This quantum circuit prepares four qubits into a decoherence-free subspace using singlet states.
```python
from qiskit import QuantumCircuit
# Create a circuit with 4 qubits and 0 classical bits
qc = QuantumCircuit(4, 0)
# Prepare qubits 0 and 1 in the singlet state (|01⟩ - |10⟩) / √2
qc.x(1) # Prepare qubit 1 in state |1⟩
qc.h(0) # Hadamard gate on qubit 0
qc.cx(0, 1) # CNOT gate with control qubit 0 and target qubit 1
qc.z(0) # Z gate on qubit 0 to introduce a relative phase
# Prepare qubits 2 and 3 in the singlet state (|01⟩ - |10⟩) / √2
qc.x(3) # Prepare qubit 3 in state |1⟩
qc.h(2) # Hadamard gate on qubit 2
qc.cx(2, 3) # CNOT gate with control qubit 2 and target qubit 3
qc.z(2) # Z gate on qubit 2 to introduce a relative phase
# The combined state of all four qubits is in a decoherence-free subspace
```'Failed to run program: \'409 Client Error: Conflict for url: https://api.quantum.ibm.com/runtime/jobs. {"errors":[{"message":"You have reached the limit of 3 pending jobs. Please wait for a job to complete or cancel one before submitting anything new.","code":3458,"solution":"Wait until some previous jobs were finished. You can cancel pending jobs to run new jobs.","more_info":"https://docs.quantum-computing.ibm.com/errors"}]}\''