⚛️ phenomenology

CP asymmetry and visible decay in $3+1$ neutrino oscillations on a quantum computer

This paper utilizes quantum simulation on an IBM quantum processor to demonstrate how visible neutrino decay modifies vacuum CP asymmetry in a $3+1$ scenario, successfully validating theoretical predictions for flavor probabilities and coherence while quantifying the significant contribution of daughter regeneration to the decay-induced shift.

Original authors: Amartya Sengupta, Sidhartha Samtani, Ani Girgvliani, Dejan Stojkovic

Published 2026-09-24

📖 5 min read🧠 Deep dive

CC BY 4.0

Original authors: Amartya Sengupta, Sidhartha Samtani, Ani Girgvliani, Dejan Stojkovic

Original paper licensed under CC BY 4.0 (http://creativecommons.org/licenses/by/4.0/). ✨ This is an AI-generated explanation of the paper below. It is not written or endorsed by the authors. For technical accuracy, refer to the original paper. Read full disclaimer

Neutrinos are the most abundant massive particles in the universe, yet they remain among the most elusive. These ghostly particles zip through matter almost entirely unimpeded, and they possess a strange ability to change their identity as they travel. A neutrino born as a muon type can transform into an electron type or a tau type before reaching a detector. This phenomenon, known as oscillation, depends on the particle's energy and the distance it travels. Crucially, the laws of physics suggest that neutrinos and their antimatter counterparts, antineutrinos, should not behave exactly the same way during these transformations. This difference, called CP violation, is a key ingredient in explaining why the universe is made of matter rather than being a void of equal parts matter and antimatter. While scientists have observed this asymmetry in other particles, measuring it in neutrinos is incredibly difficult because the signals are faint and easily confused by other effects.

A new study brings a fresh perspective to this problem by exploring a scenario where neutrinos might not just change flavor, but also decay into lighter particles while traveling. In this specific theoretical model, an unstable neutrino breaks apart into a lighter neutrino and a very light, invisible particle. The researchers investigated how this decay process alters the expected difference between matter and antimatter behavior. They found that the decay does more than just reduce the number of parent particles; it also creates a new signal from the "daughter" particles produced in the breakup. This daughter signal can interfere with the remaining particles in a way that shifts the measured asymmetry. To untangle these complex interactions, the team turned to a quantum computer, using it to simulate the quantum mechanics of the decay and oscillation process with a level of detail that is difficult to achieve with standard classical computers.

The researchers focused on a model involving four types of neutrinos: the three known active flavors and one hypothetical "sterile" type that interacts only through gravity and mixing. In their scenario, the fourth type is unstable and decays into the lighter, active types. The team asked a precise question: if this decay happens, how does it change the observed difference between how neutrinos and antineutrinos oscillate? They discovered that the decay introduces two distinct effects. First, the original neutrino is attenuated, or thinned out, as it decays, which changes the interference pattern of the surviving particles. Second, the decay produces new, lighter neutrinos that can interfere with each other. This second effect, called regeneration, adds a new layer to the asymmetry. The study showed that for a specific set of conditions, this regenerated signal accounts for nearly half of the total change in the asymmetry caused by the decay.

To verify these theoretical predictions, the team built a simulation on an IBM quantum processor. They encoded the quantum states of the neutrinos and the decay process into a small network of qubits, the basic units of quantum information. The simulation was designed to track how the probability of finding a specific neutrino flavor changes over time, accounting for the energy loss and the creation of daughter particles. A key feature of their experiment was the ability to control the "coherence" of the daughter particles. In quantum mechanics, coherence refers to the ability of particles to maintain a specific phase relationship that allows them to interfere. The researchers could tune their simulation to see what happens when this coherence is preserved versus when it is destroyed. They found that when the daughter particles lose their ability to interfere, the new asymmetry signal vanishes, confirming that the effect relies entirely on this quantum connection.

The experiment yielded a specific numerical result for the size of this regenerated asymmetry. Under their benchmark conditions, the decay-induced shift in the asymmetry was measured to be approximately 4.85 times 10 to the power of negative 4. This value represents a tiny but significant deviation from what would be observed if the neutrinos were stable. The quantum computer measurements matched the theoretical predictions within a margin of error of one standard deviation, a strong indication that the simulation accurately captured the physics. The team also ran several "null" tests, which are control experiments designed to produce zero signal if the theory is correct. For instance, they simulated a scenario where the daughter particles could not interfere, and another where the specific mixing factors required for the asymmetry were absent. In both cases, the quantum computer produced results consistent with zero, reinforcing the conclusion that the observed signal is indeed driven by the interplay of decay, coherence, and the specific mixing of neutrino types.

This work demonstrates that quantum computers can serve as powerful tools for simulating complex particle physics processes that involve open systems, where particles interact with an environment or decay. By successfully modeling the regeneration of neutrinos from a decay, the researchers proved that they can isolate and measure subtle quantum effects that are difficult to calculate using traditional methods. The study does not claim to have discovered that neutrinos actually decay in this manner; rather, it establishes a method to test what would happen if they did. It provides a clear roadmap for how future experiments could distinguish between different decay scenarios and how the quantum nature of the daughter particles influences the matter-antimatter balance. As quantum hardware continues to improve, these simulations could become essential for interpreting data from next-generation neutrino detectors, helping scientists to understand the fundamental properties of these elusive particles and the forces that govern them.

Technical Summary: CP Asymmetry and Visible Decay in 3 + 1 Neutrino Oscillations on a Quantum Computer

Problem Statement
The paper investigates how visible neutrino decay modifies the vacuum CP asymmetry in muon-to-electron ( $\nu_\mu \to \nu_e$ ) oscillations within a $3+1$ neutrino scenario (three active flavors and one sterile flavor). While the decay interaction itself is assumed to preserve CP symmetry, the decay of an unstable mass eigenstate ( $\nu_4$ ) alters the interference patterns of surviving states (parent attenuation) and introduces a new signal from regenerated daughter neutrinos at lower energies. The central challenge is to disentangle the contribution of "parent attenuation" from "daughter regeneration" to the total decay-induced shift in CP asymmetry. Specifically, the authors examine the conditions under which daughter regeneration contributes to a CP asymmetry, which requires the simultaneous presence of daughter coherence, a CP-odd mixing factor, and a non-vanishing propagation phase.

Methodology
The authors employ a combination of analytical derivation, classical numerical simulation, and quantum hardware execution:

Theoretical Framework:
- They model the system using open quantum system methods, specifically the Gorini–Kossakowski–Sudarshan–Lindblad (GKSL) formalism.
- The decay channel is defined as $\nu_4 \to \nu_j + J$ (where $J$ is a light scalar), with CP symmetry imposed on the decay dynamics. CP violation arises solely from phases in the neutrino mixing matrix $U$ .
- The evolution is described by a density matrix $\rho$ that includes both the parent energy sector and the daughter energy sector. The "environmental overlap" parameter $\zeta \in [0, 1]$ is introduced to control the distinguishability of daughter mass states; $\zeta=1$ implies indistinguishable states (maximal coherence), while $\zeta=0$ implies distinguishable states (no coherence).
- The CP asymmetry change ( $\Delta A_{CP}$ ) is decomposed into a parent term (attenuation) and a daughter term (regeneration).
Benchmark and Circuit Construction:
- An effective two-energy model is established with a source energy $E_H = 1$ GeV and a daughter energy $E_L = 0.5$ GeV.
- A five-qubit quantum circuit is constructed to simulate the full decay channel (Stinespring dilation), mapping the four-dimensional mass space to combined parent/daughter energy sectors and environment qubits.
- For the hardware experiment, a "compact" two-qubit circuit is derived to prepare the daughter state conditioned on decay. This circuit purifies the daughter density matrix $\rho_D$ using an auxiliary environment qubit, allowing for the measurement of flavor probabilities and coherence observables without simulating the full parent evolution on the quantum processor.
Experimental Execution:
- The compact circuits were executed on the IBM Quantum processor "Kingston."
- Measurements were performed for two values of environmental overlap ( $\zeta = 0$ and $\zeta = 1$ ) and six predefined "null controls" (cases where coherence or the CP-sensitive mixing factor vanishes).
- Measurement errors were mitigated using computational-basis calibration and linear inversion of the measurement matrix. Statistical uncertainties were calculated accounting for correlations between observable and calibration measurements.

Key Results

Classical Validation: The noiseless simulation of the full five-qubit circuit agrees with independent classical evolution (Liouvillian exponentiation) to within numerical precision ( $< 10^{-14}$ ).
Quantum Processor Performance:
- The predicted regenerated CP asymmetry for the benchmark is $4.85 \times 10^{-4}$ (approximately 46% of the total decay-induced shift).
- After correcting for measurement assignment errors, the quantum processor measurements for the flavor-probability estimator ( $W_P$ ) and the coherence-based estimator ( $W_Y$ ) yielded $4.979 \pm 0.125 \times 10^{-4}$ and $4.884 \pm 0.039 \times 10^{-4}$ , respectively. Both agree with the theoretical prediction within one standard error.
- The coherence measurement achieved a statistical uncertainty of 0.80% of the signal magnitude.
Null Controls: Six null control configurations (where $\zeta=0$ , or the pair is $(1,2)$ with real mixing, or $\delta_{13}=0$ ) yielded results consistent with zero within two standard errors, confirming the suppression of the regenerated signal when coherence or CP-sensitive mixing is absent.
Continuous Spectrum: Numerical integration over a continuous daughter energy spectrum (0.05–1.0 GeV) confirmed the energy-dependent structure of the regenerated asymmetry, showing a total integrated value of $3.753 \times 10^{-4}$ , distinct from the two-energy benchmark due to phase averaging.

Significance and Claims
The paper claims to demonstrate the feasibility of using quantum processors to simulate open quantum system dynamics relevant to neutrino physics, specifically the interplay between decay, coherence, and CP violation.

Validation of Decomposition: The experiment successfully isolates and measures the "daughter regeneration" contribution to CP asymmetry, distinguishing it from parent attenuation.
Coherence Control: The results verify that the regenerated CP signal is linearly dependent on the environmental overlap $\zeta$ and vanishes when the necessary quantum coherence or CP-odd mixing factors are removed.
Scalability: While the benchmark itself is classically tractable, the authors posit that the constructed decay-channel formalism provides a necessary starting point for simulating interacting many-neutrino systems where classical evolution becomes computationally prohibitive.
Modesty: The authors explicitly state that the benchmark is chosen to be classically solvable to serve as a reference for the processor's accuracy. They do not claim to have discovered new physics or to have solved a problem intractable for classical computers in this specific instance, but rather to have validated the quantum simulation methodology for future, more complex applications involving visible decay and many-body correlations.

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