Researchers at the University of Sheffield and University of Basel have, for the first time, demonstrated quantum interference using up to four photons emitted from a single quantum dot. The team overcame longstanding efficiency limitations by combining a quantum dot with a technique called deterministic demultiplexing, directly observing quantum interference fringes. Measurements reveal high interference contrasts of 93.0 ± 0.1% for two photons and 84.1 ± 1.0% for four, with the results predicting similar interference patterns will extend to even larger numbers of photons and potentially enhance quantum metrology.

Quantum Dot Source Enables Four-Photon Interference

Deterministic demultiplexing, paired with a quantum dot, enabled the direct observation of quantum interference fringes originating from up to four photons, a feat previously limited by efficiency constraints in scaling quantum dot-based interference. The team’s approach allows for the manipulation of larger photonic states, essential for advancements beyond standard single-photon applications.

The work demonstrates interference contrasts of 84. 1 ± 1. 0% for four photons, a result fully supported by a theoretical model and indicating a significant leap from the typical two-photon Hong-Ou-Mandel interference used to benchmark single-photon sources. This high contrast is made possible by the discovery of “deep fringes” unaffected by distinguishable photons, a phenomenon that renders the maximum contrast of four-photon interference highly sensitive to multi-photon emission yet robust against imperfections in photon identification.

Aligned with this, the researchers performed single-, two- and four-photon interference experiments to characterize the behavior of multi-photon interference from their source, beginning with single-photon tests by blocking input to leave photons incident on one port. The observed phase sensitivity of the interference fringes exceeds the standard quantum limit, as demonstrated by a Fisher information analysis, suggesting potential applications in quantum metrology. The team, comprised of scientists from the University of Sheffield, University of Basel and Ruhr-Universität Bochum, predicts that these phenomena will extend to the interference of even larger numbers of photons.

The researchers believe their results illustrate the vast potential of quantum dot photon sources for both multi-photon interference and optical quantum metrology experiments, with a clear path to increasing contrast, brightness and photon number. The work represents a step toward realizing more complex quantum systems and exploring their potential for advanced applications.

Deterministic Demultiplexing Boosts Multi-Photon Rates

This achievement bypasses longstanding efficiency limitations that previously restricted quantum dot-based interference experiments to a maximum of two photons at a beamsplitter. The architecture employed combines a quantum dot within an open microcavity with the deterministic demultiplexing process and standard linear optics, creating an efficient source of higher-order photon number states. A comprehensive theoretical model accurately reproduces these results, confirming that the observed harmonic structure arises from the complex interplay between multi-photon emission, photon distinguishability and system losses.

This model also reveals a surprising robustness of the maximum four-photon interference contrast against photon distinguishability, a characteristic not observed in two-photon systems. The system’s capabilities suggest potential for optimized metrology schemes, using the coherent interaction between the laser and the quantum dot transition.

High-Contrast Interference Measured for Two and Four Photons

The ability to directly observe quantum interference from up to four photons represents an advance in photonic quantum technologies, overcoming limitations that previously restricted such demonstrations to two-photon interference. This work details a method for generating and manipulating these higher-order photon states using a single quantum dot, coupled with a technique called deterministic demultiplexing.

This demultiplexing process efficiently splits photons into distinct spatial modes, an important step in enabling interference beyond the two-photon level. The observed interference patterns exhibit a harmonic structure not previously seen in these types of experiments, revealing a non-trivial relationship between photon indistinguishability and the resulting quantum interference.

Beyond simply demonstrating interference, the research shows that interference fringes from the source can exhibit phase sensitivity beyond the standard quantum limit. The system achieves maximum observed count rates of 2.08 MHz and minimums of 530 Hz. The authors write, “We have combined an efficient, on-demand QD SPS with active demultiplexing and HOM interference to experimentally realise direct quantum interference of up to four photons,” highlighting the integration of multiple advanced techniques to achieve this result. The work demonstrates a non-trivial relationship between indistinguishability and quantum interference beyond the two-photon case.

“Deep Fringes” Reveal Robustness to Photon Distinguishability

Researchers found that these deep fringes maintain clear minima even when photons are not perfectly indistinguishable, a departure from the behavior of two-photon interference where indistinguishability is paramount. This enhanced sensitivity arises from the specific structure of the interference fringes, which allows for more precise measurements of phase shifts. The team’s work shows that the alternating depth of these fringes selectively impacts every other interference minimum when indistinguishability is reduced.

The study’s theoretical model accurately predicts the observed behavior, confirming that the alternating deep and shallow fringe structure is a fundamental property of multi-photon interference, not an artifact. The researchers write, highlighting the shift in governing factors as photon number increases.

This suggests a new avenue for investigation into the relationship between multi-photon interference and photon indistinguishability, potentially unlocking new strategies for optimizing quantum systems. The analysis also indicates that, for realistic source parameters, performance is primarily limited by photon indistinguishability in two-photon interference, a constraint that appears less severe in the four-photon regime. These results reveal the existence of “deep fringes” whose minima are unaffected by distinguishable photons.

Theoretical Model Reproduces Four-Photon Fringe Structure

A theoretical model accurately reproduces complex interference patterns observed when up to four photons interact, revealing a harmonic structure previously obscured in experiments using spontaneous parametric down-conversion sources. This detailed modeling confirms that the observed interference fringes exhibit a robustness against photon distinguishability not seen in two-photon interference. Instead, the four-photon system’s contrast is more strongly influenced by the emission of multiple photons, as detailed in the study’s comprehensive theoretical framework.

The researchers measure high mean interference contrasts of 93.0 ± 0.1~% for two photons, and 84.1 ± 1.0~% for four photons, with the complex fringe structure fully reproduced by a theoretical model. These results reveal the existence of “deep fringes” whose minima are unaffected by distinguishable photons, rendering the maximum contrast of four-photon interference highly sensitive to multi-photon emission but robust against photon distinguishability.

We predict that these phenomena will extend to interference of larger numbers of photons, with relevance across a range of potential optical quantum technologies. A Fisher information analysis demonstrates that interference fringes from our source can exhibit phase sensitivity beyond the standard quantum limit, illustrating potential applications in quantum metrology.

Quantum Dot Nanostructures Generate High-Purity Single Photons

The experimental setup uses a quantum dot integrated within a photonic nanostructure, generating on-demand single photons with high brightness and purity. These photons are then directed through an interferometer, where their wave-like properties combine to create interference patterns. This is a departure from expectations based on two-photon interference, where fringe behavior is less affected by these factors. The team split one of the output fibres across three superconducting nanowire single-photon detectors using cascaded beamsplitters, ensuring approximately equal intensities at each detector for the four-photon experiment.

This careful calibration was essential for accurately capturing the interference patterns and validating the theoretical predictions. This opens possibilities for more complex quantum computations and secure communication protocols, relying on the manipulation of increasingly large photonic states.

Multi-Photon Interference Beyond the Standard Quantum Limit

The observed resilience of interference minima to distinguishable photons represents a departure from established two-photon behavior, creating a sensitivity to multi-photon emission while maintaining robustness against individual photon identification. This characteristic is not merely a technical detail. This multi-photon interference can exhibit non-intuitive behaviours, such as a non-monotonic relationship with photon distinguishability or the persistence of interference even among pairwise-distinguishable photons.

This confirms the expectation that these interference minima will persist in all equivalent interference fringes for four or more photons.

Alistair J. The analysis reveals a phase sensitivity that surpasses the conventional boundary, suggesting potential improvements in the accuracy of optical quantum metrology applications. A Fisher information analysis demonstrates that interference fringes from their source can exhibit phase sensitivity beyond the standard quantum limit, illustrating potential applications in quantum metrology.

The team predicts that these phenomena will extend to interference of larger numbers of photons, with relevance across a range of potential optical quantum technologies. The quantum dot source was combined with active demultiplexing and Hong-Ou-Mandel interference to realize this direct quantum interference.

👉 More information
🗞 Four-photon interference with a high-efficiency quantum dot source
✍️ Alistair J. Brash et al.
🧠 DOI: http://link.aps.org/doi/10.1103/kb91-8zd1

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