Erfan Abbasgholinejad, Daniel Malz, Ana Asenjo-Garcia, and Rahul Trivedi have connected the design of light sources to their potential for quantum advantage in sensing. The researchers developed a framework linking a quantum light source’s internal dynamics and emitted photon characteristics to its performance in interferometry, moving beyond analysis of highly controlled, single-mode systems.

They demonstrate that tunable optical elements with Kerr non-linearity can always implement the optimal measurement for any given source, offering a pathway to quantum advantage with existing technology. This work provides a useful tool for evaluating and designing quantum light sources for precision sensing applications.

Quantum Fisher Information Links Source Dynamics to Sensing

The quantum Fisher information (QFI) of photons emitted by a light source directly correlates to its internal dynamics, a connection previously uncharacterized outside of highly controlled experimental setups. This framework, detailed in recent work, moves beyond analysis limited to single-mode or highly symmetric multi-mode light sources, such as Fock or superradiant states, and instead focuses on “general Markovian quantum light sources”. This broadened scope allows for a wider range of quantum sensing applications by establishing a link between a source’s level structure and spectrum and its potential for quantum advantage in interferometry.

Analysis of the QFI bypasses the need to simulate entire photon wave-packets; instead, calculations can focus on the internal dynamics of the source itself, simplifying the computational burden. The researchers demonstrated this by expressing the QFI in terms of the Lindbladian, a mathematical operator describing the evolution of a quantum system, and the associated quantum channel.

This approach uses the input-output formalism and the quantum regression theorem to determine that multi-time correlation functions of the emitted photons are sufficient to characterize the source’s potential. The ability to accurately compute QFI using only internal source dynamics represents an advancement in evaluating light source designs. Tunable optical elements with Kerr non-linearity can always be harnessed to implement the optimal measurement for any given source, and general conditions under which linear optics and photodetection is enough to implement the optimal measurement are also outlined.

This finding offers a practical pathway to achieving quantum advantage, as it suggests existing technology can be harnessed to maximize sensing precision without requiring entirely new hardware. “Within the Markov approximation, we first show how to compute the quantum Fisher information (QFI) of the emitted photons in a Mach-Zehnder Interferometer (MZI),” the paper states, detailing the methodology for calculating QFI and elucidating the necessary spectral properties of photon sources.

Markovian Light Sources Enable Quantum-Enhanced Interferometry

The connection between a light source’s internal structure and its potential for quantum-enhanced interferometry is now more clearly defined, revealing that sources with multiple ground states can reach the Heisenberg limit, a threshold for optimal precision, while those with a single ground state are fundamentally limited to classical scaling. This approach allows for evaluation of a light source’s metrological potential by examining only its internal dynamics, a significant simplification over previous methods requiring complete knowledge of the emitted field.

The researchers demonstrate that this internal analysis can accurately compute the QFI, a key metric for assessing quantum enhancement, and identify measurement protocols, including both linear and non-linear setups, capable of extracting the full quantum advantage from these sources. This is particularly relevant for emerging solid-state light sources where precise control over quantum states is challenging, offering a pathway to realizing quantum advantage with realistic, rather than ideal, devices.

The analysis also reveals that supplementing photodetection with linear optical elements can optimize measurement strategies for entangled photons entering a Mach-Zehnder Interferometer (MZI). In certain instances, photodetection alone is sub-optimal, but becomes the ideal measurement when combined with these elements, providing a concrete example of how to harness existing technology for improved sensing. The framework developed by Abbasgholinejad and colleagues provides a fundamentally important tool for designing and evaluating quantum light sources for interferometry, with implications for a wide range of precision sensing applications.

Kerr Nonlinearity Optimizes Interferometry Measurements

Computing the quantum Fisher information (QFI) for interferometry has historically been limited to specific, highly symmetric multi-mode states, but new analysis demonstrates a pathway to assess a broader range of light sources for quantum sensing applications. The work reveals that Kerr nonlinearity allows for consistent implementation of the optimal measurement, regardless of the quantum light source. By computing the sensitivity of different measurements on the MZI output, researchers can supplement the upper bound provided by the QFI with a lower bound, offering a more complete picture of a light source’s potential.

Quantum Advantage Beyond Ideal Fock and NOON States

The work links the quantum Fisher information (QFI) of emitted photons to multi-time correlation functions and source dynamics, clarifying how a source’s level structure and spectrum contribute to quantum advantage, an area largely unexplored outside of ideal conditions. This framework allows for evaluating the metrological potential of sources by computing only their internal dynamics, streamlining the assessment process. Existing approaches often prioritize preparing ideal quantum states like Fock or NOON states, but these are experimentally demanding at optical frequencies and limit the number of photons achievable.

Instead of focusing on states that reach the Heisenberg limit, the researchers adopted a practical approach, asking which light sources are both simple to create and capable of delivering a quantum advantage in metrology, even if they don’t achieve the theoretical upper bound. This shift in focus acknowledges the challenges of realizing perfect quantum states and seeks viable alternatives for real-world applications.

Using quantum many-body correlations is well known to provide a quadratic advantage in sensing. The work also acknowledges the progress made in generating squeezed states through strong pumping of bulk optical nonlinearities and the theoretical analysis of superradiant decay in waveguide Dicke limits, noting that these states achieve Heisenberg-limited scaling with photon number. However, the current framework offers a more general approach, applicable to a wider range of sources and experimental conditions.

Superradiant Decay and Collective Correlations in Sensing

The framework detailed in the work allows for the computation of QFI by examining multi-time correlation functions and source dynamics, offering a new approach to evaluating the metrological potential of quantum light sources. Specifically, the researchers show that a source with a level structure, when initialized in and projected onto a state, emits photons exhibiting these advantageous correlations. This sensitivity is harnessed through a measurement scheme designed to extract the necessary correlation functions for QFI computation.

Two-Time Correlation Functions Characterize Source Potential

The ability to accurately measure a quantum light source’s potential now hinges on characterizing its multi-time correlation functions and internal dynamics, a connection detailed in new work that bypasses the need for complex wave function calculations. The analysis directly links a source’s level structure and emission spectrum to its capacity for precision sensing, a previously uncharacterized area. The resulting expression for the QFI relies on the number of photons emitted over time and two-point correlation functions, where each source is assumed to have levels driven for a specific duration, emitting photons into an output port.

👉 More information
🗞 Theory of Quantum-Enhanced Interferometry with General Markovian Light Sources
✍️ Erfan Abbasgholinejad, Daniel Malz, Ana Asenjo-Garcia and Rahul Trivedi
🧠 DOI: http://link.aps.org/doi/10.1103/dmhd-pyct

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