QuantumAlgebra.jl - quantum operator algebra in Julia

This package does quantum operator algebra (i.e., algebra with non-commuting operators) in Julia, supporting bosonic, fermionic, and two-level system operators, with arbitrary names and indices, as well as sums over any of the indices. It defines an opinionated canonical form (normal ordering plus some additional rules) to automatically simplify expressions. It is recommended to use an interface that can display LaTeX formulas (e.g., Jupyter notebooks) for convenient output formatting.

Starting from v1.4, QuantumAlgebra also interoperates with computer algebra systems (CAS) such as Symbolics.jl or SymPy.jl / SymPyPythonCall.jl, as the "scalar" prefactors of each quantum term can be arbitrary expressions provided by these systems. While such expressions do not support symbolic indices in the same way as QuantumAlgebra, they provide much more flexibility in terms of the mathematical operations and powerful manipulation functions possible on the parameters.

Example jupyter notebooks are available in the examples folder and can be viewed online with nbviewer and tried out interactively with Binder.

Release notes / changelog

Please see the release notes for a summary of changes in each version.

Overview

The basic functions to create QuantumAlgebra expressions (which are of type QuExpr) are

a(inds...) and a'(inds...) for a and a^†, the annihilation and creation operators for a bosonic mode.
f(inds...) and f'(inds...) for f and f^†, the annihilation and creation operators for a fermionic mode.
σx(inds...), σy(inds...), σz(inds...) for the Pauli matrices σ^x,y,z for a two-level system (TLS).
σp(inds...), σm(inds...) for excitation and deexcitation operators σ^± for a two-level system (TLS).
Indices: All of these functions take an arbitrary number of indices as arguments, which can be either integers (1,2,...) or symbolic, where symbolic indices must be a single unicode character, with possibly an integer subindex:
```
julia> using QuantumAlgebra

julia> a()
a()

julia> a'(:i)
a†(i)

julia> f'(1,2,:i_9)
f†(12i₉)

julia> σx(:i_1, 1, :j, :k_2, :μ_2, :◔_1, :😄_121)
σˣ(i₁1jk₂μ₂◔₁😄₁₂₁)
```
You can define your own bosonic/fermionic/two-level system operators with a set of macros:
- @boson_ops name defines new function $name() (and deprecated $(name)dag()) for bosonic species name.
- @fermion_ops name defines new function $name() (and deprecated $(name)dag()) for fermionic species name.
- @tlsxyz_ops name defines new functions $(name)x(), $(name)y() and $(name)z() for the Pauli matrices for two-level system species name.
- @tlspm_ops name defines new functions $(name)p() and $(name)m() for the two-level system excitation and deexcitation operators for species name.
Note that for @boson_ops and @fermion_ops, the deprecated $(name)dag() functions are defined for backward compatibility. These will be removed in a future version, as $(name)'() is now the preferred syntax for creating an adjoint.
```
julia> @boson_ops b
(b (QuExpr constructor), b† (QuExpr constructor))

julia> b'(:k)*b(:i)
b†(k) b(i)
```
Operators with different names are assumed to belong to different "species" and always commute. For fermions, this is not always desired, since you might want to use different named operators to refer to different kinds of states for the same species (e.g., localized and itinerant electrons). This can be achieved with the macro @anticommuting_fermion_group, which creates several fermionic operators that mutually anticommute:
```
julia> @anticommuting_fermion_group c d

julia> normal_form(c()*d() + d()*c())
0
```
param(name::Symbol,state='n',inds...) to create a named parameter. state must be one of 'r', 'n', or 'c' for purely real, non-conjugated complex, and conjugated complex parameters. More conveniently, parameters can be entered with string macros Pr"name_inds..." and Pc"name_inds..." for real and complex parameters:
```
julia> Pr"g_i,j_2,k"
g(ij₂k)

julia> Pr"g_i,j_2,k" == param(:g,'r',:i,:j_2,:k)
true

julia> Pc"α_3" == param(:α,3)
true
```
Arithmetic operations (*, +, -, ^, adjoint=') are supported (exponents must be nonnegative integers), with any Number types integrating automatically. Division by numbers is also supported.
```
julia> 5*a'(:k)*f(3)*σx(3)
5 a†(k) f(3) σˣ(3)

julia> (5//3+4im) * a'(:k)*f(3)*σx(3) + 9.4
9.4 + (5//3+4i) a†(k) f(3) σˣ(3)

julia> (a(:i)*f(:k))'
f†(k) a†(i)
```
If you explicitly need a bare number as a QuantumAlgebra expression, you can use, e.g. QuExpr(1) (which is equal to one(QuExpr)). However, most functions that take a QuExpr will also accept a bare number.
∑(ind,A::QuExpr) to represent an analytic sum over index ind. Since summed indices have no semantic meaning, the index within the expression gets replaced by a special numbered sum index #ᵢ, with i=1,2,....
```
julia> ∑(:i,a(:i))
∑₁ a(#₁)
```
normal_form(A::QuExpr) converts an expression to a well-defined "canonical" order. To achieve this canonical form, relevant commutators etc are used, so an expression written as a single product can turn into a sum of expressions. The order is essentially normal ordering (creation before annihilation operators, with σˣʸᶻ in the middle), with some additional conventions to make the normal form (hopefully) unique. In some contexts (e.g., interactive work), it can be convenient to automatically transform all expressions to normal form. This can be enabled by calling QuantumAlgebra.auto_normal_form(true). To make the setting permanent, call QuantumAlgebra.auto_normal_form(true; set_preference=true) or alternatively use Preferences.jl directly, i.e., call Preferences.set_preferences!(QuantumAlgebra,"auto_normal_form"=>true/false).
```
julia> normal_form(a(:i)*a'(:j))
δ(ij)  + a†(j) a(i)
```
expval(A::QuExpr) to represent an expectation value.
```
julia> expval(a'(:j)*a(:i))
⟨a†(j) a(i)⟩
```
expval_as_corrs(A::QuExpr) to represent an expectation value through its correlators, i.e., a cumulant expansion.
```
julia> expval_as_corrs(a'(:j)*a(:i))
⟨a†(j)⟩c ⟨a(i)⟩c  + ⟨a†(j) a(i)⟩c
```

comm(A::QuExpr,B::QuExpr) to calculate the commutator [A,B] = AB - BA.

julia> comm(a(),a'())
-a†() a() + a() a†()

julia> normal_form(comm(a(),a'()))
1

Avac(A) and vacA(A) simplify operators by assuming they are applied to the vacuum from the left or right, respectively. To be precise, Avac(A) returns A' such that A|0⟩ = A'|0⟩, while vacA(A) does the same for ⟨0|A. These functions automatically apply normal_form to assure that the operators are simplified as much as possible. Note that "vacuum" for two-level systems is interpreted as the lower state, σᶻ|0⟩ = -|0⟩.
```
julia> Avac(a())
0

julia> Avac(a(:i)*a'(:j))
δ(ij)

julia> Avac(a()*a'()*a'())
2 a†()

julia> vacA(a()*a'()*a'())
0

julia> Avac(σx())
σˣ()

julia> Avac(σz())
-1
```
Both functions can also be called with an optional second argument, Avac(A,modes_in_vacuum) or vacA(A,modes_in_vacuum), which is an iterable over operators (or a single operator) that will be assumed to be in the vacuum state, while all others are not. Note that the operators in modes_in_vacuum do not distinguish by index, i.e., if the modes have indices, all modes with the same name are assumed to be in the vacuum state. To avoid confusion, the modes_in_vacuum argument thus does not accept operators with indices.
```
julia> Avac(a(),a())
0

julia> Avac(a(),f())
a()

julia> Avac(a(:i)*a'(:j),f())
δ(ij)  + a†(j) a(i)

julia> Avac(a'()*a()*f()*f'(),f())
a†() a()

julia> @boson_ops b
julia> Avac(a'()*a()*b()*b'()^2*f()*f'(),(f(),b()))
2 a†() b†() a()
```
vacExpVal(A,S=1) calculates the vacuum expectation value ⟨0|S^†AS|0⟩, i.e., the expectation value ⟨ψ|A|ψ⟩ for the state defined by |ψ⟩=S|0⟩. The result is guaranteed to not contain any operators.
```
julia> vacExpVal(a'()*a())
0

julia> vacExpVal(a'()*a(), a'()^4/sqrt(factorial(4)))
4.000000000000001

julia> vacExpVal(a'()*a(), a'()^4/sqrt(factorial(big(4))))
4

julia> vacExpVal(σx())
0
```
Like vacA and Avac, vacExpVal also takes an optional modes_in_vacuum argument, vacExpVal(A,S,modes_in_vacuum) (since all arguments are positional, S has to be given explicitly in this case even if it is just the identity operator, i.e., vacExpVal(A,1,a())):
```
julia> @boson_ops b
julia> vacExpVal(a'()*a()*b()^2*b'()^2*f()*f'(), 1, (f(),b()))
2 a†() a()
```
heisenberg_eom(A,H,Ls=()) calculates the Heisenberg equation of motion for operator A under the action of Hamiltonian H and potential Lindblad decay terms Ls, given by dA/dt = i[H,A] + ∑_i γ_i (L_i^† A L_i - 1/2 {L_i^† L_i,A}). The Lindblad decay operators are passed as a tuple (not an array) of tuples, where each inner tuple describes one decay operator. The possible forms are (L,) for decay operator L, (γ,L) for decay operator L with rate γ, and (inds,γ,L) for decay operators summed over the given indices (note that this is different from the operator itself being a sum, seen in the example below). Finally, L can (in all three cases above) be just a single operator or a tuple of two operators L=(X,Y) to represent off-diagonal Lindblad terms L_X,Y[ρ] = X ρ Y^† - 1/2 {Y^† X,ρ}.
```
julia> H = Pr"ω"*a'()a()
julia> Ls = ((Pr"γ",a()),)
julia> heisenberg_eom(a(),H,Ls)
-1//2 γ a() - 1i ω a()

julia> H = QuExpr()
julia> Ls = ((:i,a(:i)),)
julia> heisenberg_eom(a(:i),H,Ls)
-1//2 a(i)

julia> Ls = ((∑(:i,a(:i)),),)
julia> heisenberg_eom(a(:i),H,Ls)
-1//2 ∑₁ a(#₁)

julia> Ls = (((:i,:j),(a(:i),a(:j))),)
julia> heisenberg_eom(a(:i),H,Ls)
-1//2 ∑₁ a(#₁)
```
heisenberg_eom_system(H,rhsfilt,Ls=(),ops=nothing) calculates the system of equations of motion for the expectation values of operators appearing in H and Ls (same conventions as for heisenberg_eom above). Typically, these equation systems are not closed without approximations as equations for products of n operators involve products of m>n operators, so the system has to be truncated. This is achieved with a filter function that removes higher-order terms or rewrites them (approximately) in terms of lower-order expressions. The function rhsfilt is applied to the right-hand side of the equations to filter them as desired. If rhsfilt(A::QuExpr)::QuExpr is a function, it will be applied to the calculated right-hand side of the equations. QuantumAlgebra comes with two predefined constructors for filter functions, droplen(maxorder::Int), which leads to all terms of order higher than maxorder being neglected, and dropcorr(maxorder::Int), where all terms of order higher than maxorder are rewritten in terms of lower-order expressions up to order maxorder and higher-order correlators, with those correlations being neglected (i.e., dropcorr(1) will replace ⟨a^† a⟩ = ⟨a^† a⟩_c + ⟨a^†⟩ ⟨a⟩ ≈ ⟨a^†⟩ ⟨a⟩). If rhsfilt is a number, it will be interpreted as droplen(rhsfilt). Finally, the ops argument can be used to specify the operators that should be used to "seed" the system of equations, otherwise all operators appearing in H are used.
```
julia> H = Pr"ω"*a'()*a() + Pr"χ"*a'()*(a'()+a())*a();

julia> Ls = ((Pr"γ",a()),);

julia> heisenberg_eom_system(H,2,Ls,a())
dₜ⟨a()⟩ = -1//2 γ ⟨a()⟩  - 1i ω ⟨a()⟩  - 2i χ ⟨a†() a()⟩  - 1i χ ⟨a()²⟩ 
dₜ⟨a†() a()⟩ = -γ ⟨a†() a()⟩ 
dₜ⟨a()²⟩ = -2i χ ⟨a()⟩  - γ ⟨a()²⟩  - 2i ω ⟨a()²⟩  
```
The heisenberg_eom_system function can also be passed either ExpVal or Corr as a first argument, which will give the equations of motion of the expectation values (the default) or correlators (corresponding to a cumulant expansion) of the operators.
```
julia> H = Pr"ω"*a'()*a() + Pr"χ"*a'()*(a'()+a())*a();

julia> Ls = ((Pr"γ",a()),);

julia> heisenberg_eom_system(Corr,H,1,Ls,a())
dₜ⟨a()⟩c = -1//2 γ ⟨a()⟩c  - 1i ω ⟨a()⟩c  - 2i χ ⟨a†()⟩c ⟨a()⟩c  - 1i χ ⟨a()⟩c² 
```
julia_expression(A) to obtain a julia expression that can be used to automatically build codes implementing equations derived with QuantumAlgebra. Every expectation value or correlator is treated as a separate array. Daggers are represented as ᴴ, which are valid identifiers that can appear in the array names. Note that expectation values and correlators are not distinguished, so it is best to have all expressions use the same kind.
```
julia> julia_expression(expval_as_corrs(a'(:j)*a(:i)))
:(aᴴ[j] * a[i] + aᴴa[j, i])
```
Also note that expressions are always treated as arrays, even if they have no indices (which gives zero-dimensional arrays). If you are working with scalar quantities exclusively, it might be useful to clean up the resulting expression (e.g., use MacroTools to remove the []).
```
julia> julia_expression(expval(a'()*a()*σx()))
:(aᴴaσˣ[])
```
By default, two-level system operators are represented by the Pauli matrices σˣʸᶻ, and calling σp() and σm() will give results expressed through them:
```
julia> σp()
1//2 σˣ() + 1//2i σʸ()

julia> σm()
1//2 σˣ() - 1//2i σʸ()
```
This can be changed by calling QuantumAlgebra.use_σpm(true; set_preference=true/false) (where the value of set_preference determines whether this is stored permanently using Preferences.jl). In this mode, σ⁺ and σ⁻ are the "fundamental" operators, and all expressions are written in terms of them. Note that mixing conventions within the same expression is not supported, so it is suggested to set this flag once at the beginning of any calculation.
```
julia> QuantumAlgebra.use_σpm(true)

julia> σp()
σ⁺()

julia> σx()
σ⁺() + σ⁻()

julia> σz()
-1 + 2 σ⁺() σ⁻()
```

Preferences

Several preferences changing the behavior of QuantumAlgebra can be set permanently (this uses Preferences.jl):

"define_default_ops": if this is set to false (default is true), the "default" operators a, adag, f, fdag, σx, σy, σz, σp, σm are not defined upon import. Note that changing this value requires restarting the Julia session to take effect. The setting can be changed with QuantumAlgebra.set_define_default_ops(true/false) (which will inform you whether a restart is required) or with Preferences.set_preferences!(QuantumAlgebra,"define_default_ops"=>true/false).
"auto_normal_form": Choose whether all expressions are automatically converted to normal form upon creation. The default is false. It can be changed for a single session with QuantumAlgebra.auto_normal_form(true/false), and can be made permanent with QuantumAlgebra.auto_normal_form(true/false; set_preference=true) or with Preferences.set_preferences!(QuantumAlgebra,"auto_normal_form"=>true/false). Note that this could previously be set by defining an environment variable "QUANTUMALGEBRA_AUTO_NORMAL_FORM", but this usage has been deprecated and will be removed in a future version.
"use_σpm": Choose whether for two-level systems, the "basic" operators are excitation/deexcitation operators σ⁺,σ⁻ or the Pauli matrices σˣ,σʸ,σᶻ. This can be changed in a single session by calling QuantumAlgebra.use_σpm(true/false), and can be made permanent with QuantumAlgebra.use_σpm(true/false; set_preference=true) or with Preferences.set_preferences!(QuantumAlgebra,"use_σpm"=>true/false).

Citing

If you use QuantumAlgebra in academic work, we would appreciate a citation. See CITATION.bib for the relevant references.