The negatively charged silicon vacancy (SiV) color center in diamond has recently proven its suitability for bright and stable single photon emission. However, its electronic structure so far has remained elusive. We here explore the electronic structure by exposing single SiV defects to a magnetic field where the Zeeman effect lifts the degeneracy of magnetic sublevels. The similar response of single centers and a SiV ensemble in a low strain reference sample proves our ability to fabricate almost perfect single SiVs, revealing the true nature of the defect's electronic properties. We model the electronic states using a group-theoretical approach yielding a good agreement with the experimental observations. Furthermore, the model correctly predicts polarization measurements on single SiV centers and explains recently discovered spin selective excitation of SiV defects. Negatively charged silicon vacancy (SiV − ) color centers in diamond show a typical room-temperature zero phonon line (ZPL) at 738 nm which splits into a four line fine structure centered at about 737 nm when cooled down to liquid helium temperature [1][2][3]. The origin of the fine structure splitting is attributed to a split ground and excited state [1]. One mechanism that can account for the level splitting is spin-orbit (SO) coupling, like it is present for the excited state in negatively charged nitrogen-vacancy (NV − ) centers [4]. Alternatively, Clark et al. [1] and Moliver [5] suggest a tunnel splitting whereas Goss et al.[6] assume a Jahn-Teller (JT) effect in addition to SO coupling to lift the orbital degeneracy between the electronic states which account for the presumed optical transition 2 E u → 2 E g . To form doubly degenerate 2 E many-body wave functions, at least a trigonal defect symmetry is required [7,8]. The molecular structure of the SiV center was predicted using density functional theory (DFT) to show a rather unique split vacancy configuration, exhibiting a D 3d symmetry [9]. Yet, polarization [10,11] and uniaxial stress measurements [2] evidenced lower symmetrical point groups such as C 2 or D 2 symmetry. Still, all these experimental evidences were obtained using samples that possess strongly strained environments for the defect centers. In this letter, however, we present evidence for the predicted D 3d symmetry by performing spectroscopy on SiV centers in low-strain samples.Recently published EPR measurements showed that the presumed neutral charge state SiV 0 is a S = 1 system [12]. This suggests that its negative counterpart SiV − is a paramagnetic S = 1/2 system, although this has not been confirmed by independent EPR measurements so far. Very recently, we reported direct spin-selective population of the SiV − excited states under a magnetic field, resulting in a spin-tagged resonance fluorescence pattern [13], suggesting that the SiV − shows effectively S = 1/2. In the present letter, we experimentally explore the electronic states of the SiV center by measuring Zeeman splittings and polarization orientation of t...

In force sensing, optomechanics, and quantum motion experiments, it is typically advantageous to create lightweight, compliant mechanical elements with the lowest possible force noise. Here we report wafer-scale batch fabrication and characterization of high-aspect-ratio, nanogram-scale Si$_3$N$_4$ "trampolines" having quality factors above $4 \times 10^7$ and ringdown times exceeding five minutes (1 mHz linewidth). We measure a thermally limited force noise sensitivity of 16.2$\pm$0.8 aN/Hz$^{1/2}$ at room temperature, with a spring constant ($\sim$1 N/m) 2-5 orders of magnitude larger than those of competing technologies. We also characterize the suitability of these devices for high-finesse cavity readout and optomechanics applications, finding no evidence of surface or bulk optical losses from the processed nitride in a cavity achieving finesse 40,000. These parameters provide access to a single-photon cooperativity $C_0 \sim 8$ in the resolved-sideband limit, wherein a variety of outstanding optomechanics goals become feasible.Comment: 8 pages, 4 figures, 1 tabl

Colour centres in diamond have emerged as versatile tools for solid-state quantum technologies ranging from quantum information to metrology, where the nitrogen-vacancy centre is the most studied to date. Recently, this toolbox has expanded to include novel colour centres to realize more efficient spin-photon quantum interfaces. Of these, the silicon-vacancy centre stands out with highly desirable photonic properties. The challenge for utilizing this centre is to realize the hitherto elusive optical access to its electronic spin. Here we report spin-tagged resonance fluorescence from the negatively charged silicon-vacancy centre. Our measurements reveal a spin-state purity approaching unity in the excited state, highlighting the potential of the centre as an efficient spin-photon quantum interface.

Single photons and entangled photon pairs are a key resource of many quantum secure communication and quantum computation protocols, and non-Poissonian sources emitting in the low-loss wavelength region around 1,550 nm are essential for the development of fibre-based quantum network infrastructure. However, reaching this wavelength window has been challenging for semiconductor-based quantum light sources. Here we show that quantum dot devices based on indium phosphide are capable of electrically injected single photon emission in this wavelength region. Using the biexciton cascade mechanism, they also produce entangled photons with a fidelity of 87 ± 4%, sufficient for the application of one-way error correction protocols. The material system further allows for entangled photon generation up to an operating temperature of 93 K. Our quantum photon source can be directly integrated with existing long distance quantum communication and cryptography systems, and provides a promising material platform for developing future quantum network hardware.

We have performed all-optical measurements of spin relaxation in single self-assembled InAs/ GaAs quantum dots ͑QDs͒ as a function of static external electric and magnetic fields. To study QD spin dynamics, we measure the degree of resonant absorption which results from a competition between optical spin pumping induced by the resonant laser field and spin relaxation induced by reservoirs. Fundamental interactions that determine spin dynamics in QDs are hyperfine coupling to QD nuclear spin ensembles, spin-phonon coupling, and exchange-type interactions with a nearby Fermi sea of electrons. We show that the strength of spin relaxation generated by the three fundamental interactions can be changed by up to 5 orders of magnitude upon varying the applied electric and magnetic fields. We find that the strength of optical spin pumping that we use to study the spin relaxation is determined predominantly by hyperfine-induced mixing of single-electron spin states at low magnetic fields and heavy-light hole mixing at high magnetic fields. Our measurements allow us to determine the rms value of the hyperfine ͑Overhauser͒ field to be ϳ15 mT with an electron g factor of g e = 0.6 and a hole mixing strength of ͉⑀ H ͉ 2 =5ϫ 10 −4 .

Efficient sources of individual pairs of entangled photons are required for quantum networks to operate using fiber-optic infrastructure. Entangled light can be generated by quantum dots (QDs) with naturally small fine-structure splitting (FSS) between exciton eigenstates. Moreover, QDs can be engineered to emit at standard telecom wavelengths. To achieve sufficient signal intensity for applications, QDs have been incorporated into one-dimensional optical microcavities. However, combining these properties in a single device has so far proved elusive. Here, we introduce a growth strategy to realize QDs with small FSS in the conventional telecom band, and within an optical cavity. Our approach employs ''droplet-epitaxy'' of InAs quantum dots on (001) substrates. We show the scheme improves the symmetry of the dots by 72%. Furthermore, our technique is universal, and produces low FSS QDs by molecular beam epitaxy on GaAs emitting at ∼900 nm, and metal-organic vapor-phase epitaxy on InP emitting at ∼1550 nm, with mean FSS 4× smaller than for Stranski-Krastanow QDs.

A practical way to link separate nodes in quantum networks is to send photons over the standard telecom fibre network. This requires sub-Poissonian photon sources in the telecom wavelength band around 1550 nm, where the photon coherence time has to be sufficient to enable the many interference-based technologies at the heart of quantum networks. Here, we show that droplet epitaxy InAs/InP quantum dots emitting in the telecom C-band can provide photons with coherence times exceeding 1 ns even under non-resonant excitation, more than a factor two longer than values reported for shorter wavelength quantum dots under similar conditions. We demonstrate that these coherence times enable near-optimal interference with a C-band laser qubit, with visibilities only limited by the quantum dot multiphoton emission. Using entangled photons, we further show teleportation of such qubits in six different bases with average fidelity reaching 88.3±4%. Beyond direct applications in long-distance quantum communication, the high degree of coherence in these quantum dots is promising for future spin based telecom quantum network applications.

We demonstrate electrical control of the single photon emission spectrum from chromium-based colour centres implanted in monolithic diamond. Under an external electric field the tunability range is typically three orders of magnitude larger than the radiative linewidth and at least one order of magnitude larger than the observed linewidth. The electric and magnetic field dependence of luminescence gives indications on the inherent symmetry and we propose Cr-X or X-Cr-Y type noncentrosymmetric atomic configurations as most probable candidates for these centres.Room temperature operation of diamond-based colour centres offers a unique platform to the field of quantum photonics [1] . The nitrogen-vacancy (NV) colour centre in diamond has received interest in diverse research fields ranging from spin-based quantum information [2][3][4][5] to nanoscale magnetometry [6][7][8][9] within the last decade. The impressive level of spin coherence [5] , the fast microwave spin manipulation [10] , and the recently demonstrated spin-photon interface [11] are essential to the proposed applications. However, the NV centre's broad photon spectrum is unfavourable for many of these experiments. Although emission from the zero-phonon line (ZPL) has been shown to be radiative lifetime broadened, it only accounts for about 4% of the total spectrum, owing to the dominance of phonon-assisted decay. This motivated the search for alternative centres, where potentially a similar degree of spin control could coexist with superior photonic properties. Consequently, silicon-vacancy (Si-V) [12,13] and nickel-based centres, such as NE8 [14] , were studied in the last few years for this purpose. Recently, chromium-based colour centres were reported among the brightest singlephoton emitters in the near infrared spectrum [15,16] . Here, we show that these centres have an impressive level of spectral tunability on the order of a few meVs, while