On-chip single photon filtering and multiplexing in hybrid quantum photonic circuits

Abstract

Quantum light plays a pivotal role in modern science and future photonic applications. Since the advent of integrated quantum nanophotonics different material platforms based on III-V nanostructures-, colour centers-, and nonlinear waveguides as on-chip light sources have been investigated. Each platform has unique advantages and limitations; however, all implementations face major challenges with filtering of individual quantum states, scalable integration, deterministic multiplexing of selected quantum emitters, and on-chip excitation suppression. Here we overcome all of these challenges with a hybrid and scalable approach, where single III-V quantum emitters are positioned and deterministically integrated in a complementary metal-oxide-semiconductor-compatible photonic circuit. We demonstrate reconfigurable on-chip single-photon filtering and wavelength division multiplexing with a foot print one million times smaller than similar table-top approaches, while offering excitation suppression of more than 95 dB and efficient routing of single photons over a bandwidth of 40 nm. Our work marks an important step to harvest quantum optical technologies' full potential.Combining different integration platforms on the same chip is currently one of the main challenges for quantum technologies. Here, Elshaari et al. show III-V Quantum Dots embedded in nanowires operating in a CMOS compatible circuit, with controlled on-chip filtering and tunable routing.

Fig. 1

Hybrid quantum photonic circuit. a Schematic view of the fabricated hybrid quantum photonic circuit, consisting of an InAsP QD in an InP nanowire (ruby coloured) that is integrated with a SiN waveguide (blue) and on-chip tunable ring resonator filter. The ring resonator filter is tuned by applying voltage to the gold contacts (orange). The out-of-plane laser (green) excites the QD, which emits single photons (ellipsoids) into the waveguide. b–d The CMOS compatible process flow for integrating the nanowire-based quantum light sources within the hybrid quantum photonic circuit. Using a custom-built tungsten nanomanipulation tool, the nanowires are transferred from the growth chip to the selected photonic circuit substrate. e A microscope image of an integrated single-photon source with a SiN photonic waveguide, the scale bar has length of 5 µm. Emitted photons are coupled to the SiN photonic channel with the possibility to collect both forward and backward photons independently. The photonic circuits are fabricated with respect to the transferred quantum emitters as described in the Methods section. f, g The collected forward and backward photoluminescence (PL) from the nanowire QD, respectively. T and X represent the trion and exciton emission lines, respectively

Fig. 2

On-chip single-photon routing. a A microscope image of the ring resonator filter. The filter transmission is controlled with a titanium resistor, the scale bar has length of 70 µm. Details of the fabrication process are provided in the Methods section. Graphs b and c show the through-port and drop-port transmission of the ring resonator, respectively. d Tuning of the ring resonator filter as a function of the filter voltage (V _rr).The red circles show the measured shifts while the blue line is a fit. The resonances are blue shifted by design, which is achieved by means of the large negative thermo-optic coefficient of the PMMA top cladding. e Tuning of the QD transition as a function of the ring resonator voltage. In contrast to the observed shift for the ring resonator, the QD emission red shifts for increasing voltage as expected due to the increase in temperature. Graphs f and g show the results of selectively routing a unique transition of the QD between the drop-port and through-port of the ring resonator. As we tune the filter resonance using the integrated heater, a single emission line can be tuned in and out of resonance, thus routing single photons between the through-port and drop-port

Fig. 3

On-chip filtering. a Experimental setup showing the hybrid integrated quantum circuit with electrical access to control the integrated filters. The setup allows for both in-plane (via the waveguide) and out-of-plane laser excitation. Details of out-of-chip coupling are included in the Methods section. The collected emission from the waveguide is either coupled to the APDs after filtering with a monochromator (case 1), or it can be directly coupled to the APDs with no external wavelength filtering (case 2). b Collected QD emission from the facet of the SiN waveguide in the forward direction. c By tuning the on-chip filter, a single QD transition is routed to the drop-port. d A close-up of the filtered trion (T) emission line. The QD emission wavelength is slightly different in b and c or d due to different biases applied to the ring resonator filter. Despite the presence of an intense laser for excitation and InP nanowire emission, the filtered spectrum shows only a single QD transition over a broad wavelength range (500–950 nm). e Second-order correlation measurement of the QD trion line using an off-chip commercial monochromator for filtering, resulting in a multi-photon probability of g ⁽²⁾(0) = 0.13 ± 0.04 when taking into account the finite temporal resolution of the APDs. f Second-order correlation measurement of the QD trion line at the drop-port of the ring resonator after directly coupling it to the APDs. A single-stage ring filter is capable of delivering single photons on-chip with multi-photon probability g ⁽²⁾(0) = 0.41 ± 0.05. The results show the excellent performance of the integrated ring resonator filter as compared to the bulky off-chip monochromator. g Second-order correlation measurement without any on-chip and off-chip filtering. The results show the expected Poissonian statistics of coherent (uncorrelated) emission. In e, f, the blue circles show the raw data, the green line represents a fit, and the red line represents the fit considering the finite detector response (see Supplementary Note 4 for more details)

Fig. 4

Wavelength mulitplexing of quantum emitters. Artistic image of multiplexing/demultiplexing of two quantum emitters coupled to a photonic circuit with integrated tunable filters. The two nanowires are butt-coupled to a SiN waveguide, each of them emitting photons independently with different colours (depicted as red and blue in this case). The flexibility of the process allows for the possibility of wavelength and modal multiplexing of selected single-photon sources to an already fabricated and characterized photonic circuit, thus making the process highly deterministic. Details of the fabrication and nanowire transfer process are included in the Methods section. a Collected emission from the through-port waveguide, consisting of a wavelength-multiplexed signal from QD1 and QD2. The spectra are highlighted in red and blue to indicate the individual emission from QD1 and QD2. b Selected excitonic transitions of QD1 and QD2 are filtered deterministically to the drop-port waveguide as a function of the on-chip filter tuning voltage. c Integrated intensity of QD1 and QD2 in the drop-port as a function of voltage. As the voltage is controlled, QD1 and QD2 follow the Lorentzian shape of the ring resonator transmission function

Fig. 5

In-plane excitation and large-scale integration. a Artistic image of in-plane excitation of the nanowire quantum dot. The nanowire QD emission is collected in the waveguide orthogonal to the pump waveguide. b Collected spectrum from the waveguide coupled to the nanowire. c A close-up view of the emission spectrum of the QD and a microscope image of the measured device. The spectrum mainly shows a single exciton peak from the QD emission with no measurable signal from the pump (at ~ 632 nm). Pump signal is efficiently suppressed due to the device geometry decoupling the pump photons from the nanowire emission. d–i The emission spectrum of six independently and deterministically integrated nanowire quantum emitters operating on the same photonic chip. In-plane excitation was used to excite the nanowire quantum dots. This paves the way for large-scale integration and excitation of multiple quantum emitters on-chip either using a single off-chip source or an integrated electrically pumped source with phase shifters and routers. j An artistic image of the fabricated device (shown in inset) containing six nanowires encapsulated in SiN waveguides