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Optics Express

Optimization of crystal length in a pulse-pumped up-conversion single-photon detector for decoding femtosecond time-bin qubits

Yuta Kochi, Sunao Kurimura, Junko Ishi‐Hayase

Published Dec 1, 2024• 1 citation
Publisher

Abstract

In advancing ultrafast quantum communication and computing, it is crucial to develop precise time-resolved measurement techniques for single-photon pulses. However, the measurement of photonic qubits, especially time-bin qubits, is limited by the temporal resolution of single-photon detectors, typically around tens of picoseconds. In this study, we developed a pulse-pumped up-conversion single-photon detector (UCSPD) using periodically poled Mg-doped stoichiometric lithium tantalate (PPMg:SLT) crystals of varying lengths to optimize femtosecond up-conversion. We evaluated the UCSPD’s efficiency and temporal resolution using a convolution model that accounts for group delay in nonlinear crystals. Our results demonstrate that the model calculations enable the accurate prediction of the crystal length dependence of temporal resolution and up-conversion efficiency without fitting parameters. The UCSPD achieved 415 fs resolution and 10.1 \% efficiency with a 2 mm crystal, enabling successful characterization of time-bin qubits with 800 fs pulse intervals.

DOI
10.1364/oe.542524

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In advancing ultrafast quantum communication and computing, it is crucial to develop precise time-resolved measurement techniques for single-photon pulses. However, the measurement of photonic qubits, especially time-bin qubits, is limited by the temporal resolution of single-photon detectors, typically around tens of picoseconds. In this study, we developed a pulse-pumped up-conversion single-photon detector (UCSPD) using periodically poled Mg-doped stoichiometric lithium tantalate (PPMg:SLT) crystals of varying lengths to optimize femtosecond up-conversion. We evaluated the UCSPD’s efficiency and temporal resolution using a convolution model that accounts for group delay in nonlinear crystals. Our results demonstrate that the model calculations enable the accurate prediction of the crystal length dependence of temporal resolution and up-conversion efficiency without fitting parameters. The UCSPD achieved 415 fs resolution and 10.1 % efficiency with a 2 mm crystal, enabling successful characterization of time-bin qubits with 800 fs pulse intervals.
Time-bin qubits, in which quantum information is encoded in a single photon at different times ΔΔΔt, are suitable for long-distance transmission via optical fibers. However, detection of time-bin qubits has been limited to the nanosecond range owing to the limited temporal resolution of single-photon detectors. In this study, we developed an up-conversion single-photon detector (UCSPD), using commercial nonlinear crystals of different lengths. By changing the crystal length and pump power, we quantitatively evaluated the efficiency and temporal resolution of the UCSPD and determined the optimal conditions for measuring femtosecond time-bin qubits. This detector achieved a temporal resolution of 415 fs and up-conversion efficiency of 10.1 %. Consequently, we successfully evaluated single-photon level pseudo femtosecond time-bin qubits with a pulse interval of only 800 fs.
We demonstrate a time-resolved single-photon detection technique based on ultrafast sum-frequency generation, providing femtosecond measurement capability for single photons in photonic quantum information processing. Noncollinear broadband upconversion in periodically poled MgO-doped stoichiometric lithium tantalate with an ultrafast pump and detection with a Si single-photon counter enable efficient detection of IR photons and temporal resolution of ~150 fs. We utilize the timing resolution to map the generation efficiency profile along the propagation axis of a periodically poled KTiOPO(4) crystal, revealing its local grating quality with millimeter resolution. We also apply the technique to two-photon coincidence measurements and directly demonstrate time anticorrelation between coincident-frequency entangled photons that are parametrically generated under extended phase-matching conditions.
Recently, quantum information processing and communications using photons have been attracting attention. To increase the density of transmitted photonic qubits, it is necessary to develop broadband quantum memories and detectors that can handle ultrashort pulses. Although rare-earth-ion doped quantum memories [1] have been widely studied, due to their storage bandwidth of ~GHz, they are incapable of storing ultrashort pulses. In our group, we used the photon echo (PE) method with quantum dots (QDs) to realize ultra-short femtosecond pulse storage. However, we encountered many difficulties in realizing this method such as low PE efficiency from the QDs and the insufficient temporal resolution of the avalanche single-photon detectors. To solve these problems, we have applied a new quantum control method and utilized an up-conversion single-photon detector (UCSPD) [2]. In this presentation, we will introduce the techniques “1. Time-resolving measurement by UCSPD,” “2. PE quantum memory using QDs” and “3. enhancement of PE signals.”
Frequency up-conversion technology can be used to increase detection efficiency for near infrared photons, as has been demonstrated in fiber-based quantum communication systems. In a continuous wave pumped up-conversion detector, the temporal resolution is limited by the timing jitter of the detector in the visible range, which limits the maximum clock rate of a quantum communication system. In this paper we describe a scheme to improve the temporal resolution of an up-conversion single-photon detector using multi-wavelength optical-sampling techniques, allowing for increased transmission rates in single-photon communications systems. We experimentally demonstrate our approach with an up-conversion detector using two spectrally and temporally distinct pump pulses, and show that it allows for high-fidelity single-photon detection at twice the rate supported by a conventional single-pump up-conversion detector.