Question

How are qubits entangled in superconducting quantum processors, if SPDC (spontaneous parametric down conversion) is not used? Does the resonator that the qubits are coupled to play any role in entanglement generation?

Answer 1

In superconducting quantum processors, qubits are typically entangled through a different mechanism known as the controlled-phase gate, which involves the interaction between qubits mediated by a shared resonator.

Here's a high-level explanation of how entanglement is generated in superconducting quantum processors:

1. Coupling: The individual superconducting qubits in the processor are coupled to a common microwave resonator. This resonator acts as a "bus" that enables interaction and information transfer between qubits.

2. Initialization: Initially, the qubits are prepared in a well-defined state, typically the ground state or a specific superposition state.

3. Gate Operations: Gate operations are applied to manipulate the qubits' states. These operations include single-qubit gates (e.g., rotations around different axes in the Bloch sphere) and two-qubit gates (e.g., the controlled-NOT gate or controlled-phase gate).

4. Controlled-Phase Gate: The controlled-phase gate, also known as the CPHASE gate or controlled-Z gate, is a crucial gate for generating entanglement. It imparts a phase shift of π radians on the target qubit's state when the control qubit is in the excited state (|1⟩). This phase shift creates an entangled state between the control and target qubits.

5. Measurement and Feedback: After applying gate operations, measurements are performed on the qubits to extract information about their states. These measurements can be used for feedback control and to verify the presence of entanglement.

The resonator plays a crucial role in the generation of entanglement by mediating interactions between qubits. The coupling between the qubits and the resonator enables them to exchange energy and information, allowing for the implementation of two-qubit gates such as the controlled-phase gate. The resonator helps create entangled states by entangling the qubits' states through their interaction with the shared resonator mode.

It's important to note that the specific details of the implementation and control of entanglement generation in superconducting quantum processors can vary depending on the architecture and design choices of the particular system. Ongoing research and development continue to refine and optimize these processes for improved performance and scalability.