This is a draft article. It is a work in progress open to editing by anyone. Please ensure core content policies are met before publishing it as a live Wikipedia article. Find sources: Google (books · news · scholar · free images · WP refs) · FENS · JSTOR · TWL Easy tools: Citation bot (help) | Advanced: Fix bare URLs · Article logs · Draft logs. Last edited by Ldm1954 (talk | contribs) 15 hours ago. (Update) Finished drafting? Submit for review

This article has multiple issues. Please help improve it or discuss these issues on the talk page. (Learn how and when to remove these messages) This article's factual accuracy is disputed. Relevant discussion may be found on the talk page. Please help to ensure that disputed statements are reliably sourced. (June 2026) (Learn how and when to remove this message) The topic of this draft may not meet Wikipedia's general notability guideline. Please help to demonstrate the notability of the topic by citing reliable secondary sources that are independent of the topic and provide significant coverage of it beyond a mere trivial mention. Notability should be established before publishing this draft or submitting it via the articles for creation process. Find sources: "Non-equilibrium quasiparticles" – news · newspapers · books · scholar · JSTOR (June 2026) (Learn how and when to remove this message) (Learn how and when to remove this message)

In superconductors, non-equilibrium quasiparticles, also known as "resident" quasiparticles,^[1] are Bogoliubov quasiparticles which exceed the expected number of thermal equilibrium quasiparticles by several orders of magnitude.^[1] While first observed in bulk superconductors,^[2] they are most relevant as a dominant source of energy decay and decoherence in thin-film superconducting circuits.^[3][4]

In quantum computing via superconducting qubits or Majorana qubits, non-equilibrium quasiparticles can cause decoherence and reduce qubit lifetimes, in a process known as quasiparticle poisoning.^[5][6] High-energy non-equilibrium quasiparticles can also cause correlated error bursts across several qubits at once.^[7] in a process called quasiparticle poisoning. As a result, non-equilibrium quasiparticles place a fundamental limitation on qubit coherence times.^[8]

In quantum sensing, these non-equilibrium quasiparticles can be used as signatures of various sub-meV events.^[9][10] This is useful for dark matter detection,^[11][12] and for nuclear monitoring.^[13]

Superconductivity is mediated by Cooper pairs, which are pairs of electrons that together form a boson and can move without resistance. A Bogoliubov quasiparticle is an excitation above the ground state of this system. In practice, breaking a Cooper pair requires two quasiparticles, and thus each quasiparticle generation event requires an energy of ${\textstyle 2\Delta }$ or more, where ${\textstyle \Delta }$ is the superconducting gap.

While a certain number of quasiparticles are expected based on standard BCS theory and statistical mechanics, the observed density of quasiparticles is orders of magnitude above what would be expected at operating temperatures of a superconducting circuit.^[3]

The expected density of equilibrium quasiparticles, normalized by the number of Cooper pairs, is:

$${\displaystyle x_{qp}={\sqrt {\frac {2\pi k_{B}T}{\Delta }}}\,e^{-\Delta /k_{B}T}}$$

Below the critical temperature of superconducting metals such as aluminum (${\displaystyle T_{c}\approx 1.2{\text{K}}}$) the formation of equilibrium quasiparticles is predicted to be exponentially suppressed.^[14] Yet, the observed density of quasiparticles, about ${\displaystyle 10^{-9}-10^{-5}}$ per Cooper pair,^[1] exceeds the amount predicted by thermodynamics, by 40 orders of magnitude at typical dilution fridge operating temperatures of ${\textstyle 10-20{\text{ mK}}}$.^[3][15]

In superconducting quantum circuits, quasiparticles which generate close to a Josephson junction can tunnel through the gap, which creates correlated errors in the device.^[16] Furthermore, high-energy photons can cause bursts of many non-equilibrium quasiparticles to generate.^[17] Gap engineering can suppress this effect, but does not eliminate the effects of non-equilibrium quasiparticles, and phase errors in gap-engineered devices have still been observed.^[16][18]

This is also a source of noise for topological superconducting devices such as Majorana qubits.^[19]

Non-equilibrium quasiparticles have traditionally been referred to as "hot" because they are equally likely to relax or excite the qubit.^[14] This makes them "hot" because if a QP tunneling event could either deposit or take energy from the qubit, this implies a broad energy distribution extending above ${\displaystyle \Delta +\hbar \omega _{01}}$.^[14] However, alternate explanations for such phenomena exist, and it is also possible that these quasiparticles have energy distributions in thermal equilibrium with the phonon bath, despite their densities being out-of-equilibrium.^[1]

There are several confirmed sources of non-equilibrium quasiparticles, although the exact microscopic physics of quasiparticles is not entirely understood.^[1]

Infrared radiation can cause quasiparticle generation events in single qubits, but their effects are largely localized. The effects of such radiation is amplified, as the structure of the superconducting qubit can act as an antenna for millimeter-wave radiation.^[15]

Ionizing events such as cosmic-ray muons^[4] and gamma rays can induce quasiparticles.^[20][12] These quasiparticles can cause errors in several qubits at once, because they are at such high energy compared to the background of the superconducting device.^[20] Such "QP bursts" can last for several microseconds, and increase the number of quasiparticles generated in the device by a thousandfold.^[21]

Ionizing events can also change the charge landscape near the qubit,^[22] making these generation events distinct from other sources of non-equilibrium quasiparticles.^[21]

Like ionizing radiation, phonon-only events can cause bursts of quasiparticles to generate,^[21] but unlike ionizing radiation, they are not expected to alter the charge distribution in the environment.^[23] It is not entirely understood where exactly such events originate from.

One explanation for phonon-only events is recombination. Recombination of two quasiparticles emits a phonon with energy ${\textstyle 2\Delta }$. If it does not escape the chip, this phonon may be able to break another Cooper pair, generating another quasiparticle.^[24]

Non-equilibrium quasiparticles have been used to probe for low-mass dark matter, such as axions or dark photons, using superconducting circuits.^[25]

Microwave Kinetic Inductance Detectors (MKIDs) are low-energy photon detectors. In MKIDs, photons are absorbed into a superconducting film and then produce non-equilibrium quasiparticles. These quasiparticles increase the surface inductance and resistance of a parallel LC circuit. This shifts the resonator's frequency down, which can be measured via a microwave tone.^[26]

• Bogoliubiv quasiparticles