Quantum coherence has no single lifetime: it depends on the physical system, its interaction with the environment, and how coherence is measured. For qubits, the key measures are T1, the energy-relaxation time, and T2, the phase-coherence time. The relationship between them is T2 < 2T1, but that is a limit between two metrics—not a universal number of seconds.
Why there is no single answer
Coherence describes the preservation of quantum relationships, such as the relative phase in a superposition or the correlations in an entangled state. A system remains useful for a particular quantum operation only while those relationships persist well enough for the operation to work.
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Different physical systems have different sensitivities to their surroundings. NIST notes that ion qubits can sustain superpositions for a long time, while superconducting qubit states are more fragile and shorter-lived. This contrast does not establish one platform-wide lifetime for either type: a meaningful figure must identify the device, the coherence measure, operating conditions, and measurement protocol. NIST’s overview sums up the challenge: Qubits are exquisite but fragile.
NIST’s quantum-computing explainer provides that platform-level context.
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What T1, T2, and T2* measure
| Measure | What it describes | How to interpret it |
|---|---|---|
| T1 | Energy relaxation: how quickly a qubit loses energy to its environment, for example through dissipation. | It measures energy loss, not every way phase information can fade. See the cited discussion of qubit relaxation and coherence. |
| T2 | Phase coherence: how quickly phase relationships decay. | For qubits covered by the 2020 review, relaxation limits this time to less than twice T1: T2 < 2T1. This is a relationship between measures, not an absolute lifetime. See “Materials challenges and opportunities for quantum computing hardware,” Science (2020). |
| T2* | Inhomogeneous dephasing: apparent loss of phase coherence caused in part by quasi-static differences in transition frequency. | For example, magnetic-field variations can shift spin-qubit frequencies. A measurement that includes such variation can yield a shorter T2* than a coherence measure using echo. See the 2020 Science review. |
These measures are not interchangeable. Energy relaxation contributes to coherence loss, while additional phase noise can shorten coherence further. That is why the review’s T2 < 2T1 relationship is useful but does not let you infer a universal runtime or compare devices without their conditions and protocols.
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What causes coherence to break down
Decoherence occurs when unwanted interactions with the surroundings or internal dissipation disturb a quantum state. The relevant disturbance depends on the platform and device; a mechanism found in one design should not be treated as the dominant cause in every quantum system.
Environmental disturbances
NIST identifies stray electric or magnetic fields, temperature fluctuations, and cosmic rays as disturbances that can ruin a qubit’s superposition or entanglement. Their practical effect depends on how strongly a given qubit couples to them and how effectively the device is shielded or controlled. See NIST’s explainer.
Material loss and device noise
In superconducting qubits, energy loss and phase noise can arise from device materials and interfaces. A NIST-indexed study found dielectric loss associated with two-level states to be a dominant decoherence source in the Josephson qubits it examined. An IBM Research review also discusses dielectric loss, two-level systems, and material and fabrication effects. These findings describe particular superconducting-qubit devices, not ions, photons, or all other platforms. See the NIST-indexed study and the IBM Research review.
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Low-frequency bias noise is another studied mechanism. Its contribution can depend on the qubit and the measurement or control sequence, so it is not a universal explanation for short coherence. See the NIST-indexed study of low-frequency noise and control sequences.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.How some coherence loss can be mitigated
Control techniques can reduce certain kinds of dephasing, but they do not remove every source of decoherence. A Hahn echo reverses some phase spreading caused by quasi-static inhomogeneity. Spin-echo and Rabi control sequences have also been reported as less sensitive to low-frequency noise in the studied superconducting-qubit context. The benefit depends on the noise and on the sequence; the control itself does not make all environmental disturbance or energy loss disappear. See the Science review and the NIST-indexed study.
Device improvements can target material loss or reduce how strongly a qubit responds to noise. For one specific example, a 2005 study by John M. Martinis and coauthors reported a factor-of-20 improvement in energy-relaxation rate for a redesigned phase qubit using low-loss dielectrics. That result applies to the design and comparison in that study; it is not a general multiplier for other qubits. See the 2005 Science study.
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How to compare reported coherence times
A lifetime number is meaningful only when its context is clear. Measurement errors and inconsistent procedures can make comparisons across devices and laboratories inaccurate or impossible, as NIST’s reproducibility perspective warns. See NIST’s discussion of reproducibility in quantum computing.
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One free scan finds every outdated or missing driver and matches the right update for your exact hardware.Free scan · exact hardware match- Platform and device: identify the physical qubit type and, where reported, the device design.
- Metric: distinguish T1, T2, and T2*; they describe different processes.
- Conditions: note operating environment and relevant measurement conditions.
- Protocol: say whether echo or other control was applied and how coherence was measured.
Without these details, two figures may describe different phenomena rather than a fair comparison. A platform ranking based only on the largest quoted number can therefore mislead.
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