Bell states

A Bell state is the simplest example of quantum entanglement - a two-qubit state where measuring one qubit instantly determines the outcome of the other. It's built with just two gates: a Hadamard to create superposition, followed by a CNOT to entangle the qubits.

Circuit

Code

The same circuit, shown across all four supported languages:

OpenQASMQiskitCirqQ#

OPENQASM 2.0;
include "qelib1.inc";

qreg q[3];
creg c[3];

h q[0];
cx q[0], q[1];

from qiskit import QuantumRegister, ClassicalRegister, QuantumCircuit
from numpy import pi

qreg_q = QuantumRegister[3, 'q']
creg_c = ClassicalRegister[3, 'c']
circuit = QuantumCircuit[qreg_q, creg_c]

circuit.h[qreg_q[0]]
circuit.cx[qreg_q[0], qreg_q[1]]

import cirq
import math

q = cirq.LineQubit.range[3]
circuit = cirq.Circuit[]
circuit.append[cirq.H[q[0]]]
circuit.append[cirq.CNOT[q[0], q[1]]]
print[circuit]

namespace QompileCircuit {
    open Microsoft.Quantum.Canon;
    open Microsoft.Quantum.Intrinsic;
    open Microsoft.Quantum.Math;
    open Microsoft.Quantum.Convert;

    operation Circuit() : Result[] {
        use q = Qubit[3];
        mutable c = [Zero, size = 3];

        H(q[0]);
        CNOT(q[0], q[1]);

        ResetAll(q);
        return c;
    }
}

See Build your circuit with code for details on the bracket-vs-parentheses quirk in the Qiskit and Cirq views.

What you'll see

• Probabilities - two equal bars, each at 50%, for the two correlated outcomes
• Q-Sphere - two points on the sphere, one for each basis state in the superposition
• Statevector - two non-zero amplitudes of equal magnitude (≈0.707), confirming an equal-superposition entangled state