Cross Section in the Circuits

#1

Hello,
For this circuit:
modernchip

I first write this code (Thanks to @Tom_Bromley :slight_smile: )

prog = sf.Program(modes)
with prog.context as q:
Ket(initial_state) | q # Initial state preparation

BSgate() | (q[0], q[1])
BSgate() | (q[2], q[3])

#BSgate() | (q[1], q[2])

Rgate((np.pi)/(8)) | q[0]
Rgate((np.pi)/(8)) | q[2]

BSgate() | (q[0], q[1])
BSgate() | (q[2], q[3])

Rgate(0) | q[0]
Rgate((np.pi)/2) | q[2]

BSgate() | (q[0], q[1])
BSgate() | (q[2], q[3])
#MeasureFock() | q[0]
#MeasureFock() | q[2]
MeasureFock() | q

results = eng.run(prog)

However I am quite confused with cross sections like the following figure:
sktr

And do you think that I should follow which mode goes to where
For instance in my circuit
0 mode goes to 2nd mode
1st mode goes to 0 mode
2nd mode goes to 3rd mode and finally
3rd mode goes to 1st mode
So should I change the indices after first beam splitter like that:
prog = sf.Program(modes)
with prog.context as q:
Ket(initial_state) | q # Initial state preparation

    BSgate() | (q[0], q[1])  #it is ok
    BSgate() | (q[2], q[3])  #it is ok
    
    #BSgate() | (q[1], q[2])
    
    Rgate((np.pi)/(8)) | q[1]   # here they are going different places
    Rgate((np.pi)/(8)) | q[0]   # same
    
    BSgate() | (q[1], q[3])      
    BSgate() | (q[2], q[0])
    
    Rgate(0) | q[3]
    Rgate((np.pi)/2) | q[2]
    
    BSgate() | (q[3], q[1]) 
    BSgate() | (q[2], q[0])
    #MeasureFock() | q[0]
    #MeasureFock() | q[2]
    MeasureFock() | q

results = eng.run(prog)
#2

Hi @nisq,

However I am quite confused with cross sections like the following figure

To me this looks like a typical beamsplitter gate. Since it doesn’t have a box in front, it would suggest that the beamsplitter is taking some default value - which I would guess to be a simple 50:50 beamsplitter. Where did you get the diagram? Perhaps you could confirm with the people who provided it to you.

And do you think that I should follow which mode goes to where

If I understand your question correctly - no, you do not need to try to permute the mode labels to follow the circuit. You should treat each horizontal line as a fixed mode label. Perhaps a photon will switch modes due to a beamsplitter, but this will just show up when comparing your input state to output state.

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#3

Hi @Tom_Bromley

To me this looks like a typical beamsplitter gate. Since it doesn’t have a box in front, it would suggest that the beamsplitter is taking some default value - which I would guess to be a simple 50:50 beamsplitter. Where did you get the diagram? Perhaps you could confirm with the people who provided it to you.

I totally agree and I even asked the people who are providing me the diagram and they just told that “No waveguides are just crossing over eachself” let’s wait. Sometimes these things become clearer with time and maybe they will tell me a better explanations later. That is why I thought maybe I should change the index then. Because if I do not have beam splitter, how can they cross over theirself and I tried to change parameters but this time I needed to update all parameter and it is useless…

If I understand your question correctly - no, you do not need to try to permute the mode labels to follow the circuit. You should treat each horizontal line as a fixed mode label. Perhaps a photon will switch modes due to a beamsplitter, but this will just show up when comparing your input state to output state.

Great! otherwise, I could not manage the situation. Now we have just 4 modes and suppose that we have many modes and no I can’t be a detective

#4

Hi @nisq,

I totally agree and I even asked the people who are providing me the diagram and they just told that “No waveguides are just crossing over eachself” let’s wait. Sometimes these things become clearer with time and maybe they will tell me a better explanations later. That is why I thought maybe I should change the index then. Because if I do not have beam splitter, how can they cross over theirself and I tried to change parameters but this time I needed to update all parameter and it is useless…

If you look at the BSGate in Strawberry Fields and the theory behind it, you can make the beamsplitter’s action be to “cross over” the waveguides by setting the values of theta and phi. For example, a value of theta = pi/2 and phi = 0 should do the trick.

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