🔬 mesoscale physics

Temporal magnon-qubit Mach-Zehnder interferometer

This paper proposes a temporal magnon-qubit Mach-Zehnder interferometer that utilizes a pulsed magnetic field as a beam splitter to entangle microwave qubits and magnonic states, enabling the independent measurement of single magnon decoherence rates to advance fundamental quantum studies and applications.

Original authors: Cody Trevillian, Steven Louis, Vasyl Tyberkevych

Published 2026-02-24

📖 5 min read🧠 Deep dive

CC BY 4.0

Original authors: Cody Trevillian, Steven Louis, Vasyl Tyberkevych

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

Imagine you have two tiny, invisible dancers in a microscopic ballroom. One is a Qubit (a quantum bit, the basic unit of quantum computing), and the other is a Magnon (a ripple of magnetism, like a tiny wave in a magnetic field).

The paper you shared proposes a clever new way to watch these two dancers interact, not by watching them move across a room, but by watching them interact over time. The authors call this a "Temporal Magnon-Qubit Mach-Zehnder Interferometer."

That sounds like a mouthful, so let's break it down using a simple story.

The Story: The Quantum Dance-Off

1. The Setup: Two Dancers, One Stage

Usually, to see interference (like when two waves of water crash into each other to make a pattern), you split a beam of light into two paths, send them around different obstacles, and then smash them back together.

In this new experiment, the "paths" aren't physical distances. Instead, the "paths" are time.

The Qubit is like a skilled dancer who can hold a pose (an excited state).
The Magnon is a partner who is usually asleep (in the ground state).

2. The "Beam Splitter": A Magnetic Pulse

In a normal light experiment, a "beam splitter" is a piece of glass that splits a light beam in half.

In this experiment, the "beam splitter" is a quick pulse of a magnetic field.

The Trick: The scientists turn on this magnetic pulse for a split second. This pulse acts like a "handshake" between the Qubit and the Magnon.
The Result: The Qubit shares its energy with the Magnon. They become entangled. Think of this as the two dancers linking arms and spinning together. Now, they are no longer two separate people; they are a single, linked unit.

3. The "Free Evolution": The Solo Dance

After the magnetic pulse stops, the dancers are separated again. The magnetic field is turned off (or changed) so they can't talk to each other for a while.

During this quiet time, the Magnon is left to dance alone.
The Problem: In the real world, the Magnon is fragile. It might bump into a stray atom (decoherence) or lose its rhythm (dephasing). It's like a dancer getting tired or distracted by the crowd.
The Goal: The scientists want to know exactly how the Magnon gets tired. Does it lose energy? Or does it just lose its timing?

4. The "Recombination": The Second Pulse

After a specific amount of time, the scientists hit the dancers with a second magnetic pulse.

This pulse is the reverse of the first one. It tries to "un-link" the dancers.
If the Magnon danced perfectly and kept its rhythm, the un-linking works smoothly, and the Qubit ends up in a specific state.
If the Magnon got distracted or lost energy during the "solo" time, the un-linking is messy. The Qubit ends up in a different state.

5. The Result: Reading the Pattern

Finally, the scientists check the Qubit: "Are you excited or not?"

By repeating this experiment many times and changing how long the Magnon danced alone, they see a pattern (like the stripes on a zebra).
This pattern tells them the secret history of the Magnon's dance.

Why is this a Big Deal?

1. It's a "Time Machine" for Quantum Physics
Usually, to study quantum particles, you need to build huge, complex machines to separate them in space. This paper says, "Why build a big room? Just use time!" It's like watching a movie in slow motion to see the details of a fight scene, rather than trying to freeze the actors in place.

2. It Solves a Mystery: "How do particles die?"
Quantum particles are notoriously fragile. They "decohere" (lose their quantum magic) very fast. Scientists have two main theories on how this happens:

Amplitude Noise: The particle loses energy (like a dancer falling down).
Phase Noise: The particle keeps its energy but gets out of sync (like a dancer spinning the wrong way).

The cool thing about this new "Time Interferometer" is that it can tell these two problems apart.

If the pattern gets fainter but stays centered, it's Phase Noise (timing issues).
If the pattern shrinks and disappears, it's Amplitude Noise (energy loss).

The Takeaway

This paper proposes a new tool to study the tiniest building blocks of magnetism. By using magnetic pulses as "switches" to link and unlink a quantum bit and a magnetic wave, they created a way to measure exactly how and why these tiny waves lose their quantum magic.

It's like having a high-speed camera that can take a picture of a hummingbird's wingbeat, but instead of a camera, they use magnetic fields and time to capture the invisible dance of the quantum world. This could help us build better quantum computers by teaching us how to keep these fragile particles stable for longer.

1. Problem Statement

The paper addresses the challenge of studying single-magnon decoherence in hybrid quantum systems. While Mach-Zehnder Interferometers (MZIs) are well-established tools for probing quantum phenomena using photons, electrons, and phonons, applying them to magnons (quanta of spin waves) presents unique difficulties.

Limitations of Existing Approaches: Traditional spatial MZIs for magnons rely on physically separated paths, which can be difficult to implement and control at the single-quantum level.
The Specific Gap: There is a need for a method to independently characterize different decoherence channels (specifically amplitude damping and phase dephasing) for single magnons. Understanding these mechanisms is critical for developing practical quantum technologies that utilize finite magnon number states.

2. Methodology

The authors propose a temporal-domain MZI scheme that operates in the quantum regime by entangling a microwave qubit and a single magnon mode. Instead of using spatial beam splitters, the system uses time-dependent magnetic field pulses to act as "temporal beam splitters" (TBS).

System Architecture

Hybrid System: A coupled system consisting of a single magnon mode ( $|m\rangle$ ) with frequency $\omega_m$ and a qubit ( $|q\rangle$ ) with frequency $\omega_q$ .
Coupling Mechanism: The two subsystems are coupled with a rate $g$ . The interaction is controlled by dynamically tuning the frequency detuning $\Delta\omega = \omega_m - \omega_q$ via an external pulsed magnetic field $B(t)$ .
Operational Regimes:
- Resonant ( $\Delta\omega \approx 0$ ): Strong coupling regime where coherent exchange of quantum information occurs (entanglement).
- Detuned ( $\Delta\omega \gg g$ ): Weak coupling regime where subsystems evolve independently.

The Interferometric Protocol

The protocol mimics a standard MZI but in the time domain:

Initialization: The system starts in the unentangled state $|1\rangle_q \otimes |0\rangle_m$ (qubit excited, magnon ground).
First Beam Splitter (TBS1): A magnetic pulse brings the magnon and qubit into resonance for a duration $\tau$ . This creates an entangled superposition state: $\alpha|1,0\rangle + \beta|0,1\rangle$ . A "balanced" TBS achieves $|\alpha| = |\beta| = 1/\sqrt{2}$ .
Free Evolution: The system is detuned by a large frequency gap $\Omega$ for a time $T$ . During this period, the subsystems evolve independently, but the global state remains entangled. This is where decoherence (damping or dephasing) occurs.
Second Beam Splitter (TBS2): A second pulse applies the inverse operation to recombine the amplitudes, effectively "unentangling" the system.
Measurement: The final probability of the qubit being in the excited state ( $P_q$ ) is measured. The resulting interference pattern encodes information about the magnon dynamics during time $T$ .

Simulation Framework

Noise-Free: Modeled using the Schrödinger equation with a time-dependent Hamiltonian $\hat{H}(t)$ .
Noisy: Modeled using the Gorini–Kossakowski–Sudarshan–Lindblad (GKSL) master equation to simulate two specific decoherence channels:
- Amplitude Noise ( $L_{amplitude} = \hat{m}$ ): Represents energy loss (e.g., magnon scattering into a phonon).
- Phase Noise ( $L_{phase} = \hat{m}^\dagger \hat{m}$ ): Represents pure dephasing without energy loss.

3. Key Contributions

Temporal MZI Concept: Introduction of a novel interferometric scheme where "beam splitting" is achieved via temporal control of coupling rather than spatial path separation.
Decoupling Condition: Identification of the critical condition for free evolution. The authors demonstrate that a frequency gap of $\Omega \gtrsim 5g$ is sufficient to suppress residual off-resonant coupling, ensuring that the evolution is effectively "free" and the interference visibility remains high.
Independent Decoherence Characterization: The scheme allows for the independent extraction of the amplitude damping rate ( $\Gamma_{amplitude}$ ) and the phase dephasing rate ( $\Gamma_{phase}$ ) from a single set of interference measurements.
Robustness to Pulse Shape: Numerical simulations confirm that the ideal rectangular pulse assumption holds even with realistic trapezoidal pulses (finite rise/fall times), with negligible impact on the required pulse duration.

4. Results

Interference Visibility: The simulation shows clear interference fringes in the final qubit population ( $P_q$ ) as a function of free evolution time $T$ or frequency gap $\Omega$ .
Distinct Decoherence Signatures:
- Amplitude Noise: Causes the average population ( $P_{avg}$ ) of the interference pattern to decay exponentially with time $T$ , while the visibility ( $\nu$ ) remains relatively high.
- Phase Noise: Keeps the average population constant (at 50%) but causes the visibility ( $\nu$ ) to decay exponentially with time $T$ .
- Mixed Noise: Results in a decay of both average population and visibility.
Analytical Formulas: The authors derived explicit formulas to calculate the decoherence rates directly from the measured extrema ( $P_{max}$ $P_{ma x}$ and $P_{min}$ $P_{min}$ ) of the interference pattern:
- $\Gamma_{amplitude} \propto \ln(1 / (P_{max} + P_{min}))$
- $\Gamma_{phase} \propto \ln((P_{max} + P_{min}) / (P_{max} - P_{min})^2)$
Optimal Measurement Mode: The paper suggests operating in a fixed-evolution-time ( $T$ ) mode while varying the frequency gap ( $\Omega$ ). This simplifies data analysis because the decoherence envelope becomes independent of $\Omega$ , affecting only the fringe periodicity.

5. Significance

Fundamental Physics: This work provides a powerful tool to investigate quasi-particle decoherence at the single-quantum level, a regime that is difficult to access with current techniques.
Quantum Technology: By enabling the independent measurement of damping and dephasing, this scheme aids in the development of robust hybrid quantum systems (combining magnons with superconducting qubits). This is essential for creating magnon-based quantum memories and transducers.
Versatility: The temporal nature of the scheme makes it adaptable to various physical implementations and potentially applicable to other hybrid quantum systems beyond magnon-qubit pairs.

In summary, the paper proposes a theoretically robust and experimentally feasible method to use a microwave qubit as a probe for single-magnon quantum dynamics, offering a precise way to disentangle and quantify the specific mechanisms causing decoherence in magnonic systems.