Curved GaAs cantilever waveguides for the vertical coupling to photonic integrated circuits.

We report the nanofabrication and characterization of optical spot-size converter couplers based on curved GaAs cantilever waveguides. Using the stress mismatch between the GaAs substrate and deposited Cr-Ni-Au strips, single-mode waveguides can be bent out-of-plane in a controllable manner. A stable and vertical orientation of the out-coupler is achieved by locking the spot-size converter at a fixed 90 ∘ angle via short-range forces. The optical transmission is characterized as a function of temperature and polarization, resulting in a broad-band chip-to-fiber coupling extending over 150 nm wavelength bandwidth at cryogenic temperatures, with the lower bound for the coupling efficiency into the TE mode being 16±2% in the interval 900-1050 nm. The methods reported here are fully compatible with quantum photonic integrated circuit technology with quantum dot emitters, and open opportunities to design novel photonic devices with enhanced functionality.

Independent Electrical Control of Two Quantum Dots Coupled through a Photonic-Crystal Waveguide.

Efficient light-matter interaction at the single-photon level is of fundamental importance in emerging photonic quantum technology. A fundamental challenge is addressing multiple quantum emitters at once, as intrinsic inhomogeneities of solid-state platforms require individual tuning of each emitter. We present the realization of two semiconductor quantum dot emitters that are efficiently coupled to a photonic-crystal waveguide and individually controllable by applying a local electric Stark field. We present resonant transmission and fluorescence spectra in order to probe the coupling of the two emitters to the waveguide. We exploit the single-photon stream from one quantum dot to perform spectroscopy on the second quantum dot positioned 16 µm away in the waveguide. Furthermore, power-dependent resonant transmission measurements reveal signatures of coherent coupling between the emitters. Our work provides a scalable route to realizing multiemitter collective coupling, which has inherently been missing for solid-state deterministic photon emitters.

Integration of GaAs waveguides on a silicon substrate for quantum photonic circuits.

We report a method for integrating GaAs waveguide circuits containing self-assembled quantum dots on a Si/SiO2 wafer, using die-to-wafer bonding. The large refractive-index contrast between GaAs and SiO2 enables fabricating single-mode waveguides without compromising the photon-emitter coupling. Anti-bunched emission from individual quantum dots is observed, along with a waveguide propagation loss <7 dB/mm, which is comparable with the performance of suspended GaAs circuits. These results enable the integration of quantum emitters with different material platforms, towards the realization of scalable quantum photonic integrated circuits.

Scalable integrated single-photon source.

Photonic qubits are key enablers for quantum information processing deployable across a distributed quantum network. An on-demand and truly scalable source of indistinguishable single photons is the essential component enabling high-fidelity photonic quantum operations. A main challenge is to overcome noise and decoherence processes to reach the steep benchmarks on generation efficiency and photon indistinguishability required for scaling up the source. We report on the realization of a deterministic single-photon source featuring near-unity indistinguishability using a quantum dot in an "on-chip" planar nanophotonic waveguide circuit. The device produces long strings of >100 single photons without any observable decrease in the mutual indistinguishability between photons. A total generation rate of 122 million photons per second is achieved, corresponding to an on-chip source efficiency of 84%. These specifications of the single-photon source are benchmarked for boson sampling and found to enable scaling into the regime of quantum advantage.

Spin-photon interface and spin-controlled photon switching in a nanobeam waveguide.

The spin of an electron is a promising memory state and qubit. Connecting spin states that are spatially far apart will enable quantum nodes and quantum networks based on the electron spin. Towards this goal, an integrated spin-photon interface would be a major leap forward as it combines the memory capability of a single spin with the efficient transfer of information by photons. Here, we demonstrate such an efficient and optically programmable interface between the spin of an electron in a quantum dot and photons in a nanophotonic waveguide. The spin can be deterministically prepared in the ground state with a fidelity of up to 96%. Subsequently, the system is used to implement a single-spin photonic switch, in which the spin state of the electron directs the flow of photons through the waveguide. The spin-photon interface may enable on-chip photon-photon gates, single-photon transistors and the efficient generation of a photonic cluster state.