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arXiv:1707.02094 · Quantum Science and Technology

Quantum-dot based photonic quantum networks

Peter Lodahl

Published Oct 6, 2017• 7 citations
arXivHTMLPDF

Abstract

Abstract Quantum dots (QDs) embedded in photonic nanostructures have in recent years proven to be a very powerful solid-state platform for quantum optics experiments. The combination of near-unity radiative coupling of a single QD to a photonic mode and the ability to eliminate decoherence processes imply that an unprecedent light–matter interface can be obtained. As a result, high-cooperativity photon-emitter quantum interfaces can be constructed opening a path-way to deterministic photonic quantum gates for quantum-information processing applications. In the present manuscript, I review current state-of-the-art on QD devices and their applications for quantum technology. The overarching long-term goal of the research field is to construct photonic quantum networks where remote entanglement can be distributed over long distances by photons.

DOI
10.1088/2058-9565/aa91bb

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Abstract Quantum dots (QDs) embedded in photonic nanostructures have in recent years proven to be a very powerful solid-state platform for quantum optics experiments. The combination of near-unity radiative coupling of a single QD to a photonic mode and the ability to eliminate decoherence processes imply that an unprecedent light–matter interface can be obtained. As a result, high-cooperativity photon-emitter quantum interfaces can be constructed opening a path-way to deterministic photonic quantum gates for quantum-information processing applications. In the present manuscript, I review current state-of-the-art on QD devices and their applications for quantum technology. The overarching long-term goal of the research field is to construct photonic quantum networks where remote entanglement can be distributed over long distances by photons.
In this paper, exciting progress of quantum optics in solid state is reviewed. The focus is on semiconductor microcavities with self-assembled quantum dots embedded in the active layer. Due to enormous progress in semiconductor nanotechnology, such photonic structures have become a model system for the study of quantum optics on a scalable and integrable technology platform with high potential for future applications in quantum information technology. Quantum optical phenomena have become accessible due to 3-D confinement of light and matter on the length scale of their wavelength in state-of-the-art semiconductor micro- and nanostructures. This confinement leads to a quantization of the associated photonic and electronic energy levels and requires a quantum mechanical description of the system in the framework of cavity quantum electrodynamics (cQED). This approach considers the dipole interaction between two-level quantum emitters and discrete photonic states of a microcavity. Within the well-known Jaynes-Cummings model, the dipole interaction is described in terms of a coherent exchange of energy between the emitter and the resonator mode. This coherent interaction in the so-called strong coupling regime of cQED is reflected in a normal mode splitting, the vacuum Rabi splitting, of the involved modes and represents a central feature of quantum optics in solid state. Another important example of quantum optics in semiconductor nanostructures is the generation of nonclassical light in specific quantum devices. Of particular interest is the realization of Fock states which represent states containing a well-defined number of photons. Single-photon sources, for instance, allow for the generation of single photons on demand, which is highly desirable for quantum communication systems. In this context, this review paper will present recent experimental studies of quantum optics in solid state. This paper is meant for readers who would like to become familiar with this topic and for experts being interested in the progress in this field. It will cover a broad range of studies ranging from examples of fundamental light-matter interaction in the quantum limit to devices capable of emitting single photons and entangled photon pairs on demand.
Entanglement-based quantum key distribution (e-QKD) is a cornerstone of quantum-secure communication, offering advantages in device-independent security, compatibility with quantum networks, and resilience to specific attacks. While the probabilistic nature of entangled photon generation has slowed down experimental adoption compared to prepare-and-measure approaches, deterministic solid-state emitters—particularly epitaxial quantum dots (QDs)—have recently emerged as promising candidates for scalable, high-rate implementations. In this review, we survey the main e-QKD protocols along with their practical requirements and assess current technologies for entangled photon pair generation. We place particular emphasis on the state of the art in QD-based entangled photon sources, analyzing design strategies and performance metrics that directly impact secure key generation. Recent experimental advances demonstrating entanglement distribution and e-QKD with QDs over both fiber and free-space channels are discussed in detail. We conclude with an outlook on remaining challenges in source engineering, transmission capacity, and system integration, highlighting the potential of QDs to enable next-generation quantum communication technologies.
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.
Over the past twenty-five years, intensive research on epitaxial semiconductor quantum dots (QDs) has transformed these systems from random quantum emitters embedded in a noisy solid-state environment into one of the most efficient platforms for generating not only single photons but also multiple entangled photons in highly pure quantum states. Today, QD-photon interfaces often outperform their natural-atom counterparts in both quantum-state purity and photon-generation rates while also offering a clear route toward scalable integration and industrial deployment. Building on these remarkable achievements, this Essay outlines a path toward a not-so-distant future in which large numbers of quantum-dot spins are entangled to generate quantum states of light with unprecedented numbers of entangled photons. Just as lasers evolved from laboratory curiosities into ubiquitous enabling technologies, semiconductor sources of large-scale photonic entanglement could become indispensable resources for both emerging quantum technologies and fundamental research. Part of a series of Essays in Physical Review Letters which concisely present author visions for the future of their field.
Non-classical states of light play a fundamental role in quantum technology. From photonic quantum computers and simulators, to quantum communication and sensing, quantum states of light enable performing tasks that may outperform their best classical counterparts. Semiconductor quantum dots embedded in photonic nanostructures offer the most advanced classes of quantum light sources. Importantly, the underlying physics processes determining device performance are today fully understood, and dedicated engineering projects are currently advancing these sources towards real-world quantum technology applications. We review the performance of deterministic single-photon sources based on quantum dots in photonic crystal waveguides, the approach with the highest performance specs since it intrinsically combines suppression of leaky modes and Purcell enhancement to slow-light waveguide mode. Furthermore, we present prototype data from sources that today are commercially available and with performance metrics approaching the ideal.