Electro-Optic Frequency Beam Splitters and Tritters for High-Fidelity Photonic Quantum Information Processing

Abstract

We report experimental realization of high-fidelity photonic quantum gates for frequency-encoded qubits and qutrits based on electro-optic modulation and Fourier-transform pulse shaping. Our frequency version of the Hadamard gate offers near-unity fidelity (0.99998 ± 0.00003), requires only a single microwave drive tone for near-ideal performance, functions across the entire C-band (1530-1570 nm), and can operate concurrently on multiple qubits spaced as tightly as four frequency modes apart, with no observable degradation in the fidelity. For qutrits we implement a 3 × 3 extension of the Hadamard gate: the balanced tritter. This tritter-the first ever demonstrated for frequency modes-attains fidelity 0.9989 ± 0.0004. These gates represent important building blocks toward scalable, high-fidelity quantum information processing based on frequency encoding.Introduction.-The coherent translation of quantum states from one frequency to another via optical nonlinearites has been the focus of considerable research since the early 1990s [1]; yet only fairly recently have such processes been explored in the more elaborate context of time-frequency quantum information processing (QIP), where optical frequency is not just the carrier of quantum information but the information itself. Important examples include the quantum pulse gate [2,3], which uses nonlinear mixing with shaped classical pulses for selective conversion of the time-frequency modes of single photons [4][5][6], and demonstrations of frequency beamsplitters based on both χ (2) [7,8] and χ (3) [9][10][11] nonlinearities, which interfere two wavelength modes analogously to a spatial beamsplitter. These seminal experiments have shown key primitives in frequency-based QIP, but many challenges remain. For example, optical filters and/or low temperatures are required to remove background noise due to powerful optical pumps, either from the sources themselves or Raman scattering in the nonlinear medium. And achieving the necessary nonlinear mixing for arbitrary combinations of modes will require additional pump fields, as well as properly engineered phase-matching conditions.Recently we proposed a fundamentally distinct platform for frequency-bin manipulations, relying on electrooptic phase modulation and Fourier-transform pulse shaping for universal QIP [12]. Our approach requires no optical pump fields, is readily parallelized, and scales well with the number of modes. In this Letter, we apply this paradigm to experimentally demonstrate the first electro-optic-based frequency beamsplitter. Our frequency beamsplitter attains high fidelity, operates in * lougovskip@ornl.gov parallel on multiple two-mode subsets across the entire optical C-band, and retains excellent performance at the single-photon level. Moreover, by incorporating an additional harmonic in the microwave drive signal, we also realize a balanced frequency tritter, the threemode extension of the beamsplitter. This is the first frequency tritter demonstrated on any platform, and establishes our electro...

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