Abstract Long distance entanglement distribution is an important problem for quantum information technologies to solve. Current optical schemes are known to have fundamental limitations. A coherent photon-to-spin interface built with quantum dots (QDs) in a direct bandgap semiconductor can provide a solution for efficient entanglement distribution. QD circuits offer integrated spin processing for full Bell state measurement (BSM) analysis and spin quantum memory. Crucially the photo-generated spins can be heralded by non-destructive charge detection techniques. We review current schemes to transfer a polarization-encoded state or a time-bin-encoded state of a photon to the state of a spin in a QD. The spin may be that of an electron or that of a hole. We describe adaptations of the original schemes to employ heavy holes which have a number of attractive properties including a g -factor that is tunable to zero for QDs in an appropriately oriented external magnetic field. We also introduce simple throughput scaling models to demonstrate the potential performance advantage of full BSM capability in a QD scheme, even when the quantum memory is imperfect, over optical schemes relying on linear optical elements and ensemble quantum memories.
Abstract Long distance entanglement distribution is an important problem for quantum information technologies to solve. Current optical schemes are known to have fundamental limitations. A coherent photon-to-spin interface built with quantum dots (QDs) in a direct bandgap semiconductor can provide a solution for efficient entanglement distribution. QD circuits offer integrated spin processing for full Bell state measurement (BSM) analysis and spin quantum memory. Crucially the photo-generated spins can be heralded by non-destructive charge detection techniques. We review current schemes to transfer a polarization-encoded state or a time-bin-encoded state of a photon to the state of a spin in a QD. The spin may be that of an electron or that of a hole. We describe adaptations of the original schemes to employ heavy holes which have a number of attractive properties including a g -factor that is tunable to zero for QDs in an appropriately oriented external magnetic field. We also introduce simple throughput scaling models to demonstrate the potential performance advantage of full BSM capability in a QD scheme, even when the quantum memory is imperfect, over optical schemes relying on linear optical elements and ensemble quantum memories.
InAs quantum dots (QDs) can be used as optically coupled quantum storage devices for quantum information applications. The QD can be charged with a single electron, where the spin state (up or down) provides a long lived quantum bit (qubit). The QD's optically excited states are used to initialize, manipulate, and read out the electron spin state with laser pulses. However, most practical quantum information applications require many interacting qubits, forming a quantum network. Since QDs are based on semiconductor technology, and are compatible with standard nano-fabrication processing, there is promise that they can provide a solid state platform where a scalable quantum information architecture is realizable. We focus on scaling the QD system to multiple qubits using intermediate spin-photon entangled states. In this work, experimental and theoretical techniques are developed to study the QD-light matter interaction at the single photon level. Resonance fluorescence from a single QD is experimentally realized, and the single photon nature of the scattered radiation is verified through intensity correlation experiments. Transient fluorescence measurements on resonantly excited QDs are performed using time correlated single photon counting techniques to measure the excited state lifetime. High speed electro-optic modulators are used to time gate narrow bandwidth lasers, so that a QD can be driven under step-wise excitation, allowing for the direct observation of time-dependent optical Rabi oscillations. From these measurements, we are able to extract a decoherence rate which is consistent with the lifetime limit, indicating that pure dephasing is negligible in this system. These techniques are applied to the QD spin system to demonstrate a spin-photon entangled state, by performing correlation measurements on the spin and photon state in two bases. A lower bound on the entanglement fidelity of 0.59(4) is achieved, which exceeds the classical limit of 0.5 by more than two standard deviations. The entanglement fidelity is limited primarily by the finite timing resolution of available single photon detectors. Taking this into account, we achieve 84% of the apparatus limited fidelity. These spin-entangled photons can be used to mediate entanglement between distant QD spins, providing the basis of an optically coupled QD spin network.
Abstract The realization of a functional quantum repeater is one of the major research goals in long-distance quantum communication. Among the different approaches that are being followed, the one relying on quantum memories interfaced with deterministic quantum emitters is considered as one of the most promising solutions. In this work, we focus on the hardware to implement memory-based quantum-repeater schemes that rely on semiconductor quantum dots (QDs) for the generation of polarization entangled photons. Going through the most relevant figures of merit related to efficiency of the photon source, we select significant developments in fabrication, processing and tuning techniques aimed at combining high degree of entanglement with on-demand pair generation, with a special focus on the progress achieved in the representative case of the GaAs system. We proceed to offer a perspective on integration with quantum memories, both highlighting preliminary works on natural–artificial atomic interfaces and commenting a wide choice of currently available and potentially viable memory solutions in terms of wavelength, bandwidth and noise-requirements. To complete the overview, we also present recent implementations of entanglement-based quantum communication protocols with QDs and highlight the next challenges ahead for the implementation of practical quantum networks.
The ultimate goal of quantum information science is to build quantum networks for distributed quantum computing or for the secure sharing of information between spatially remote parties [1]. Quantum networks utilize static (matter) quantum bits (qubits) to store and process quantum information at local nodes, and photons as flying qubits for long-distance quantum state transmission between different nodes. To realize a quantum network, it is crucial to achieve light-matter entanglement and reversible quantum-state transfer between light and matter, i.e., the light-matter quantum interface, and the quantum repeater for large-scale quantum communications [2]. Recent experiments have shown that both electrons and holes confined in semiconductor quantum dots (QDs) have long spin relaxation time (T1e, T1h ~ ms) and long spin coherence time (T2e ~ s, T2h > 100 ns). Moreover, fast spin cooling and ultra-fast spin manipulation as well as spin echoes to preserve the spin coherence have also been demonstrated. Undoubtedly these rapid progresses imply that the QD spin is a good candidate for matter qubit in quantum information processing. Furthermore, QD-based single photon sources have been also developed. Therefore, semiconductor QDs offer a good platform for solid-state quantum networking. Here we present two types of conditional quantum gates, i.e., the photon-spin entangling gates [3-4] using a single QD spin in a single-sided or double-sided optical microcavity. Both gates are universal and deterministic (if they are optimized). The spin-selective coherent photon-spin interaction enhanced by the cavity QED lead to giant circular birefringence, which allows us to build these gates. We will show some of the key applications [3-5], including: (i) single-shot quantum non-demolition measurement of spin; (ii) single-photon based spin manipulations and spin echoes to preserve the spin coherence; (iii) photon-spin, spin-spin and photon-photon entanglement generation with high fidelity and high efficiency; (iv) deterministic photon-spin interface and perfect spin memory; (v) complete and loss-resistant Bell-state analyzer, which could increase the distance of state-of-the-art quantum communications by one order of magnitude (from current 100 km to over 1000 km) [5]; (vi) the full quantum repeater combining quantum memory, entanglement swapping and purifications for arbitrary long distance communications [5]; (vii) spin-controlled single photon sources. All these schemes are feasible with current semiconductor technology [6], and we have recently seen conditional phase shifts in uncharged QD-cavity systems [7]. The versatile spin-cavity systems can be applied in all aspects of quantum information science and technology, not only for large-scale quantum communication networks, but also for scalable quantum computing with either photons or spins as qubits. References [1] H.J. Kimble, Nature (London) 453, 1023 (2008). [2] H.-J. Briegel et al., Phys. Rev. Lett. 81, 5932 (1998). [3] C.Y. Hu et al., Phys. Rev. B 78, 085307 (2008); ibid. 78, 125318 (2008). [4] C.Y. Hu et al., Phys. Rev. B 80, 205326 (2009). [5] C.Y. Hu and J.G. Rarity, ArXiv: quant-ph 1005.5545 (2010). [6] S. Reitzenstein et al., Appl. Phys. Lett. 90, 251109 (2007). [7] A.B. Young et al., ArXiv: quant-ph 1011.0384 (2010).
Quantum entanglement emerges naturally in interacting quantum systems and plays a central role in quantum information processing. Remarkably, it is possible to generate entanglement even in the absence of direct interactions: provided that which path information is erased, weak spin-state dependent light scattering can be used to project two distant spins onto a maximally entangled state upon detection of a single photon. Even though this approach is necessarily probabilistic, successful generation of entanglement is heralded by the photon detection event. Here, we demonstrate heralded quantum entanglement of two quantum dot heavy-hole spins separated by 5 meters using single-photon interference. Thanks to the long coherence time of hole spins and the efficient spin-photon interface provided by self-assembled quantum dots embedded in leaky microcavity structures, we generate 2300 entangled spin pairs per second, which represents an improvement approaching three orders of magnitude as compared to prior experiments. Delayed two-photon interference scheme we developed allows for efficient verification of quantum correlations. Our results lay the groundwork for the realization of quantum networks in semiconductor nanostructures. Combined with schemes for transferring quantum information to a long-lived memory qubit, fast entanglement generation we demonstrate could also impact quantum repeater architectures.
Coherent control of solid-state qubits lies at the heart of most quantum information architectures. In quantum dots (QDs), optical fields are an attractive medium for qubit manipulation and readout. The entanglement between a QD spin qubit and an emitted photonic qubit allows for the transport of quantum information between distant quantum memories via decoherence resistant photon channels. I will present recent experimental work showing the entanglement between a single electron spin confined to an InAs QD and its spontaneously emitted photon. This entanglement is significant for the further development of quantum information technologies using QDs and forms the foundation of on-chip technologies using photonic crystal pathways. In addition, I will discuss on-going work on teleportation of information from a single photon generated in a spontaneous parametric down conversion (SPDC) process to a QD spin through intermediate interference between the SPDC photon and the dot’s emitted photon. The ability to integrate two quantum information platforms is not only exciting in its own right, but this technique could allow for an entanglement swapping bridge between other matter-qubit (ions, NV centers, etc.) based quantum memories.
A central challenge for scalable quantum networks is the realization of coherent interfaces between stationary qubits and telecom-band photonic qubits for long-distance entanglement distribution. Semiconductor quantum dots emitting at telecom wavelengths present a promising spin-photon platform, and a precise understanding of the properties of the confined spin is crucial for optimizing its interplay with the photonic qubit. Here, we simultaneously benchmark the electron and hole g-factors and coherence properties of a droplet epitaxy QD, solely from time and polarization resolved photon correlations. These measurements identify the hole as the preferable qubit for spin-photon entanglement in quantum network nodes. We then perform full state tomography of the confined hole ground state to reveal subtle anisotropies in the spin precession, providing essential diagnostics for minimizing phase errors critical for deterministic multiphoton entanglement generation.