Classical Bit Operations

Spinach lets you manipulate classical bits directly using logical operations. This is useful for post-processing measurement results inside the circuit itself — for example, combining two measured bits before using the result in a conditional gate.

All classical operations require classical bits declared with b.

Declaring classical bits

b0 : b 0
b1 : b 1
b2 : b 2

SET — assign a constant

Force a classical bit to a fixed value:

b0 -> SET(0)   # force b0 = 0
b0 -> SET(1)   # force b0 = 1

NOT — invert a bit

b1 -> NOT        # b1 = NOT b1  (in-place)
b1 -> NOT(b0)    # b1 = NOT b0

COPY — copy one bit to another

b1 -> COPY(b0)   # b1 = b0

AND, OR, XOR — binary logic

b2 -> AND(b0, b1)   # b2 = b0 AND b1
b2 -> OR(b0, b1)    # b2 = b0 OR b1
b2 -> XOR(b0, b1)   # b2 = b0 XOR b1

Combining quantum and classical operations

Classical bit operations can appear anywhere in a program and mix freely with quantum gates:

q0 : q 0
q1 : q 1
b0 : b 0
b1 : b 1
bout : b 2

q0 -> H
q0 -> M          # result → b0
q1 -> M          # result → b1
bout -> AND(b0, b1)   # bout = 1 only if both measured 1

Using bit results in conditional actions

Classical bit operations pair naturally with conditional actions:

b0     : b 0
b1     : b 1
bparity : b 2
q2     : q 2

bparity -> XOR(b0, b1)   # compute parity of two bits
q2 -> Z if bparity        # flip phase only if parity is odd

Repeat count

Like any action, classical bit operations support a repeat count:

b0 : b 0
b0 -> 3 NOT   # NOT three times = net identity (still 0)

Reference

Operation	Syntax	Effect
SET	b -> SET(0) or SET(1)	Assign constant value
NOT (in-place)	b -> NOT	b = NOT b
NOT (copy)	b -> NOT(src)	b = NOT src
COPY	b -> COPY(src)	b = src
AND	b -> AND(b0, b1)	b = b0 AND b1
OR	b -> OR(b0, b1)	b = b0 OR b1
XOR	b -> XOR(b0, b1)	b = b0 XOR b1