Challenges
Datasets
Workspaces
Papers
Discussions
Leaderboard
Log inSign up
Challenges
Datasets
Workspaces
Papers
Discussions
Leaderboard
Blog
Job Board
Q3AS

© 2026 Aqora Quantum S.A.S.

TermsPrivacyLegal Notice

Optical and Quantum Sensing and Precision Metrology II

Quantum-enhanced sensing of time-varying signals at the nanometer scale

Colin P. Lualdi, Joshua Rapp, Paul G. Kwiat

Published Mar 5, 2026• 0 citations
Publisher

Abstract

The resilience of quantum two-photon interference against optical loss and background sets it apart from classical single-photon interference for the purpose of sensing. However, achieving fast measurements at the nanometer scale with two-photon interference is challenging, as attaining high resolution typically requires either ultrabroadband photons or many probe photons. To realize nanometer-scale measurements in a matter of seconds or less, we greatly increase the per-photon information content via highly non-degenerate energy entanglement. Our fast measurements thereby enable us to probe time-varying signals at the nanometer scale. To further develop our capabilities, we have drawn on single-photon imaging techniques to implement a robust protocol for reconstructing periodic signals from our relatively sparse detection data, complete with amplitude and frequency information. With this analysis regimen, we have successfully extracted complex multi-frequency signals as well as kHz-range acoustic vibrations. We review our method and present the results of an example measurement to illustrate our capabilities.

DOI
10.1117/12.3079847

Related papers

Entanglement-Enhanced Quantum Nano-Vibrometry

Colin P. Lualdi, Joshua Rapp, Spencer J. Johnson +2 more

The study of dynamic systems at the nanometer scale can benefit from the loss and background resilience offered by quantum two-photon interference. However, fast measurements with the required resolution are difficult to realize. As a solution, we introduce extreme energy entanglement between the photons undergoing interference. Using a flux probing analysis technique, we recover vibrational signals with frequencies as high as 21 kHz. Along with validating nanometer-scale precision and accuracy, we observe a significant quantum advantage when measuring in the presence of loss and background.

quant-pharXiv:2512.05961•Dec 8, 2025•0 citations

Toward vibration measurement via frequency-entangled two-photon interferometry

Spencer J. Johnson, Colin P. Lualdi, Andrew J Conrad +3 more

We present progress toward measuring nanometer-scale vibrations via a frequency-entangled two-photon interferometer. Unlike classical interference, two-photon – or Hong-Ou-Mandel – interference allows for optical metrology with resilience against imbalanced loss, dispersion, and optical background. However, the resolution of traditional degenerate frequency two-photon interference is limited by the photons’ bandwidths, requiring large bandwidths or long integration times to achieve nanometer-scale resolution. We have implemented a twophoton interferometer utilizing highly non-degenerate frequency-entangled photon pairs at 810 nm and 1550 nm, drastically increasing measurement sensitivity while retaining the advantages of two-photon interference. This enhancement comes via a beat note with frequency proportional to the photon detuning of 177 THz. The resulting measurement saturates the quantum Cram´er-Rao bound, maximizing the information extracted per photon. We have demonstrated a measurement resolution of 2.3 nm with fewer than 18,000 detected photon pairs, orders of magnitude better than previous results. By reflecting one photon from the pair off a target surface, we may use our system to study small-scale vibrations.

Optical and Quantum Sensing and Precision Metrology II•Mar 8, 2023•5 citations

Fast quantum interferometry at the nanometer and attosecond scales with energy-entangled photons

Colin P. Lualdi, Spencer J. Johnson, Michael Vayninger +4 more

In classical optical interferometry, loss and background complicate achieving fast nanometer-resolution measurements with illumination at low light levels. Conversely, quantum two-photon interference is unaffected by loss and background, but nanometer-scale resolution is physically difficult to realize. As a solution, we enhance two-photon interference with highly nondegenerate energy entanglement featuring photon frequencies separated by 177 THz. We observe measurement resolution at the nanometer (attosecond) scale with only O (10 4 ) photon pairs, despite the presence of background and loss. Our nondestructive thickness measurement of a metallic thin film agrees with atomic force microscopy, which often achieves better resolution via destructive means. With contactless, nondestructive measurements in seconds or faster, our instrument enables metrological studies in optically challenging contexts where background, loss, or photosensitivity are factors.

arXiv:2505.15956•Science Advances•May 21, 2025•13 citations

Attosecond Quantum Metrology via Highly Non-Degenerate Frequency-Entangled Photons

Colin P. Lualdi, Spencer J. Johnson, Kristina A. Meier +1 more

We have demonstrated highly non-degenerate frequency-entangled two-photon interference. A timing resolution of 7.3 attoseconds was observed with only 24,000 coincidence detections. Our system enables fast, single-photon-level nanometer-scale measurements with loss, background, and dispersion tolerance.

Frontiers in Optics•Jan 1, 2022•1 citation

Quantum Optical Metrology Using Two-Photon Interference

Kyle Jordan

Optical quantum metrology uses specially-prepared states of light to probe sensitive samples. Choosing a suitable probe state allows one to extract more information per probe photon about an unknown parameter than is possible using any classical technique. In effect, these techniques improve the sensitivity of the measurement whenever the number of probe photons is constrained in some way. This thesis presents experimental and theoretical results on the use of pairs of time-frequency entangled photons in precision measurements. It begins with an overview of necessary background knowledge about quantum optics and quantum information. This is followed by a theoretical discussion on the estimation of a single delay using two-photon interference. We show that constraints on the probe photon's bandwidth complicate the measurement problem, requiring suitable optimization of the two-photon quantum state to avoid losing precision. We then consider the use of two-photon interference for linear spectroscopic measurements. Linear spectroscopy relates the absorption and phase spectrum of a sample to unknown electronic parameters. We describe a proof-of-principle experiment demonstrating that two-photon interferometric spectroscopy can be used for simultaneously measurement of both absorption and phase. This is followed by an information-theoretic discussion of the precision of this technique. For low losses, we show that the method provides phase sensitivity similar to classical interferometry while approaching the optimal absorption precision of any possible measurement. If the absorption and phase values are both related to a single unknown parameter, then our technique provides improved sensitivity to this parameter compared to both classical measurements and other quantum techniques in the presence of realistic levels of loss. We conclude with a discussion of quantum-optical coherence tomography, which is a two-photon interferometric method for determining the material structure of a sample. We present an ongoing experiment which aims to apply the method to tissue-like samples, where the quantum measurement is limited by a poor signal-to-noise ratio. Preliminary results demonstrate that this noise can be largely reduced by suitable frequency-domain filtering. This allows us to precisely determine the position of a scattering center using a few hundred detected photon pairs despite the presence of large amounts of noise. We follow this with a theoretical discussion of signal artifacts which are known to complicate the measured interferogram. We show these artifacts are due to a distinct interferometric effect, which can be distinguished from the signal of interest using the relative time delay between detected photons. We conclude with a brief discussion on possible techniques for artifact removal.

•Jan 22, 2026•0 citations

Optical Sensing near the Quantum Limit with Enhanced Dynamic Range by Resolving the Spectra of Interfering Photons

Russell Brooks, Luca Maggio, Thomas Jaeken +4 more

Optical sensing schemes based on two-photon interference offer a powerful platform for precision metrology, but are usually limited by a trade-off between dynamic range and measurement precision. Here, we overcome this limitation by resolving the frequency of two photons impinging on a beam splitter and combining these measurements with a new estimation strategy. This increases the usable dynamic range by up to a factor of 20 compared with conventional frequency nonresolved schemes. We implement the method with independent photon pair sources and characterize its performance in the presence of finite-resolution frequency-resolved detectors, which approach the quantum limit in the lossless regime. Our approach enables scan-free, nanometer resolution depth sensing over millimeter scale distances, with potential applications in biological and nanomaterial imaging.

arXiv:2509.22791•Physical Review Letters•Jan 20, 2026•0 citations
The resilience of quantum two-photon interference against optical loss and background sets it apart from classical single-photon interference for the purpose of sensing. However, achieving fast measurements at the nanometer scale with two-photon interference is challenging, as attaining high resolution typically requires either ultrabroadband photons or many probe photons. To realize nanometer-scale measurements in a matter of seconds or less, we greatly increase the per-photon information content via highly non-degenerate energy entanglement. Our fast measurements thereby enable us to probe time-varying signals at the nanometer scale. To further develop our capabilities, we have drawn on single-photon imaging techniques to implement a robust protocol for reconstructing periodic signals from our relatively sparse detection data, complete with amplitude and frequency information. With this analysis regimen, we have successfully extracted complex multi-frequency signals as well as kHz-range acoustic vibrations. We review our method and present the results of an example measurement to illustrate our capabilities.
The study of dynamic systems at the nanometer scale can benefit from the loss and background resilience offered by quantum two-photon interference. However, fast measurements with the required resolution are difficult to realize. As a solution, we introduce extreme energy entanglement between the photons undergoing interference. Using a flux probing analysis technique, we recover vibrational signals with frequencies as high as 21 kHz. Along with validating nanometer-scale precision and accuracy, we observe a significant quantum advantage when measuring in the presence of loss and background.
We present progress toward measuring nanometer-scale vibrations via a frequency-entangled two-photon interferometer. Unlike classical interference, two-photon – or Hong-Ou-Mandel – interference allows for optical metrology with resilience against imbalanced loss, dispersion, and optical background. However, the resolution of traditional degenerate frequency two-photon interference is limited by the photons’ bandwidths, requiring large bandwidths or long integration times to achieve nanometer-scale resolution. We have implemented a twophoton interferometer utilizing highly non-degenerate frequency-entangled photon pairs at 810 nm and 1550 nm, drastically increasing measurement sensitivity while retaining the advantages of two-photon interference. This enhancement comes via a beat note with frequency proportional to the photon detuning of 177 THz. The resulting measurement saturates the quantum Cram´er-Rao bound, maximizing the information extracted per photon. We have demonstrated a measurement resolution of 2.3 nm with fewer than 18,000 detected photon pairs, orders of magnitude better than previous results. By reflecting one photon from the pair off a target surface, we may use our system to study small-scale vibrations.
In classical optical interferometry, loss and background complicate achieving fast nanometer-resolution measurements with illumination at low light levels. Conversely, quantum two-photon interference is unaffected by loss and background, but nanometer-scale resolution is physically difficult to realize. As a solution, we enhance two-photon interference with highly nondegenerate energy entanglement featuring photon frequencies separated by 177 THz. We observe measurement resolution at the nanometer (attosecond) scale with only O (10 4 ) photon pairs, despite the presence of background and loss. Our nondestructive thickness measurement of a metallic thin film agrees with atomic force microscopy, which often achieves better resolution via destructive means. With contactless, nondestructive measurements in seconds or faster, our instrument enables metrological studies in optically challenging contexts where background, loss, or photosensitivity are factors.
Optical quantum metrology uses specially-prepared states of light to probe sensitive samples. Choosing a suitable probe state allows one to extract more information per probe photon about an unknown parameter than is possible using any classical technique. In effect, these techniques improve the sensitivity of the measurement whenever the number of probe photons is constrained in some way. This thesis presents experimental and theoretical results on the use of pairs of time-frequency entangled photons in precision measurements. It begins with an overview of necessary background knowledge about quantum optics and quantum information. This is followed by a theoretical discussion on the estimation of a single delay using two-photon interference. We show that constraints on the probe photon's bandwidth complicate the measurement problem, requiring suitable optimization of the two-photon quantum state to avoid losing precision. We then consider the use of two-photon interference for linear spectroscopic measurements. Linear spectroscopy relates the absorption and phase spectrum of a sample to unknown electronic parameters. We describe a proof-of-principle experiment demonstrating that two-photon interferometric spectroscopy can be used for simultaneously measurement of both absorption and phase. This is followed by an information-theoretic discussion of the precision of this technique. For low losses, we show that the method provides phase sensitivity similar to classical interferometry while approaching the optimal absorption precision of any possible measurement. If the absorption and phase values are both related to a single unknown parameter, then our technique provides improved sensitivity to this parameter compared to both classical measurements and other quantum techniques in the presence of realistic levels of loss. We conclude with a discussion of quantum-optical coherence tomography, which is a two-photon interferometric method for determining the material structure of a sample. We present an ongoing experiment which aims to apply the method to tissue-like samples, where the quantum measurement is limited by a poor signal-to-noise ratio. Preliminary results demonstrate that this noise can be largely reduced by suitable frequency-domain filtering. This allows us to precisely determine the position of a scattering center using a few hundred detected photon pairs despite the presence of large amounts of noise. We follow this with a theoretical discussion of signal artifacts which are known to complicate the measured interferogram. We show these artifacts are due to a distinct interferometric effect, which can be distinguished from the signal of interest using the relative time delay between detected photons. We conclude with a brief discussion on possible techniques for artifact removal.
Optical sensing schemes based on two-photon interference offer a powerful platform for precision metrology, but are usually limited by a trade-off between dynamic range and measurement precision. Here, we overcome this limitation by resolving the frequency of two photons impinging on a beam splitter and combining these measurements with a new estimation strategy. This increases the usable dynamic range by up to a factor of 20 compared with conventional frequency nonresolved schemes. We implement the method with independent photon pair sources and characterize its performance in the presence of finite-resolution frequency-resolved detectors, which approach the quantum limit in the lossless regime. Our approach enables scan-free, nanometer resolution depth sensing over millimeter scale distances, with potential applications in biological and nanomaterial imaging.