Dynamic circuits basics with OpenQASM 3¶

Variables¶

Let’s learn more about the variable support in OpenQASM 3. Variables are used to assign values within a program. Variables representing any classical type can be initialized on declaration.

To declare a qubit:

qubit $0;
qubit $1;

Other declaration of classical variables:

int[32] a;
bit[3] c = "001";
bool my_bool = false;

for an integer variable a, a register of three bits width c, and a Boolean variable my_bool.

Control flow¶

OpenQASM 3 also supports looping and branching.

bool target = false;
qubit $0;
h $0;
bit output = measure $0;

// example of branching
if (target == output) {
   // do something
} else {
   // do something else
}

if_example = """
OPENQASM 3.0;

bit target = 1;
qubit $0;
x $0;
bit check = measure $0;
if (check) {
   x $0;
}

bit[1] result;
result[0] = measure $0;

"""

job = run_qasm3_program(if_example)
counts = job.result().get_counts()
print(f"Measurement results: {counts}")

Runtime job id: ceqq6jne2dgblp3mf2c0
Job Status: job has successfully run
Measurement results: {'0': 10, '10': 978, '100': 1, '110': 11}

int a = 1;

// here step=1
for i in [0 : 4] {
   a = i;
}

// here step=2
for i in [0 : 2 : 4] {
   a = i
}

for_loop = """
OPENQASM 3.0;

qubit $0;
for i in [0 : 4] {
   U(1.57079632679, 0.0, 3.14159265359) $0;
}

bit[1] result;
result[0] = measure $0;

"""

# U(1.57079632679, 0.0, 3.14159265359) applies a Hadamard gate

job = run_qasm3_program(for_loop)
counts = job.result().get_counts()
print(f"Measurement results: {counts}")

Runtime job id: ceqq6lve2dgblp3mf2f0
Job Status: job has successfully run
Measurement results: {'0': 519, '1': 481}

Classical compute¶

OpenQASM 3 supports classical computation on individual bits. These bits may be assigned by the program, or as a result of qubit measurement.

Individual bits are declared with the bit keyword, or registers of multiple bits may be declared using bit[size] where size indicates the number of bits in the register. For example,

bit a;
bit b;
bit c;

declares three individual bits with variable names a, b, and c, while

bit[3] result;

declares a three-bit register with the name result;.

Bits and bit registers may be assigned by the program

bit a = 0;
a = 1;

bit[3] result = "101";
result = "010";

or they may store individual qubit measurement results.

measure_register_bits = """\
OPENQASM 3.0;
qubit $0;
qubit $1;
qubit $2;
bit a;
a = measure $0;

bit[3] result;
result[0] = measure $0;
result[1] = measure $1;
result[2] = measure $2;
"""

job = run_qasm3_program(measure_register_bits)
counts = job.result().get_counts()
print(f"Measurement results: {counts}")

Runtime job id: ceqq6nrhsot3802d22bg
Job Status: job has successfully run
Measurement results: {'0': 967, '1': 1, '10': 1, '11': 12, '100': 8, '1000': 11}

The current dynamic circuits implementation does not support quantum registers; therefore, storing the result of the measurement of a quantum register

qubit[3] qubits;
bits[3] result;
result = measure qubits;

is not yet supported.

Classical computation may be applied to bits and bit registers. The current implementation supports computing bitwise operators applied to bits or bit registers. These include:

bit a;
bit b;
bit c;

Or:

c = a | b;

And:

c = a & b;

Xor:

c = a ^ b;

These may be included as part of if statements, and grouped using parentheses:

grouped_bitwise = """\
OPENQASM 3.0;

qubit $0;

bit a = 0;
bit b = 1;
bit c;

if ((a | b)) {
   c = 0;
}

if ((a | b) ^ c) {
   c = 1;
}

bit d;
x $0;
d = measure $0;

"""

job = run_qasm3_program(grouped_bitwise)
counts = job.result().get_counts()
print(f"Measurement results: {counts}")

Runtime job id: ceqq6prhsot3802d22eg
Job Status: job has successfully run
Measurement results: {'0': 30, '1000': 970}

Taking the logical not of a bit is currently supported with the form:

logical_not = """\
OPENQASM 3.0;

qubit $0;

bit a;
bit b;
bit c;

a = !a;

if (a | b) {
  c = 1;
}

bit d;
x $0;
d = measure $0;

"""

job = run_qasm3_program(logical_not)
counts = job.result().get_counts()
print(f"Measurement results: {counts}")

Runtime job id: ceqq6s76fpt7053qr8sg
Job Status: job has successfully run
Measurement results: {'0': 21, '1000': 979}

At present time, taking the bit negation ~ is not supported.

Bit registers support conditions on individual bits:

bit_comparison = """\
OPENQASM 3.0;

qubit $0;

bit[3] result;
bit c;

if (result[0]) {
   c = 0;
}

bit d;
x $0;
d = measure $0;
"""

job = run_qasm3_program(bit_comparison)
counts = job.result().get_counts()
print(f"Measurement results: {counts}")

Runtime job id: ceqq6u3hsot3802d22kg
Job Status: job has successfully run
Measurement results: {'0': 35, '10000': 965}

bitwise Boolean expressions:

bitwise_boolean = """\
OPENQASM 3.0;

qubit $0;

bit[3] result1;
bit[3] result2;
bit c;

if ((result1 | result2)) {
   c = 0;
}

if ((result1 & result2)) {
   c = 1;
}

bit d;
x $0;
d = measure $0;
"""

job = run_qasm3_program(bitwise_boolean)
counts = job.result().get_counts()
print(f"Measurement results: {counts}")

Runtime job id: ceqq71oc7aa76lano0n0
Job Status: job has successfully run
Measurement results: {'0': 27, '10000000': 973}

and comparison:

bitwise_comparison = """\
OPENQASM 3.0;

qubit $0;

bit[3] result1 = "010";
bit[3] result2 = "101";
bit c;

if (result1 == result2)  {
   c = 1;
}

if (result1 > result2) {
   c = 1;
}

bit d;
x $0;
d = measure $0;
"""

job = run_qasm3_program(bitwise_comparison)
counts = job.result().get_counts()
print(f"Measurement results: {counts}")

Runtime job id: ceqq73oc7aa76lano0p0
Job Status: job has successfully run
Measurement results: {'0': 12, '10000000': 988}

The usage of bit-string literals is currently only supported when initializing bit registers. Therefore, expressions such as

if ((result1 | result2) == "000") {
   c = 0;
}

are not yet supported.

Conditional measurement¶

Classical computation, Boolean expressions, and conditionals may be combined with qubit measurement to implement conditional measurement. Here the result of qubit measurements may be used to conditionally select additional qubit measurements.

qubit $0
qubit $1
bit r1;
bit r2;

r1 = measure $0;
if (r1 == "1") {
    r2 = measure $1;
}

else branches may be used to dynamically select individual qubits at runtime:

conditional_measurement_program = """\
OPENQASM 3.0;

qubit $0;
qubit $1;
qubit $2;

bit r0;
bit r1;
bit r2;

x $0;

r0 = measure $0;
if (r0) {
    r1 = measure $1;
} else {
    r2 = measure $2;
}
"""

job = run_qasm3_program(conditional_measurement_program)
counts = job.result().get_counts()
print(f"Measurement results: {counts}")

Runtime job id: ceqq77rhsot3802d22v0
Job Status: job has successfully run
Measurement results: {'0': 27, '1': 973}

Current system limitations prevent accurate reporting of the conditional measurement results once the program terminates. The result of the measurement is stored and used during the program; however, the measurement results will not be reported to the user unless there are subsequent measurements. If there are subsequent measurements, then the conditional measurement will always be reported as 0 to the user. We expect to add support for reporting conditional measurement results in 2023.

import qiskit.tools.jupyter
%qiskit_version_table