Conditional Execution on H-Series

Quantinuum H-Series systems are capable of executing conditional workflows. This notebook presents how to do this in pytket. For more information, see Conditional Execution and Classical and conditional operations.

Classical assignment of registers or bits

The value of classical registers and bits be assigned as illustrated below. Note that you can assign classical values at any point as well as re-assign values.

from pytket.circuit.display import render_circuit_jupyter
from pytket.circuit import (
    Circuit,
    BitRegister,
    if_bit,
    if_not_bit,
    reg_eq,
    reg_geq,
    reg_gt,
    reg_leq,
    reg_lt,
    reg_neq,
)

circuit = Circuit(name="Conditional Example")

qreg = circuit.add_q_register("q", 1)
reg_a = circuit.add_c_register("a", 10)
reg_b = circuit.add_c_register("b", 10)
reg_c = circuit.add_c_register("c", 10)

circuit.add_c_setbits([1], [reg_a[0]])  # a[0] = 1
circuit.add_c_setreg(2, reg_a)  # a = 2
circuit.add_c_setreg(3, reg_b)  # b = 3

Binary operators

Bitwise binary operators are avilable and can be applied to entire classical registers or bits. This allows one to update values based on mid-circuit measurement results as well as allow more advanced classical register comparisons.

circuit.add_classicalexpbox_register(reg_a ^ reg_b, reg_c)  # c = a ^ b
circuit.add_classicalexpbox_bit(reg_a[0] ^ reg_b[0], [reg_c[0]])  # c[0] = a[0] & b[0]
circuit.add_classicalexpbox_register(reg_a & reg_b, reg_c)  # c = a & b
circuit.add_classicalexpbox_register(reg_a | reg_b, reg_c)  # c = a | b

Compound Logical Expressions

Comparison operators in addition to the == operator are available and you can evaluate bits. Note, the != operator can be useful in identifying if measurement results were trivial (for example, meas!=0) or not.We can operate a quantum gate on a quantum circuit when such a logical formula is satisfied as below.

circuit.X(qreg[0], condition=reg_a[0]) # if(a[0]==1) x q[0], evaluation of a bit

circuit.X(
    qreg[0], condition=if_bit(reg_a[0])
)  # if(a[0]==1) x q[0], evaluation, same function as above
circuit.X(qreg[0], condition=if_not_bit(reg_a[0]))  # if(a[0]==0) x q[0]
circuit.X(qreg[0], condition=reg_eq(reg_a, 1))  # if(a==1) x q[0]
circuit.X(qreg[0], condition=reg_neq(reg_a, 1))  # if(a!=1) x q[0]
circuit.X(qreg[0], condition=reg_gt(reg_a, 1))  # if (reg_a > 1)
circuit.X(qreg[0], condition=reg_lt(reg_a, 1))  # if (reg_a < 1)
circuit.X(qreg[0], condition=reg_geq(reg_a, 1))  # if (reg_a >= 1)
circuit.X(qreg[0], condition=reg_leq(reg_a, 1))  # if (reg_a <= 1)

Conditional Classical Assignments

You can not only use the option condition to apply quantum gates but to make classical assignments as well. Note, this allows one to dynamically set flags.

circuit.add_c_setreg(1, reg_b, condition=reg_eq(reg_a, 10))  # if (a==10) b=1
circuit.add_c_setreg(1, reg_b, condition=if_not_bit(reg_a[0]))  # if (a[0]==0) b=1

Pytket enables you to visualize where conditional operators are used within a circuit.

render_circuit_jupyter(circuit)