Mid-infrared microscopy via position correlations of undetected photons.

Quantum imaging with undetected photons (QIUP) has recently emerged as a new powerful imaging tool. Exploiting the spatial entanglement of photon pairs, it allows decoupling of the sensing and detection wavelengths, facilitating imaging in otherwise challenging spectral regions by leveraging mature silicon-based detection technology. All existing implementations of QIUP have so far utilised the momentum correlations within the biphoton states produced by spontaneous parametric downconversion. Here, for the first time, we implement and examine theoretically and numerically the complementary scenario - utilising the tight position correlations formed within photon pairs at birth. This image plane arrangement facilitates high resolution imaging with comparative experimental ease, and we experimentally show resolutions below 10 µm at a sensing wavelength of 3.7 µm. Moreover, we present a quantitative numerical model predicting the imaging capabilities of QIUP for a wide range of parameters. Finally, by imaging mouse heart tissue at the mid-IR to reveal morphological features on the cellular level, we further demonstrate the viability of this technique for the life sciences. These results offer new perspectives on the capabilities of QIUP for label-free widefield mid-IR microscopy, enabling real-world biomedical as well as industrial imaging applications.

Mid-IR spectroscopy with NIR grating spectrometers.

Mid-infrared (mid-IR) spectroscopy is a crucial workhorse for a plethora of analytical applications and is suitable for diverse materials, including gases, polymers or biological tissue. However, this technologically significant wavelength regime between 2.5-10 µm suffers from technical limitations primarily related to the large noise in mid-IR detectors and the complexity and cost of bright, broadband mid-IR light sources. Here, using highly non-degenerate, broadband photon pairs from bright spontaneous parametric down-conversion (SPDC) in a nonlinear interferometer, we circumvent these limitations and realise spectroscopy in the mid-IR using only a visible (VIS) solid-state laser and an off-the-shelf, commercial near-infrared (NIR) grating spectrometer. With this proof-of-concept implementation, covering a broad range from 3.2 µm to 4.4 µm we demonstrate short integration times down to 1 s and signal-to-noise ratios above 200 at a spectral resolution from 12 cm-1 down to 1.5 cm-1 for longer integration times. Through the analysis of polymer samples and the ambient CO2 in our laboratory, we highlight the potential of this measurement technique for real-world applications.

Low random duty-cycle errors in periodically poled KTP revealed by sum-frequency generation.

Low-noise quantum frequency conversion in periodically poled nonlinear crystals has proved challenging when the pump wavelength is shorter than the target wavelength. This is-at least in large part-a consequence of the parasitic spontaneous parametric downconversion of pump photons, whose efficiency is increased by fabrication errors in the periodic poling. Here we characterize the poling quality of commercial periodically poled bulk potassium titanyl phosphate (ppKTP) by measuring the sum-frequency generation (SFG) efficiency over a large phase mismatch range from 0 to more than 400π. Over the probed range, the SFG efficiency behaves nearly ideally and drops to a normalized efficiency of 10-6. Our results demonstrate that any background pedestal that would be formed by random duty-cycle errors in ppKTP is substantially reduced when compared to periodically poled lithium niobate. The standard deviation of the random duty-cycle errors can be estimated to be smaller than 2% of the domain length. From this, we expect a noise spectral density that is at least 1 order of magnitude smaller than that of current state-of-the-art single-step frequency converters.

Multiparticle Interference of Pairwise Distinguishable Photons.

One of the central principles of quantum mechanics is that if there are multiple paths that lead to the same event and there is no way to distinguish between them, interference occurs. It is often assumed that distinguishing information in the preparation, evolution, or measurement of a system is sufficient to destroy interference. However, it is still possible for photons in distinguishable, separable states to interfere due to the indistinguishability of paths corresponding to possible exchange processes. Here we experimentally measure an interference signal that depends only on the multiparticle interference of four photons in a four-port interferometer despite pairs of them occupying distinguishable states.

Ultra-broadband SPDC for spectrally far separated photon pairs.

Photon pairs from spontaneous parametric down-conversion (SPDC) are important for a wide range of quantum optics experiments with spectral properties such as their bandwidths often being a crucial concern. Here we show the generic existence of particular phase-matching conditions in quasi-phase-matched KTP, MgO:LN, and SLT crystals that lead to ultra-broadband, widely nondegenerate photon pairs. It is based on the existence of group-velocity-matched, far apart wavelength pairs and, for 2 mm long crystals, results in SPDC bandwidths between 15 and 25 THz for photon pairs with the idler photon in the technologically relevant mid-IR range of 3-5 µm and the signal photon in the near-IR below 1100 nm. We experimentally demonstrate this type of broadband phase matching in ppKTP crystals for photon pairs centered at 800 and 3800 nm and measure a bandwidth of 15 THz. This novel method of generating broadband photon pairs will be highly beneficial for SPDC-based imaging, spectroscopy, refractometry, and optical coherence tomography with undetected mid-IR photons.

Efficient and pure femtosecond-pulse-length source of polarization-entangled photons.

We present a source of polarization entangled photon pairs based on spontaneous parametric downconversion engineered for frequency uncorrelated telecom photon generation. Our source provides photon pairs that display, simultaneously, the key properties for high-performance quantum information and fundamental quantum science tasks. Specifically, the source provides for high heralding efficiency, high quantum state purity and high entangled state fidelity at the same time. Among different tests we apply to our source we observe almost perfect non-classical interference between photons from independent sources with a visibility of (100 ± 5)%.

Microscopy with undetected photons in the mid-infrared.

Owing to its capacity for unique (bio)-chemical specificity, microscopy with mid-infrared (IR) illumination holds tremendous promise for a wide range of biomedical and industrial applications. The primary limitation, however, remains detection, with current mid-IR detection technology often marrying inferior technical capabilities with prohibitive costs. Here, we experimentally show how nonlinear interferometry with entangled light can provide a powerful tool for mid-IR microscopy while only requiring near-IR detection with a silicon-based camera. In this proof-of-principle implementation, we demonstrate widefield imaging over a broad wavelength range covering 3.4 to 4.3 µm and demonstrate a spatial resolution of 35 µm for images containing 650 resolved elements. Moreover, we demonstrate that our technique is suitable for acquiring microscopic images of biological tissue samples in the mid-IR. These results form a fresh perspective for potential relevance of quantum imaging techniques in the life sciences.

Heralded quantum steering over a high-loss channel.

Entanglement is the key resource for many long-range quantum information tasks, including secure communication and fundamental tests of quantum physics. These tasks require robust verification of shared entanglement, but performing it over long distances is presently technologically intractable because the loss through an optical fiber or free-space channel opens up a detection loophole. We design and experimentally demonstrate a scheme that verifies entanglement in the presence of at least 14.8 ± 0.1 dB of added loss, equivalent to approximately 80 km of telecommunication fiber. Our protocol relies on entanglement swapping to herald the presence of a photon after the lossy channel, enabling event-ready implementation of quantum steering. This result overcomes the key barrier in device-independent communication under realistic high-loss scenarios and in the realization of a quantum repeater.