Field-Resolved Infrared Spectroscopy (Dr. I. Pupeza) / A cavity-mediated photon-photon quantum gate (S. Welte)

"Field-Resolved Infrared Spectroscopy" (Dr. I. Pupeza)

The majority of molecular assemblies exhibit fundamental vibrational and rotational modes in the mid-infrared (MIR) spectral range between 2 and 30 µm. MIR vibrational spectroscopy thus provides information on the molecular composition, structure and conformation, affording tremendous potential for advances in fields ranging from fundamental research over security and environmental applications to biology and medicine.

Traditionally, broadband measurements in the molecular fingerprint region are carried out in the frequency domain. For a certain spectral element, the absorption of a sample is determined from the attenuation of the source intensity when placing the sample in the beam path. This brings about two severe limitations for the detection of small absorptions, and of small absorption differences. First, intensity noise of the source directly affects the ability of the spectrometer to detect intensity changes induced by small changes in the absorption. Second, the detection dynamic range necessary to simultaneously resolve the full power of the source and small absorption changes restricts power scaling and ultimately limits the smallest detectable change induced by an absorption.

In this talk we introduce field-resolved spectroscopy (FRS) in the molecular fingerprint region, as a time-domain method able to mitigate the above-mentioned limitations and providing several major advantages over state-of-the-art broadband infrared vibrational spectroscopy schemes. FRS takes advantage of recently developed sources of phase-coherent, broadband femtosecond MIR pulses [1] and the ability to sample their electric fields in time with electro-optical sampling (EOS) [1,2]. The phase-coherent superposition of all MIR frequency components to an ultrashort pulse enables the confinement of the vibration excitation event to a time window of a few hundred femtoseconds only, which is significantly shorter than the typical decay times of the excited vibrations under scrutiny. Sampling the resulting electric field therefore allows for a temporal separation of the response emerging from the sample (free induction decay, FID) from the high-power ultrashort excitation. Thus, the FID is detected in a MIR-background-free manner such that source intensity noise only affects the absolute value of the determined absorption but not the instrument’s ability to detect small absorptions. Furthermore, the dynamic range of detection is adjustable to the strength of the time-domain signal, rendering the scheme power-scalable up to the deformation/damage of the sample. Finally, the heterodyne detection via EOS readily provides the full phase information of the FID.

First benchmarking measurements in the wavelength range between 8 and 12 µm will be presented and an outlook towards measurements with an unparalleled sensitivity and specificity over the entire molecular fingerprint region will be given.

References:
[1] I. Pupeza et al., Nature Photonics 9, 721 (2015)
[2] S. Keiber et al., Nature Photonics 10, 159 (2016)

"A cavity-mediated photon-photon quantum gate" (S. Welte)

Photons are promising candidates for applications in quantum information processing and quantum communication. However, the direct interaction between two photons is negligible in free space, which is a drawback when it comes to the implementation of quantum logic gates between them. A solution to this problem was offered by Duan and Kimble [1] who proposed that a strongly coupled atom in an optical cavity [2] could mediate an effective interaction between two photons. We experimentally demonstrate that an implementation of this proposal is indeed possible [3]. To this end, the universal CNOT operation of the gate as well as its capability to entangle two separable input photons are characterized. We will discuss details of our experimental implementation and present intriguing implications of our gate for photonic quantum information processing.

References:
[1] L.-M. Duan, H.J.Kimble, Phys. Rev. Lett. 92, 127902 (2004)
[2] A.Reiserer, G.Rempe, Rev. Mod. Phys. 87, 1379 (2015)
[3] B.Hacker, S.Welte, G.Rempe, S.Ritter, Nature 536, 193 (2016)