Generation of quantum light with tunable Wigner negativity with a semiconductor quantum dot

AI summary

Overview

Research area: Continuous-variable (CV) quantum optics and semiconductor quantum light sources — specifically, generating non-Gaussian, Wigner-negative states of light from a single quantum dot in a micropillar cavity.

Technical level: Advanced. The paper assumes familiarity with phase-space representations (Wigner functions), homodyne/weak-local-oscillator tomography, and cavity quantum electrodynamics. The summary below translates the core ideas, but the underlying methods are specialist material.

Scope (one sentence): The authors use coherent control of a single InGaAs quantum dot to generate vacuum–one-photon superposition states with tunable single-photon population, reconstruct their Wigner functions point-by-point via homodyne tomography, and quantify the resulting negativity, purity, fidelity and squeezing after correcting for optical losses up to the emitter.

What This Paper Is About

Wigner-negative light is a required resource for reaching quantum advantage in continuous-variable optical quantum computing, but producing it deterministically (rather than probabilistically via frequency conversion and photon detection) requires a strong single-photon nonlinearity. A single quantum dot in a cavity provides exactly that nonlinearity: a short resonant laser pulse prepares a superposition of the dot's ground and excited states, and spontaneous emission maps that coherence onto the emitted light field as a superposition of zero and one photon. The goal of this work is to generate these states with a tunable amount of Wigner negativity, and to characterize them rigorously in phase space to show how close they come to the ideal states once propagation and collection losses are accounted for.

Key Contributions

1. Deterministic generation of tunable Wigner-negative states from a semiconductor device. Using a neutrally charged InGaAs quantum dot in a near-non-birefringent micropillar cavity at 5.5 K, the authors produce states of the form √(1−p)|0⟩ + √p·exp(iφ)|1⟩, with the single-photon population p = sin²(θ/2) set by the pulse area θ of 10 ps resonant pulses at an 80 MHz repetition rate.

2. Point-by-point Wigner tomography using a weak local oscillator and zero-photon counting. The state interferes with a spectrally shaped weak LO on a fiber beam splitter; the displacement is calibrated, the phase is extracted from oscillations in the count rate of one SNSPD, and the photon-number statistics of the displaced state are reconstructed from zero-photon probabilities measured on a second SNSPD behind a calibrated variable attenuator.

3. Loss-corrected reconstruction with high purity and fidelity across the full tuning range. Six wavepackets at different pulse areas were reconstructed in a single realignment-free run using 11 LO strengths, with reconstructed fidelities to the ideal target states and quantum purities both above 95% after correcting for optical losses up to the quantum dot.

4. Quantification of negativity and squeezing approaching ideal limits. The paper reports the minimum of the Wigner function, the Wigner-negative volume, and the quadrature squeezing, and compares each to the theoretical optimum for an ideal vacuum–one-photon qubit.

Main Findings

• Source quality at θ = π: single-photon purity 1−g⁽²⁾(0) = 97.9(4)%, indistinguishability M_s = 95.9(2)%, and photon-number coherence C₀ = 96.1(8)%. Collection probability per excitation pulse is B = 37(2)% at the collection lens and B_F = 13(1)% at the fiber output.

• Wigner negativity at π-excitation: min(W(α)) = −0.579⁺⁰·¹²²₋₀.₀₀₉ and Wigner-negative volume N_W = 0.385⁺⁰·⁰⁰¹₋₀.₀₄₃, compared with ideal single-photon limits of −0.637 and 0.426 respectively.

• Photon-number composition at zero displacement: 95.1%⁺³·²%₋₁.₇% single photon, around 3% vacuum, and 1% two-photon population; the two-photon component arises from reexcitation dynamics of the two-level system under finite pulse excitation and produces a rotational asymmetry in phase space.

• Continuous transition from Gaussian to non-Gaussian: as p increases, the reconstructed Wigner functions evolve from the Gaussian vacuum state (N_W = 0 at p = 0) to a strongly non-Gaussian single-photon-like state, with the negativity increasing monotonically with p.

• Near-optimal squeezing: at p = 0.25 the emission shows maximal quadrature squeezing of s_dB = 1.244⁺⁰·⁰⁴³₋₀.₀₀₃ dB, or 24.8%⁺⁰·²%₋₁.₀% below the shot-noise level, close to the 25% optimum for an ideal vacuum–single-photon qubit.

• Comparison with earlier work: the authors note that the pioneering work of Schulte et al. achieved about 40% of the ideal squeezing value, which they attribute to stronger decoherence for quantum dots in bulk.

• Displacement fidelity: the signal–LO overlap is M = 68 ± 1%, with a measured temporal overlap of at most 84 ± 2%, limited by the complex temporal wavepacket profile of the dot emission with imprinted quantum beats between its two excitonic states. The measured interference visibility agrees with expectations for M = 0.68(1) across all p, indicating the state occupies an identical mode for every p.

Methodology in Plain English

The team takes a single artificial atom — a semiconductor quantum dot — and places it inside a tiny optical cavity that makes it emit light efficiently and with low noise. They hit it with a short laser pulse whose area sets how much of the time the dot ends up in its excited state. When the dot then relaxes, it emits a light pulse that is a quantum superposition of containing zero photons and one photon, with the balance controlled by the pulse area.

To see the quantum nature of that light, they interfere it with a deliberately weak, carefully shaped reference laser (the local oscillator) on a fiber beam splitter. Changing the reference's amplitude and phase shifts the state in phase space; measuring how often zero photons arrive at a detector after a calibrated attenuator then lets them recover the photon-number statistics of the shifted state. By repeating this at many phase-space coordinates and many attenuator settings, they rebuild the full Wigner function.

A practical complication is that photons are lost between the dot and the detectors, which blurs the reconstruction. The authors therefore run an iterative maximum-likelihood reconstruction that corrects for losses up to the solid-state device, so the reported Wigner functions describe what the dot actually emitted rather than what survived the optics.

Why This Matters

Impact on research. The work shows that a mature semiconductor platform — quantum dots in cavities, already used for single-photon and entangled-photon sources at clock rates up to GHz — can deterministically produce non-Gaussian states with tunable negativity and near-ideal purity and squeezing after loss correction. This links the discrete-variable quantum-dot community (where the first optical quantum computing prototypes have been built) with continuous-variable approaches that require Wigner-negative resources for quantum advantage.

Potential applications (as framed by the paper):

• Continuous-variable optical quantum computing, where Wigner-negative states are identified as a key resource for quantum advantage.
• Generation of more complex non-Gaussian states, such as cat states, following demonstrations with trapped atoms but here using semiconductor integrated quantum-dot–cavity devices.
• Deterministic single-photon nonlinearity for universal quantum operations, as an alternative to probabilistic measurement-induced interactions that carry substantial resource overhead.
• High-rate operation, with the paper pointing to quantum-dot devices operating at up to GHz rates.

Industry relevance. The devices come from a semiconductor growth and fabrication line (the work was conducted within the QDLight joint laboratory between C2N and Quandela), and one author is affiliated with Quandela. The paper's own outlook is explicitly engineering-oriented: it identifies polarization-selective, high-quality-factor cavities as the route to raising device efficiency and obtaining Wigner negativity at the device output rather than only after loss correction.

Future Directions

• Moving negativity to the device output. The present demonstration relies on loss correction up to the emitter; the paper notes that high photon-number coherence requires a neutral quantum dot with two linear optical dipoles, which limits source efficiency to 50% in an unpolarized cavity. Polarization-selective, high-quality-factor cavities that reduce laser-rejection losses are proposed as the fix.

• Generating more complex states. Extending the scheme to cat states and other non-Gaussian states beyond the vacuum–one-photon qubit.

• Pushing to higher rates. Leveraging quantum-dot devices operating at up to GHz repetition rates, in contrast to the 80 MHz used here, for scalable continuous-variable architectures.

• Suppressing the residual two-photon component. The two-photon population (1% at θ = π) from reexcitation dynamics is identified as the main factor degrading squeezing below the 25% ideal and modifying the phase-space symmetry; reducing it would tighten agreement with ideal states.

Target Audience

Researchers and graduate students in quantum optics, quantum information and photonics who work on non-Gaussian state generation, continuous-variable quantum computing, or semiconductor quantum light sources. It is also relevant to device engineers interested in the practical limits of quantum-dot–cavity sources, and to readers who already know the earlier Wigner-function measurement on a quantum-dot single photon and want to see the extension to tunable superpositions with quantified negativity, purity and squeezing.

Authors’ abstract

Wigner-negative states of light are key resources for quantum advantage in continuous-variable optical quantum computing. They can be produced probabilistically via frequency conversion and photon detection, or deterministically by exploiting the single-photon nonlinearity of a quantum emitter. Here, we generate such Wigner-negative states of light with a tunable negativity using a single semiconductor quantum dot in a cavity. Optical coherent control of the quantum-dot transition allows to generate a quantum superposition of zero and one photon. The generated quantum states are characterized point-by-point in the phase space through homodyne measurement. The emergence of Wigner-function negativity is observed as the excitation power is increased. After correcting for collection and propagation losses, we obtain fidelities to the ideal state and quantum purities above 95%, together with near-optimal quadrature squeezing. The ability to generate a wide variety of Wigner-negative states with mature semiconductor technology opens promising opportunities for continuous-variable optical quantum technologies.