |g{\rangle}} and Failed to parse (MathML with SVG or PNG fallback (recommended for modern browsers and accessibility tools): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle |e{\rangle}} .

As shown by this example, the physical picture behind the master equation is not so complicated, even though the mathematics (used in all its glory) can be overwhelming.

Putting it together: the optical Bloch equations

The equation of motion for ${\dot {\rho }}$ we have obtained is very close to the classical Einstein equations we began with, as we can see by writing out equations of motion for the individual components of Failed to parse (MathML with SVG or PNG fallback (recommended for modern browsers and accessibility tools): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle \rho} . Explicitly, and including Hamiltonian evolution under the classical field Jaynes-Cummings interaction, we find

Failed to parse (MathML with SVG or PNG fallback (recommended for modern browsers and accessibility tools): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle \begin{array}{rcl} \dot{\rho_{ee}} &=& i\Omega(\rho_{eg}-\rho_{ge}) - \Gamma \rho_{ee} \\ \dot{\rho_{ge}} &=& i\omega_0\rho_{ge} - i\Omega(\rho_{ee}-\rho_{gg}) - \frac{\Gamma}{2} \rho_{ge} \,, \end{array}}

where the other two components are given by Failed to parse (MathML with SVG or PNG fallback (recommended for modern browsers and accessibility tools): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle \rho_{gg}=1-\rho_{ee}} and $\rho _{eg}=\rho _{ge}^{*}$. These differential equations are known as the Optical Bloch Equations, and we will base a great deal of our study of atoms and light forces on this quantum description of open system dynamics of a spontaneously emitting atom driven by a classical field.