How do qubits interact with each other in a quantum computer?

Qubits interact in a quantum computer through quantum gates, which manipulate their states and create entanglement—a unique quantum property where qubits become correlated. These interactions are enabled by physical mechanisms specific to the hardware platform, such as superconducting circuits, trapped ions, or photonic systems. For example, a controlled-NOT (CNOT) gate entangles two qubits by flipping the state of one qubit (the target) based on the state of the other (the control). This gate is fundamental to building quantum circuits, as entanglement allows qubits to share information in ways classical bits cannot. The interaction strength and timing are carefully controlled to maintain coherence and minimize errors during computations.

The physical implementation determines how qubits interact. In superconducting quantum computers (like those from IBM or Google), qubits are coupled via microwave resonators or tunable couplers. Applying microwave pulses to one qubit induces magnetic interactions with neighboring qubits, enabling gate operations. For trapped-ion systems (used by companies like IonQ), qubits are ions suspended in electromagnetic fields. Lasers manipulate their electronic states and induce interactions via their shared vibrational motion. Photonic qubits, meanwhile, interact through optical components like beam splitters or waveguides, where photons interfere to create entanglement. Each approach has trade-offs: superconducting systems scale more easily but face coherence time limits, while trapped ions offer high-fidelity interactions but are harder to scale.

A key challenge in qubit interactions is managing errors caused by decoherence and noise. Longer interaction times increase the risk of environmental interference, which degrades qubit states. For example, superconducting qubits typically have coherence times in the microsecond-to-millisecond range, limiting the number of operations before errors occur. Cross-talk—unwanted interactions between nearby qubits—is another issue, especially in densely packed architectures. Companies like Rigetti use tunable couplers to reduce cross-talk by dynamically adjusting qubit coupling strengths. Error correction techniques, such as surface codes, aim to mitigate these issues but require additional qubits and complex control. As hardware improves, optimizing interaction fidelity while scaling to hundreds or thousands of qubits remains a critical focus for practical quantum computing.