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JSAP-OSA Joint Symposia

Femtosecond Regime Photon Echo with Quantum Dots Evaluated by Up-conversion Single-photon Detector

Yuta Kochi, Sunao Kurimura, Kouichi Akahane, Junko Ishi‐Hayase

Published Jan 1, 2024• 0 citations
Publisher

Abstract

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.”

DOI
10.1364/jsapo.2024.20a_c43_1

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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.”
We developed a THz bandwidth quantum memory using a photon echo technique in InAs quantum dot ensemble with a large inhomogeneous broadning. To improve photon echo generation efficiency, we applied Adiabatic Rapid Passage (ARP) technique to quantum manipulation of quantum memory. In addition, we developed pulse pumped up conversion single photon detector (UCSPD) which enables us to achieve the femtosecond scale temporal resolution and we successfully demonstrated femtosecond time bin photon echo signals.
We have developed a quantum memory using quantum dots with a photon echo method for the storage of ultraweak femtosecond pulses. This quantum memory has a large bandwidth of 7.2 THz, which can be achieved due to the large inhomogeneous broadening of quantum dots. We successfully demonstrated femtosecond timebin pulse transfer to photon-echo-based quantum memory using quantum dots. We also succeeded in measuring the retrieved time-bin pulses as a photon echo from the quantum memory using a pulse-pumped frequency upconversion single-photon detector (UCSPD) with a temporal resolution of 429 fs. It is found that the retrieved time-bin photon echo pulse maintains the sub-picosecond time duration and the relative phase.
In quantum information technology, it is necessary to develop a light-matter quantum interface that transfers and stores quantum information. As a bandwidth of quantum entangled photon pairs used for quantum information increases, a quantum interface with broad bandwidth will be required. The combination of quantum dot (QD) ensemble and photon echo (PE) method is one of promising methods for broadband quantum interface. Since the bandwidth of the quantum interface using this method is limited only by the inhomogeneous width of the QDs, it is possible to implement a quantum interface with the bandwidth of 10 THz at telecommunication wavelength. However, in the PE method, the spatial inhomogeneousity of the laser intensity and the inhomogeneousity of the resonance frequency of the QDs result in the uniform quantum control of excitons in QDs. As a result, the regeneration efficiency of the PE light is significantly deteriorated. To solve this problem, it is effective to introduce a quantum control method using chirped pulses (Adaptive Rapid Passages; ARPs) which is robust to inhomogeneousities. In this study, we demonstrate that the regeneration efficiency of PE in inhomogeneous QDs can be improved by ARPs using femtosecond pulses. By performing numerical simulation and optical experiments, it was found that the regeneration efficiency improves as the chirp amount and the pulse area increase, and saturates at a certain condition.
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.
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.