Electromagnetically Induced Transparency of On-demand Single Photons in a Hybrid Quantum Network
Lucas Schweickert, Klaus D. Jöns, Mehdi Namazi, Guodong Cui, Thomas Lettner, Katharina D. Zeuner, Lara Scavuzzo, Saimon Filipe Covre da Silva, Marcus Reindl, Huiying Huang, Rinaldo Trotta, Armando Rastelli, Val Zwiller, Eden V. Figueroa
Long range quantum communication and quantum information processing require the development of light-matter interfaces for distributed quantum networks. Even though photons are ideal candidates for network links to transfer quantum information, the system of choice for the realization of quantum nodes has not been identified yet. Ideally, one strives for a hybrid network architecture, which will consist of different quantum systems, combining the strengths of each system. However, interfacing different quantum systems via photonic channels remains a major challenge because a detailed understanding of the underlying light-matter interaction is missing. Here, we show the coherent manipulation of single photons generated on-demand from a semiconductor quantum dot using a rubidium vapor quantum memory, forming a hybrid quantum network. We demonstrate the engineering of the photons' temporal wave function using four-level atoms and the creation of a new type of electromagnetic induced transparency for quantum dot photons on resonance with rubidium transitions. Given the short lifetime of our quantum dot transition the observed dynamics cannot be explained in the established steady-state picture. Our results play a pivotal role in understanding quantum light-matter interactions at short time scales. These findings demonstrate a fundamental active node to construct future large-scale hybrid quantum networks.
Long range quantum communication and quantum information processing require the development of light-matter interfaces for distributed quantum networks. Even though photons are ideal candidates for network links to transfer quantum information, the system of choice for the realization of quantum nodes has not been identified yet. Ideally, one strives for a hybrid network architecture, which will consist of different quantum systems, combining the strengths of each system. However, interfacing different quantum systems via photonic channels remains a major challenge because a detailed understanding of the underlying light-matter interaction is missing. Here, we show the coherent manipulation of single photons generated on-demand from a semiconductor quantum dot using a rubidium vapor quantum memory, forming a hybrid quantum network. We demonstrate the engineering of the photons' temporal wave function using four-level atoms and the creation of a new type of electromagnetic induced transparency for quantum dot photons on resonance with rubidium transitions. Given the short lifetime of our quantum dot transition the observed dynamics cannot be explained in the established steady-state picture. Our results play a pivotal role in understanding quantum light-matter interactions at short time scales. These findings demonstrate a fundamental active node to construct future large-scale hybrid quantum networks.
Long-range quantum communication requires the development of in-out light-matter interfaces to achieve a quantum advantage in entanglement distribution. Ideally, these quantum interconnections should be as fast as possible to achieve high-rate entangled qubits distribution. Here, we demonstrate the coherent quanta exchange between single photons generated on-demand from a GaAs quantum dot and atomic ensemble in a 87Rb vapor quantum memory. Through an open quantum system analysis, we demonstrate the mapping between the quantized electric field of photons and the coherence of the atomic ensemble. Our results play a pivotal role in understanding quantum light-matter interactions at the short time scales required to build fast hybrid quantum networks.
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.
Quantum memories play a pivotal role in establishing long-distance communication by entanglement swapping operations within quantum repeater nodes. Constructing such a quantum memory involves Electromagnetically Induced Transparency (EIT) within atomic vapors at room temperatures to store highly attenuated coherent light pulses down to the level of single photons. The photons may be generated from quantum nodes containing stationary quantum systems, such as atoms or semiconductor quantum dots (QDs). QDs serve as a potent source of quantum light, furnishing bright, precisely timed single photons of exceptional purity. While previous endeavors have demonstrated the integration of QDs with atomic vapors through techniques like “slow light,” the development of a dedicated quantum memory for QDs remains unmatched. In our study, we introduce an EIT quantum memory hosted within warm cesium vapor. Our approach exhibits a good efficiency in storing faint coherent light pulses at the single photon level. Moreover, the measured bandwidth of around 200 MHz approaches the Fourier-limited emission characteristics of QDs. We present initial efforts to match the emission from QDs with our quantum memory and discuss application scenarios of room temperature EIT quantum memories.
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.
Future quantum networks will need flying qubits and stationary nodes. As for the generation of single photons which may act as flying qubits, resonantly excited single-semiconductor quantum dots are ideal in terms of their on-demand single-photon emission, indistinguishability, and brightness. Atomic systems can effectively act as mediators for photon–photon interactions, storage media, or building blocks for stationary qubits. Here, we hybridize these two systems and investigate the non-classical interference of spectral Lorentzian-shaped photons, fine-tuned between the cesium (Cs)-D1 hyperfine resonances. The temporal delay in the dispersive hot atomic cesium vapor amounts up to 50 times the photons’ initial width and reveals beats on the single quanta. The photons’ indistinguishability is preserved even after atomic-enabled delay. This proves that the interaction with the Cs vapor conserves the photons’ coherence. The role of spectral diffusion in the solid-state emitter is studied in single- and two-photon experiments in light of the strong frequency dependence of the atomic medium. Our results pave the way for efficient hybrid interfaces between quantum dots and hot atomic vapors as storage media in future quantum networks.
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