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arXiv:1708.01705 · Optics Express

Photonic temporal-mode multiplexing by quantum frequency conversion in a dichroic-finesse cavity

Dileep V. Reddy, Michael G. Raymer

Published Oct 11, 2018• 14 citations
arXivHTMLPDF

Abstract

Photonic temporal modes (TMs) form a field-orthogonal basis set representing a continuous-variable degree of freedom that is in principle infinite dimensional, and create a promising resource for quantum information science and technology. The ideal quantum pulse gate (QPG) is a device that multiplexes and demultiplexes temporally orthogonal optical pulses that have the same carrier frequency, spatial mode, and polarization. The QPG is the chief enabling technology for usage of orthogonal temporal modes as a basis for high-dimensional quantum information storage and processing. The greatest hurdle for QPG implementation using nonlinear-optical, parametric processes with time-varying pump or control fields is the limitation on achievable temporal mode selectivity, defined as perfect TM discrimination combined with unity efficiency. We propose the use of pulsed nonlinear frequency conversion in an optical cavity having greatly different finesses for different frequencies to implement a nearly perfectly TM-selective QPG in a low-loss integrated-optics platform.

DOI
10.1364/oe.26.028091

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Photonic temporal modes (TMs) form a field-orthogonal basis set representing a continuous-variable degree of freedom that is in principle infinite dimensional, and create a promising resource for quantum information science and technology. The ideal quantum pulse gate (QPG) is a device that multiplexes and demultiplexes temporally orthogonal optical pulses that have the same carrier frequency, spatial mode, and polarization. The QPG is the chief enabling technology for usage of orthogonal temporal modes as a basis for high-dimensional quantum information storage and processing. The greatest hurdle for QPG implementation using nonlinear-optical, parametric processes with time-varying pump or control fields is the limitation on achievable temporal mode selectivity, defined as perfect TM discrimination combined with unity efficiency. We propose the use of pulsed nonlinear frequency conversion in an optical cavity having greatly different finesses for different frequencies to implement a nearly perfectly TM-selective QPG in a low-loss integrated-optics platform.
Temporal modes (TM) are a new basis for storage and retrieval of quantum information in states of light. The full TM manipulation toolkit requires a practical quantum pulse gate (QPG), which is a device that unitarily maps any given TM component of the optical input field onto a different, easily separable subspace or degree of freedom. An ideal QPG must "separate" the selected TM component with unit efficiency, whilst avoiding crosstalk from orthogonal TMs. All attempts at implementing QPGs in pulsed-pump traveling-wave systems have been unable to satisfy both conditions simultaneously. This is due to a known selectivity limit in processes that rely on spatio-temporally local, nonlinear interactions between pulsed modes traveling at independent group velocities. This limit is a consequence of time ordering in the quantum dynamical evolution, which is predicted to be overcome by coherently cascading multiple stages of low-efficiency, but highly TM-discriminatory QPGs. Multi-stage interferometric quantum frequency conversion in nonlinear waveguides was first proposed for precisely this purpose. TM-nonselective cascaded frequency conversion, also called optical Ramsey interferometry, has recently been demonstrated with continuous-wave (CW) fields. Here, we present the first experimental demonstration of TM-selective optical Ramsey interferometry and show a significant enhancement in TM selectivity over single-stage schemes.
The temporal-mode (TM) basis is a prime candidate to perform high-dimensional quantum encoding. Quantum frequency conversion has been employed as a tool to perform tomographic analysis and manipulation of ultrafast states of quantum light necessary to implement a TM-based encoding protocol. While demultiplexing of such states of light has been demonstrated in the quantum pulse gate (QPG), a multiplexing device is needed to complete an experimental framework for TM encoding. In this work we demonstrate the reverse process of the QPG. A dispersion-engineered difference frequency generation in nonlinear optical waveguides is employed to imprint the pulse shape of the pump pulse onto the output. We study experimentally the process by spectrally shaping the first five orders of Hermite-Gauss modes of various bandwidths. Finally, we establish and model the limits of practical, reliable spectral pulse shaping operation.
A new, and thus far only, method to overcome a selectivity barrier in parametrically pumped quantum pulse gates is proposed and experimentally demonstrated for the first time, using frequency conversion of optical temporal modes in second-order nonlinear waveguides. Temporal modes and quantum pulse gates are defined and their utilities are explored. Pulsed operation of three-field and four-field, parametric, optical processes are modeled and numerically investigated. A maximum limit to achievable selectivity for quantum pulse gating in uniform media is discovered and theoretically explained. An interferometric means of overcoming said limit and asymptotically approaching unit selectivity is proposed. The principle is experimentally verified by double-passing specifically shaped optical pulses derived from an ultrafast Ti:sapphire laser through a periodically-poled lithium niobate waveguide phasematched for sum-frequency generation. Further improvements and future implications for quantum technologies are discussed.
In this work, we demonstrate a quantum pulse gate (QPG) in thin-film lithium niobate. QPGs enable the selective manipulation and detection of temporal modes of quantum light and form the basis of numerous applications in photonic quantum technologies. To date, their widespread adaption is held back by two main limitations: restricted wavelength and polarization combinations of the involved fields and low normalized conversion efficiencies. We overcome these limitations through developing a QPG in thin-film lithium niobate. We design a waveguide geometry that provides the required dispersion properties for a highly efficient type-0 sum-frequency generation.We verify our design through mapping of the phase matching intensity, and demonstrate high-quality QPG operation by measuring a temporal-mode selectivity of (96.8$\pm1.7)1.7)% on par with existing QPGs. Thanks to the strong confinement in thin-film lithium niobate, we succeed in demonstrating an internal conversion efficiency of (89.61.7)\pm0.1)0.1)% for a pump power of only 20mW in front of our sample. This yields a lower-bound estimate for the normalized conversion efficiency of (18100.1)\pm10)10)10)\mathrm{W}^{-1}\mathrm{cm}^{-2}$, three orders of magnitude higher than in previous QPGs. Our results establish thin-film lithium niobate as ideal platform for high-performance QPGs and are a major step towards practical QPGs for photonic quantum technologies.
Temporal modes (TMs) are field-orthogonal broadband wave-packet states of light occupying a common frequency band, and they can encode information in a higher-dimensional alphabet compared to, say, photon polarization. The key—yet still missing—ingredient for the full implementation of a system deploying TMs is a highly selective quantum pulse gate—a multiplexing device that can route photonic packets according to their temporal shape with high temporal-mode discrimination and high efficiency, figures of merit that together we call high selectivity. Here, we present the first implementation of a highly selective quantum pulse gate. The method is a generalization of all-optical Ramsey interferometry, so far demonstrated only for continuous-wave light fields [Phys. Rev. Lett.117, 223601 (2016)PRLTAO0031-900710.1103/PhysRevLett.117.223601]. As applied to temporal modes, we refer to the method as temporal-mode interferometry.
Encoding quantum information in the photon temporal mode (TM) offers a robust platform for high-dimensional quantum protocols. The main practical challenge, however, is to design a device that operates on single photons in specific TMs and all coherent superpositions. The quantum pulse gate (QPG) is a mode-selective sum-frequency generation designed for this task. Here, we perform a full modal characterization of a QPG using weak coherent states in well-defined TMs. We reconstruct a full set of measurement operators, which show an average fidelity of 0.85 to a theoretically ideal device when operating on a seven-dimensional space. Then we use these characterized measurement operators of the QPG to calibrate the device. Using the calibrated device and a tomographically complete set of measurements, we show that the QPG can perform high-dimensional TM state tomography with 0.99 fidelity.