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Quantum Communications and Quantum Imaging

New progress in quantum imaging with undetected photons (Conference Presentation)

Markus Gräfe, Marta Gilaberte Basset, Josué R. León Torres

Published Sep 10, 2019• 0 citations
Publisher

Abstract

Since the second quantum revolution, the growing exploitation of quantum states led to many sophisticated and novel applications [Phil. Trans. R. Soc. Lond. A 361, 1655 (2003). Former, mainly academic research is more and more transferred into real-world quantum technology ready to serve practical tasks. Today, the quantum computer is within reach, satellite based quantum communication already started, and the field of sensing and imaging was revolutionized, too. Non-classical states of light promise a phase sensitivity beyond any classical possibility [PRL 112, 103604 (2014)] and super-resolution capability [Nature Commun. 8, 14786 (2017)]. Moreover, based on quantum correlations, which are rooted in the very heart of quantum mechanics, the imaging of samples with photons that have never interacted with the object is feasible. This science fiction like phenomenon was first investigated by Mandel [PRA 44, 4614 (1991)] and later implemented for actual imaging purposes in the Zeilinger group [Nature 512, 409 (2014)]. We are going to present the transfer of this approach into applicable quantum technology within the realm of the Fraunhofer Key Research Initiative Quantum Methods for Advanced Imaging Solutions (QUILT). The method itself is based on induced coherence without induced emission [PRA 44, 4614 (1991)]. We built a novel compact implementation of this scheme based on only one nonlinear crystal. The crystal is coherently pumped by a laser from two sides such that signal and idler photon pairs can be collinearly emitted in either of two opposite directions (to the right or to the left). On the right side the idler beam will be separated by a dichroic mirror (DM) and interacts with an object (directly placed in front of a mirror or reflective object). On the left side the signal beam will be separated by another dichroic mirror and hit the camera, which is either a sCMOS or an EMCCD. Additional lenses provide the particular imaging. Sine the two possibilities for the idler beam – going right and interact with the object or going left from the nonlinear crystal – interfere the object could be observed by idler detection. However, the same interference can be observed for the signal beams although the never interacted with the object. Hence, the object can be imaged by detecting the signal light only. The crucial point here is the lack of wich-path information of the idler beams in this type of Michelson interferometric setup. The following points should be emphasized: (i) the signal channels never interact with the object, (ii) there is no induced emission due to the idler beam that interacted with the object, and (iii) there is no coincidence detection involved in this scheme. The obvious advantage of this technique is that the wavelength of the idler photons can be tailored to match the interesting spectral range for interaction with the object. At the same time, the signal photons, which are actually detected, can stay in the VIS range where, e.g., Si-based detectors are optimized. Besides the application for life science imaging, we are comparing the quantum imaging properties utilizing momentum correlations or spatial correlations, where the ladder is a modus that was never investigated before. As an outlook we work on significantly enhance SPDC sources in scope of quantum performance and wavelength separation. Obviously, pushing the short-wavelength idler photons further into the deep UV or XUV broadens the range of applications, but must be implemented by non-SPDC photon pair sources.

DOI
10.1117/12.2527876

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Since the second quantum revolution, the growing exploitation of quantum states led to many sophisticated and novel applications [Phil. Trans. R. Soc. Lond. A 361, 1655 (2003). Former, mainly academic research is more and more transferred into real-world quantum technology ready to serve practical tasks. Today, the quantum computer is within reach, satellite based quantum communication already started, and the field of sensing and imaging was revolutionized, too. Non-classical states of light promise a phase sensitivity beyond any classical possibility [PRL 112, 103604 (2014)] and super-resolution capability [Nature Commun. 8, 14786 (2017)]. Moreover, based on quantum correlations, which are rooted in the very heart of quantum mechanics, the imaging of samples with photons that have never interacted with the object is feasible. This science fiction like phenomenon was first investigated by Mandel [PRA 44, 4614 (1991)] and later implemented for actual imaging purposes in the Zeilinger group [Nature 512, 409 (2014)]. We are going to present the transfer of this approach into applicable quantum technology within the realm of the Fraunhofer Key Research Initiative Quantum Methods for Advanced Imaging Solutions (QUILT). The method itself is based on induced coherence without induced emission [PRA 44, 4614 (1991)]. We built a novel compact implementation of this scheme based on only one nonlinear crystal. The crystal is coherently pumped by a laser from two sides such that signal and idler photon pairs can be collinearly emitted in either of two opposite directions (to the right or to the left). On the right side the idler beam will be separated by a dichroic mirror (DM) and interacts with an object (directly placed in front of a mirror or reflective object). On the left side the signal beam will be separated by another dichroic mirror and hit the camera, which is either a sCMOS or an EMCCD. Additional lenses provide the particular imaging. Sine the two possibilities for the idler beam – going right and interact with the object or going left from the nonlinear crystal – interfere the object could be observed by idler detection. However, the same interference can be observed for the signal beams although the never interacted with the object. Hence, the object can be imaged by detecting the signal light only. The crucial point here is the lack of wich-path information of the idler beams in this type of Michelson interferometric setup. The following points should be emphasized: (i) the signal channels never interact with the object, (ii) there is no induced emission due to the idler beam that interacted with the object, and (iii) there is no coincidence detection involved in this scheme. The obvious advantage of this technique is that the wavelength of the idler photons can be tailored to match the interesting spectral range for interaction with the object. At the same time, the signal photons, which are actually detected, can stay in the VIS range where, e.g., Si-based detectors are optimized. Besides the application for life science imaging, we are comparing the quantum imaging properties utilizing momentum correlations or spatial correlations, where the ladder is a modus that was never investigated before. As an outlook we work on significantly enhance SPDC sources in scope of quantum performance and wavelength separation. Obviously, pushing the short-wavelength idler photons further into the deep UV or XUV broadens the range of applications, but must be implemented by non-SPDC photon pair sources.
We present a tutorial on the phenomenon of induced coherence without induced emission, and specifically its application to imaging and metrology. It is based on a striking effect where two nonlinear crystals, by sharing a coherent pump and one or two output beams, can induce coherence between the other two output beams. This can be thought of as a type of quantum-erasure effect, where the “welcher-weg” (which-way), or in this case, “which-source,” information is erased when the shared beams are aligned. With the correct geometry, this effect can allow an object to be imaged using only photons that have never interacted with the object—in other words, the image is formed using undetected photons. Interest in this and related setups has been accelerating in recent years due to a number of desirable properties, mostly centered around the fact that the fields for detection and imaging (since separate) may have different optical properties, entailing significant advantages for various applications. The purpose of this tutorial is to introduce researchers to this area of research, to provide practical tools for setting up experiments as well as understanding the underlying theory, and also to provide a comprehensive overview of the sub-field as a whole.
We obtained the phase and intensity images of an object by detecting classical light which never interacted with it. With a double passage of a pump and a signal laser beams through a nonlinear crystal, we observe interference between the two idler beams produced by stimulated parametric down conversion. The object is placed in the amplified signal beam after its first passage through the crystal and the image is observed in the interference of the generated idler beams. High contrast images can be obtained even for objects with small transmittance coefficient due to the geometry of the interferometer and to the stimulated parametric emission. Like its quantum counterpart, this three-color imaging concept can be useful when the object must be probed with light at a wavelength for which detectors are not available.
Quantum imaging with undetected photons relies on the principle of induced coherence without induced emission and uses two sources of photon pairs with a signal- and an idler photon. Each pair shares strong quantum correlations in both position and momentum, which allows us to image an object illuminated with idler photons by just measuring signal photons that never interact with the object. In this work, we theoretically investigate the transverse resolution of this nonlocal imaging scheme through a general formalism that treats propagating photons beyond the commonly used paraxial approximation. We hereby prove that the resolution of quantum imaging with undetected photons is fundamentally diffraction limited to the longer wavelength of the signal and idler pairs. Moreover, we conclude that this result is also valid for other nonlocal two-photon imaging schemes.
Quantum imaging with undetected photons allows for imaging an object by detecting photons that never interacted with it, using the correlation between photon pairs generated in the process of spontaneous parametric down conversion (SPDC) [1] . Through SPDC in a χ (2) material, a pump (P) photon of wavelength λ P can split into a pair of signal (S) and idler (I) photons, such that 1/ λ P = 1/ λ S + 1/ λ I . In such imaging schemes, the object’s optical properties at λ I , such as its absorption profile, can be inferred by only imaging the signal photons. Thus, one can image in challenging spectral ranges, such as mid-IR for λ I , using detectors in the visible for λ S [1] . In this scheme, the resolution is fundamentally diffraction-limited to the wavelength that interacts with the object, λ I [2] . We show that one can go beyond this diffraction limit by placing the object in the near-field of a subwavelength thin SPDC source, as shown schematically in Fig. 1(a) , through transferring the near-field information at λ I into propagating modes at λ S that can be detected in the far field. In this scheme, the resolution of imaging the object at λ I is determined by the detected signal wavelength λ S . As an application, we numerically demonstrate how two closely-spaced small scatterers at mid-IR can be resolved with a visible-range resolution.
Abstract. The field of Quantum Imaging exploits the quantum nature of light and the intrinsic parallelism of optical signals to devise novel techniques for optical imaging and for parallel information processing at the quantum level (see [1] and references quoted therein). In this presentation, we will shortly discuss two topics in this area: the first is the so-called ghost imaging which, however, is not necessarily quantum; the second is the detection of faint amplitude objects with a sensitivity beyond the standard quantum limit, and in this case we are fully in the quantum domain. Both topics are related to the phenomenon of optical parametric down-conversion (PDC), in which a fraction of the pump photons of a laser beam, injected into a crystal with a quadratic non-linearity, are down-converted to a pair of signal and idler photons, with conservation of total energy and total momentum. A feature of paramount importance is that the signal and idler beams are spatially correlated both in the near field (position correlation) and in the far field (momentum correlation).The simultaneous presence of position and momentum correlation implies quantum entanglement, as it has been also observed experimentally [2]. 1. Ghost Imaging