Conference on Lasers and Electro-Optics Europe & European Quantum Electronics Conference (CLEO/Europe-EQEC)
Quantum networks based on single atoms in optical cavities
Stephan Ritter, Christian Nölleke, Carolin Hahn, Andreas Reiserer, Andreas Neuzner, Manuel Uphoff, Martin Mücke, Eden V. Figueroa, Joerg Bochmann, Gerhard Rempe
The distribution of quantum information as well as the utilization of non-locality are at the heart of quantum networks, which show great promise for future applications like quantum communication, distributed quantum computing and quantum metrology. Their practical realization however is a formidable challenge. On the one hand, the state of the respective quantum system has to be under perfect control, ideally in all degrees of freedom. This requires low decoherence, i.e. minimal interaction with the environment. But at the same time, strong, tailored interactions are required to enable all envisioned processing tasks. In this respect, single atoms and photons are the ideal building blocks of a quantum network. Atoms can act as stationary nodes as they are long-lived and their interaction with the environment is weak, while their external and internal degree of freedom can precisely be controlled and manipulated. Photons can be transmitted over larger distances using existing fiber technology and do not mutually interact.
The distribution of quantum information as well as the utilization of non-locality are at the heart of quantum networks, which show great promise for future applications like quantum communication, distributed quantum computing and quantum metrology. Their practical realization however is a formidable challenge. On the one hand, the state of the respective quantum system has to be under perfect control, ideally in all degrees of freedom. This requires low decoherence, i.e. minimal interaction with the environment. But at the same time, strong, tailored interactions are required to enable all envisioned processing tasks. In this respect, single atoms and photons are the ideal building blocks of a quantum network. Atoms can act as stationary nodes as they are long-lived and their interaction with the environment is weak, while their external and internal degree of freedom can precisely be controlled and manipulated. Photons can be transmitted over larger distances using existing fiber technology and do not mutually interact.
Quantum physics allows a new approach to information processing. A grand challenge is the realization of a quantum network for long-distance quantum communication and large-scale quantum simulation. This paper highlights a first implementation of an elementary quantum network with two fibre-linked high-finesse optical resonators, each containing a single quasi-permanently trapped atom as a stationary quantum node. Reversible quantum state transfer between the two atoms and entanglement of the two atoms are achieved by the controlled exchange of a time-symmetric single photon. This approach to quantum networking is efficient and offers a clear perspective for scalability. It allows for arbitrary topologies and features controlled connectivity as well as, in principle, infinite-range interactions. Our system constitutes the largest man-made material quantum system to date and is an ideal test bed for fundamental investigations, e.g. quantum non-locality.
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.
Harnessing quantum mechanics to revolutionize various fields of established technology has fueled research activities in recent years. Especially the prospect of inherently secure quantum communication channels has become increasingly desirable to businesses, politicians and society as a whole to protect sensitive information. \nAt heart, quantum communication relies on the distribution of entangled quantum states that make the communication impervious to eavesdroppers. Naturally single photons or photon pairs are an excellent choice for distributing entangled states at the speed-of-light through existing fiber-networks. \nOne of the most promising quantum light sources constitute epitaxial quantum dots. The high oscillator strength renders them exceptionally bright, while still emitting nearly indistinguishable single photons with quantum efficiencies close to unity, an important prerequisite for high-fidelity photonic quantum applications. By carefully manufacturing the semiconductor heterostructure, the optical environment can be individually tailored, utilizing Purcell enhancement, by embedding the emitter in a semiconductor cavity or exploiting wave guiding properties in form of micropillars or nanowires to enhance the extraction efficiency. \nInevitable optical attenuation in fiber networks however necessitates the overall communication channel length to be divided into subsections with nodes that can temporarily store the quantum information. Naturally this requires a quantum memory which can efficiently store the quantum state for a sufficiently long time and subsequently recreate the stored photon on demand. \nIn this framework, atomic memories represent the established benchmark, unrivaled by quantum dots spin states which remain intrinsically limited by the decoherence-inducing interaction with the solid state environment. Combining the excellent single-photon source of a quantum dot with the superior coherence properties of alkali quantum memories in a hybrid system at each quantum node offers the best of both worlds, promising exponential speed-up of truly secure communication. \nThis PhD thesis focuses on the requirements imposed on the quantum dot in such a hybrid quantum network and shows how these challenges can be overcome. \nThe first part of the introduction aims to give a detailed overview on the underlying quantum communication protocol of a hybrid quantum network and how it fares against the more established DLCZ protocol. Next, single-photon sources, and quantum dots in particular, will be outlined and the growth mechanism and optical properties of epitaxial GaAs quantum dots discussed in detail. Lastly, to illustrate the framework in which a quantum dot can efficiently be paired with alkali atoms and to understand the challenges that arise, the mode of operation and attributes of the state-of-art broadband quantum memory will be summarized in chapter 2. \nThe third chapter investigates the optical properties of an epitaxial GaAs quantum dot spectrally matched to rubidium. By means of strain-tuning, the quantum dot can address all hyperfine transitions of the rubidium D2 line and a first interaction with atomic vapors is shown in a transmission measurement. In conjunction with other optical measurements, true Fourier-limited emission of single photons is demonstrated. Furthermore, we establish a possible route to overcome the bandwidth mismatch of the two systems in form of the coherent-scattering regime. \nWhile this coherent-scattering regime offers quantum dot single-photons with sub-natural bandwidths, in form of elastically scattered single photons that predominately retain the small linewidth of the excitation laser, the emission is highly probabilistic and relies on continuous-wave excitation or weak, resonant laser pulses of durations exceeding the exciton lifetime. The fourth chapter demonstrates the generation of true on-demand single photons with tailored temporal waveform envelopes between 14 and 245 ns, overcoming the temporal limitations imposed by the exciton two-level system. The photonic bandwidth is reduced by almost one order of magnitude. \nIn the following, the decay dynamics of a positively charged exciton in an GaAs quantum dot will be investigated by time-resolved photolumincescence and resonance fluorescence measurements (chapter 5). In Chapter 6 the optical properties of GaAs quantum dots in 500 nm thick membranes are characterized. Finally, an outlook into future developments and the solutions to remaining challenges will be presented. \n
Summary form only given. Entanglement - the property that particles can share a single quantum state - is arguably the most counterintuitive yet potentially most powerful element of quantum physics. Future quantum networks may harness the unique features of entanglement in a range of exciting applications, such as quantum computation and simulation, secure communication, enhanced metrology for astronomy and time-keeping as well as fundamental tests of nature. To fulfill these promises, a strong worldwide effort is ongoing to gain precise control over multi-particle nodes and to wire them up using quantum-photonic channels. Diamond spins associated with NV centers are promising building blocks for such a network as they combine a coherent electron-optical interface [1] (similar to that of trapped atomic qubits) with a local register of robust and well-controlled nuclear spin qubits [2]. Here I will introduce the field of quantum networks and present an overview of the latest progress, including the first loophole-free violation of Bell's inequalities [3,4] and the first primitive network experiments on a pair of spatially separated two-qubit nodes.
The realization of a large-scale quantum network of interconnected quantum nodes is one of the most promising directions in quantum technology. Such a network distributes entanglement between remote matter qubits via single photons in optical fibers, enabling quantum repeaters and long-distance secure communication and supporting emerging applications such as distributed quantum computing and network-enhanced metrology. To realize this vision, proposed architectures require quantum nodes that provide an efficient quantum light-matter interface for entanglement generation, serve as a quantum memory allowing long-lived storage of quantum states, and have the possibility to connect to telecommunication fiber-based photonic channels. This thesis presents the development and experimental realization of a quantum network node based on a single Rb-87 atom. A single atom is confined in a tightly focused optical dipole trap, and entanglement is generated between the polarization of an emitted photon and the Zeeman state of the atom. To enable long-lived quantum storage, we coherently map the atomic qubit to a basis less sensitive to magnetic fields using Zeeman-state-selective Raman transfer, reducing magnetic field sensitivity by a factor of 500. Such hyperfine qubit control is implemented via a two-photon Raman process driven by a laser field that is amplitude modulated at the qubit splitting using a fiber-based Mach-Zehnder interferometer. With this approach, we achieve a 1/e storage lifetime of 10 ms, extending coherence by a factor of 30 compared with the initial qubit encoding. Residual dephasing arises primarily from position-dependent effective magnetic fields caused by the tightly focused optical dipole trap beam together with atomic motion, and the differential AC-Stark shifts come from the trap detuning. While the first can be fully canceled via a standing-wave dipole trap, which needs to change the trap setup. We mitigate the second effects with a spin-echo pulse, extending coherence by a factor of two. Nonetheless, the significantly longer coherence time already allows high-fidelity entanglement distribution over fiber links with a length of a few hundred kilometers. To reduce fiber attenuation for long-distance distribution, we convert the emitted 780 nm single photons to the low-loss telecom S band using difference-frequency generation in a periodically poled lithium niobate waveguide, pumped at 1607 nm to produce 1517 nm photons. By combining long-lived quantum memory with a telecom interface, we demonstrated long-distance atom-photon entanglement distribution through 50 km and 101 km of spooled fiber with fidelities of 85% and 71%, respectively, establishing a key component for scalable quantum communication. These results establish neutral atom nodes as promising building blocks for metropolitan-scale quantum networking and outline a scalable path toward repeater protocols based on multiplexing memories and Rydberg-mediated interactions.