Quantum coherence has no single lifetime: it depends on the physical system, its surroundings, and how coherence is measured. For qubits, the key measures are T1, the time for energy relaxation, and T2, the time for phase coherence. A useful bound is T2 < 2T1, but it does not give every qubit the same lifetime.
Why quantum coherence has no universal lifetime
Coherence is the preservation of quantum phase relationships that allow a system to exhibit effects such as superposition and entanglement. A coherent state may persist for very different periods in different physical platforms and operating conditions. NIST notes that ion qubits can sustain superpositions for a long time, while superconducting-qubit states are more fragile and shorter-lived. Those descriptions do not establish a directly comparable duration or a current record for either platform. NIST’s quantum-computing overview puts it succinctly: “Qubits are exquisite but fragile.”
To interpret a reported lifetime, identify the platform, the coherence metric, the environment and the measurement or control protocol. Without those details, a number cannot reliably answer how long coherence lasts in another system.
What T1, T2 and T2* measure
| Metric | What it measures | How to interpret it |
|---|---|---|
| T1 | Energy-relaxation time: how quickly a qubit loses energy to its environment. | Describes energy decay, for example through dissipation. Science’s 2020 review discusses its relationship to coherence. |
| T2 | Phase-coherence time: how quickly phase relationships decay. | For the qubits covered by the review, relaxation limits T2 to less than twice T1: T2 < 2T1. This is a relationship between measures, not a universal lifetime. |
| T2* | Inhomogeneous dephasing time, which can be shortened by quasi-static differences in transition frequency. | For example, magnetic-field variations can affect spin qubits. An echo sequence can cancel some of these quasi-static effects. |
The distinction matters: a qubit can lose phase coherence without first losing energy. T1 and T2 therefore describe related but different ways a quantum state can degrade. NIST’s explanation of qubit relaxation provides further context for energy loss.
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What causes coherence to break down
Decoherence results when a quantum system interacts with its surroundings or experiences internal dissipation. Such interactions can disturb the state or leave information about it in the environment, eroding the phase relationships required for coherent behavior.
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 impact depends on the platform and the conditions in which the qubit operates; the list is not a claim that each disturbance affects every device equally.
Material loss and device noise
For superconducting qubits, loss and 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, materials and fabrication effects. These findings describe particular superconducting devices, not a universal ranking of decoherence mechanisms across quantum platforms.
Low-frequency bias noise is another studied mechanism. Its importance likewise depends on the device and operating conditions.
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Control techniques can reduce certain kinds of dephasing, but they do not remove every source of decoherence.
- Hahn echo: A control pulse can cancel some dephasing caused by quasi-static inhomogeneity, such as slowly varying differences in transition frequency.
- Spin-echo and Rabi sequences: A NIST-indexed bias-noise study reports that these control sequences are less sensitive to low-frequency noise.
- Materials and design improvements: Reducing material loss or a qubit’s sensitivity to noise can help. In a 2005 study, 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 that study and design, not to qubits generally.
Echo and control sequences change how a qubit responds to some disturbances; they do not make the surrounding environment disappear. Their benefit also depends on the noise being targeted and the measurement protocol used.
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How to compare reported coherence times
Two lifetime figures are meaningful to compare only when their key conditions are clear. NIST warns that measurement errors can make comparisons between devices and laboratories inaccurate or impossible. Check these details before interpreting a result:
- Platform: Identify whether the system is an ion, superconducting, spin or another kind of qubit.
- Metric: Check whether the figure is T1, T2 or T2*; these are not interchangeable.
- Conditions: Note the operating environment and relevant device conditions.
- Protocol: Establish how long and by what measurement or control sequence the value was obtained, including whether echo was applied.
Without matched conditions and protocols, ranking platforms by a single coherence-time number can be misleading. A broadly comparable current table of best coherence times across major platforms is not established by the cited sources.
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