Quantum entanglement emerges naturally in interacting quantum systems and plays a central role in quantum information processing [1][2][3][4] . But the generation of entanglement does not require direct interactions: single-photon detection in spin-flip Raman scattering projects two distant spins onto a maximally entangled state, provided that it is impossible to determine the source of the detected photon 5 . Here, we demonstrate such heralded quantum entanglement [6][7][8][9] of two quantum-dot hole spins separated by 5 m using single-photon interference. Thanks to fast spin initialization in 10 ns, hole-spin coherence lasting ∼40 ns and e cient photon extraction from dots [10][11][12] embedded in leaky microcavity structures, we generate 2,300 entangled spin pairs per second, which represents a 1,000-fold improvement as compared to previous experiments 13. The delayed two-photon interference scheme we developed allows the e cient verification of quantum correlations. Combined with schemes for transferring quantum information to a long-lived memory qubit 14 , fast entanglement generation could impact quantum repeater architectures.In contrast to previous experiments demonstrating electron spin photon entanglement [10][11][12] , our experiments are based on heavyhole pseudo-spins in self-assembled quantum dots (QD) that have been shown to exhibit long coherence times [15][16][17][18] . Figure 1a depicts our experimental set-up, incorporating two QDs separated by 5 m that are resonantly driven by weak 3.2 ns-long pulses from a Ti:Sapphire laser, termed the entanglement laser. Additional diode laser pulses ensure that each QD is optically charged with a single excess heavy hole and that the hole pseudo-spin is prepared in the requisite state. The QDs are embedded in distributed Bragg reflector (DBR) structures 19 which, together with a ZnO solid immersion lens, allow efficient (∼20%) collection of the generated resonance fluorescence. Figure 1b shows the relevant energy-level diagram as well as the allowed optical transitions for single-hole charged QDs when an external magnetic field (B x ) is applied perpendicular to the growth direction (Voigt geometry; refs 20,21). The initial states of the optical transitions in the single-hole charged regime are metastable states identified by the orientation of the heavy-hole pseudo-spin, with |⇑ (|⇓ ) denoting +3/2 (−3/2) hole angular momentum projection. The presence of B x = 0 yields a finite splitting of the pseudo-spin states due to heavy-light hole mixing 22 . Spontaneous emission of a V (H) polarized photon at frequency ω blue (ω diag1 ) from the trion state |T b at rate Γ /2 brings the QD back into the |⇓ (|⇑ ) state. Owing to these selection rules, addressing any of the four allowed transitions with a single laser will efficiently transfer the spin population into the opposite ground state within 10 ns (see Supplementary information). As the intensity of the entanglement laser is chosen to be well below saturation, the ensuing optical transitions lead to either V-...

No abstract

A quantum interface between a propagating photon used to transmit quantum information and a long-lived qubit used for storage is of central interest in quantum information science. A method for implementing such an interface between dissimilar qubits is quantum teleportation. Here we experimentally demonstrate transfer of quantum information carried by a photon to a semiconductor spin using quantum teleportation. In our experiment, a single photon in a superposition state is generated using resonant excitation of a neutral dot. To teleport this photonic qubit, we generate an entangled spin-photon state in a second dot located 5 m away and interfere the photons from the two dots in a Hong-Ou-Mandel set-up. Thanks to an unprecedented degree of photon-indistinguishability, a coincidence detection at the output of the interferometer heralds successful teleportation, which we verify by measuring the resulting spin state after prolonging its coherence time by optical spin-echo.

Single photon emitters (SPEs) in low-dimensional layered materials have recently gained a large interest owing to the auspicious perspectives of integration and extreme miniaturization offered by this class of materials. However, accurate control of both the spatial location and the emission wavelength of the quantum emitters is essentially lacking to date, thus hindering further technological steps towards scalable quantum photonic devices. Here, we evidence SPEs in high purity synthetic hexagonal boron nitride (hBN) that can be activated by an electron beam at chosen locations. SPE ensembles are generated with a spatial accuracy better than the cubed emission wavelength, thus opening the way to integration in optical microstructures. Stable and bright single photon emission is subsequently observed in the visible range up to room temperature upon non-resonant laser excitation. Moreover, the low-temperature emission wavelength is reproducible, with an ensemble distribution of width 3 meV, a statistical dispersion that is more than one order of magnitude lower than the narrowest wavelength spreads obtained in epitaxial hBN samples. Our findings constitute an essential step towards the realization of top-down integrated devices based on identical quantum emitters in 2D materials.

In this Letter we investigate a low dimensional semiconductor system, in which the light-matter interaction is enhanced by the cooperative behavior of a large number of dipolar oscillators, at different frequencies, mutually phase locked by Coulomb interaction. We experimentally and theoretically demonstrate that, owing to this phenomenon, the optical response of a semiconductor quantum well with several occupied subbands is a single sharp resonance, associated with the excitation of a bright multisubband plasmon. This effect illustrates how the whole oscillator strength of a two-dimensional system can be concentrated into a single resonance independently from the shape of the confining potential. When this cooperative excitation is tuned in resonance with a cavity mode, their coupling strength can be increased monotonically with the electronic density, allowing the achievement of the ultrastrong coupling regime up to room temperature.

The strength of the light-matter interaction depends on the number of dipoles that can couple with the photon trapped in an optical cavity. The coupling strength can thus be maximized by filling the entire cavity volume with an ensemble of interacting dipoles. In this work this is achieved by inserting a highly doped semiconductor layer in a subwavelength plasmonic resonator. In our system the ultra-strong light-matter coupling occurs between a collective electronic excitation and the cavity photon. The measured coupling strength is 73% of the matter excitation energy, the highest ever reported for a light-matter coupled system at room temperature. We experimentally and theoretically demonstrate that such an ultra-strong interaction modifies the optical properties on a very wide spectral range (20-250 meV), and results in the appearance of a photonic gap of 38 meV, independently of the light polarization and angle of incidence. Light-matter ultra-strong coupling can thus be exploited to conceive metasurfaces with an engineered reflectivity band.Content from this work may be used under the terms of the Creative Commons Attribution 3.0 licence. Any further distribution of this work must maintain attribution to the author(s) and the title of the work, journal citation and DOI. R matter greater than 0.5 at 4 K [20]. This increase of the relative coupling strength is associated with the uniform filling of the cavity mode volume with electrons. In this work, we further increase the strength of the coupling with respect to previous values in the literature by filling the entire cavity space with a highly doped semiconductor New J. Phys. 16 (2014) 043029 B Askenazi et al New J. Phys. 16 (2014) 043029 B Askenazi et al 4 New J. Phys. 16 (2014) 043029 B Askenazi et al 7

Single-shot read-out of individual qubits is typically the slowest process among the elementary single-and two-qubit operations required for quantum information processing. Here, we use resonance fluorescence from a single-electron charged quantum dot to read-out the spin-qubit state in 800 nanoseconds with a fidelity exceeding 80%. Observation of the spin evolution on longer timescales reveals quantum jumps of the spin state: we use the experimentally determined waitingtime distribution to characterize the quantum jumps.PACS numbers: 03.67. Lx, 73.21.La, A fundamental difficulty in quantum information processing is the need for isolation of individual quantum systems from their noisy environment on the one hand, and the requirement for information extraction by selective coupling of qubits to classical (noisy) detectors on the other hand [1]. The requisite one-and two-qubit operations, as well as initialization of each qubit can be carried out by using classical out-of-equilibrium external fields, such as lasers or microwaves; the lack of a need for heralding the successful completion of these operations ensures that they can be accomplished in short timescales. In contrast, quantum measurements are typically slow since information extraction by a classical observer is in many cases hindered by the need to protect the qubit from the external fluctuations. While ingenious schemes for fast qubit measurements have been developed, the timescales required for a high fidelity qubit measurement remains at least an order of magnitude longer than those required for coherent operations in practically all quantum information processing schemes [2][3][4]. In the case of spin qubits in optically active quantum dots (QD), the predicament is even more striking since while optical excitation allows for fast turn on/off of light-matter interaction enabling spin read-out, it at the same time allows for an additional fast channel for spin relaxation. In fact, with the exception of a slow coupled QD scheme requiring a designated read-out QD [5], it has not been possible to carry out single-shot spin measurements on isolated optically active spin qubits [6].In this Letter, we overcome the predicament underlying single-shot spin read-out by enhancing the collection efficiency of resonance fluorescence (RF) from spindependent recycling transitions that are ubiquitous to single-electron charged QDs. The photon collection efficiency of 0.45% that we achieve allows us to obtain a single-shot spin read-out fidelity exceeding 80% in a measurement time of 800 ns. This result corresponds to an enhancement of the spin read-out time by almost three orders of magnitude as compared to the prior measurements on coupled QDs [5]. Continuous monitoring of the spin state enabled by single-shot read-out reveals quantum jumps of the observed spin stemming either from the finite T 1 spin lifetime or spin pumping induced by the resonant read-out laser. A theoretical analysis of quantum jumps using the waiting time distribution (W (τ )) was presented ear...

Articles you may be interested inCoexistence of guided mode resonance and extraordinary optical transmission in metal/dielectric/metal photonic crystal slab GaN hybrid microcavities in the strong coupling regime grown by metal-organic chemical vapor deposition on sapphire substrates