Qubits, which are the fundamental units of quantum information, can be put into a quantum superposition through a process called quantum state preparation or initialization. The method of preparing qubits in a superposition depends on the specific physical system used to implement them. Let's explore two common approaches:
- Superconducting Qubits: In superconducting quantum processors, qubits are typically implemented using superconducting circuits. Superconducting qubits, such as transmon qubits, are manipulated using microwave pulses. To prepare a superconducting qubit in a superposition, a combination of single-qubit operations, such as rotations, are applied.
For example, a single-qubit gate like the Hadamard gate (H gate) can be applied to a superconducting qubit to create a superposition. The Hadamard gate transforms the qubit's state from the |0⟩ state (ground state) to an equal superposition of the |0⟩ and |1⟩ states, represented as (|0⟩ + |1⟩)/√2.
By applying appropriate sequences of pulses and gates, superconducting qubits can be prepared in a wide range of superposition states.
- Trapped Ion Qubits: Trapped ion qubits rely on the manipulation of internal energy levels of trapped ions to store and process quantum information. To put trapped ion qubits into a superposition, laser pulses are typically used.
For instance, a common technique is the application of a π/2-pulse, also known as a "pi over 2" or "pi/2" pulse. This pulse resonantly excites the qubit from its ground state to a specific superposition state. By controlling the parameters of the laser pulse, such as its duration and frequency, trapped ion qubits can be prepared in various superposition states.
These are just two examples of how qubits can be put into superposition states. Different physical systems and architectures may employ distinct methods, but the underlying principle is to apply appropriate quantum operations that manipulate the qubit's state to achieve the desired superposition.