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arXiv:2211.05693 · PRX Quantum

Realization of a Multi-Output Quantum Pulse Gate for Decoding High-Dimensional Temporal Modes of Single-Photon States

Laura Serino, Jano Gil-López, Michael Stefszky, Raimund Ricken, Christof Eigner, Benjamin Brecht, Christine Silberhorn

Published Apr 12, 2023• 58 citations
arXivHTMLPDF

Abstract

Temporal modes (TMs) of photons provide an appealing high-dimensional encoding basis for quantum information.While techniques to generate TM states have been established, high-dimensional decoding of single-photon TMs remains an open challenge.In this work, we experimentally demonstrate demultiplexing of five-dimensional TMs of single photons with an average fidelity of 0.96 ± 0.01, characterized via measurement tomography.This is achieved with use of a newly developed device, the multi-output quantum pulse gate (MQPG).We demonstrate a proof-of-principle complete decoder based on the MQPG that operates on any basis from a set of six five-dimensional mutually unbiased bases and is therefore suitable as a receiver for high-dimensional quantum key distribution.Furthermore, we confirm the highquality operation of the MQPG by performing resource-efficient state tomography with an average fidelity of 0.98 ± 0.02.

DOI
10.1103/prxquantum.4.020306

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Temporal modes (TMs) of photons provide an appealing high-dimensional encoding basis for quantum information.While techniques to generate TM states have been established, high-dimensional decoding of single-photon TMs remains an open challenge.In this work, we experimentally demonstrate demultiplexing of five-dimensional TMs of single photons with an average fidelity of 0.96 ± 0.01, characterized via measurement tomography.This is achieved with use of a newly developed device, the multi-output quantum pulse gate (MQPG).We demonstrate a proof-of-principle complete decoder based on the MQPG that operates on any basis from a set of six five-dimensional mutually unbiased bases and is therefore suitable as a receiver for high-dimensional quantum key distribution.Furthermore, we confirm the highquality operation of the MQPG by performing resource-efficient state tomography with an average fidelity of 0.98 ± 0.02.
Accessing a high dimensional Hilbert space spanned by temporal modes (TMs) of single-photons is an intriguing idea for quantum information protocols, though it is experimentally challenging. The quantum pulse gate (QPG), a group-velocity matched sum-frequency process, can perform arbitrary TM resolved measurement. We present a thorough characterisation of measurement operators of a QPG and we show that this device is capable of performing full state tomography with 99% fidelity. Moreover, we use this device for TM tomography of pure and mixed heralded single-photon states in 7-dimensional Hilbert space.
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.
We demonstrate a high-dimensional mode-sorter for single photons based on a multi-output quantum pulse gate, which we can program to switch between different temporal-mode encodings including pulse modes, frequency bins, time bins, and their superpositions. This device can facilitate practical realizations of quantum information applications such as high-dimensional quantum key distribution and thus enables secure communication with enhanced information capacity. We characterize the mode-sorter through a detector tomography in 3 and 5 dimensions and find a fidelity up to 0.958 ± 0.030 at the single-photon level.
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.
Spectrally pure and indistinguishable single photons are essential for quantum network platforms, where high-visibility interference underpins many quantum information protocols. However, most practical single-photon sources emit spectrally multimode states with reduced purity. Conventional spectral intensity filtering can partially improve purity but cannot select a well-defined temporal mode (TM). Here, we demonstrate coherent temporal filtering of a multimode parametric down-conversion (PDC) source using a quantum pulse gate (QPG) and benchmark its performance against conventional intensity filtering. The generated PDC photons exhibit strong spectral correlations, rendering extraction of pure heralded photons from the pair a challenge. We demonstrate that QPG filtering consistently generates heralded photons with purities above 0.90 regardless of the filter shape. Contrariwise, using spectral intensity filters yields mixed photons. Photon purities are probed with chronocyclic Q-function tomography. Furthermore, we demonstrate the versatility of QPG filtering by extracting structured TMs, including superposition of picosecond time bins. These results establish the QPG as a practical coherent filtering tool for quantum network applications.