The process by which a qubit, through interactions with its environment, loses the phase and energy information of its superposition and its quantum properties break down. Its speed is described by the energy relaxation time T1 and the dephasing time T2.
Japanese name: デコヒーレンス
Decoherence appears in 1 practice question across 1 topic, including Quantum Computing Hardware.
Field: Quantum Computing Hardware
T1 (the energy relaxation time) is the time constant for a qubit in |1⟩ to lose energy and return to |0⟩. T2 (the dephasing time) is the time until the information on the relative phase of a superposition α|0⟩ + β|1⟩ is lost; because 1/T2 = 1/(2T1) + 1/Tφ, T2 ≤ 2T1 (Tφ is the pure dephasing time). T1 is set by energy dissipation and Tφ by fluctuations in the qubit frequency, and both are caused by small interactions with the environment.
In superconducting qubits, the main causes include dielectric loss from two-level-system (TLS) defects in amorphous oxides at surfaces and interfaces, thermal noise entering through the wiring, flux and charge noise, and quasiparticles created when superconductivity is broken. Radiation such as cosmic rays has also been reported to cause errors simultaneously over large areas of a chip. In silicon spin qubits, the nuclear spins of ²⁹Si and charge noise are problems, and in ions and neutral atoms, laser noise and magnetic field fluctuations. Improvements are therefore needed throughout the hardware, including materials, fabrication processes, shielding and wiring filters.
What matters is the ratio of the coherence time to the gate time. With a T2 of 100 µs and a gate time of 50 ns, the ratio is 2,000, which gives a rough upper bound on the number of operations that can be performed without error correction (in practice the errors of the gates themselves add to this). Quantum error correction is the technology for continuing a computation while detecting and correcting errors caused by decoherence, and it requires the physical error rate to be well below the threshold.
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.
No. To control and measure qubits, they must be connected to the outside world, and through those connections they are also affected by the environment. Efforts to extend coherence times are therefore combined with quantum error correction to fix the remaining errors.
That alone does not decide it. Trapped ions can have coherence times of seconds or longer, but their gates are also slow, so what should be compared is the ratio of coherence time to gate time, along with gate fidelity.
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.
Which correctly describes the energy relaxation time T1 of …As an Amazon Associate I earn from qualifying purchases.
Qubit Time constant Quantum superposition Dilution refrigerator Ion trap Josephson junction NISQ Post-quantum cryptography Quantum entanglement Quantum error correction