How do quantum systems maintain entanglement across qubits?

Quantum systems maintain entanglement across qubits by carefully controlling their interactions, minimizing environmental interference, and using error-correction techniques. Entanglement occurs when qubits are placed in a shared quantum state, such that measuring one instantly determines the state of the other, even at a distance. This is typically achieved through operations like controlled-NOT (CNOT) gates, which create correlations between qubits. However, maintaining entanglement requires isolating qubits from external noise (like heat or electromagnetic fields) that could disrupt their fragile quantum states. For example, superconducting qubits are kept at near-zero temperatures to reduce thermal noise, while trapped-ion qubits use vacuum chambers to limit particle collisions.

To preserve entanglement, quantum systems rely on error correction and physical design. Quantum error correction codes, such as the surface code, encode logical qubits across multiple physical qubits, allowing errors to be detected and corrected without collapsing the entangled state. For instance, a logical qubit might be spread across five physical qubits, with parity checks monitoring for decoherence. Additionally, hardware designs minimize unintended interactions—superconducting qubits are spaced to reduce crosstalk, and control pulses are finely tuned to avoid overcoupling. In trapped-ion systems, lasers precisely manipulate ions to entangle specific pairs while leaving others unaffected. These strategies ensure that entanglement persists long enough for computations to proceed.

Finally, real-time calibration and control systems play a critical role. Quantum processors continuously adjust parameters like microwave pulse timing or magnetic field strength to counteract environmental drift. For example, IBM’s quantum computers use automated calibration routines to maintain gate fidelities (accuracy of quantum operations) above 99% for key entangling gates like the CNOT. Similarly, dynamic decoupling techniques apply sequences of pulses to qubits to cancel out low-frequency noise. If a qubit’s coherence time (duration it retains quantum information) starts to drop, the system might reroute operations to spare qubits or adjust error-correction protocols. These methods, combined with advances in materials and fabrication, allow modern quantum systems to sustain entanglement for milliseconds to seconds—sufficient for executing multi-step algorithms.