HOME

PDF (letter size)

PDF (legal size)

my Quantum Mechanics cheat sheet

Nasser M. Abbasi

June 28, 2025   Compiled on June 28, 2025 at 9:33am

Table 1: QM cheat sheet
Position Operator \(X\) Momentum operator \(P\) Hamiltonian operator \(H\)
Eigenvalue eigenvector relation
\(X \ket {x} = x \ket {x}\) where \(x\) is the eigenvalue (size) of the \(\ket {x}\) which is the position vector associated with \(x\) measured. \(P \ket {\phi _p} = p \ket {\phi _p}\) where \(p\) is the momentum of the particle. \(H \ket {\Psi _{E_i}} = E_i \ket {\Psi _{E_i}}\) where \(E_i\) is the energy level of the particle.
Normalization relation \(\int _{-\infty }^{\infty } \ket {x} \bra {x} \,dx = 1\) \(\int _{-\infty }^{\infty } \ket {\phi _p} \bra {\phi _p} \,dp = 1\) \(\int _{-\infty }^{\infty } \ket {\Psi _{E_i}} \bra {\Psi _{E_i}} \,dE = 1\)
orthogonality \(\braket {x|x'}=\delta {(x-x')}\) \(\braket {\phi _p|\phi _{p'}}=\delta {(p-p')}\) \(\braket {\Psi _{E_i}|\Psi _{E_j}}=\delta {(E_i-E_j)}\)
Matrix element of operator
\(\Braket {x|X|x'}= x' \delta {(x-x')}\). Operator \(X\) is diagonal matrix. \(\braket {x|P|x'}=-i \hbar \delta {(x-x')} \frac {d}{d x'}\) where momentum operator \(P\) is expressed in position operator \(\ket {x}\) basis. Note that operator \(P\) is not a diagonal matrix. \(\Braket {x|H|x'}= ?\)
Function form of the state function \(\ket {\Psi }\)
N/A ?
\begin{align*} P \ket {\phi _p} &= p \ket {\phi _p}\\ \int P \ket {x'} \braket {x'|\phi _p} \, dx &= p \int \ket {x'} \braket {x'|\phi _p} \, dx\\ \int \braket {x|P|x'} \braket {x'|\phi _p} \, dx &= p \int \braket {x|x'} \braket {x'|\phi _p} \, dx\\ \int -i \hbar \delta {(x-x')} \frac {d}{d x'} \phi _p(x') \, dx &= p \int \delta {(x-x')} \phi _p(x') \, dx\\ &= \frac {2}{L} \frac {L}{2}\\ &=1 \end{align*}
\begin{align*} \braket {\Psi |\Psi } &= \int _{-\infty }^{\infty } \braket {\Psi |x} \braket {x|\Psi } \,dx\\ &= \int _{-\infty }^{\infty } \braket {x|\Psi } \braket {x|\Psi } \,dx\\ &= \int _{-\infty }^{\infty } \Psi ^*(x) \Psi (x) \,dx\\ &= \int _{0}^{L} \left ( \sqrt {\frac {2}{L}} \sin {\frac {n \pi x}{L}} \right )^2 \,dx\\ &= \frac {2}{L} \frac {L}{2}\\ &=1 \end{align*}
Vector form to function form
\(\Braket {x|\Psi }= \Psi (x)\) \(\Braket {x|\phi _p}= \phi _p(x)\) \(\Braket {x|\Psi _{E}}=\Psi _{E}(x)\)
Expansion of state vector \(\ket {\Psi }\)
\(\ket {\Psi } = \int _{-\infty }^{\infty } \ket {x} \braket {x|\Psi } \,dx\) \(\ket {\Psi } = \int _{-\infty }^{\infty } \ket {\phi _p} \braket {\phi _p|\Psi } \,dp\) \(\ket {\Psi } = \int _{-\infty }^{\infty } \ket {E_i} \braket {E_i|\Psi } \,di\)
State function \(\ket {\Psi }\) For infinite potential deep well of width \(x<0<L\)
todo todo todo
Probability of measurement
1.
Probability of measuring \(x\) given system is in state \(\ket {\Psi }\) is \(|\braket {\Psi |\Psi }|^2\). For infinite potential deep well of width \(x<0<L\) this becomes

\begin{align*} \braket {\Psi |\Psi } &= \int _{-\infty }^{\infty } \braket {\Psi |x} \braket {x|\Psi } \,dx\\ &= \int _{-\infty }^{\infty } \braket {x|\Psi } \braket {x|\Psi } \,dx\\ &= \int _{-\infty }^{\infty } \Psi ^*(x) \Psi (x) \,dx\\ &= \int _{0}^{L} \left ( \sqrt {\frac {2}{L}} \sin {\frac {n \pi x}{L}} \right )^2 \,dx\\ &= \frac {2}{L} \frac {L}{2}\\ &=1 \end{align*}
2.
Probability of measuring \(x\) given system is in state \(\phi _p\)

\(\ket {\Psi } = \int _{-\infty }^{\infty } \ket {\phi _p} \braket {\phi _p|\Psi } \,dp\) \(\ket {\Psi } = \int _{-\infty }^{\infty } \ket {E_i} \braket {E_i|\Psi } \,di\)