Frédéric Bouchard, Duncan England, Philip J. Bustard, Kate L. Fenwick, Ebrahim Karimi, Khabat Heshami, and Benjamin Sussman,
Achieving Ultimate Noise Tolerance in Quantum Communication,
Phys. Rev. Applied 15, 024027 (2021)

At the fundamental level, quantum communication is ultimately limited by noise. For instance, quantum signals cannot be amplified without the introduction of noise in the amplified states. Furthermore, photon loss reduces the signal-to-noise ratio, accentuating the effect of noise. Thus, most of the efforts in quantum communications have been directed towards overcoming noise to achieve longer communication distances, larger secret key rates, or to operate in noisier environmental conditions. Here, we propose and experimentally demonstrate a platform for quantum communication based on ultrafast optical techniques. In particular, our scheme enables the experimental realization of high rates and quantum signal filtering approaching a single spectrotemporal mode, resulting in a dramatic reduction in channel noise. By experimentally realizing a 1-ps optically induced temporal gate, we show that ultrafast time filtering can result in an improvement in noise tolerance by a factor of up to 1200 compared to a 2-ns electronic filter, enabling daytime quantum key distribution or quantum communication in bright fibers.

A. Röder, A. Skov, A. E. Boguslavskiy, R. Lausten and A. Stolow,
VUV Excited-state dynamics of cyclic ethers as a function of ring size,
Phys. Chem. Chem. Phys. 22, 26241-26254 (2020)

The vacuum ultraviolet (VUV) absorption spectra of cyclic ethers consist primarily of Rydberg ← n transitions. By studying three cyclic ethers of varying ring size (tetrahydropyran, tetrahydrofuran and trimethylene oxide, n = 6–4), we investigated the influence of ring size on the VUV excited-state dynamics of the 3d Rydberg manifold using time-resolved photoelectron spectroscopy (TRPES), time-resolved mass spectroscopy (TRMS) and ab initio electronic structure calculations. Whereas neither the electronic characters nor the term energies of the excited-states are substantially modified when the ring-size is reduced from n = 6 to 5 to 4, the excited-state lifetimes concomitantly decrease five-fold. TRPES and TRMS allow us to attribute the observed dynamics to a Rydberg cascade from the initially excited d-Rydberg manifold via the p-Rydberg manifold to the s-Rydberg state. Cuts through potential energy surfaces along the C–O bond reveal that a nσ* state crossing brings the s-Rydberg state along a path to the ring-opened ground state. The observed difference in excited-state lifetimes is attributed to an increasing slope along the repulsive C–O bond coordinate as ring size decreases.

We demonstrate a novel method to measure the temporal electric field evolution of ultrashort laser pulses. Our technique is based on the detection of transient currents in air plasma. These directional currents result from subcycle ionization of air with a short pump pulse and the steering of the released electrons with the pulse to be sampled. We assess the validity of our approach by comparing it with different state-of-the-art laser-pulse characterization techniques. Notably, our method works in ambient air and facilitates a direct measurement of the field waveform, which can be viewed in real time on an oscilloscope in a similar way as a radio frequency signal.

E Scott Goudreau, Connor Kupchak, Benjamin J Sussman, Robert W Boyd, Jeff S Lundeen,
Theory of four-wave mixing of cylindrical vector beams in optical fibers,
Journal of the Optical Society of America B 37,  1670-1682 (2020)

Cylindrical vector (CV) beams are a set of transverse spatial modes that exhibit a cylindrically symmetric intensity profile and a variable polarization about the beam axis. They are composed of a non-separable superposition of orbital and spin angular momenta. Critically, CV beams are also the eigenmodes of optical fiber and, as such, are of widespread practical importance in photonics and have the potential to increase communications bandwidth through spatial multiplexing. Here, we derive the coupled amplitude equations that describe the four-wave mixing (FWM) of CV beams in optical fibers. These equations allow us to determine the selection rules that govern the interconversion of CV modes in FWM processes. With these selection rules, we show that FWM conserves the total angular momentum, the sum of orbital and spin angular momenta, in the conversion of two input photons to two output photons. When applied to spontaneous FWM, the selection rules show that photon pairs can be generated in CV modes directly and can be entangled in those modes. Such quantum states of light in CV modes could benefit technologies such as quantum key distribution with satellites.

C. Marceau, J. B. Bertrand, Peng Peng, H. J. Wörner, P. B. Corkum, D. M. Villeneuve,
Simultaneous Measurements of Strong-Field Ionization and High Harmonic Generation in Aligned Molecules,
Journal Of Physics B: Atomic, Molecular And Optical Physics 53, 084006 (2020)

High harmonic spectroscopy relies on high harmonic generation (HHG) in aligned molecules. The first step of HHG is the ionization of the molecule in the intense femtosecond laser field. Here we present measurements of both ionization yield and high harmonic yield as a function of molecular angle in N2 and CO2 molecules. Measurements were done at two wavelengths, 800 and 1200 nm, and for a range of laser intensities, to study the sensitivity of laser conditions on both processes. The behavior of N2 was relatively insensitive to laser conditions. However in CO2, a minimum in high harmonic emission was observed that was sensitive to both laser intensity and wavelength, and was attributed to interference in emission from the HOMO and HOMO-2 orbitals.

F. Hufnagel, A. Sit, F. Bouchard, Y. Zhang, D. England, K. Heshami, B. J Sussman, and E. Karimi,
Investigation of underwater quantum channels in a 30 meter flume tank using structured photons,
New Journal of Physics 22, 093074 (2020)

Underwater quantum communication has recently been explored using polarization and orbital angular momentum (OAM). Here, we show that spatially structured modes, e.g., a coherent superposition of beams carrying both polarization and OAM, can also be used for underwater quantum cryptography. We also use the polarization degree of freedom to investigate the impact of the channel length on key rates for quantum communication applications. The underwater channel proves to be a difficult environment for establishing quantum communication as underwater optical turbulence results in significant beam wandering and distortions. However, the errors associated to the turbulence do not result in error rates above the threshold for establishing a positive key in a quantum communication link with both the polarization and spatially structured photons. The impact of the underwater channel on the spatially structured modes is also investigated at different distances using polarization tomography.

Peng Peng, Claude Marceau, Marius Hervé, P. B. Corkum, A. Yu Naumov, D. M. Villeneuve,
Symmetry of Molecular Rydberg States Revealed by XUV Transient Absorption Spectroscopy,
Nature Communications 10, 5269 (2019)

Transient absorption spectroscopy is utilized extensively for measurements of bound- and quasibound-state dynamics of atoms and molecules. The extension of this technique into the extreme ultraviolet (XUV) region with attosecond pulses has the potential to attain unprecedented time resolution. Here we apply this technique to aligned-in-space molecules. The XUV pulses are much shorter than the time during which the molecules remain aligned, typically < < 100 fs. However, transient absorption is not an instantaneous probe, because long-lived coherences re-emit for picoseconds to nanoseconds. Due to dephasing of the rotational wavepacket, it is not clear if these coherences will be evident in the absorption spectrum, and whether the properties of the initial excitations will be preserved. We studied Rydberg states of N2 2 and O2 2 from 12 to 23 eV. We were able to determine the polarization direction of the electronic transitions, and hence identify the symmetry of the final states.

Xiaoyan Ding, R. Forbes, M. Kübel, Kevin F. Lee, M. Spanner, A. Yu. Naumov, D. M. Villeneuve, A. Stolow, P. B. Corkum, A. Staudte,
Threshold Photodissociation Dynamics of NO2 Studied by Time-Resolved Cold Target Recoil Ion Momentum Spectroscopy,
The Journal Of Chemical Physics 151, 174301 (2019)

We study the near-threshold photodissociation dynamics of NO2 by a kinematically complete femtosecond pump-probe scheme using a cold target recoil ion momentum spectrometer. We excite NO2 to the optically bright Ã2B2 state with a 400 nm pulse and probe the ensuing dynamics via strong field single and double ionization with a 25 fs, 800 nm pulse. The pump spectrum spans the NO(X2Π) + O(3P) dissociation channel threshold, and therefore, following internal conversion, excited NO2 is energetically prepared both “above threshold” (dissociating) and “below threshold” (nondissociating). Experimentally, we can clearly discriminate a weak two-photon pump channel from the dominant single-photon data. In the single ionization channel, we observe NO+ fragments with nonzero momentum at 200 fs delay and an increasing yield of NO+ fragments with near-zero momentum at 3.0 ps delay. For double ionization events, we observe a time-varying Coulombic kinetic energy release between the NO+ and O+ fragments impulsively created from the evolving “hot” neutral ground state. Supported by classical trajectory calculations, we assign the decreasing Coulombic kinetic energy release at longer time delays to the increasing average NO–O distances in the ground electronic state during its large amplitude phase space evolution toward free products. The time-resolved kinetic energy release in the double ionization channel probes the large amplitude ground state evolution from a strongly coupled “inner region” to a loosely coupled “outer region” where one O atom is on average much further away from the NO. Both the time evolution of the kinetic energy release and the NO+ angular distributions support our assignments.

F. Hufnagel, A. Sit, F. Grenapin, F. Bouchard, K. Heshami, D. England, Y. Zhang, B. J. Sussman, R. W. Boyd, G. Leuchs, and E. Karimi,
Characterization of an underwater channel for quantum communications in the Ottawa River,
Optics Express 27, 26346 (2019)

We examine the propagation of optical beams possessing different polarization states and spatial modes through the Ottawa River in Canada. A Shack-Hartmann wavefront sensor is used to record the distorted beam’s wavefront. The turbulence in the underwater channel is analysed, and associated Zernike coefficients are obtained in real-time. Finally, we explore the feasibility of transmitting polarization states as well as spatial modes through the underwater channel for applications in quantum cryptography.

Claude Marceau, Varun Makhija, Peng Peng, Marius Hervé, P. B. Corkum, A. Yu. Naumov, A. Stolow, D. M. Villeneuve,
Non-Born-Oppenheimer electronic wave packet in molecular nitrogen at 14 eV probed by time-resolved photoelectron spectroscopy,
Phys. Rev. A 99, 023426 (2019)

We report the observation of a 4.1±0.2 THz quantum beat in the photoelectron spectrum of molecular nitrogen following coherent optical excitation by 14 eV photons and probed by two photons at 800 nm. The intermediate states in the two pathway interference are the valence state b1Σ+uv=13 and Rydberg state c41Σ+uv=4. The amplitude of the coherent oscillations decays in about 3 ps and does not revive. Opposite phases in the photoelectron yield corresponding to different vibrational levels of the final cationic states X2Σ+gv=0,A2Πuv=1, and X2Σ+gv=2,3,4 are reported. Simulation results show that the interference originates from the strongly mixed character of the two simultaneously excited eigenstates, causing large population oscillations in the zeroth-order Born-Oppenheimer diabatic basis. No quantum beat was observed when probing with single photon at 400 nm, suggesting a resonance with a state just below the ionization potential which acts as a filter, improving the quantum beat contrast.