Optical quantum memories will enable technologies including long distance quantum communication and modular quantum computing. Rare earth ion doped crystals provide an excellent solid state platform for optical quantum memories. Among rare earths, erbium is particularly appealing due to its long-lived telecom-wavelength resonance, allowing integration with silicon photonics and with existing optical communication technology and infrastructure.
We present an on-chip all-optical quantum memory at telecom wavelengths using a nanobeam photonic crystal cavity fabricated directly in erbium-167 doped yttrium orthosilicate. Using an atomic frequency comb protocol, we store coherent pulses for memory times as long as 10 µs, albeit with low efficiency. For shorter memory times, we achieve a memory efficiency of 0.4%, which is limited by the coupling rate between the resonator and the ensemble of ions. By working at dilution refrigerator temperatures, we are able to access a regime where the ions have long optical coherence times and good spectral holeburning properties using only a moderate magnetic field applied with permanent magnets. We characterize the multimode properties and fidelity of the quantum memory in this device, and outline a path toward higher efficiency.
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.
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.
Quantum light-matter interfaces that reversibly map the quantum state of photons onto the quantum states of atoms, are essential components in the quantum engineering toolbox with applications in quantum communication, computing, and quantum-enabled sensing. In this talk I present our progress towards developing on-chip quantum light-matter interfaces based on nanophotonic resonators fabricated in rare-earth-doped crystals known to exhibit the longest optical and spin coherence times in the solid state. We recently demonstrated coherent control of neodymium (Nd3+) ions coupled to yttrium orthosilicate Y2SiO5 (YSO) photonic crystal nano-beam resonator. The coupling of the Nd3+ 883 nm 4I9/2-4F3/2 transition to the nano-resonator results in a 40 fold enhancement of the transition rate (Purcell effect), and increased optical absorption (~80%) - adequate for realizing efficient optical quantum memories via cavity impedance matching. Optical coherence times T2 up to 100 μs with low spectral diffusion were measured for ions embedded in photonic crystals, which are comparable to those observed in unprocessed bulk samples. This indicates that the remarkable coherence properties of REIs are preserved during nanofabrication process. Multi-temporal mode photon storage using stimulated photon echo and atomic frequency comb (AFC) protocols were implemented in these nano-resonators. Our current technology can be readily transferred to Erbium (Er) doped YSO devices, therefore opening the possibility of efficient on-chip optical quantum memory at 1.5 μm telecom wavelength. Integration with superconducting qubits can lead to devices for reversible quantum conversion of optical photons to microwave photons.
The integration of rare-earth ions in an on-chip photonic platform would enable quantum repeaters and scalable quantum networks. While ensemble-based quantum memories have been routinely realized, implementing single rare-earth ion qubit remains an outstanding challenge due to its weak photoluminescence. Here we demonstrate a nanophotonic platform consisting of yttrium vanadate (YVO) photonic crystal nanobeam resonators coupled to a spectrally dilute ensemble of Nd ions. The cavity acts as a memory when prepared with spectral hole burning, meanwhile it permits addressing of single ions when high-resolution spectroscopy is employed. For quantum memory, atomic frequency comb (AFC) protocol was implemented in a 50 ppm Nd:YVO nanocavity cooled to 480 mk. The high-fidelity quantum storage of time-bin qubits is demonstrated with a 80% efficient WSi superconducting nanowire single photon detector (SNSPD). The small mode volume of the cavity results in a peak atomic spectral density of <10 ions per homogeneous linewidth, suitable for probing single ions when detuned from the center of the inhomogeneous distribution. The high-cooperativity coupling of a single ion yields a strong signature (20%) in the cavity reection spectrum, which could be detected by our efficient SNSPD. We estimate a signal-to-noise ratio exceeding 10 for addressing a single Nd ion with its 879.7nm transition. This, combines with the AFC memory, constitutes a promising platform for preparation, storage and detection of rare-earth qubits on the same ship.
Rare-earth-ion doped crystals are state-of-the-art materials for optical quantum memories and quantum transducers between optical and microwave photons. Here we describe our progress towards a nanophotonic quantum memory based on a rare-earth (Neodymium) doped yttrium orthosilicate (YSO) photonic crystal resonator. The Purcell-enhanced coupling of the 883 nm transitions of Neodymium (Nd3+) ions to the nano-resonator results in increased optical depth, which could in principle facilitate highly efficient photon storage via cavity impedance matching. The atomic frequency comb (AFC) memory protocol can be implemented in the Nd:YSO nano-resonator by efficient optical pumping into the long-lived Zeeman state. Coherent optical signals can be stored and retrieved from the AFC memory. We currently measure a storage efficiency on par with a bulk crystal Nd:YSO memory that is millimeters long. Our results will enable multiplexed on-chip quantum storage and thus quantum repeater devices using rare-earth-ions.
With an assortment of narrow line-width transitions spanning the visible and IR spectrum and long spin coherence times, rare-earth doped crystals are the leading material system for solid-state quantum memories. Integrating these materials in an on-chip optical platform would create opportunities for highly integrated light-matter interfaces for quantum communication and quantum computing. Nano-photonic resonators with high quality factors and small mode volumes are required for efficient on-chip coupling to the small dipole moment of rare-earth ion transitions. However, direct fabrication of optical cavities in these crystals with current nanofabrication techniques is difficult and unparallelized, as either exotic etch chemistries or physical milling processes are required. We fabricated hybrid devices by mechanically transferring a nanoscale membrane of gallium arsenide (GaAs) onto a neodymium-doped yttrium silicon oxide (Y2SiO5) crystal and then using electron beam lithography and standard III-V dry etching to pattern nanobeam photonic crystal cavities and ring resonator cavities, a technique that is easily adapted to other frequency ranges for arbitrary dopants in any rare earth host system. Single crystalline GaAs was chosen for its low loss and high refractive index at the transition wavelength. We demonstrated the potential to evanescently couple between the cavity field and the 883 nm 4I9/2- 4F3/2 transition of nearby neodymium impurities in the host crystal by examining transmission spectra through a waveguide coupled to the resonator with a custom-built confocal microscope. The prospects and requirements for using this system for scalable quantum networks are discussed.
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