States
TODO: INTRO
Basic State
Describes the Fock state of \(n\) indistinguishable photons over \(m\) modes.
See reference perceval.utils.BasicState
for detailed information.
A Fock state, called BasicState in Perceval coding language, is represented by n_1,n_2,...,n_m>
notation where n_k
is the number of photons in mode k
.
Example code:
>>> bs = pcvl.BasicState("0,1>") # Creates a twomode Fock state with 0 photons in first mode, and 1 photon in second mode.
>>> print(bs) # Prints out the created Fock state
0,1>
>>> bs.n # Displays the number of photons of the created Fock state
1
>>> bs.m # Displays the number of modes of the created Fock state
2
>>> bs[0] # Displays the number of photons in the first mode of the created Fock state ( note that the counter of the number of modes starts at 0 and ends at m1 for an mmode Fock state)
0
>>> print(pcvl.BasicState([0,1])*pcvl.BasicState([2,3])) # Tensors the 0,1> and 2,3> Fock states, and prints out the result (the Fock state 0,1,2,3>)
0,1,2,3>
Annotated Basic State
AnnotatedBasicState
extends BasicState
and describes state of \(n\) annotated photons over \(m\) modes.
See reference perceval.utils.AnnotatedBasicState
for detailed information.
Annotation
Annotation
distinguishes individual photons and is represented generically as a map of key \(\rightarrow\) values  where key are
userdefined labels, and values can be string or complex numbers.
Special predefined annotations are:
P for polarization used in circuit with polarization operators  see Polarization
t used in time circuit with an integer value is defining the period from where the photon is generated (default
0
meaning that the photon is coming from current period).
Photons with annotations are represented individually using python dictionary notation:
{key_1:value_1,...,key_k:value_k}
key_i
will be referred to as an annotation key, and represents for example a degree of freedom labelled i ( time, polarization,…) that a photon can have,
value_i
is the value on this degree of freedom.
Note that a photon can have a set of annotation keys, representing different degrees of freedom, each with its own value.
Note
Two photons are indistinguishable if they share the same values on all their common annotation keys, or if they have no common annotation keys. For instance, for the following three photons,
\(p_1=\)
{a1:1}
\(p_2=\)
{a2:1}
\(p_3=\)
{a1:2,a2:2}
\(p_1\) and \(p_3\) are distinguishable because their a1 annotation keys have different values (1 for p_1 as opposed to 2 for p_3). \(p_2\) and \(p_3\) are also distinguishable because the values of their annotation key a2 do not agree. However, \(p_1\) and \(p_2\) are indistinguishable, because they share no common annotation keys.
Use of Annotation in AnnotatedBasicState
A AnnotatedBasicState
notation extends the BasicState
notation as following:
AP_(1:1)...AP_(1:n_1),...,AP_(m:1)...AP_(m:n_m)>
where AP_(k:i)
is the representation of the i
th photon in mode k
, n_i
is the number of photons in mode i
.
To simplify the notation, for each mode, annotated photons with same annotations can be grouped and represented prefixed by
their count: e.g. 2{...}
. Absence of photons is represented by 0
, and non annotated photons are just represented by
their count as for BasicState
.
For instance the following are representing different BasicStates with 2 photons having polarization annotations (here
limited to H
/V
:
{P:H},{P:V}>
corresponding to an annotated1,1>
BasicState
.2{P:H},0>
corresponding to an annotated2,0>
BasicState
where the 2 photons in the first mode are annotated with the same annotation.{P:H}{P:V},0>
corresponding also to an annotated``2,0>``BasicState
where the two photons in the first mode have different annotations.1,{t:1}>
corresponding to an annotated1,1>
BasicState
, the degree of freedom being time, with the photon in mode 2 coming from previous period, and the photon in mode 1 is not annotated.
Example code:
>>> print(pcvl.AnnotatedBasicState("0,1>"))
0,1>
>>> a_bs = pcvl.AnnotatedBasicState("{P:H}{P:V},0>") # Creates an annotated state 2,0> , with two photons in the first mode, one having a horizontal polarization, and the other a vertical polarization.
>>> print(a_bs)
{P:H}{P:V},0>
>>> a_bs[0] # prints the photons in the first mode
({"P":"H"},{"P":"V"})
>>> print(a_bs.clear()) #prints the nonannotated Basic state corresponding to a_bs
2,0>
State Vector
StateVector
extends AnnotatedBasicState
to represents state superpositions.
See reference perceval.utils.StateVector
for detailed information.
StateVector
instances are constructed through addition and linear combination operations.
>>> st1 = pcvl.StateVector("1,0>") # write basic states or annotated basic states with the 'StateVector' command in order to enable creating a superposition using the '+' command
>>> st2 = pcvl.StateVector("0,1>")
>>> st3= st1 + st2
>> print(len(st3))
2
>>> print(st3)
1/sqrt(2)*1,0>+1/sqrt(2)*0,1>
>>> st3[0] # outputs the first state in the superposition state st3
1,0>
>>> st3[1] # outputs the second state in the superposition st3
0,1>
>>> st4 = alpha*st1 + beta*st2
Warning
StateVector
will normalize themselves so it will add normalization terms to any combination.
State Vector Distribution
SVDistribution
is a recipe for constructing a mixed state using BasicState and/or
StateVector
commands (which themselves produce pure states).
For example, The following SVDistribution









results in the mixed state 1/20,1><0,1+1/4(1/sqrt(2)*1,0>+1/sqrt(2)*0,1>)(1/sqrt(2)*<1,0+1/sqrt(2)*<0,1)+1/41,0><1,0
TimeSVDistribution
TimedSVDistribution
is representing a time sequence distribution of StateVector
.