A complete and always up-to-date list of publications can also be found on my Google Scholar page.
The negatively charged silicon-vacancy (SiV−) color center in diamond has recently emerged as a promising system for quantum photonics. Its symmetry-protected optical transitions enable the creation of indistinguishable emitter arrays and deterministic coupling to nanophotonic devices. Despite this, the longest coherence time associated with its electronic spin achieved to date (∼250 ns) has been limited by coupling to acoustic phonons. We demonstrate coherent control and suppression of phonon-induced dephasing of the SiV− electronic spin coherence by 5 orders of magnitude by operating at temperatures below 500 mK. By aligning the magnetic field along the SiV− symmetry axis, we demonstrate spin-conserving optical transitions and single-shot readout of the SiV- spin with 89% fidelity. Coherent control of the SiV− spin with microwave fields is used to demonstrate a spin coherence time T2 of 13 ms and a spin relaxation time T1 exceeding 1 s at 100 mK. These results establish the SiV− as a promising solid-state candidate for the realization of quantum networks.
Solid-state quantum registers that can store quantum information in a long-lived memory and efficiently interface with optical photons constitute the building blocks of a scalable quantum network. To date, no solid-state quantum-emitter has yet been able to satisfy both of these requirements. The negatively-charged silicon-vacancy (SiV–) color center in diamond gained recent attention due to its symmetry-protected optical transitions which enable deterministic spin-photon interfaces in nanophotonic devices. As a quantum memory, the coherence time of the SiV– electronic spin is limited to 100 ns at 4K by acoustic phonons. Here, we suppress phonon-induced dephasing of the SiV– spin by operating at temperatures below 500 mK. We use microwave fields to coherently control the SiV– spin and demonstrate a spin coherence time T2 of 13 ms and a spin relaxation time T1 exceeding 1 s at 100 mK. By aligning the magnetic field along the SiV– symmetry axis, we obtain spin-conserving optical transitions and demonstrate single-shot readout of the SiV– spin with 89% fidelity. These results render SiV– the first solid-state quantum emitter with excellent optical properties and long spin coherence.
Color centers in diamond provide a promising platform for quantum optics in the solid state, with coherent optical transitions and long-lived electron and nuclear spins. Building upon recent demonstrations of nanophotonic waveguides and optical cavities in single-crystal diamond, we now demonstrate on-chip diamond nanophotonics with a high-efficiency fiber-optical interface achieving >90% power coupling at visible wavelengths. We use this approach to demonstrate a bright source of narrow-band single photons based on a silicon-vacancy color center embedded within a waveguide-coupled diamond photonic crystal cavity. Our fiber-coupled diamond quantum nanophotonic interface results in a high flux (approximately 38 kHz) of coherent single photons (near Fourier limited at < 1 GHz bandwidth) into a single-mode fiber, enabling possibilities for realizing quantum networks that interface multiple emitters, both on chip and separated by long distances.
A solid-state system combining a stable spin degree of freedom with an efficient optical interface is highly desirable as an element for integrated quantum-optical and quantum-information systems. We demonstrate a bright color center in diamond with excellent optical properties and controllable electronic spin states. Specifically, we carry out detailed optical spectroscopy of a germanium-vacancy (GeV) color center demonstrating optical spectral stability. Using an external magnetic field to lift the electronic spin degeneracy, we explore the spin degree of freedom as a controllable qubit. Spin polarization is achieved using optical pumping, and a spin relaxation time in excess of 20 μs is demonstrated. We report resonant microwave control of spin transitions, and use this as a probe to measure the Autler-Townes effect in a microwave-optical double-resonance experiment. Superposition spin states were prepared using coherent population trapping, and a pure dephasing time of about 19 ns was observed at a temperature of 2.0 K.
We demonstrate an all-optical thermometer based on an ensemble of silicon-vacancy centers (SiVs) in diamond by utilizing a temperature dependent shift of the SiV optical zero-phonon line transition frequency of 6.8 GHz/K. Using SiVs in bulk diamond, we achieve 70 mK precision at room temperature with a sensitivity of 360 mK per sqrt(Hz). Finally, we use SiVs in 200 nm nanodiamonds as local temperature probes with 521 mK per sqrt(Hz) sensitivity. These results open up new possibilities for nanoscale thermometry in biology, chemistry, and physics, paving the way for control of complex nanoscale systems.
We demonstrate a quantum nanophotonics platform based on germanium-vacancy (GeV) color centers in fiber-coupled diamond nanophotonic waveguides. We show that GeV optical transitions have a high quantum efficiency and are nearly lifetime broadened in such nanophotonic structures. These properties yield an efficient interface between waveguide photons and a single GeV center without the use of a cavity or slow-light waveguide. As a result, a single GeV center reduces waveguide transmission by 18±1% on resonance in a single pass. We use a nanophotonic interferometer to perform homodyne detection of GeV resonance fluorescence. By probing the photon statistics of the output field, we demonstrate that the GeV–waveguide system is nonlinear at the single-photon level.
The controlled creation of defect centre—nanocavity systems is one of the outstanding challenges for efficiently interfacing spin quantum memories with photons for photon-based entanglement operations in a quantum network. Here we demonstrate direct, maskless creation of atom-like single silicon vacancy (SiV) centres in diamond nanostructures via focused ion beam implantation with ∼32 nm lateral precision and sub-50 nm positioning accuracy relative to a nanocavity. We determine the Si+ ion to SiV centre conversion yield to be ∼2.5% and observe a 10-fold conversion yield increase by additional electron irradiation. Low-temperature spectroscopy reveals inhomogeneously broadened ensemble emission linewidths of ∼51 GHz and close to lifetime-limited single-emitter transition linewidths down to 126±13 MHz corresponding to ∼1.4 times the natural linewidth. This method for the targeted generation of nearly transform-limited quantum emitters should facilitate the development of scalable solid-state quantum information processors.
Efficient interfaces between photons and quantum emitters form the basis for quantum networks and enable optical nonlinearities at the single-photon level. We demonstrate an integrated platform for scalable quantum nanophotonics based on silicon-vacancy (SiV) color centers coupled to diamond nanodevices. By placing SiV centers inside diamond photonic crystal cavities, we realize a quantum-optical switch controlled by a single color center. We control the switch using SiV metastable states and observe optical switching at the single-photon level. Raman transitions are used to realize a single-photon source with a tunable frequency and bandwidth in a diamond waveguide. By measuring intensity correlations of indistinguishable Raman photons emitted into a single waveguide, we observe a quantum interference effect resulting from the superradiant emission of two entangled SiV centers.
In one or more exemplary embodiments, an optical switch comprises an optical cavity and an optically active defect comprising three or more energy levels, wherein a first optical transition between two of the three or more energy levels of the optically active defect is coupled to a cavity mode of the optical cavity and a second optical transition between another two of the three or more energy levels of the optically active defect is uncoupled to the cavity, wherein the cavity at least one of reflects or scatters at least an additional portion of a probe light when the first transition of the optically active defect is active and the cavity transmits at least an additional portion of the probe light when the second transition of the optically active defect is active, and wherein the optically active defect is controllable between the first transition being active and the second transition being active.
The negatively charged silicon-vacancy (SiV−) center in diamond is a bright source of indistinguishable single photons and a useful resource in quantum-information protocols. Until now, SiV− centers with narrow optical linewidths and small inhomogeneous distributions of SiV− transition frequencies have only been reported in samples doped with silicon during diamond growth. We present a technique for producing implanted SiV− centers with nearly lifetime-limited optical linewidths and a small inhomogeneous distribution. These properties persist after nanofabrication, paving the way for the incorporation of high-quality SiV− centers into nanophotonic devices.
In an exemplary embodiment, a structure comprises a plurality of deterministically positioned optically active defects, wherein each of the plurality of deterministically positioned optically active defects has a linewidth within a factor of one hundred of a lifetime limited linewidth of optical transitions of the plurality of deterministically positioned optically active defects, and wherein the plurality of deterministically positioned optically active defects has an inhomogeneous distribution of wavelengths, wherein at least half of the plurality of deterministically positioned optically active defects have transition wavelengths within a less than 8 nm range. In a further exemplary embodiment, method of producing at least one optically active defect comprises deterministically implanting at least one ion in a structure using a focused ion beam; heating the structure in a vacuum at a first temperature to create at least one optically active defect; and heating the structure in the vacuum at a second temperature to remove a plurality of other defects in the structure, wherein the second temperature is higher than the first temperature.
The dual-emissive properties of solid-state difluoroboron β-diketonate-poly(lactic acid) (BF2bdkPLA) materials have been utilized for biological oxygen sensing. In this work, BF2dbm(X)PLA materials were synthesized, where X = H, F, Cl, Br, and I. The effects of changing the halide substituent and PLA polymer chain length on the optical properties in dilute CH2Cl2 solutions and solid-state polymer films were studied. These luminescent materials show fluorescence, phosphorescence, and lifetime tunability on the basis of molecular weight, as well as lifetime modulation via the halide substituent. Short BF2dbm(Br)PLA (6.0 kDa) and both short and long BF2dbm(I)PLA polymers (6.0 or 20.3 kDa) have fluorescence and intense phosphorescence ideal for ratiometric oxygen sensing. The lighter halide-dye polymers with hydrogen, fluorine, and chlorine substitution have longer phosphorescence lifetimes and can be utilized as ultrasensitive oxygen sensors. Photostability was also analyzed for the polymer films.
Aggregation-induced emission (AIE) is an important photophysical phenomenon in molecular materials and has found broad applications in optoelectronics, bioimaging, and chemosensing. Currently, the majority of reported AIE-active molecules are based on either propeller-shaped rotamers or donor–acceptor molecules with strong intramolecular charge-transfer states. Here, we report a new design motif, where a fluorophore is covalently tethered to a quencher, to expand the scope of AIE-active materials. The fluorophore–quencher dyad (FQD) is nonemissive in solutions due to photoinduced electron-transfer quenching but becomes luminescent in the solid state. The intrinsic emission lifetimes are found to be within the microseconds domain at both room and low temperatures. We performed single-crystal X-ray diffraction measurement for each of the FQDs as well as theoretical calculations to account for the possible origin of the long-lived AIE. These FQDs represent a new class of AIE-active molecules with potential applications in organic optoelectronics.
We report the observation of stable optical transitions in nitrogen-vacancy (NV) centers created by ion implantation. Using a combination of high temperature annealing and subsequent surface treatment, we reproducibly create NV centers with zero-phonon lines (ZPL) exhibiting spectral diffusion that is close to the lifetime-limited optical linewidth. The residual spectral diffusion is further reduced by using resonant optical pumping to maintain the NV- charge state. This approach allows for placement of NV centers with excellent optical coherence in a well-defined device layer, which is a crucial step in the development of diamond-based devices for quantum optics, nanophotonics, and quantum information science.
A synthetic diamond material comprising one or more spin defects having a full width half maximum intrinsic inhomogeneous zero phonon line width of no more than 100 MHz. The method for obtain such a material involves a multi-stage annealing process.
The realization of efficient optical interfaces for solid-state atom-like systems is an important problem in quantum science with potential applications in quantum communications and quantum information processing. We describe and demonstrate a technique for coupling single Nitrogen Vacancy (NV) centers to suspended diamond photonic crystal cavities with quality factors up to 6,000. Specifically, we present an enhancement of the NV center's zero-phonon line fluorescence by a factor of ~7 in low temperature measurements.
Difluoroboron β-diketone complexes are versatile light-emitting molecules that exhibit tunable emission in both solution and the solid state. Among this class of dyes, difluoroboron dibenzoylmethane-polylactide (BF2dbmPLA) polymers have been investigated for their molecular weight dependent fluorescence where the polymer chain plays an important role in BF2dbm solid-state emission. Here the substituent effects were further examined with a lipid chain replacing polylactide. Surprising process dependent and reversible mechanochromic fluorescence was discovered for the boron dodecane complex (BF2dbmOC12H25). A thermally annealed spin-cast film of the lipid dye on glass exhibited blue fluorescence under UV light but after shearing or scratching, the mechanically perturbed region turned yellow-green. The blue coloration could be rapidly recovered by thermal treatment of the film. The phenomena were investigated by steady-state fluorescence spectroscopy at room, low, and high temperatures, in situfluorescence microscopy, fluorescence lifetime measurements, and X-ray diffraction. Consistent with previous findings, the ordered-to-amorphous structural change that occurs upon mechanical perturbation may increase molecular rotational freedom, allowing for more efficient excimer emission, which typically occurs at longer wavelengths.
Difluoroboron β-diketonate−polymer conjugates have remarkable solid-state luminescent properties that are useful in a variety of fields including multiphoton microscopy, cell biology, and tumor hypoxia imaging. Despite the successful applications of these systems, the role of boron in these polymeric materials has not been thoroughly investigated. Here we explore a boron-free model system with dibenzoylmethane chromophores in poly(lactic acid) (PLA) for comparison. The hydroxyl-functionalized aromatic diketone, dibenzoylmethane (dbmOH), is weakly fluorescent in the solid state and nonfluorescent in solution while its difluoroboron complex (BF2dbmOH) is highly emissive in both states. Using dbmOH and BF2dbmOH as initiators, well-defined end-functionalized polylactides, dbmPLA and BF2dbmPLA, were obtained via tin-catalyzed controlled ring-opening polymerization. Boronation of the dbmOH initiator affects the polymerization kinetics and the photophysical properties of the resulting BF2dbmPLA material. Both dbmPLA and BF2dbmPLA are dual emissive in the solid state, exhibiting both fluorescence and room-temperature phosphorescence (RTP), whereas only BF2dbmPLA is luminescent in solution. These results suggest that boron plays two roles: (1) as a protecting group in the polymerization and (2) as an emission enhancer. Finally, the presence of dual emission for both polymers indicates that it may be the diketone core structure rather than the difluoroboron that is essential for RTP in a rigid PLA matrix.
Aromatic difluoroboron β-diketonate complexes were synthesized and their photophysical properties were investigated in CH2Cl2 and poly (lactic acid)(PLA). Small symmetrical dyes exhibit π− π* transitions and comparable luminescence in CH2Cl2 and PLA, but dyes with larger arene rings (eg, naphthyl or anthracyl) also show intramolecular charge transfer (ICT) and greater medium sensitivity in PLA. The results involving substituent effects and ICT are supported by computational chemistry.
A class of aryl trifluoromethyl-containing β-diketones were synthesized via one step Claisen condensation. These π-conjugated diketones exhibit strong solvatochromism from intramolecular donor-acceptor charge transfer (CT). In addition, fluorescence quantum yields and lifetimes were measured in different solvents. Diketones exhibit bathochromic shifts in emission spectra with increasing solvent polarity. Fluorescence changes upon Group II metal binding were also studied. Despite the relatively simple structure, the anthracene-CF3 diketone, atm, has strong binding affinity for Mg2+. A 70 nm blue shift and sixfold increase in intensity were observed upon addition of only one equivalent MgCl2 in ethanol solution. It also shows selectivity for Mg2+ binding even in the presence of excess Ca2+. Association constant calculations suggest atm has two orders of magnitude stronger chelation for divalent magnesium than for calcium. These findings make atm an attractive starting point for molecular probe and light emitting material design.
Fluorescence spectroscopy has been widely used to monitor different polymer processes such as polymerization kinetics, chain entanglements, and thermal transitions. The solvent-free controlled ring-opening polymerization (ROP) of lactide is significant both commercially and for research; thus, monitoring this process with a simple fluorescence method can be very useful. Here, a fluorescent dye, difluoroboron 4-methoxydibenzoylmethane (BF2dbmOMe) is employed to probe lactide bulk ROP by measuring the emission from solidified reaction aliquots at room temperature. It was found that, through the course of polymerization, the fluorescence of BF2dbmOMe in the solid-state aliquots exhibited a systematic shift from yellow to green and then to blue, accompanied by a gradual reduction in the decay lifetime. The fluorescence color change is sensitive to the monomer percent conversion, not the polymer molecular weight. On the basis of these observations and experimental data, we propose that the long-wavelength emission with perceivably longer lifetimes arises from BF2dbmOMe dye aggregates (ground and/or excited states), while the dissolved individual dye molecules are responsible for the blue fluorescence with a shorter lifetime. This demonstration of the utility of BF2dbmOMe as a fluorescent probe for lactide polymerization could have important practical implications.
We investigate theoretically the feasibility of an experimental test of the Bell-type inequality derived by Mermin for correlated spins larger than 1/2. Using the Schwinger representation, we link the output fields of two two-mode squeezers in order to create correlated effective spins between two observers. Spin measurements will be performed by photon-number-resolved photodetection, which has recently come of age. We examine the effect of nonideal detection quantum efficiency---and any other optical loss---on the violation margin of Mermin's inequality. We find that experimental violation is well accessible for spins larger than 1 for quantum efficiencies compatible with the current state of the art.
Since Bell’s original paper in 1964, a wide variety of experimental tests have overwhelmingly supported the completeness of quantum mechanics over local hidden-variable theories. However, relatively little effort has focused on systems of spins larger than 1/2; generalizing Bell’s result to higher dimensions is difficult, and the experiments needed to test these high-spin Bell inequalities are exacting. New advances in high effciency photon-number-resolving detectors suggest that experimental tests of these inequalities should be possible in the Schwinger representation, using the continuous-variable entangled (two-mode squeezed) fields produced by an optical parametric oscillator below threshold. In this paper, we explore the realistic experimental implementation of this proposal to violate Mermin’s high-spin inequalities. We demonstrate that violation for spin values greater than 1 should be attainable under a range of feasible experimental conditions that include finite squeezing and nonideal detection effciency.