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International Conference on Numerical Simulation of Optoelectronic Devices (NUSOD)

Physical model of an optical memory cell with coupling quantum dots

Lei Fan, Fangmin Guo

Published Aug 1, 2012• 0 citations
Publisher

Abstract

The physical model was founded by Crosslight Apsys software for new type of photonic memory cell based on a quantum dot (QD)-quantum well (QW) hybrid structure. The physical mechanisms involved such as interband optical transition of quantum dots. The scan conditions and iterative algorithm was also set up to finish solving. Photon storage process has well proved based on I-V curve and transient time response obtained from the model. These are crucial in the signal readout-circuit design afterward.

DOI
10.1109/nusod.2012.6316505

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The physical model was founded by Crosslight Apsys software for new type of photonic memory cell based on a quantum dot (QD)-quantum well (QW) hybrid structure. The physical mechanisms involved such as interband optical transition of quantum dots. The scan conditions and iterative algorithm was also set up to finish solving. Photon storage process has well proved based on I-V curve and transient time response obtained from the model. These are crucial in the signal readout-circuit design afterward.
In forthcoming quantum networks various quantum systems might be involved to accomplish individual tasks, including storage of quantum states, quantum logic operations, error correction, or entanglement distillation. An interface between a single photon emitter and a potential photon storage could provide one fundamental building block of such a hybrid quantum system. Semiconductor quantum dots (QD) and atomic vapors have already been brought together by single photon spectroscopy of cesium and rubidium [1,2], narrow filtering of QD resonance fluorescence in cesium [3], slowing down QD photons by dispersion between the 6.8 GHz hyperfine-split rubidium D2transitions [4] and in the middle of the four cesium D1 transitions [5] (at around 5.0 GHz in Fig. 1 b).In our experiment the neutrally charged excitonic transition of a strain-tunable InGaAs QD is pumped either resonantly or non-resonantly by a pulsed laser. The QD emission is set between two cesium D1 transitions at 894 nm, which are hyperfine-split only by 1.2 GHz, with a very strong dispersion compared to preceding studies [4,5]. This allows for delay times of a few nanoseconds even at low optical densities, resulting in a propagation velocity at the order of 1/20 of the vacuum speed of light. Such a single photon - atom interface might lay the foundations for a low-loss quantum memory in a future hybrid quantum network.
A quantum network is believed to be the next generation of quantum technology [1]. In the past decades, individual quantum systems—for example, quantum dot (QD) [2–4], etc.—have been studied in depth and the technologies are mature; therefore, the consideration of combining different systems to construct a quantum network (especially a hybrid one) is timely. Moreover, a quantum network is absolutely necessary in the fulfilment of a general quantum task, which needs the cooperation of many different kinds of quantum units. On a larger scale, the local quantum networks can be further linked to construct a global quantum network, namely the quantum Internet [1]. With this technology at hand, distributed quantum computation can be realized, making scalable quantum information processing possible in multiple sites over long distances. The primary challenge of the construction of a quantum network is the state distribution among different nodes. An elementary quantum network based on two identical nodes each constituted by a single atom in the optical cavity has been realized in an experiment [5]. However, here we focus on the quantum network comprising heterogeneous nodes, which is more general and can combine advantages of different quantum systems to complete more complicated quantum tasks. The construction of a hybrid quantum network is more difficult, since heterogeneous quantum systems have different properties that are not naturally matched, such as the different emission (or absorption) wavelengths and different bandwidths [8], etc. To establish the hybrid interface, QD is a good candidate. QD can act as a deterministic single [2] or entangled [3] photon source and is also a promising physical carrier for solid-state spin qubit [4]. QD has been interfaced with hot atoms [6] and single ions [7] in recent years. Almost at the same time, Li's group experimentally demonstrated the first optical connection between QD and a solid-state quantum memory [8]. Distinguishing from the former works, the all-solid-state character of this work makes the whole architecture more stable and easier to be manipulated and integrated. During this work, the sample was grown by Niu's group [8]. A special sample design moves the emission band of QDs to approximately 879.7 nm—the memory band of the solid-state memory [9]. Since the fine-tuning range is only several 0.1 nm, hundreds of QDs on the sample were detected by Li et al. and finally one of them was selected, whose emission line is located at 879.5 nm. Using a local-heating technique, the researchers precisely tuned the emission wavelength of this QD to the solid-state memory's memory band, whose bandwidth is as narrow as hundreds of MHz [9]. The quantum memory in this experiment was based on the atomic frequency comb (AFC) protocol [10]. The memory efficiency is as high as 20% when the storage time is 40 ns, and the non-collinear configuration for the pump light and the signal photons can substantially reduce the noise from the pump light. To further reduce the detection noise, a superconducting single-photon detector (SSPD), whose dark count rate can be as low as 1.5 per second, is used instead of the silicon single-photon avalanche diode. With these conditions, Li's experiment clearly shows the stored and retrieved single photons that are originally generated from the QD. The signal-to-noise ratio (SNR) is 9:1. The second-order retrieved single photons are even seen in the data, which further validates the experimental result. Another advantage of Li's work is the demonstration of multi-temporal-mode quantum memory with up to 100 narrow deterministic-single-photon pulses [8]. This result is directly associated with the construction of a quantum repeater [11]—one of the essential components for a long-distance quantum network. The primary characteristics of this experiment, namely the deterministic single-photon memory (differing from the previous works) and the multimode memory, are both critically important in the improvement of a quantum repeater's efficiency [12]. Also, Li's further work finished the quantum memory of 51 spatial modes using the orbital angular momentum (OAM) modes of herald single photons [9]. By combining both the temporal and spatial multiplexing, and the deterministic single-photon source, more efficient quantum repeaters can be constructed. We hope that the recent studies [8,9] will pave the way for the construction of quantum repeaters, and furthermore quantum networks—both in the short term and the long term—based on all-solid-state devices. As a special application of a quantum network, quantum key distribution (QKD) can also benefit from Li's work both in security and transmission distance (key rate). QKD utilizes the principles of quantum mechanics to guarantee secure communication between two authorized parties (or quantum nodes, for instance) and the famous protocols including BB84 (proposed by Bennett and Brassard in 1984) [13]), etc. However, the commonly used weak-coherent-pulse photon source (typically attenuated laser) will decrease the security of QKD because of its multiphoton probability, which means that QKD suffers from several certain attacks, such as the photon number splitting (PNS) attack. Although a decoy state has been introduced to achieve potential solutions to this problem, a substantial shrinking of the sifted key will be required and the complexity of the protocol may contain loopholes [14,15]. Therefore, the unconditional security guaranteed by quantum mechanics still requires a single-photon source, such as QD (experimental work regarding QKD on the basis of a QD single-photon source can be referred to in [14,15]), etc. The deterministic-single-photon pulse train generated by QD in Li's experiment [8] can provide a single-photon-source node (in the network) and keep the security of QKD. Moreover, because of both the elimination of multiphoton probability and the multi-temporal-mode single-photon generation, the key rate of QKD will be improved, which is associated with a longer transmission distance. The quantum repeater that benefited from Li's work will also further improve the transmission distance of QKD.
Summary form only given. Quantum networks consist of many quantum memory nodes that are interconnected via photonic links, transporting single photons carrying quantum information. In the future, such quantum networks may enable: high-speed quantum cryptography for unconditionally secure communication; large-scale quantum computers; and quantum simulators that will allow for exponential speed-up in solving specific complex problems. A promising route towards functional quantum network nodes is the heterogeneous approach [1], where different and separately optimized physical systems are used for single photon generation and storage. For example semiconductor quantum dots may be used as efficient, fast and deterministic single photon sources, while atomic ensembles allow for efficient storage of these photons.We demonstrate a photonic memory in warm Rb vapour with on-demand storage and retrieval, based on electromagnetic induced transparency (EIT). The memory is suitable for storing single photons emitted by a GaAs droplet quantum dots [2] embedded into a state-of-the-art photonic structures [3]. With our experiments we close the gap between low speed quantum memories with acceptance bandwidth well below 100 MHz and ultra-high speed memories with acceptance bandwidth above 1 GHz. We find that in this intermediate regime vapour cell memories offer an excellent compromise between storage efficiency, storage time and experimental complexity.
Quantum memories can substantially increase the efficiency of long-distance communications by synchronizing entanglement swapping operations in quantum repeater nodes. To build a quantum memory, electromagnetically induced transparency (EIT) in atomic vapors can be exploited to coherently store light pulses even at room temperatures. As a quantum source of light, semiconductor quantum dots (QD) offer bright on-demand single photons with high purity.4 Interfacing QDs with atomic vapors has been shown by “slow light” but a quantum memory for QDs is yet to be demonstrated. In this work, we develop an EIT quantum memory hosted in warm cesium vapor. Storage of faint coherent light pulses on the single photon level shows high storage efficiency. A measured bandwidth in the order of 200 MHz makes the memory compatible with the Fourier-limited emission of QDs embedded in micropillar cavities. We show the first attempts to interface the emission from a QD-micropillar with our quantum memory by finetuning the emission wavelength of the emitters to one of the hyperfine transitions in Cs, where the EIT memory takes place. This work sets the base for a hybrid quantum memory based on atomic ensembles for an on-demand semiconductor single-photon source.