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1Clear out junk files and repair common Windows errors2Fix the driver behind crashes, sound loss and screen glitches3Repair Windows errors before they cause bigger problems“Stable” has no single meaning for a quantum state. It can mean that an excited energy level decays slowly, or that a superposition retains the phase relationships needed for quantum interference. Those are different properties, so a useful comparison always specifies what is being preserved, in which system, and over what timescale.
What makes a quantum state stable?
Ask what kind of change you mean. An atom’s excited energy level can decay to a lower level; a qubit’s superposition can lose coherence and stop producing useful interference. These processes can occur in the same physical system, but energy lifetime and coherence time are not interchangeable measures.
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- Energy stability: how resistant an energy level is to transitions to lower-energy levels.
- Coherence stability: how long a superposition preserves the phase relationships that let its alternatives interfere.
Stable and unstable energy states
Ground states and excited states
The ground state is the lowest-energy state of a specified system. An excited state has more energy and may transition to a lower level, for example by emitting radiation. NIST defines an atomic level’s radiative lifetime in terms of the transition probabilities from that level to lower-energy levels: NIST’s reference on atomic lifetimes.
A longer lifetime means slower decay under the relevant conditions; it does not mean the state is the ground state or that it will last forever. The term “unstable” in this context describes a state that can decay, not necessarily one that disappears immediately.
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What is a metastable quantum state?
A metastable state is an excited state that persists comparatively longer than other available states under the particular transition conditions. It remains higher in energy than the ground state and can still decay. “Metastable” is therefore a statement about relative lifetime, not permanence.
Coherence, superposition, and decoherence
A superposition combines quantum alternatives with phase relationships that can produce interference. In a qubit, retaining those relationships is important for quantum operations. NIST’s explanation of quantum computing notes that stray fields and temperature changes can disturb qubits; interactions with the environment can spoil a superposition or entanglement.
Decoherence is the loss of a coherent superposition’s ability to interfere. The system behaves more like a classical mixture for the relevant observations, as described in the National Academies Press chapter “Quantum Information with Light and Atoms”. This describes a loss of coherence; it does not by itself establish that the system lost energy. Energy relaxation and decoherence are distinct questions, and their relevant timescales can differ.
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Different technologies have different strengths and trade-offs. NIST describes trapped-ion qubits as capable of sustaining superpositions for a long time but performing computations relatively slowly. Superconducting qubits can compute quickly, but their states are more fragile and shorter-lived. This is a broad comparison of technology families, not a set of directly comparable lifetime measurements for every device.
Environmental sensitivity also depends on the system. Potential disturbances include stray electric or magnetic fields, temperature fluctuations, thermal noise, radio-frequency radiation, mechanical instability, and phonons. There is no meaningful universal winner unless the comparison identifies the system, the operating conditions, and whether it concerns energy lifetime or coherence time.
How the environment can change an excited-state lifetime
A JILA experiment reported by NIST shows that an excited state’s lifetime can depend on its surroundings. In a specific ultracold, degenerate strontium Fermi gas, the atoms’ organized quantum motion made it harder for an excited atom to emit a photon and recoil. NIST reported that an atom remained excited, on average, about 10% longer than usual under the experiment’s conditions. Photon emission fell by up to 50% in a narrow scattering angle; the natural five-nanosecond excited-state lifetime was too short to measure directly, so photon scattering served as an indirect indicator. These figures describe that experiment, not a general effect for all atoms or quantum states. NIST’s account of the JILA experiment quotes NIST/JILA Fellow Jun Ye: “Pauli blocking uses well-organized quantum motional states of a Fermi sea to block the recoil of an atom that wants to decay, thus prohibiting spontaneous decay.”
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How to compare two quantum states
- Name the system: specify the atom, molecule, qubit, or other system and the conditions in which it operates.
- Choose the property: use energy lifetime for decay from an excited level, or coherence time for retaining useful phase relationships.
- Keep the comparison like-for-like: compare the same property under stated conditions. Do not treat a long energy lifetime as proof of long coherence, or vice versa.
- State the timescale and scope: say whether a result is an experimental measurement, a typical technology-family description, or a lifetime under a particular setup.
Experiments also show that decoherence depends on system details: a 2000 trapped-atom study reported a decoherence rate scaling with the square of a quantity describing superposition amplitude. That result applies to the study’s setting, not as a universal law for every quantum system. NIST’s publication record for the study.
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