We exploit the non-dissipative dynamics of a pair of electrons in a large square quantum dot to perform singlet-triplet spin measurement through a single charge detection and show how this may be used for entanglement swapping and teleportation. The method is also used to generate the AKLT ground state, a further resource for quantum computation. We justify, and derive analytic results for, an effective charge-spin Hamiltonian which is valid over a wide range of parameters and agrees well with exact numerical results of a realistic effective-mass model. Our analysis also indicates that the method is robust to choice of dot-size and initialization errors, as well as decoherence introduced by the hyperfine interaction.
A basic problem of quantum communication is to generate entangled states between distant sites. Using entanglement swapping, we are able to generate an entangled state of the desired distance from connecting many short distance entangled states. We investigate the entanglement swapping of the Valence-Bond Solid state with un-identical local filtering operations. It is found that not only a long distance entangled state can be generated from the VBS state, but also there is a trade-off between the probability and the degree of the entanglement in the resultant state. The results are also genera…
We theoretically investigate the efficiency of an entanglement swapping procedure based on the use of quantum dots as sources of entangled photon pairs. The four-photon interference that affects such efficiency is potentially limited by the fine-structure splitting and by the time correlation between cascaded photons, which provide which-path information. The effect of spectral inhomogeneity is also considered, and a possible quantum eraser experiment is discussed for the case of identical dots.
We propose a deterministic and scalable scheme to construct a two-qubit controlled-NOT (CNOT) gate and realize entanglement swapping between photonic qubits using a quantum-dot (QD) spin in a double-sided optical microcavity. The scheme is based on spin selective photon reflection from the cavity and can be achieved in a nondestructive and heralded way. We assess the feasibility of the scheme and show that the scheme can work in both the weak coupling and the strong coupling regimes. The scheme opens promising perspectives for long-distance photonic quantum communication and distributed quantu…
Photonic entanglement swapping, the procedure of entangling photons without any direct interaction, is a fundamental test of quantum mechanics and an essential resource to the realization of quantum networks. Probabilistic sources of non-classical light can be used for entanglement swapping, but quantum communication technologies with device-independent functionalities demand for push-button operation that, in principle, can be implemented using single quantum emitters. This, however, turned out to be an extraordinary challenge due to the stringent requirements on the efficiency and purity of …
We present a scheme for efficient state teleportation and entanglement swapping using a single quantum-dot spin in an optical microcavity based on giant circular birefringence. State teleportation or entanglement swapping is heralded by the sequential detection of two photons, and is finished after the spin measurement. The spin-cavity unit works as a complete Bell-state analyzer with a built-in spin memory allowing loss-resistant repeater operation. This device can work in both the weak coupling and the strong coupling regime, but high efficiencies and high fidelities are only achievable when…
We numerically study the hyperfine induced nuclear spin dynamics in a system of two coupled quantum dots in zero magnetic field. Each of the electron spins is considered to interact with an individual bath of nuclear spins via homogeneous coupling constants (all coupling coefficients being equal). In order to lower the dimension of the problem, the two baths are approximated by two single long spins. We demonstrate that the hyperfine interaction enables to utilize the nuclear baths for quantum information purposes. In particular, we show that it is possible to swap the nuclear ensembles on tim…
Transferring entangled states between photon pairs is essential for quantum communication technologies. Semiconductor quantum dots are the most promising candidate for generating polarization-entangled photons deterministically. Recent improvements in photonic quality and brightness now make them suited for complex quantum optical purposes in practical devices. Here we demonstrate for the first time swapping of entangled states between two pairs of photons emitted by a single quantum dot. A joint Bell measurement heralds the successful generation of the Bell state $Ψ^+$ with a fidelity of up t…
Bell-state analysis (BSA) is essential in quantum communication, but it is impossible to distinguish unambiguously the four Bell states in the polarization degree of freedom (DOF) of two-photon systems with only linear optical elements, except for the case in which the BSA is assisted with hyperentangled states, the simultaneous entanglement in more than one DOF. Here, we propose a scheme to distinguish completely the 16 hyperentangled Bell states in both the polarization and the spatial-mode DOFs of two-photon systems, by using the giant nonlinear optics in quantum dot-cavity systems. This sc…
Tomographic analysis demonstrates that the polarization state of pairs of photons emitted from a biexciton decay cascade becomes entangled when spectral filtering is applied. The measured density matrix of the photon pair satisfies the Peres criterion for entanglement by more than 3 standard deviations of the experimental uncertainty and violates Bell's inequality. We show that the spectral filtering erases the ``which path'' information contained in the photons color and that the remanent information in the quantum dot degrees of freedom is negligible.
We propose a Bell measurement free scheme to implement a quantum repeater in GaAs/AlGa double qunatum dot systems.we prove the four pairs of double quantum dots compose an entanglement unit, given the the initial state is singlet states. Our shceme differs from the famous Duan-Lukin-Cirac-zoller(DLCZ) protocol in that Bell measurements are unneccessary for the entanglement swapping,which provides great advantages and conveniences in experimental implementaion. Our scheme significantly improve the success probability of quantum repeaters based on solid state quantum devices.
Quantum gates that temporarily increase singlet-triplet splitting in order to swap electronic spins in coupled quantum dots, lead inevitably to a finite double-occupancy probability for both dots. By solving the time-dependent Schrödinger equation for a coupled dot model, we demonstrate that this does not necessarily lead to quantum computation errors. Instead, the coupled dot ground state evolves quasi-adiabatically for typical system parameters so that the double-occupancy probability at the completion of swapping is negligibly small. We introduce a measure of entanglement which explicitly t…
Semiconductor quantum dots in photonic integrated circuits enable scaling quantum-information processing to many single photons and quantum-optical gates. On-chip spectral filters are essential to achieve high-purity and coherent photon emission from quantum dots embedded in waveguides, without resorting to free-space optics. Such spectral filters should be tunable, to compensate for the inhomogeneous spectral distribution of the quantum dots transitions. Here, we report an on-chip filter monolithically integrated with quantum dots, that uses nanomechanical motion for tuning its resonant wavel…
Inspired by recent developments in the control and manipulation of quantum dot nuclear spins, which allow for the transfer of an electron spin state to the surrounding nuclear-spin ensemble for storage, we propose a quantum repeater scheme that combines individual quantum dot electron spins and nuclear-spin ensembles, which serve as spin-photon interfaces and quantum memories respectively. We consider the use of low-strain quantum dots embedded in high-cooperativity optical microcavities. Quantum dot nuclear-spin ensembles allow for the long-term storage of entangled states, and heralded entan…
A quantum dot can be used as a source of one- and two-photon states and of polarisation entangled photon pairs. The emission of such states is investigated from the point of view of frequency-resolved two-photon correlations. These follow from a spectral filtering of the dot emission, which can be achieved either by using a cavity or by placing a number of interference filters before the detectors. The combination of these various options is used to iteratively refine the emission in a "distillation" process and arrive at highly correlated states with a high purity. So-called "leapfrog process…
We propose a scheme for the complete differentiation of 64 three-photon hyperentangled GHZ states in both polarization and spatial-mode degrees of freedoms using the quantum-dot cavity system. The three-photon hyperentangled-GHZ-state-analysis scheme can also be used to generate 3-photon hyperentangled GHZ states. This proposed hyperentangled analysis and generation device can serve as crucial components of the high-capacity, long-distance quantum communication. We use quantum swapping as an example to show the application of this device in quantum communication with polarization and spatial-m…
Semiconductor quantum dots are promising building blocks for quantum communication applications. Although deterministic, efficient, and coherent emission of entangled photons has been realized, implementing a practical quantum repeater remains outstanding. Here we explore the statistical limits for entanglement swapping with sources of polarization-entangled photons from the commonly used biexciton-exciton cascade. We stress the necessity of tuning the exciton fine structure, and explain why the often observed time evolution of photonic entanglement in quantum dots is not applicable for large …
Among the different platforms for quantum information processing, individual electron spins in semiconductor quantum dots stand out for their long coherence times and potential for scalable fabrication. The past years have witnessed substantial progress in the capabilities of spin qubits. However, coupling between distant electron spins, which is required for quantum error correction, presents a challenge, and this goal remains the focus of intense research. Quantum teleportation is a canonical method to transmit qubit states, but it has not been implemented in quantum-dot spin qubits. Here, w…
We consider a generalized quantum teleportation protocol for an unknown qubit using non-maximally entangled state as a shared resource. Without recourse to local filtering or entanglement concentration, using standard Bell-state measurement and classical communication one cannot teleport the state with unit fidelity and unit probability. We show that using non-maximally entangled measurements one can teleport an unknown state with unit fidelity albeit with reduced probability, hence probabilistic teleportation. We also give a generalized protocol for entanglement swapping using non-maximally e…
Hybrid interfaces between distinct quantum systems play a major role in the implementation of quantum networks. Quantum states have to be stored in memories to synchronize the photon arrival times for entanglement swapping by projective measurements in quantum repeaters or for entanglement purification. Here, we analyze the distortion of a single photon wave packet propagating through a dispersive and absorptive medium with high spectral resolution. Single photons are generated from a single In(Ga)As quantum dot with its excitonic transition precisely set relative to the Cesium D1 transition. …
Single photons produced by fundamentally dissimilar physical processes will in general not be indistinguishable. We show how photons produced from a quantum dot and by parametric down-conversion in a nonlinear crystal can be manipulated to be indistinguishable. The measured two-photon coalescence probability is 16%, and is limited by quantum-dot decoherence. Temporal filtering to the quantum dot coherence time and accounting for detector time response increases this to 61% while retaining 25% of the events. This technique can connect different elements in a scalable quantum network.
We classify multipartite entangled states in the 2 x 2 x n (n >= 4) quantum system, for example the 4-qubit system distributed over 3 parties, under local filtering operations. We show that there exist nine essentially different classes of states, and they give rise to a five-graded partially ordered structure, including the celebrated Greenberger-Horne-Zeilinger (GHZ) and W classes of 3 qubits. In particular, all 2 x 2 x n-states can be deterministically prepared from one maximally entangled state, and some applications like entanglement swapping are discussed.
We introduce a filter using a noise-free quantum buffer with large optical bandwidth that can both filter temporal-spectral modes, as well as inter-convert them and change their frequency. We show that such quantum buffers optimally filter out temporal-spectral noise; producing identical single-photons from many distinguishable noisy single-photon sources with the minimum required reduction in brightness. We then experimentally demonstrate a noise-free quantum buffer in a warm atomic system that is well matched to quantum dots and can outperform all intensity (incoherent) filtering schemes for…
Single-photon sources play a key role in photonic quantum technologies. Semiconductor quantum dots can emit indistinguishable single photons under resonant excitation. However, the resonance fluorescence technique typically requires cross-polarization filtering which causes a loss of the unpolarized quantum dot emission by 50%. To solve this problem, we demonstrate a method to generate indistinguishable single photons with optically controlled polarization by two laser pulses off-resonant with neutral exciton states. This scheme is realized by exciting the quantum dot to the biexciton state an…
Highly Tunable Two-Qubit Interactions in Si/SiGe Quantum Dots by Interchanging the Roles of Qubit-Defining Gates
NextQuantum, Department of Physics and Astronomy, and Institute of Applied Physics, Seoul National University（首尔大学）；QuTech and Kavli Institute of Nanoscience, Delft University of Technology（代尔夫特理工大学）
Experimental zero-added-loss multiplexing Bell-pair source for long-haul quantum networks
Advanced ICT Research Institute, National Institute of Information and Communications Technology (NICT)（国家信息通信技术研究所先进ICT研究院）；Department of Electronics and Electrical Engineering, Keio University（庆应义塾大学电子电气工程系）；Graduate School of Engineering Science, The University of Osaka（大阪大学工学研究科）；Center for Quantum Information and Quantum Biology, The University of Osaka（大阪大学量子信息与量子生物学中心）
Narrow-band filtering of light is widely used in optical spectroscopy. Atomic filters, which rely on the Faraday effect, allow for GHz-wide transmission spectra, which are intrinsically matched to an atomic transition. We present an experimental realization and a theoretical study of a Faraday anomalous dispersion optical filter (FADOF) based on cesium and its D$_1$-line-transition ($6^2S_{1/2}\rightarrow6^2P_{1/2}$) around 894nm. We also present the prospects and visions for combining this filter with the single photon emission of a single quantum dot (QD), which matches with the atomic trans…
We demonstrate a hybrid device architecture where the charge states in a double quantum dot (DQD) formed in a Si/SiGe heterostructure are read out using an on-chip superconducting microwave cavity. A quality factor Q = 5,400 is achieved by selectively etching away regions of the quantum well and by reducing photon losses through low-pass filtering of the gate bias lines. Homodyne measurements of the cavity transmission reveal DQD charge stability diagrams and a charge-cavity coupling rate g_c/2pi = 23 MHz. These measurements indicate that electrons trapped in a Si DQD can be effectively couple…
Photon pairs produced by parametric down-conversion or four-wave mixing can interfere with each other in multiport interferometers, or carry entanglement between distant nodes for use in entanglement swapping. This requires the photons be spectrally pure to ensure good interference, and have high heralding efficiency to know accurately the number of photons involved and to maintain high rates as the number of photons grows. Spectral filtering is often used to remove noise and define spectral properties. For heralded single photons high purity and heralding efficiency is possible by filtering t…
A simple calculation method for photon statistics of frequency-filtered fields is proposed. This method, based on eigenvalue decompositions of superoperators, allows us to study effects on the photon statistics of spectral filtering by various types of filters, such as Gaussian and rectangular filters as well as Lorentzian filters, which is not possible by conventional approaches. As an example, this method is applied to a simulation of quantum dot single-photon emitters, where we found the efficient choice of the filter types to have pure single photons depends on the excitation conditions i.…
Applying high power continuous-wave resonant s-shell excitation to a single self-assembled In- GaAs quantum dot, we demonstrate the generation of post-selected single-indistinguishable photons from the Mollow triplet sidebands. A sophisticated spatial filtering technique based on a double Michelson interferometer enabled us to separate the spectrally close lying individual Mollow components and perform almost background free two-photon interference measurements. Showing high consistency with the results of an independent determination of the emission coherence of 250 +/- 30 ps, our analysis re…
We review progress on the spintronics proposal for quantum computing where the quantum bits (qubits) are implemented with electron spins. We calculate the exchange interaction of coupled quantum dots and present experiments, where the exchange coupling is measured via transport. Then, experiments on single spins on dots are described, where long spin relaxation times, on the order of a millisecond, are observed. We consider spin-orbit interaction as sources of spin decoherence and find theoretically that also long decoherence times are expected. Further, we describe the concept of spin filteri…
We investigate pump-induced exciton inversion in a quantum-dot cavity system with continuous wave drive. Using a polaron-based master equation, we demonstrate excited-state populations above 0.9 for an InAs dot at a phonon bath temperature of 4K. In an exciton-driven system, the dominant mechanism is incoherent excitation from the phonon bath. For cavity driving, the mechanism is phonon-mediated switching between ground- and excited-state branches of the ladder of photon states, as quantum trajectory simulations clearly show. The exciton inversion as a function of detuning is found to be quali…
We show how "single" quantum dots, each hosting a singlet-triplet qubit, can be placed in arrays to build a spin quantum cellular automaton. A fast ($\sim 10$ ns) deterministic coherent singlet-triplet filtering, as opposed to current incoherent tunneling/slow-adiabatic based quantum gates (operation time $\sim 300$ ns), can be employed to produce a two-qubit gate through capacitive (electrostatic) coupling that can operate over significant distances. This is the coherent version of the widely discussed charge and nano-magnet cellular automata and would offer speed, reduce dissipation, perform…
The development of scalable sources of non-classical light is fundamental to unlock the technological potential of quantum photonics\cite{Kimble:Nat2008}. Among the systems under investigation, semiconductor quantum dots are currently emerging as near-optimal sources of indistinguishable single photons. However, their performances as sources of entangled-photon pairs are in comparison still modest. Experiments on conventional Stranski-Krastanow InGaAs quantum dots have reported non-optimal levels of entanglement and indistinguishability of the emitted photons. For applications such as entangle…
Entanglement-based quantum key distribution can enable secure communication in trusted node-free networks and over long distances. Although implementations exist both in fiber and in free space, the latter approach is often considered challenging due to environmental factors. Here, we implement a quantum communication protocol during daytime for the first time using a quantum dot source. This technology presents advantages in terms of narrower spectral bandwidth -- beneficial for filtering out sunlight -- and negligible multiphoton emission at peak brightness. We demonstrate continuous operati…
Quantum interference between spectral bandwidth mismatched photons
Faculty of Physics, University of Warsaw（华沙大学物理学院）；Integrated Quantum Optics, Institute for Photonic Quantum Systems (PhoQS), Paderborn University（帕德博恩大学光子量子系统研究所（PhoQS）集成量子光学）
Semiconductor quantum dots embedded in micro-pillar cavities are excellent emitters of single photons when pumped resonantly. Often, the same spatial mode is used to both resonantly excite a quantum dot and to collect the emitted single photons, requiring cross-polarization to reduce the uncoupled scattered laser light. This inherently reduces the source brightness to 50 %. Critically, for some quantum applications the total efficiency from generation to detection must be over 50 %. Here, we demonstrate a resonant-excitation approach to creating single photons that is free of any cross-polariz…
This paper presents an augmented Markovian system model for non-Markovian quantum systems. In this augmented system model, ancillary systems are introduced to play the role of internal modes of the non-Markovian environment converting white noise to colored noise. Consequently, non-Markovian dynamics are represented as resulting from direct interaction of the principal system with the ancillary system. To demonstrate the utility of the proposed augmented system model, it is applied to design whitening quantum filters for non-Markovian quantum systems. Examples are presented to illustrate how w…
Entanglement resources are key ingredients of future quantum technologies. If they could be efficiently integrated into a semiconductor platform a new generation of devices could be envisioned, whose quantum-mechanical functionalities are controlled via the mature semiconductor technology. Epitaxial quantum dots (QDs) embedded in diodes would embody such ideal quantum devices, but QD structural asymmetries lower dramatically the degree of entanglement of the sources and hamper severely their real exploitation in the foreseen applications. In this work, we overcome this hurdle using strain-tuna…
We demonstrate that semiconductor quantum dots can be excited efficiently in a resonant three-photon process, whilst resonant two-photon excitation is highly suppressed. Time-dependent Floquet theory is used to quantify the strength of the multi-photon processes and model the experimental results. The efficiency of these transitions can be drawn directly from parity considerations in the electron and hole wavefunctions in semiconductor quantum dots. Finally, we exploit this technique to probe intrinsic properties of InGaN quantum dots. In contrast to non-resonant excitation, slow relaxation of…
Semiconductor quantum dot spin qubits are promising candidates for quantum computing. In these systems, the dynamically corrected gates offer considerable reduction of gate errors and are therefore of great interest both theoretically and experimentally. They are, however, designed under the static-noise model and may be considered as low-frequency filters. In this work, we perform a comprehensive theoretical study of the response of a type of dynamically corrected gates, namely the {\sc supcode} for singlet-triplet qubits, to realistic $1/f$ noises with frequency spectra $1/ω^α$. Through rand…
We present rigorous and intuitive master equation models to study on-demand single photon sources from pulse-excited quantum dots coupled to cavities. We consider three methods of source excitation: resonant pi-pulse, off-resonant phonon-assisted inversion, and two-photon excitation of a biexciton-exciton cascade, and investigate the effect of the pulse excitation process on the quantum indistinguishability, efficiency, and purity of emitted photons. By explicitly modelling the time-dependent pulsed excitation process in a manner which captures non-Markovian effects associated with coupling to…
We demonstrate a high-purity source of indistinguishable single photons using a quantum dot embedded in a nanophotonic waveguide. The source features a near-unity internal coupling efficiency and the collected photons are efficiently coupled off-chip by implementing a taper that adiabatically couples the photons to an optical fiber. By quasi-resonant excitation of the quantum dot, we measure a single-photon purity larger than 99.4% and a photon indistinguishability of up to 94+-1% by using p-shell excitation combined with spectral filtering to reduce photon jitter. A temperature-dependent stud…
The key challenge to scalable optical quantum computing, boson sampling, and quantum metrology is sources of single photons with near-unity system efficiency and simultaneously near-perfect indistinguishability in all degrees of freedom (including spectral, temporal, spatial, and polarization). However, previous high-indistinguishability solid-state single-photon sources had to rely on polarization filtering that reduced the system efficiency by at least 50%. Here, we overcome this challenge by developing a new single-photon source based on a coherently driven quantum dot embedded in an ellipt…
We devise a mathematical framework for assessing the fidelity of multi-photon entangled states generated by a single solid-state quantum emitter, such as a quantum dot or a nitrogen-vacancy center. Within this formalism, we theoretically study the role of imperfections present in real systems on the generation of time-bin encoded Greenberger-Horne-Zeilinger and one-dimensional cluster states. We consider both fundamental limitations, such as the effect of phonon-induced dephasing, interaction with the nuclear spin bath, and second-order emissions, as well as technological imperfections, such a…
Spin-1/2 electrons are scattered through one or two diamond-like loops, made of quantum dots connected by one-dimensional wires, and subject to both an Aharonov-Bohm flux and (Rashba and Dresselhaus) spin-orbit interactions. With some symmetry between the two branches of each diamond, and with appropriate tuning of the electric and magnetic fields (or of the diamond shapes) this device completely blocks electrons with one polarization, and allows only electrons with the opposite polarization to be transmitted. The directions of these polarizations are tunable by these fields, and do not depend…
Integration of single-photon sources and detectors to silicon-based photonics opens the possibility of complex circuits for quantum information processing. In this work, we demonstrate integration of quantum dots with a silicon photonic add-drop filter for on-chip filtering and routing of telecom single photons. A silicon microdisk resonator acts as a narrow filter that transfers the quantum dot emission and filters the background over a wide wavelength range. Moreover, by tuning the quantum dot emission wavelength over the resonance of the microdisk we can control the transmission of the emit…
