⚛️ quantum physics

Accessing which-path information in the absorption and emission of light by a quantum dot in a Ramsey sequence

This paper experimentally demonstrates how which-path information extracted from a quantum dot's absorption during a Ramsey sequence progressively degrades interference contrast and subsequently governs the emission of coherent light, quantitatively illustrating the fundamental link between quantum correlations and light-matter energy exchanges.

Original authors: I. Maillette de Buy Wenniger, M. Maffei, S. C. Wein, S. P. Prasad, H. Lam, D. Fioretto, A. Lemaître, I. Sagnes, C. Antón-Solanas, P. Senellart, A. Auffèves

Published 2026-03-16

📖 5 min read🧠 Deep dive

CC BY 4.0

Original authors: I. Maillette de Buy Wenniger, M. Maffei, S. C. Wein, S. P. Prasad, H. Lam, D. Fioretto, A. Lemaître, I. Sagnes, C. Antón-Solanas, P. Senellart, A. Auffèves

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 are trying to listen to a secret conversation between two friends, Alice and Bob, who are playing a game of "hide and seek" with a single photon of light. This is the story of a quantum dot (a tiny, artificial atom) and how scientists tried to figure out exactly when and how much of the secret was revealed, even before the game was over.

Here is the story of the paper, broken down into simple concepts.

The Setup: The Quantum "Ping-Pong" Game

Think of the Quantum Dot (QD) as a tiny, super-fast ping-pong ball that can be in two states at once: "Up" (excited) or "Down" (ground).

The scientists use two laser pulses to play a game with this ball, similar to a Ramsey Interferometer. You can think of this as a cosmic version of a "double-slit" experiment, but instead of a ball going through two holes, the ball is taking two different paths through time.

The First Pulse (The Split): The first laser pulse hits the dot. It puts the dot into a "superposition"—a magical state where it is both Up and Down at the same time.
The Wait (The Secret): The scientists wait for a tiny moment. During this time, the dot might spontaneously let go of a photon (a particle of light).
- If the dot was "Down," it stays quiet.
- If the dot was "Up," it might drop a photon.
- The Catch: Because the dot was in a mix of Up and Down, the light it emits is also a mix of "nothing" and "one photon."
The Second Pulse (The Reveal): A second laser pulse hits the dot later. This pulse tries to "erase" the history and make the dot interfere with itself again, creating a pattern of light and dark (fringes).

The Problem: The "Spy" in the Room

In quantum physics, there is a golden rule: If you know the path, you lose the magic.

If you can tell which path the particle took, the interference pattern (the magic) disappears. This is called Which-Path (WP) Information.

In this experiment, the first burst of light (emitted during the "Wait" phase) acts like a spy.

If the dot emits a photon, the spy says, "I saw it was Up!"
If the dot stays silent, the spy says, "I didn't see anything, so it might be Down."

Even though the scientists aren't looking at the spy, the existence of the spy changes the game. The more the spy knows, the less "quantum magic" (interference) remains.

The Experiment: Two Ways to Check the Spy

The researchers wanted to measure exactly how much information the spy had at two different moments. They used two clever tricks:

1. The "Absorption Test" (Before the Second Pulse)

Imagine the second laser pulse is a door. The dot tries to walk through it.

If the dot has no memory of the first pulse (the spy knows nothing), it walks through the door perfectly, absorbing or emitting light in a predictable, wavy pattern.
If the spy is watching (the first light bin has information), the dot gets nervous. It can't walk through the door as smoothly. The "wave" pattern gets fuzzy.

The Result: By measuring how much light the dot absorbed, the scientists could calculate exactly how much "spy information" was sitting in the first time bin. The longer they waited, the more the spy knew, and the fuzzier the pattern became.

2. The "Self-Homodyne Test" (After the Second Pulse)

This is the really cool part. After the second pulse, the dot emits a second burst of light.

Normally, this second burst should be a perfect wave.
But because the first burst (the spy) is still holding onto secrets about the dot's state, it "taints" the second burst.
The scientists used a special mirror setup (a Mach-Zehnder interferometer) to make the second burst of light interfere with itself.

The Analogy: Imagine you are trying to sing a perfect note (the second light). But your friend (the first light) is whispering a secret to you that changes your pitch. Even though you are singing the note now, the whisper from before ruins the harmony.

By measuring how "out of tune" the second burst was, they could figure out exactly how much information the first burst was still holding.

The Big Discovery

The team found a perfect mathematical link between the two measurements:

How much the first light "spied" (reduced the absorption of the second pulse).
How much the second light "suffered" (reduced the interference pattern).

They proved that information is a physical thing. The moment the first light bin "learned" something about the dot's path, it physically altered the energy exchange of the dot for the rest of the experiment.

Why This Matters

This isn't just about tiny dots; it's about the fundamental rules of the universe.

Complementarity: You can't have perfect waves (interference) and perfect particles (knowing the path) at the same time. This experiment showed exactly how that trade-off happens in real-time.
Quantum Energy: It shows that "knowing" something (information) actually costs or changes energy. It's like a quantum version of the saying, "Knowledge is power," but here, knowledge actually changes the physics of the system.

In a Nutshell

The scientists played a game with a tiny atom and two flashes of light. They discovered that the first flash of light acted like a spy that peeked at the atom's secrets. Even though they didn't look at the spy, the mere fact that the spy had the information ruined the atom's ability to perform a magic trick later. They measured exactly how much the spy knew and proved that information and energy are deeply linked in the quantum world.

1. Problem Statement

The paper addresses the fundamental quantum mechanical concept of which-path (WP) information and its relationship to wave-particle duality and quantum coherence.

The Core Conflict: In interferometry, the presence of information regarding the path taken by a quantum system (distinguishability) reduces the visibility of interference fringes (coherence). This is the principle of complementarity.
The Gap: While WP information has been studied in optical interferometers (using atoms as slits) and atomic interferometry (Ramsey fringes), there is a lack of quantitative understanding of how WP information is dynamically generated, stored, and erased during the absorption and emission cycles of a single quantum emitter (a quantum dot) in a time-domain Ramsey sequence.
Specific Challenge: The authors aim to quantify how the spontaneous emission of a photon (acting as a "meter") correlates with the quantum dot's (QD) energy state, thereby extracting WP information, and how a subsequent control pulse modifies this information and the system's ability to absorb/emit light coherently.

2. Methodology

The researchers utilized a self-assembled semiconductor quantum dot (QD) acting as an effective two-level atom (ground state $|g\rangle$ and excited state $|e\rangle$ ) embedded in a cryostat at 5 K.

Experimental Setup:

Ramsey Sequence: The QD is driven by two classical $\pi/2$ -pulses with a tunable relative phase ( $\phi_R$ ) and a variable time delay ( $\Delta t$ ).
Time Bins: The delay $\Delta t$ $Δ t$ is long enough to allow for spontaneous emission. This creates two distinct time bins:
1. First Time Bin ( $\tau_1$ ): Occurs between the two pulses. The QD spontaneously decays, emitting a coherent superposition of vacuum ( $|0\rangle$ ) and one-photon ( $|1\rangle$ ) Fock states. This emitted field acts as a quantum meter correlated with the QD's energy state.
2. Second Time Bin ( $\tau_2$ ): Occurs after the second pulse.
Measurement 1: Absorption Contrast (Before the 2nd Pulse):
- The team measured the energy absorbed by the QD during the second pulse as a function of $\phi_R$ .
- The reduction in the Ramsey fringe contrast (visibility) directly quantifies the WP information extracted by the first time bin ( $1 - w^-$ , where $w^-$ is the overlap of the meter states).
Measurement 2: Self-Homodyne Interference (After the 2nd Pulse):
- To measure the WP information remaining in the first time bin after the second pulse, they used a path-unbalanced Mach-Zehnder Interferometer (MZI).
- The MZI overlaps the photonic field emitted in the first time bin of one sequence with the field emitted in the first time bin of the next sequence (delayed by the repetition period).
- The visibility of the resulting interference fringes ( $v_2$ ) in the second time bin is sensitive to the WP information encoded in the first bin.

Theoretical Framework:

The system is modeled using a joint QD-light field state.
The WP information is quantified by the distinguishability of the photonic states correlated with $|g\rangle$ and $|e\rangle$ .
Key equations relate the QD dipole moment ( $\sigma_-$ ) to the overlap of meter states ( $w$ ), linking absorption/emission power to the degree of path distinguishability.

3. Key Contributions

Quantitative Mapping of WP Information: The paper provides a rigorous, quantitative link between the spontaneous emission of a single photon and the loss of coherence in a Ramsey interferometer. It demonstrates that the first time bin acts as a "which-path detector" for the QD.
Dynamic Control of WP Information: The study shows that WP information is not static; it evolves over time.
- It increases as the QD decays (emitting a photon that reveals its state).
- It can be partially erased or modified by a second coherent pulse, depending on the phase $\phi_R$ and the initial population.
Unified Absorption/Emission View: The authors derive and experimentally verify a simple analytical expression (Eq. 7 in the paper) connecting the WP information before the second pulse ( $w^-$ ) to the WP information after the second pulse ( $w^+$ ). This bridges the gap between absorption (driven by the second pulse) and emission (spontaneous decay).
Experimental Verification of Quantum Energetics: The work serves as a textbook example for the emerging field of quantum energetics, demonstrating how quantum correlations (entanglement between the QD and the field) directly impact energy exchange flows.

4. Results

Fringe Contrast Reduction: As the delay $\Delta t$ increases, the QD has more time to decay, increasing the distinguishability of the paths. This leads to a measurable reduction in the Ramsey fringe contrast ( $\sigma_-$ ), matching theoretical predictions for a two-level system with spontaneous decay.
Extraction of WP Information: The extracted WP information ( $1 - w^-$ ) increases from 0 (at $\Delta t = 0$ ) to a maximum of $1 - 1/\sqrt{2}$ (for large $\Delta t$ ). The data confirms that the first time bin cannot carry complete WP information because the vacuum state can arise from either a ground-state QD or an excited QD that hasn't decayed yet.
Post-Pulse Modification:
- For specific delays (e.g., $\gamma \Delta t \approx 1.42$ ), the second pulse can erase the WP information ( $w^+ \to 1$ ), restoring indistinguishability.
- For other delays (e.g., $\gamma \Delta t \approx 0.33$ ), the second pulse can enhance the distinguishability ( $w^+ < w^-$ ), effectively extracting more information.
Self-Homodyne Visibility: The time-resolved visibility in the MZI showed two plateaus. The ratio of the second plateau ( $v_2$ ) to the first ( $v_1$ ) allowed the direct extraction of $w^+$ . The experimental data for $w^+$ as a function of $\phi_R$ showed remarkable agreement with the theoretical predictions derived from Eq. 7.
Spin Decoherence: The authors identified that spin-induced decoherence (due to the Overhauser field) slightly reduced the overall visibility but did not alter the fundamental WP dynamics, allowing them to isolate the pure radiative effects.

5. Significance

Fundamental Physics: The study offers a deep insight into the quantum measurement problem and the trade-off between information and coherence in a controlled, single-photon regime. It validates the theoretical framework of "quantum erasers" in a time-domain setting.
Quantum Technologies: The ability to quantify and control WP information is crucial for quantum computing and communication, particularly for protocols involving quantum memories, quantum repeaters, and error correction where path distinguishability must be managed.
Quantum Thermodynamics: By linking WP information to energy absorption and emission, the paper lays the groundwork for defining an energy-based entanglement witness. This is significant for understanding how quantum features drive energy flows in nanoscale systems, a key area in quantum thermodynamics.
Material Quality: The high degree of agreement between experiment and theory (accounting for spectral diffusion and spin decoherence) demonstrates the high quality and coherence of the semiconductor quantum dots used, reinforcing their viability as robust quantum emitters.

In summary, this paper successfully demonstrates that the spontaneous emission of a quantum dot acts as a real-time which-path detector, and that this information can be dynamically manipulated by subsequent coherent control pulses, with the effects precisely quantifiable through both absorption and emission interference measurements.