The implementation of the Pauli matrices and their algebra were reviewed during 2018, including the algebraic manipulation of nested commutators, resulting in faster computations using simpler and more flexible input. As it frequently happens, improvements of this type suddenly transform research problems presented in the literature as untractable in practice, into tractable.

As an illustration, we tackle below the derivation of the coefficients entering the Zassenhaus formula shown in section 4 of [1] for the Pauli matrices up to order 10 (results in the literature go up to order 5). The computation presented can be reused to compute these coefficients up to any desired higher-order (hardware limitations may apply). A number of examples which exploit this formula and its dual, the Baker-Campbell-Hausdorff formula, occur in connection with the Weyl prescription for converting a classical function to a quantum operator (see sec. 5 of [1]), as well as when solving the eigenvalue problem for classes of mathematical-physics partial differential equations [2].  
To reproduce the results below - a worksheet with this contents is linked at the end - you need to have your Maple 2018.2.1 updated with the Maplesoft Physics Updates version 280 or higher.

References

Unassign  as done above in two opportunities, to avoid interference of the results just computed.

Indicate more summation indices in the sequence  in (19), as many as the maximum value of n in .

Have in mind that the growth in size and complexity is significant, with each  taking significantly more time than the computation of all the previous ones.

Re-execute the input line (23) and the loop (24).

Coherent states are among the most relevant representations for the state of a quantum system. These states, that form an overcomplete basis, minimize the quantum uncertainty between position x and momentum p (they satisfy the Heisenberg uncertainty principle with equality and their expectation values satisfy the classical equations of motion). Coherent states are widely used in quantum optics and quantum mechanics in general; they also mathematically characterize the concept of Planck cells. Part of this development is present in Maple 2018.2.1. To reproduce what you see below, however, you need a more recent version, as the one distributed within the Maplesoft Physics Updates (version 276 or higher). A worksheet with this contents is linked at the end of this post.

Definition and the basics

Set a quantum operator  and corresponding annihilation / creation operators

In what follows, on the left-hand sides the product operator used is `*`, which properly represents, but does not perform the attachment of Bras Kets and operators. On the right-hand sides the product operator is `.`, that performs the attachments. Since the introduction of Physics in the Maple system, we have that

New development during 2018: coherent states, the eigenstates of the annihilation operator , with all of their properties, are now understood as such by the system

 is an eigenket of  but not of  

The norm of these states is equal to 1

These states, however, are not orthonormal as the occupation number states  are, and since  is not Hermitian, its eigenvalues are not real but complex numbers. Instead of (1.6) , we now have

At ,

Their scalar product with the occupation number states , using the inert %Bracket on the left-hand side and the active Bracket on the other side:

The expansion of coherent states into occupation number states, first representing the product operation using `*`, then performing the attachments replacing `*` by `.`

Hide now the ket label. When in doubt, input show to see the Kets with their labels explicitly shown

Define eigenkets of the annihilation operator, with two different eigenvalues for experimentation

Because the properties of coherent states are now known to the system, the following computations proceed automatically. The left-hand sides use the `*`, while the right-hand sides use the `.`

Properties of Coherent states

The basic ideas and design implemented

Tensor product notation and the hideketlabel option

Entangled States and the Bell basis

Entangled States, Operators and Projectors

The basic ideas and design implemented

Suppose  and  are quantum operators and  are, respectively, their eigenkets. The following works since the introduction of the Physics package in Maple

In previous Maple releases, all quantum operators are supposed to act on the same Hillbert space. New: suppose that  and  act on different, disjointed, Hilbert spaces.

1) To represent that situation, a new keyword in Setup , , is introduced. With it you can indicate the quantum operators that act on a Hilbert space, say as in  with the meaning that the operator  acts on one Hilbert space while  acts on another one.

The Hilbert space thus has no particular name (as in 1, 2, 3 ...) and is instead identified by the operators that act on it. There can be one or more, and operators acting on one space can act on other spaces too. The disjointedspaces keyword is a synonym for hilbertspaces and hereafter all Hilbert spaces are assumed to be disjointed.

NOTE: noncommutative quantum operators acting on disjointed spaces commute between themselves, so after setting - for instance - , automatically,  become quantum operators satisfying (see comment (ii) on page 156 of ref.[1])

2) Product of Kets and Bras that belong to different Hilbert spaces, are understood as tensor products satisfying (see footnote on page 154 of ref. [1]):

while

3) All the operators of one Hilbert space act transparently over operators, Bras and Kets of other Hilbert spaces. For example

4) Every other quantum operator, set as such using Setup , and not indicated as acting on any particular Hilbert space, is assumed to act on all spaces.

5) Notation:

Tensor product notation and the hideketlabel option

According to the design section, set now two disjointed Hilbert spaces with operators  acting on one of them and  on the other one (you can think of  )

Consider a tensor product of Kets, each of which belongs to one of these different spaces, note the new notation using

You see that in the product of Bras, and also in the product of Kets, A comes first, then B.

Remark: some textbooks prefer a diadic style for sorting the operands in products of Bras and Kets that belong to different spaces, for example,  instead of the projector sorting style of  (2.3). Both reorderings of Kets and Bras are mathematically equal.

The display for (2.3) is now

Important: this new option only hides the label while displaying the Bra or Ket. The label, however, is still there, both in the input and in the output. One can "see" what is behind this new display using show, that works the same way as it does in the context of   CompactDisplay . The actual contents being displayed in (2.5) is thus (2.3)

Operators of each of these spaces act on their eigenkets as usual. Here we distribute over both sides of an equation, using `*` on the left-hand side, to see the product uncomputed, and `.` on the right-hand side to see it computed:

Release the product

The same operation but now using the dot product `.` operator. Start by delaying the operation

Recalling that this product is mathematically the same as (2.11), and that

by releasing the delayed product (2.12) we have

Reset hideketlabel

Entangled States and the Bell basis

With the introduction of disjointed Hilbert spaces in Maple it is possible to represent entangled quantum states in a simple way, basically as done with paper and pencil.

Recalling the Hilbert spaces set at this point are,

where  acts on the tensor product of the spaces where  and  act. A state of  can then always be written as

where  is a matrix of complex coefficients. Bra  states of  are formed as usual taking the Dagger

Example: the Bell basis for a system of two qubits

Consider a 2-dimensional space of states acted upon by the operator , and let  act upon another, disjointed, Hilbert space that is a replica of the Hilbert space on which  acts. Set the dimensions of ,  and  respectively equal to 2, 2 and 2x2 (see Setup)

The system C with the two subsystems A and B represents the a two qubits system. The standard basis for C can be constructed in a natural way from the basis of  Kets of A and B, , by taking their tensor products:

Set a more mathematical display for the imaginary unit

The four entangled Bell states also form a basis of C and are given by

There is no standard notation for denoting a Bell state (the linar combinations of the right-hand sides above). The convention used here relates to the definition of the Bell states related to the Pauli matrices shown below. Regardless fo the convention used, this basis is orthonormal. That can be verified by taking dot products, for example:

In steps, perform the same operation but using the star (`*`) operator, so that the contraction is represented but not performed

Evaluate now the result at `*` = `.`, that is transforming the star product into a dot product

The Bell basis and its relation with the Pauli matrices

The Bell basis can be constructed departing from  using the Pauli matrices . For that purpose, using a Vector representation for ,

Multiplying by each of the  Pauli Matrices and performing the matrix operations we have

In this result we see that  and  flip the state, transforming  into ,  also multiplies the state by the imaginary unit , while  leaves the state  unchanged.

We can rewrite all that by removeing from (3.19) the Vector representations of (3.17). For that purpose, create a list of substitution equations, replacing the Vectors by the Kets

So the action of  in  is given by

For , the same operations result in

To obtain the other three Bell states using the results (3.21) and (3.24), indicate to the system that the Pauli matrices operate in the subspace where  operates

Multiplying  given in (3.7) by each of the three  we get the other three Bell states

Substitute in this result the first equations of (3.21) and (3.24)

This is  defined in (3.8)

Multiplying now by  and substituting  using the  equations of (3.21) and (3.24) we get

The above is  defined in (3.9)

Finally, multiplying  by

Substituting

We get

which is

Entangled States, Operators and Projectors

Consider a fourth operator, , that is Hermitian and acts on the same space of , and then it has the same dimension,

To operate in a practical way with these operators, Bras and Kets, however, bracket rules reflecting their relationship are necessary. From the definition of  as acting on the tensor product of  spaces where  and  act (see (3.2)) and taking into account the dimensions specified for ,  and  we have

The bracket rules for ,  and  are the first two of these; Set these rules, so that the system can take them into account

If we now recompute (4.5), the left-hand side is also computed

Suppose now that you want to compute with the Hermitian operator , that operates on the same space as , both using C using the operators  and , as in

where  =  when  is a product (not entagled) state.

For  it suffices to set a bracket rule

After that, the system operates taking the rule into account

Regarding , since  belongs to the tensor product of spaces A and B, it can be an entangled operator, one that you cannot represent just as a product of one operator acting on A times another one acting on B. A computational representation for the operator  (that is not just itself or as abstract) is not possible in the general case. For that you can use a different feature: define the action of the operator  on Kets of  and .

Basically, we want:

A program sketch for that would be:

if H is applied to a Ket of A or B then

    if H itself is indexed then
        return H accumulating its indices, followed by the index of the Ket
    else

        return H indexed by the index of the Ket;
otherwise
    return the dot product operation uncomputed, unevaluated

In Maple language, that program-sketch becomes

Let's see it in action. Start erasing the Physics performance remember tables, that remember results like  computed before the definition of

Recalling that  is Hermitian,

Note that the definition of  as a procedure does not interfer with the setting of an bracket rule for it with , that is still working

but the definition takes precedence, so if in it you indicate what to do with a  Ket, that will be taken into account before the bracket rule. Finally, In the typical case, the first four results, (4.11), (4.12), (4.13) and (4.14) are operators while (4.15) is a scalar; you can always represent the scalar aspect by substituing the noncommutative operator  by a related scalar, say H.