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Superconducting Silicon-Photonic Chip Developed For Quantum Communication

This question is answered affirmatively by a team led jointly by Xiao-Song Ma and Labao Zhang from Nanjing University, and Xinlun Cai from Sun Yat-sen University, China. As reported in Advanced Photonics, the team realizes quantum communication using a chip based on silicon photonics with a superconducting nanowire single-photon detector (SNSPD). The excellent performance of this chip allows them to realize optimal time-bin Bell state measurement and to significantly enhance the key rate in quantum communication.

Superconducting Silicon-Photonic Chip Developed for Quantum Communication

The single photon detector is a key element for quantum key distribution (QKD) and highly desirable for photonic chip integration to realize practical and scalable quantum networks. By harnessing the unique high-speed feature of the optical waveguide-integrated SNSPD, the dead time of single-photon detection is reduced by more than an order of magnitude compared to the traditional normal-incidence SNSPD. This in turn allows the team to resolve one of the long-standing challenges in quantum optics: optimal Bell-state measurement of time-bin encoded qubits.

This advance is important not only to the field of quantum optics from a fundamental perspective, but also to quantum communications from the application perspective. The team employs the unique advantages of the heterogeneously integrated, superconducting silicon-photonic platform to realize a server for measurement-device-independent quantum key distribution (MDI-QKD). This effectively removes all possible detector side-channel attacks and thus significantly enhances the security of quantum cryptography. Combined with a time multiplex technique, the method obtains an order-of-magnitude increase in MDI-QKD key rate.

(a) Schematic of the experiment setup. A superconducting silicon-photonic chip that performs optimal Bell-state measurements is used as the server for MDI-QKD, which allows Alice and Bob to exchange secure keys without detector side-channel attacks. (b) Destructive and constructive interference in coincidence counts when Alice and Bob send the same states (blue dots), or different states (red dots). (c) Secure key rate under different losses. Credit: Zheng et al., doi 10.1117/1.AP.3.5.055002

"This work shows that integrated quantum-photonic chips provide not only a route to miniaturization, but also significantly enhance the system performance compared to traditional platforms. Combined with integrated QKD transmitters, a fully chip-based, scalable, and high-key-rate metropolitan quantum network should be realized in the near future," says Ma.

Quantum information processing holds great promise for communicating and computing data efficiently. However, scaling current photonic implementation approaches to larger system size remains an outstanding challenge for realizing disruptive quantum technology. Two main ingredients of quantum information processors are quantum interference and single-photon detectors. Here we develop a hybrid superconducting-photonic circuit system to show how these elements can be combined in a scalable fashion on a silicon chip. We demonstrate the suitability of this approach for integrated quantum optics by interfering and detecting photon pairs directly on the chip with waveguide-coupled single-photon detectors. Using a directional coupler implemented with silicon nitride nanophotonic waveguides, we observe 97% interference visibility when measuring photon statistics with two monolithically integrated superconducting single-photon detectors. The photonic circuit and detector fabrication processes are compatible with standard semiconductor thin-film technology, making it possible to implement more complex and larger scale quantum photonic circuits on silicon chips.

Advanced nanofabrication techniques have proven invaluable for ensuring scalability of electronic components used in classical information technology12. The corresponding complementary metal oxide semiconductor (CMOS) fabrication recipes have recently also been employed for realizing both nanophotonic waveguides13, as well as superconducting single-photon detectors (SSPD)14 on silicon chips. As most linear optics quantum logic schemes rely on non-classical interference and single-photon detection15,16 it is crucial to realize both of these ingredients on a common scalable platform. Here we demonstrate such a quantum information processing platform by combining SSPDs with integrated silicon nitride photonic circuits to measure high-visibility quantum interference directly on-chip.

The integration of photonic circuits and detectors on a silicon chip for demonstrating quantum interference has previously been attempted with surface plasmon polariton devices. Two-plasmon quantum interference with 93% visibility on a beam splitter has been demonstrated with off-chip detectors and for photons at visible wavelengths35, which are not compatible with existing optical communication networks. Notably, the integration of plasmonic directional couplers with superconducting detectors on the same chip proved challenging and reduced the interference contrast below the classical limit36.

Here we integrate low-noise niobium titanium nitride (NbTiN) nanowire SSPDs with dielectric silicon nitride (SiN) photonic circuits on a silicon chip. Using photons from spontaneous parametric down conversion (SPDC) we measure quantum interference with 97% visibility directly on-chip. Our circuit-detector approach is fully compatible with scalable, high-yield semiconductor microfabrication processes.

The fact that we observe a visibility slightly lower than 100% at d=0 is thus mainly due to detection events from independent pairs that were created within the coincidence detection window, a slight imbalance in the splitting ratio of our on-chip directional coupler and the statistical photon counting noise. We anticipate that fine tuning of our fabrication recipes will improve the performance of our photonic circuits and detectors to allow for even higher interference visibilities, which comply with fault-tolerant quantum operations41.

Recent experiments in quantum optics3,4,5,43 manifest an ever more pressing need for a scalable solution to integrate photonic circuits and single-photon detectors. In particular, single-photon detection and high-visibility quantum interference have been identified as the two essential requirements for realizing scalable linear optic quantum computation15. Here we have shown how SSPDs embedded with nanophotonic circuits address these needs and achieve the requirements for scalable quantum technology on silicon chips. We observe high-visibility quantum interference of photons produced in SPDC with waveguide-coupled SSPDs and demonstrate that photon distinguishability is under accurate control in this architecture. All of the fabrication techniques used here can in principle be adapted to scalable technology developed for the CMOS industry, even at the front end of a CMOS line44. We anticipate that fine tuning of superconducting film and photonic circuit parameters and the implementation of photon-number resolving architectures45,46 will further increase the functionality of our integrated quantum photonic system.

Recent progress in realizing sources of non-classical light directly on silicon chips ideally complements the integration of single-photon detectors and photonic circuits described here. Such integrated quantum light sources were realized employing spontaneous four wave mixing31,47 and the excitation of waveguide-coupled quantum emitters48,49 but could also be realized via SPDC in III-nitride waveguides50,51. Combining nanophotonic sources, circuits and single-photon detectors on a silicon chip will allow for generating, processing and detecting quantum information all on one scalable platform.

How to cite this article: Schuck, C. et al. Quantum interference in heterogeneous superconducting-photonic circuits on a silicon chip. Nat. Commun. 7:10352 doi: 10.1038/ncomms10352 (2016).

A harmonic oscillator is a ubiquitous tool in various disciplines of engineering and physics for sensing and energy transduction. The degrees of freedom, low noise oscillation, and efficient input-output coupling are important metrics when designing sensors and transducers using such oscillators. The ultimate examples of such oscillators are quantum mechanical oscillators coherently transducing information or energy. Atoms are oscillators whose degrees of freedom can be controlled and probed coherently by means of light. Elegant techniques developed during the last few decades have enabled us to use atoms, for example, to build exquisite quantum sensors such as clocks with the precision of

Rare earth (RE) ions in crystals have been identified as robust optical centers and promising candidates for quantum communication and transduction applications. Lithium niobate (LN), a novel crystalline host of RE ions, is considered as a viable material for photonic system integration because of its electro-optic and integration capability. This thesis first experimentally reports the activation and characterization of LN crystals implanted with Yb and Er ions and describes their scalable integration with a silicon photonic chip with waveguide and resonator structures. The evanescent coupling of light emitted from Er ions with optical modes of waveguide and microcavity and modified photoluminescence (PL) of Er ions from the integrated on-chip Er:LN-Si-SiN photonic device with quality factor of 104 have been observed at room temperature. This integrated platform can ultimately enable developing quantum memory and provide a path to integrate more photonic components on a single chip for applications in quantum communication and transduction.

Our goal is to generate complex quantum communication and computation (QCC) systems using silicon photonic integrated circuits; we are specifically interested in creating demo high-bit-rate quantum key distribution and scalable quantum information processing systems. 041b061a72


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