Quantum Computing Hardware
A qubit in the state |0⟩ has a Hadamard (H) gate applied once and is then measured. What is the probability of getting 1?
- A0%
- B50%
- C100%
Show answer and explanation
Answer: B. 50%
The H gate turns |0⟩ into (|0⟩ + |1⟩)/√2. The amplitude of |1⟩ is 1/√2, so the probability of getting 1 is (1/√2)² = 1/2 = 50%. The H gate is often used at the start of quantum algorithms to create an equal superposition of all basis states. Applying the H gate twice in a row returns the qubit to |0⟩, and a measurement then always gives 0.
- A. This is the value when the H gate is applied twice (returning to |0⟩).
- B. Correct. (1/√2)² = 1/2.
- C. This is the value for the X gate, which turns |0⟩ into |1⟩.