Combining parametric driving and photon-atomic memory coupling within one optical cavity, we describe a scheme for in-situ generation of multimode photon-memory entanglement. We find that precise cavity impedance matching is neither required nor optimal to achieve high-rate entanglement for quantum networks. This protocol can be realized with existing technologies based on on-chip photonic cavities integrated with a rare-earth-ion doped quantum memory. The proposed scheme shows significant advantages in entanglement generation rates compared with start-of-the-art quantum memory protocols and experiments, with predicted Ebit generation rates of tens of MHz without ideal operating conditions. Such a photon-memory entanglement system offers a versatile resource for quantum interconnect applications.

Quantum memory is an essential device for quantum communication and network. We are working to realize a SiC-based micro-cavity source with integrated quantum memory at the telecom band. A low loss SiC photonic devices are fabricated using high-quality-factor micro-ring resonators for the efficient generation of entangled photon pairs, and atomic frequency comb quantum memories based on spin-initialized 167Er:YSO crystals realize the storage of photons. Such memories are expected to generate remote entanglement between nodes in quantum networks.

We present general guidelines for finding solid-state systems that could serve as coherent electron spin-photon interfaces even at relatively high temperatures, where phonons are abundant but cooling is easier, and show that transition metal ions in various crystals could comply with these guidelines. As an illustrative example, we focus on divalent nickel ions in magnesium oxide. We perform electron spin resonance spectroscopy and polarization-sensitive magneto-optical fluorescence spectroscopy of a dense ensemble of these ions and find that (i) the ground-state electron spin stays coherent at liquid-helium temperatures for several microseconds, and (ii) there exists energetically well-isolated excited states which can couple to two ground state spin sub-levels via optical transitions of orthogonal polarizations. The latter implies that fast, coherent optical control over the electron spin is possible. We then propose schemes for optical initialization and control of the ground-state electron spin using polarized optical pulses, as well as two schemes for implementing a noise-free, broadband quantum-optical memory at near-telecom wavelengths in this material system.

Center, this document is a roadmap for quantum interconnects research and its impact for quantum information science and technology.It is the outcome of the collective work of a large team of Q-NEXT members and participants from academia, industry and DOE national laboratories.The roadmap addresses the role of quantum interconnects in three emerging areas of quantum information: computing, communication and sensing.It reviews the materials, components and systems used for these purposes; summarizes relevant scientific questions and issues; and addresses the most pressing research needs.The document then distills these considerations into recommendations for strategic science and technology research imperatives for the next decade.In addition to informing Q-NEXT's internal activities, the roadmap has also been created with a broader objective of developing a guide for key issues and research needed over the next decade for the worldwide scientific and engineering community interested in quantum information.

Qudit entanglement is an indispensable resource for quantum information processing since increasing dimensionality provides a pathway to higher capacity and increased noise resilience in quantum communications, and cluster-state quantum computations. In continuous-variable time–frequency entanglement, encoding multiple qubits per photon is only limited by the frequency correlation bandwidth and detection timing jitter. Here, we focus on the discrete-variable time–frequency entanglement in a biphoton frequency comb (BFC), generating by filtering the signal and idler outputs with a fiber Fabry–Pérot cavity with 45.32 GHz free-spectral range (FSR) and 1.56 GHz full-width-at-half-maximum (FWHM) from a continuous-wave (cw)-pumped type-II spontaneous parametric downconverter (SPDC). We generate a BFC whose time-binned/frequency-binned Hilbert space dimensionality is at least 324, based on the assumption of a pure state. Such BFC’s dimensionality doubles up to 648, after combining with its post-selected polarization entanglement, indicating a potential 6.28 bits/photon classical-information capacity. The BFC exhibits recurring Hong–Ou–Mandel (HOM) dips over 61 time bins with a maximum visibility of 98.4% without correction for accidental coincidences. In a post-selected measurement, it violates the Clauser–Horne–Shimony–Holt (CHSH) inequality for polarization entanglement by up to 18.5 standard deviations with an S -parameter of up to 2.771. It has Franson interference recurrences in 16 time bins with a maximum visibility of 96.1% without correction for accidental coincidences. From the zeroth- to the third-order Franson interference, we infer an entanglement of formation ( E of ) up to 1.89 ± 0.03 ebits—where 2 ebits is the maximal entanglement for a 4 × 4 dimensional biphoton—as a lower bound on the 61 time-bin BFC’s high-dimensional entanglement. To further characterize time-binned/frequency-binned BFCs we obtain Schmidt mode decompositions of BFCs generated using cavities with 45.32, 15.15, and 5.03 GHz FSRs. These decompositions confirm the time–frequency scaling from Fourier-transform duality. Moreover, we present the theory of conjugate Franson interferometry—because it is characterized by the state’s joint-temporal intensity (JTI)—which can further help to distinguish between pure-state BFC and mixed state entangled frequency pairs, although the experimental implementation is challenging and not yet available. In summary, our BFC serves as a platform for high-dimensional quantum information processing and high-dimensional quantum key distribution (QKD).

Coherent quantum devices play a central role in quantum science and engineering. Rare-earth ions such as erbium in solids feature numerous 4f- intra-shell transitions that are effectively shielded from their crystalline surroundings by closed outer shells, allowing for long spin coherence times and narrow optical transitions. Recent ensemble experiments have established the rare-earth doped crystals as the leading materials for optical quantum memories. However, the development of large-scale quantum devices based on rare-earth doped materials has remained a challenge. Here we describe a scalable quantum photonic platform based on epitaxially grown oxide thin films doped with trivalent ions on silicon substrates. The platform builds upon the recent developments of high quality epitaxial rare-earth doped oxides on silicon, and highlights the integration of quantum coherent materials with existing silicon technologies for ultimate device scalability. The proposed device platform builds upon our capability to epitaxially grow high quality, near-lattice-matched Y2O3 thin films on silicon. The epitaxial thin film is of high structural quality and offers several distinct advantages: (1) significantly less optical loss and better quantum emitter coherence; (2) atomic precision placement of rare-earth emitters (via delta doping) with respect to the photonic mode for controllable ion-photon coupling; (3) scalable top-down device fabrication using standard lithography and etching techniques; (4) heterogeneous integration with silicon takes full advantage of existing silicon photonics technologies for routing, tuning, and modulation of the quantum emission from rare-earth ions.

Quantum interconnects allow disparate quantum systems to be entangled, leading to more powerful integrated quantum technology and increases in scalability. The foundation for such technology, including photonic quantum memories and coherent microwave-to-optical (M2O) transducers, have already been developed in rare-earth ion (REI) crystals. Here we demonstrate improved REI quantum device functionality in an on-chip platform that dramatically strengthens the ions’ interactions with optical fields and integrates with planar microwave technology. Using a photonic crystal nanobeam fabricated in a Nd-doped yttrium vanadate (YVO) crystal, we harness the enhanced ion-photon interactions that create single photon Rabi frequencies as large as 60 MHz. In particular, the large AC Stark shift is used to control an ensemble of approximately 4000 ions for photonic quantum memory applications. We demonstrate AC Stark shift control of the storage time in the atomic frequency comb protocol as well as the possibility of memories based on an all-optical variation of the hybrid photon echo rephasing protocol. The spin state of the REIs can also be addressed directly through the integration of microwave striplines and coplanar waveguide cavities. The achievement of optically detected magnetic resonance in on-chip waveguides and nanophotonic cavities in Nd:YVO will be presented along with the initial progress of achieving coherent M2O conversion using Raman heterodyne spectroscopy. With photonic quantum memories and sources, single ion qubits, and quantum M2O all feasible in the one integrated platform, REI technology is a promising platform for enabling large scale integration of diverse quantum resources.

Ensembles of solid-state optical emitters enable broadband quantum storage and transduction of photonic qubits, with applications in high-rate optical quantum networks for secure communications, global time-keeping, and interconnecting future quantum computers. To realize coherent quantum information transfer using ensembles, spin rephasing techniques are currently used to mitigate fast decoherence resulting from inhomogeneous broadening. Here we use a dense ensemble of neodymium rare-earth ions strongly coupled to a nanophotonic resonator to demonstrate that decoherence of a single photon excitation is near-completely suppressed via cavity protection- a new technique for accessing the decoherence-free subspace of collective coupling. The protected Rabi oscillations between the cavity field and the atomic superradiant state thereby enable ultra-fast transfer of photonic frequency qubits ( 50 GHz bandwidth) into the ions, followed by retrieval with 98.7 fidelity. By coupling the superradiant excitation to other long-lived rare-earth spin states, this technology will enable broadband, always-ready quantum memories and fast optical-to-microwave transducers.

Rare-earth ions doped in crystals are renowned for their excellent coherence properties and large inhomogeneous broadening, which make them ideal for quantum interfaces with broadband photons. These properties have made them one of the leading technologies in quantum optical memories and a promising candidate for optical-to-microwave conversion. To take advantage of the full bandwidth of the rare-earth ensemble, one must overcome the decoherence of a broadband collective excitation due to inhomogeneous broadening. To this end, techniques based on controllable rephasing, such as atomic frequency comb (AFC) or controlled reversible inhomogeneous broadening (CRIB) memories, have been developed with great success. Recently, an alternative method was proposed to suppress the decoherence of an inhomogeneous ensemble via strong coupling to a cavity, a phenomenon called cavity protection. This technique has been demonstrated in the microwave domain with an NV spin ensemble, but has not been demonstrated in the optical domain. Here, we demonstrate cavity protection in the optical domain at the single photon level using an ensemble of rare earths ions coupled to a nanophotonic resonator. The reduction in decoherence due to the cavity-protection effect enables transfer of ultrafast (~50 GHz) frequency qubits into the collective ion excitation and retrieval with 98.7% fidelity. Building on these results to transfer these excitations to long-lived spin states would enable broadband, on-demand quantum memories. Furthermore, this works compliments the work done coupling rare-earths to superconducting resonators in the microwave regime with potential for applications in optical-to-microwave transducers.

Rare earth quantum light-matter interfaces (QLMIs) are uniquely suited for various quantum communication applications, including quantum memories and quantum optical to microwave transducers. Among rare earths, erbium QLMIs are particularly appealing due to erbium’s long lived telecom wavelength resonance, allowing integration with existing optical communication technology and infrastructure. Micro-resonator QLMIs have various advantages over bulk rare earth crystal memories. They provide the opportunity for on-chip integration; for example, optical resonators can be integrated with microwave resonators for quantum optical-microwave transduction. For spectral hole-burning based quantum memories, coupling rare earth ions to a resonator can provide improved memory initialization via Purcell enhancement of optical lifetimes, while impedance matching the resonator to the ions can raise the theoretical memory efficiency to 100%. We present hybrid nanoscale quantum light matter interfaces in the form of amorphous silicon ring resonators on yttrium orthosilicate (YSO) substrate doped with erbium ions. While working with rare earth crystal hosts can be challenging, the fabrication process for these devices is simple and robust, using traditional thin film fabrication technologies. Our devices have measured quality factors of over 105 in the 11 µm diameter rings, and evanescent coupling to an ensemble of erbium ions characterized by a cooperativity of 0.54. We present simulation and experimental results of the optical properties of these cavities, and their coupling to erbium ions, including a demonstration of Purcell enhancement of the erbium telecom transition. We then analyze their potential as quantum memories and in optical to microwave transducers.

Objectives Because of its excellent biocompatibility, non-toxicity and biodegradability, polylactide (PLA) is a natural choice for various types of biomaterials, for example as a drug carrier. However, the use of PLA as a base substance for nitric oxide (NO) used as the donor has not been previously reported. On the other hand, high pressure gas foaming preparation of porous polylactide for tissue engineering or other biomedical applications has been developed for decades and proven to be a successful method to manipulate PLA. We employed nitric oxide as the pressure gas injected into amorphous amino acid modified-PLA solid to obtain a gas-solid solution, while the amino acids provided the amino group for EDTA modification, which may serve as a candidate for donating NO. Methods We synthesized poly-(la-serine)-esters to introduce the diazo-group on the side chain, which can be loaded with NO and acts as a NO donor (diazeniumdiolate). The bulk poly-(la-serine)-ester was formed by high pressure NO and CO2 gas to obtain a NO-loaded porous scaffold (Figure 1). Results The block ratio of LA:serine 50:50 is feasible for fine products with both good properties and NO capacity. Thermal and mechanical characterization shows good thermal, tensile and processing properties. SEM observation confirms the porous structure induced by gas foaming. The Griess method applied to measure NO release in PBS (pH 7.4) showed 1082 mol/mg release in 7 hours. Conclusions The results indicate this methodology is feasible. The block-polyester we synthesized shows a practical NO capacity with a convenient loading process, while good thermal and mechanical properties are maintained, and therefore may serve as a candidate for donating NO. Acknowledgments This work was financially supported by the Innovation Cultivation Project of Zhuhai College of Jilin University (Grant No. 2016XJCQZD06).

We demonstrate coherent optical control of Nd rare-earth ions coupled to a nano-photonic resonator. Optical storage in the nano-resonator is demonstrated using multi-mode photon echo and atomic frequency comb protocols. The long optical and spin coherence of cavity-coupled rare-earth ions indicate that these are promising systems for on-chip quantum light-matter interfaces. Rare-earth-ions (REIs) are promising candidates for implementing solid-state quantum memories and quantum repeater devices [1]. Moreover, their high spectral stability, long coherence times, and small inhomogeneous broadening [2] make REIs a good choice for integration in an on-chip quantum nano-photonic platform. Here we demonstrate photon storage in an Yttrium orthosilicate Y2SiO5 (YSO) photonic crystal nano-beam resonator with mode volume of 1.6 cubic wavelengths. The coupling of the 883 nm 4I9/2-4F3/2 transition of Neodymium (Nd) ions to the nano-resonator results in a 40 fold enhancement of the transition rate (Purcell effect), and increased optical absorption (~80%) adequate to realize efficient photon storage via cavity impedance matching [3]. Optical coherence times T2 up to 100 μs with low spectral diffusion were measured using the photon echo technique for ions embedded in the nano-beams, which are comparable to those observed in unprocessed bulk samples. This indicates that the remarkable coherence properties of REIs are preserved during nanofabrication process. Thanks to cavity impedance matching, enhancement of echo intensity by 12 times was observed compared to a nano-beam waveguide without a cavity. Multi-temporal mode photon storage using stimulated photon echo and atomic frequency comb (AFC) [4] protocols were implemented in these devices. The triangular nano-beam resonator was fabricated in Nd:YSO using focused ion beam milling. A resonance of Q=4400 and mode volume V = 1.6(λ / n) at 878 nm were tuned to the 883 nm Nd:YSO transition. When coupled, enhanced photoluminescence and reduced lifetime from 250 μs to 90 μs were observed, as shown in Fig. 1 (d). Figure 1 (a) SEM image of a Nd:YSO nano-beam resonator fabricated using FIB. (b) Efficient vertical in/out coupling of the cavity is through angled reflectors milled in the YSO substrate. (c) Simulated cavity mode profiles in the beam cross-section and along the beam length. (d) Reduction of spontaneous emission lifetime from 250 μs (blue) at off resonance to 90 μs (red) when the cavity is on resonance with Nd ions. Two-pulse photon echo experiments were performed to extract T2 optical coherence time of the Nd ions in the cavity. Shown in Fig. 2, with the 0.2% doped high-density nano-resonator, we measured T2 = 2.8 ± 0.4 μs at 3.5 K temperature with a 500 mT applied magnetic field, which agrees well with a T2 = 3.2 ± 0.4 μs measured in the bulk sample. In 0.003% doped low-density nano-beams, the measured T2 was 100.6 μs . Using the 0.2% doped Nd:YSO device, we measured an enhanced absorption by the ions in the cavity, indicated by a ~80% reduction of the cavity transmission at the center of a ~16 GHz inhomogeneous linewidth. This can be compared to <5% absorption by the same ensemble of ions in a nano-beam waveguide without a cavity. We then demonstrated multi-mode optical storage in our nanophotonic cavity based on stimulated photon echoes scheme. An arbitrary 3-bit pulse sequence (i.e. “101” or “110”) followed by a write, then a read pulse were sent into the cavity. As shown in Fig. 3, multiple photon echoes resembling the time-reversed input pulse sequence were emitted after the read pulse. An additional echo at ~7 μs was produced by the write and read pulses, which did not contribute to the data storage. Current data pulse widths were limited to 200 ns by our acousto-optic modulators (AOM). Much narrower pulses (~10 ns), thus longer bit length (~6 bit) storage can be, in principle, implemented using our device. These results point towards excellent prospects for implementing on-chip nano-photonic optical quantum memories based on rare-earth doped crystals. Figure 3: Optical data storage in the Nd:YSO nanocavity based on stimulated photon echoes. Ignoring the last echo at 7 μs, the echo amplitudes represent a time-reserved sequence of the input signals. References [1] A. I. Lvovsky, B. C. Sanders, and W. Tittel, “Optical quantum memory,” Nature Photonics 3, 706-714 (2009). [2] D. L. McAuslan and J. J. Longdell, “Cavity QED using rare-earth-metal-ion dopants in monolithic resonators: What you can do with a weak oscillator,” Phys. Rev. A 80, 062307 (2009). [3] M. Afzelius and C. Simon, “Impedance-matched cavity quantum memory,” Phys. Rev. A 82, 022310 (2010). [4] H. de Riedmatten, M. Afzelius, M. U. Staudt, C. Simon, and N. Gisin, “A solid-state light-matter interface at the single photon level,” Nature 456, 773-777 (2008). delay (7s) 1 1.5 2 2.5 3 Lo g of e ch o in te ns ity 7 7.5 8 8.5 9 9.5 10 10.5 11 11.5 12 12.5 nano-cavity bulk Figure 2: Two-pulse photon echoes in 0.2% doped high-density Nd:YSO bulk sample and a nano-resonator.

The binary (one-bit-per-photon) encoding that most existing quantum key distribution (QKD) protocols employ puts a fundamental limit on their achievable key rates, especially under high channel loss conditions associated with long-distance fiber-optic or satellite-to-ground links. Inspired by the pulse-position-modulation (PPM) approach to photon-starved classical communications, we design and demonstrate the first PPM-QKD, whose security against collective attacks is established through continuous-variable entanglement measurements that also enable a novel decoy-state protocol performed conveniently in post processing. We achieve a throughput of 8.0 Mbit/s (2.5 Mbit/s for loss equivalent to 25 km of fiber) and secret-key capacity up to 4.0 bits per detected photon, thus demonstrating the significant enhancement afforded by high-dimensional encoding. These results point to a new avenue for realizing high-throughput satellite-based or long-haul fiber-optic quantum communications beyond their photon-reception-rate limits.