I would like to write the annihilation and creation operators for the harmonic oscillator, and see how they act on basis states of the form $\lvert n\rangle$.

What's the best approach to implement this in Mathematica?

In particular, this would require to implement the commutation relations $[a,a^\dagger] = 1$, and to see how $|n\rangle$ changes when we apply the creation and destruction operators $a^\dagger$ and $a$.

  • 2
    $\begingroup$ I'm voting to close this question as off-topic because there is no well-posed question in this post; it is simply a rant about missing features that the OP would like to have. $\endgroup$ – m_goldberg Mar 16 '17 at 15:23
  • 8
    $\begingroup$ Leave it open. I might have something that can be useful for this. I'll try to dig it out tonight. $\endgroup$ – rcollyer Mar 16 '17 at 17:52
  • 1
    $\begingroup$ for how to implement the commutation rules on bosonic operators see here $\endgroup$ – glS Mar 16 '17 at 18:01
  • 2
    $\begingroup$ @rco here, have another reopen vote $\endgroup$ – LLlAMnYP Mar 17 '17 at 6:04
  • 3
    $\begingroup$ @JasonB. you're right, if it is a finite basis of dimension, k, it is simple: a[k_] := DiagonalMatrix[ConstantArray[1, k-1], k, -1] and $a^\dagger$ is then automatically defined. $\endgroup$ – rcollyer Mar 17 '17 at 12:15

Here is a simple implementation:

Protect[qCO, qDO];

qOperatorQ[expr_] := MatchQ[expr, qCO | qDO | Ket[n_Integer]];

(* take scalars out *)
CenterDot[left___, Times[scalar_?NumericQ, op_?qOperatorQ], right___] := Times[
    CenterDot[left, op, right]

(* Implement commutation relations *)
CenterDot[left___, qDO, qCO, right___] := Plus[
    CenterDot[left, qCO, qDO, right],
    CenterDot[left, right]

(* Allow to use powers of operators *)
CenterDot[left___, Power[op : (qCO | qDO), n_Integer], right___] := CenterDot[
    Sequence @@ ConstantArray[op, n],

(* effective OneIdentity attribute *)
CenterDot[op_?qOperatorQ] := op;

(* implement action on Fock states *)
CenterDot[left___, qDO, Ket[0]] := 0;
CenterDot[left___, qCO, Ket[n_Integer]] := Times[
    Sqrt[n + 1],
    CenterDot[left, Ket[n + 1]]
CenterDot[left___, qDO, Ket[n_Integer]] := Times[
    CenterDot[left, Ket[n - 1]]

I used CenterDot to denote the operator product (note that you can write a CenterDot product with the shortcut [esc].[esc]), qCO to denote the creation operator $a^\dagger$, qDO for the destruction operator $a$, and the builting symbol Ket[n] to denote the Fock basis state $\lvert n \rangle$ (which can be inserted with [esc]ket[esc]).

You can of course change this notation as you like. Here is a couple of examples of how to use the above (I here keep the full and ugly description of the operators to more clearly show what symbols are being used. Note that you can insert the SuperDagger with a[ctrl+^][esc]dg[esc]):

a = qDO;
SuperDagger[a] = qCO;




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