We develop a high-parametric-gain SU(1,1) interferometer based on an aperiodically poled potassium titanyl phosphate (apKTP) crystal, enabling frequency-domain low-coherence interferometry with undetected mid-infrared photons. The system achieves a signal-to-noise ratio as high as 40 dB and axial resolution of 30 μm, with a 3 μm-centered idler beam. By increasing the poling-period range, we also improve the axial resolution to 17 μm, demonstrating a straightforward route to enhance the performance by working on the crystal design.
Editor's Pick
Quantum light drives electrons strongly at metal needle tips
Jonas Heimerl,
Andrei Rasputnyi,
Jonathan Pölloth,
Stefan Meier,
Maria Chekhova,
Peter Hommelhoff
Nature Physics
21
1899-1904
(2025)
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Attosecond science relies on driving photoemitted electrons with the strong optical field of a laser pulse, which represents an intense classical coherent state of light. Bright squeezed vacuum is a quantum state of light that is also intense enough to drive strong-field physics. However, its mean optical electric field is zero, suggesting that, in a semi-classical view, electrons should not experience strong driving. The question arises if and how this quantum state of light can generate signatures of attosecond dynamics in strong-field photoemission. Here we show that the key signatures of strong-field physics—the high energy plateau and subsequent cut-off—also appear under driving of a needle tip by bright squeezed vacuum, but only when we post-select electron energy spectra on the individual photon number of each pulse. When averaging over many shots, we observe broad energy spectra without a plateau. This suggests that electrons driven by bright squeezed vacuum behave as if driven by an ensemble of coherent states of light. Our findings bridge strong-field physics and quantum optics, offering insights into bright squeezed vacuum and other quantum light states, and suggest the use of strongly driven electrons as quantum light sensors.
Fano interference of photon pairs from a metasurface
Jiho Noh,
Tomás Santiago-Cruz,
Chloe F. Doiron,
Hyunseung Jung,
Jaeyeon Yu,
Sadhvikas J. Addamane,
Maria Chekhova,
Igal Brener
Two-photon interference, a quantum phenomenon arising from the principle of indistinguishability, is a powerful tool for quantum state engineering and plays a fundamental role in various quantum technologies. These technologies demand robust and efficient sources of quantum light, as well as scalable, integrable, and multifunctional platforms. In this regard, quantum optical metasurfaces (QOMs) are emerging as promising platforms for the generation and engineering of quantum light, in particular pairs of entangled photons (biphotons) via spontaneous parametric down-conversion (SPDC). Due to the relaxation of the phase-matching condition, SPDC in QOMs allows different channels of biphoton generation, such as those supported by overlapping resonances, to occur simultaneously. In previously reported QOMs, however, SPDC was too weak to observe such effects. Here, we develop QOMs based on [110]-oriented GaAs that provide an order-of-magnitude enhancement in SPDC rate, after accounting for the spectral bandwidth, compared to any other QOMs studied to date. This boosted efficiency allows the QOMs to support the simultaneous generation of SPDC from several spectrally overlapping optical modes. Using a linear polarizer, we intentionally erase the distinguishability between the biphotons from a high-Q quasi-bound-state-in-the-continuum resonance and a low-Q Mie resonance, which results in the first-time observation of two-photon interference, shown in the form of a Fano contour, in the spectrum of biphotons. This quantum interference can enrich the generation of entangled photons in metasurfaces. Their advanced multifunctionality, improved nonlinear response, ease of fabrication, and compact footprint of [110]-GaAs QOMs position them as promising platforms to fulfill the requirements of photonic quantum technologies.
Generation of photon pairs through spontaneous four-wave mixing in subwavelength nonlinear films
Changjin Son,
Samuel Peana,
Owen Matthiessen,
Artem Kryvobok,
Alexander Senichev,
Alexandra Boltasseva,
Vladimir M. Shalaev,
Maria Chekhova
Pairs of entangled photons are crucial for photonic quantum technologies. The demand for integrability and multi-functionality suggests flat platforms—ultrathin layers and metasurfaces—as sources of photon pairs. Despite the success in the demonstration of spontaneous parametric downconversion (SPDC) from such sources, there are almost no works on spontaneous four-wave mixing (SFWM)—an alternative process to generate photon pairs. Meanwhile, SFWM can be implemented in any nanostructures, including ones made of isotropic materials, which are easier to fabricate than crystalline SPDC sources. Here, we investigate photon pair generation through SFWM in subwavelength films of amorphous silicon nitride (SiN) with varying nitrogen content. For all samples, we demonstrate two-photon quantum correlations, indicated by the normalized second-order correlation function g(2)(0): it exceeds 2 and decays as the pump power increases. By observing two-photon interference between SFWM from the SiN films and the fused silica (FS) substrate, we find the third-order susceptibilities of films with different nitrogen content.
Editors' Pick
A simple model for entangled photon generation in resonant structures
Nicholas J. Sorensen,
Vitaliy Sultanov,
Maria Chekhova
Optics Express
33
13946-13960
(2025)
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The ability to engineer pairs of entangled photons is essential to quantum information science, and generating these states using spontaneous parametric down-conversion (SPDC) in nano- and micrometer-scale materials offers numerous advantages. To properly engineer such sources, a reliable model describing nano- and micrometer-scale SPDC is necessary; however, such a theoretical description remains a challenge. Here, we propose and derive a simplified model to describe SPDC in resonant structures, which considers the generation of photon pairs and the resonant enhancement of spectral bands to be separate processes, even though they actually occur simultaneously. We compare our simplified model to both the rigorous theory of SPDC in an etalon – a simple example of a resonant structure – and our experiments on SPDC in etalons and find agreement for low-gain SPDC. By simplifying the calculations required to generate photon pairs, our model promises to make designing complex resonant structures easier, and it promises to hasten the iteration of designs across the field of quantum state engineering.
Simultaneous measurement of multimode squeezing through multimode phase-sensitive amplification
Ismail Barakat,
Mahmoud Kalash,
Dennis Scharwald,
Polina Sharapova,
Norbert Lindlein,
Maria Chekhova
Multimode squeezed light is increasingly popular in photonic quantum technologies, including sensing, imaging, and computation. Existing methods for its characterization are technically complex, often reducing the level of squeezing and typically addressing only a single mode at a time. Here, for the first time, we employ optical parametric amplification to characterize multiple squeezing eigenmodes simultaneously. We retrieve the shapes and squeezing degrees of all modes at once through direct detection followed by modal decomposition. This method is tolerant to inefficient detection and does not require a local oscillator. For a spectrally and spatially multimode squeezed vacuum, we characterize the eight strongest spatial modes, obtaining squeezing and anti-squeezing values of up to −5.2 ± 0.2 dB and 8.6 ± 0.3 dB, respectively, despite 50% detection loss. This work, being the first exploration of an optical parametric amplifier’s multimode capability for squeezing detection, paves the way for real-time multimode squeezing detection.
Kontakt
Forschungsgruppe Maria Chekhova
Max-Planck-Institut für die Physik des Lichts Staudtstr. 2 91058 Erlangen
