Quantum Computing Hardware
Which correctly describes the energy relaxation time T1 of a qubit?
- AThe time taken by one quantum gate operation
- BThe characteristic time for the excited state |1⟩ to lose its energy to the environment and return to the ground state |0⟩
- CThe time the refrigerator takes to cool the qubits from room temperature to the operating temperature
Show answer and explanation
Answer: B. The characteristic time for the excited state |1⟩ to lose its energy to the environment and return to the ground state |0⟩
T1 is the time constant of the process in which a qubit prepared in |1⟩ loses energy and returns to |0⟩; the probability of being in |1⟩ decays roughly as exp(−t/T1). In superconducting qubits, the main cause is energy leaking into the wiring, the substrate and defects at surfaces. Together with T2, the time until the phase information of a superposition is lost, it indicates how long a qubit can hold information. T1 and T2 are finite because of decoherence, in which the qubit interacts with its environment and loses its quantum properties, and correct computation requires gate times much shorter than T1 and T2.
- A. The gate time is a separate quantity from T1. The shorter the gate time compared with T1, the more operations can be performed.
- B. Correct. The probability of being in |1⟩ decays roughly as exp(−t/T1).
- C. It is not the cooling time but a measure of how long the state of the qubit itself is preserved.