LETTERS https://doi.org/10.1038/s41566-018-0317-y Electronically programmable photonic molecule Mian Zhang 1,7, Cheng Wang1,2,7, Yaowen Hu1,3, Amirhassan Shams-Ansari1,4, Tianhao Ren1,5, Shanhui Fan6 and Marko Lončar 1* Physical systems with discrete energy levels are ubiquitous the time required to drive the system from one state to the other. in nature and acre fundamental building blocks of quantum On the one hand, large optical systems with optical amplifiers24 can technology. Realizing controllable artificial atom- and mole- emulate a classical two-level system, but the quantum coherence of cule-like systems for light would enable coherent and dynamic the optical photons is destroyed in the process. On the other hand, control of the frequency, amplitude and phase of photons1–5. conventional integrated photonic platforms have not been able to In this work, we demonstrate a ‘photonic molecule’ with two meet the requirements of long photon lifetime and fast modulation distinct energy levels using coupled lithium niobate microring simultaneously. For example, fast phase modulators7,25 can gener- resonators and control it by external microwave excitation. We ate new optical frequencies, but they do not support distinct and show that the frequency and phase of light can be precisely long-lived optical modes. On-chip resonators with ultra-high qual- controlled by programmed microwave signals, using concepts ity factors (Q) have traditionally been realized in passive materi- 26,27 28,29 of canonical two-level systems including Autler–Townes split- als, such as silicon dioxide (SiO2) and silicon nitride (Si3N4) , ting, Stark shift, Rabi oscillation and Ramsey interference. which can only be controlled electrically using slow thermal effects. Through such coherent control, we show on-demand optical Electrically active photonic platforms18–22 based on silicon18, III–V storage and retrieval by reconfiguring the photonic molecule materials15,20, plasmonic19, graphene22 and polymers21 allow for fast into a bright–dark mode pair. These results of dynamic control electro-optic modulation at gigahertz frequencies, but suffer from of light in a programmable and scalable electro-optic system much shorter photon lifetimes than passive platforms. To circum- open doors to applications in microwave signal processing6, vent this problem, out-of-plane picosecond optical pulses have been quantum photonic gates in the frequency domain7 and explor- used to induce a broadband electro-optic tuning in silicon via ultra- ing concepts in optical computing8 and topological physics3,9. fast free carriers16,30. Purely electrical tuning is still highly desirable Photonic analogues of condensed-matter systems have resulted as narrowband microwave signals could offer much better control- in discoveries such as photonic crystals5, parity–time symmet- lability, minimal added noise and scalability. ric systems2 and topological photonic systems3,9, and have led to In this work, we overcome the existing performance trade-off technologies including quantum ground-state cooling of nanome- and realize a programmable photonic two-level system that can be chanical systems10, new classes of sensors11,12 and one-way lasers4. dynamically controlled by gigahertz microwave signals (Fig. 1a). A photonic analogue of a two-level system could allow full control Specifically, we create a microwave-addressable photonic molecule over the energy and phase of photons by using the concept of two- using a pair of integrated lithium niobate microring resonators, level systems control in atomic or molecular systems, where the 80 μ m in radius, patterned close to each other. The low optical loss31 state of the electron can be controlled and functionalized by exter- and efficient co-integration of optical waveguides and microwave nal electromagnetic fields. Such a photonic system would enable electrodes32,33 allow us to simultaneously achieve large electrical the investigation of complex physical phenomena2–4,7 and unique bandwidth (> 30 GHz), strong modulation efficiency (0.5 GHz V–1) functionalities, including on-demand photon storage and retrieval, and long photon lifetime (~2 ns). coherent optical frequency shift and optical quantum information The photonic molecule supports a pair of well-defined optical processing at room temperature7,13,14. Although it is straightforward energy levels, which are evident from the optical transmission to realize a photonic device with discrete energy levels, for example measured using a tunable telecom-wavelength laser (Fig. 1c; see by using modes of an optical resonator, controlling such a system Supplementary Information for full spectrum). The two optical dynamically (that is, inducing transitions between the levels) is chal- energy levels are formed by the evanescent coupling of light from lenging as it requires mixing of optical frequencies through strong one resonator to another through a 500-nm gap. When the optical nonlinear processes. As a result, coherent coupling between discrete coupling strength μ exceeds the optical loss rate γ of each cavity, photon energy modes has only been studied using all-optical meth- the coupling leads to a normal mode splitting resulting in a fre- ods14–17 and has limited design parameter space, configurability quency doublet consisting of a lower-frequency symmetric (S) and and scalability. a higher-frequency antisymmetric (AS) optical mode (Fig. 1a and Electro-optic methods6,18–23 are ideally suited for the dynamic c). The S (AS) mode spatially spans both optical cavities, with the control of photonic two-level systems, since they have fast response, light in the two cavities being in-phase (out-of-phase). The two new can be programmed and allow for large-scale integration. To realize eigenmodes, separated in frequency by 2μ = 2π ×​ 7 GHz, are the two coherent electro-optic control of a photonic two-level system, the energy levels of the photonic molecule. In our case, the two opti- photon lifetime of each energy state needs to be much longer than cal modes have cavity linewidths of γ = 2π ×​ 96 MHz, corresponding 1John A. Paulson School of Engineering and Applied Sciences, Harvard University, Cambridge, MA, USA. 2Department of Electronic Engineering, City University of Hong Kong, Kowloon, Hong Kong, China. 3Department of Physics, Tsinghua University, Beijing, China. 4Department of Electrical Engineering and Computer Science, Howard University, Washington, DC, USA. 5University of Electronic Science and Technology of China, Chengdu, China. 6Ginzton Laboratory and Department of Electrical Engineering, Stanford University, Palo Alto, CA, USA. 7These authors contributed equally: Mian Zhang, Cheng Wang. *e-mail: [email protected] 36 NatURE PHOTONICS | VOL 13 | JANUARY 2019 | 36–40 | www.nature.com/naturephotonics NATURE PHOTONICS LETTERS a b Microwave Microwave (2) χ Optical Phase advance 40 µm coupling Phase delay c d NT 9 1 1.0 6 0.8 3 0.6 0 NT 0.4 –3 2μ = = 0.2 QL = –6 ω1 ω2 2π × 7 GHz 6 –1 0 1.9 10 Laser detuning (GHz) 0.5 GHz V 0 × –9 –10 –5 015 0 –15 –10 –5 0510 15 Laser detuning (GHz) Bias voltage (V) Fig. 1 | Microwave-controlled photonic molecule. a, The photonic molecule is realized by a pair of identical coupled optical microring resonators (resonant frequency ω1 =​ ω2). The system has two distinct energy levels—a symmetric and an antisymmetric optical mode (indicated here by blue/blue shading for the symmetric and red/blue for the antisymmetric mode) that are spatially out of phase by π​. The microwave field can interact coherently with the two- level system through the strong Pockels effect (χ(2)) of lithium niobate. b, False-coloured scanning electron microscope image of the coupled microring resonators. c, Measured transmission spectrum of the photonic two-level system. The two optical modes are separated by 2μ =​ 2π​ × 7 GHz​ with linewidths of γ =​ 2π​ ×​ 96 MHz corresponding to a loaded optical quality factor of 1.9 ×​ 106. d, The resulting transmission spectra from an applied d.c. field show an anticrossing curve due to the finite optical coupling between the two rings, which is analogous to the d.c. Stark effect in a canonical two-level system. NT, normalized transmission. 6 to a loaded quality factor of QL =​ 1.9 ×​ 10 (intrinsic quality factor Next, we use a continuous-wave coherent microwave field to 6 Qi =​ 2.5 ×​ 10 ), thus forming a well-resolved two-level system (Fig. 1c). control our photonic two-level system. This situation is similar to We induce photonic transitions in the two-level system using an atomic two-level system under a strong coherent excitation, with high-frequency electro-optic phase modulation of the two modes. the important difference that in our case the number of photons that The phase modulation is realized through the Pockels effect (χ(2)) could populate each of the two levels is not limited to one. When the for lithium niobate, in which the optical refractive index can be microwave frequency matches the energy difference of the two lev- changed by an applied electric field, with a response time on the els, an effective coupling between the two initially decoupled S and femtosecond scale. To enable strong overlap between microwave AS modes is introduced, leading to a second-order mode splitting in and optical fields without significantly increasing the optical loss, the coupled ring resonators. The exact splitting frequency (Ω) can be we place gold microelectrodes 2.5 μ​m away from the edge of the precisely controlled up to several gigahertz by controlling the ampli- rings that form the photonic molecule (Fig. 1b). The microwave tude of the microwave signals. This microwave-induced photonic circuit layout is designed to induce a phase delay in one ring and mode splitting is a dissipative coupling between the optical modes a phase advance in the other ring, therefore introducing coupling in analogy to the Autler–Townes splitting (Rabi splitting) in elec- between the spatially orthogonal S and AS modes (Fig. 1a and tronic systems (Fig.