Recently, the team of Academician Guo Guangcan and Professor Zhou Qiang from the Institute of Basic and Frontier Research at the University of Electronic Science and Technology of China, in collaboration with researcher You Lixing from the Shanghai Institute of Microsystems, Chin

Recently, The team of Academician Guo Guangcan and Professor Zhou Qiang of the Institute of Basic and Frontier Research of the University of Electronic Science and Technology of China collaborated with Researcher Yu Lixing of the Shanghai Institute of Microsystems, Chinese Academy of Sciences, to propose and principle-verify the lithium niobate on insulator (LNOI) chip. Spectrum multiplexed announced single photon source (HSPS) realizes spectrum multiplexed announced single photon generation in the telecommunications band for the first time. This achievement breaks the formal trade-off between single-photon yield and purity, paving the way to achieve on-chip scalable and high-performance HSPS .

The related research results were published in the 2022 Issue 6 of the Photonics Research journal under the title "Indistinguishable Single Photon Generation for Spectrum Multiplexing in Telecommunications Bands" [1].

Single-photon sources are the basis of light quantum information technology. Ideally, they are capable of emitting single photons in a pure, deterministic and indistinguishable manner and have a variety of potential applications: Fundamental research on the quantum properties of light , secure communications, exponentially enhanced computing, and high-precision measurements. In order to make the emission of single photons more suitable for widespread deployment in quantum technologies, there are two paths in engineering:

• The first is based on a single emitter. Able to emit single photons with certainty, but the manufacturing process is complex and difficult to operate at room temperature;

  
  

• The second approach is based on a spontaneous nonlinear parameterization process, the declarative single photon source (HSPS) - which is based on spontaneous parametric down conversion (SPDC) or The correlated photon pair produced by spontaneous four-wave mixing detects one photon (ie, the declared photon) in the photon pair to indicate the existence of its twin photon. HSPS is convenient for experiments and has flexible emission wavelengths, but it also has unavoidable probabilistic properties; at the same time, in order to achieve higher single-photon purity, the HSPS system must operate at low power levels, which limits the announced single-photon (HSP) yield and practical applications of HSPS.

HSPS requires a trade-off between single-photon yield and single-photon purity. To break this trade-off, multiplexing techniques can be applied on different degrees of freedom, thereby significantly improving the performance of HSPS. Therefore, the team proposed a 1.5μm chip-scale HSPS to demonstrate proof-of-principle experiments based on discrete fiber optic components on lithium niobate on insulator (LNOI) by employing spectral multiplexing and active feedforward spectrum manipulation.

On-chip spectrum multiplexing HSPS principle based on LNOI. (a) LNOI-based spectrum multiplexing HSPS, all basic components are integrated on the LNOI photonic chip, including: (I) photon pair generation module, (II) filtering and detection module, and (III) feedforward sum frequency Move module. (b) Energy conservation diagram of SHG and SPDC. (c) Illustration of frequency domain multiplexing. The photon pair generation module (I) generates broadband correlation signals and idle photons through cascaded SHG and SPDC processes. The idle photons are filtered into different spectral modes and detected by the SNSPD in the filtering and detection module (II). The detection signal is sent to the logic circuit, which in turn sends the frequency-shifted signal to the feedforward and frequency-shifting module (III), where the signal photons are shifted into a common spectrum pattern. LNOI, lithium niobate on insulator; PPLN, periodically polarized lithium niobate; LN, lithium niobate; EOM, electro-optical phase modulator; SNSPD, superconducting nanowire single photon detector ; SHG, second harmonic Generate; SPDC, spontaneous parametric down conversion. Experimental setup for

spectrum multiplexing HSPS. PPLN, periodically poled lithium niobate; DWDM, dense wavelength division multiplexer; EOM, electro-optical phase modulator; TNF, tunable narrowband filter; BS, beam splitter; PC, polarization controller; TDC, time -Digital converter; SNSPD, superconducting nanowire single photon detector.

The team then verified the spectral characteristics of the announced signal photons with and without frequency-shifted signals.Signal photons are selected in the frequency domain by changing the filtering window of a tunable narrowband filter (TNF) with a bandwidth of 12.5 GHz; coincidence events between signal photons and declared photons are counted by a coincidence logic circuit. Through continuous wave laser pumping and multiplexing of three spectral modes, the results show that spectral multiplexing increases the announced single-photon yield by nearly three times . Experimental results of multiplexing of three spectrum modes

. The blue diamond is the multiplexed light source. (a) HSP rate versus pump power. The HSP rate of the multiplexed source is 2.80±0.12 at low pump power, which is a multiple larger than that of the single spectrum mode; (b) At a fixed HSP rate, the CAR of the multiplexed source is improved compared to the performance of a single HSPS ; (c) Comparison of the team's selected spectral pattern s0 with its multiplexed counterpart, showing that a single light source theoretically increases nearly 3 times more than multiplexed sources in low HSP rate regions.

To further characterize the non-classical properties and indistinguishability of spectrally multiplexed HSPS, the team demonstrated the Hong-Ou-Mandel (HOM) interference experiment between multiplexed HSPS and independent weakly coherent single photon sources. For ordinary photon pairs, a single signal mode/announcement mode does not provide nonclassical photon number statistics, and therefore visibility above 50% is used as a criterion for determining whether nonclassicality occurs in HOM interference.

In this experiment, the HOM interference experiment was performed by emitting balanced field strengths to the two input ports: when the two input fields are emitted to the two input ports of the beam splitter, the average number of photons is equal. The true triple visibility of the HOM interference between the multiplexed HSPS and the weak coherence source is close to the 66.67% upper limit, indicating that the multiplexed single photon source emits highly indistinguishable photons; if a true single photon source is used instead of the weak coherence source , the theoretical visibility of HOM interference is expected to be 91.5%. The slight decrease in visibility in the

experiment is mainly caused by noise in the detector and residual multiphoton events in the spectral multiplexed HSPS.

HOM interference between multiplexed light sources and weakly coherent light sources. The red circles are double coincidences measured without the prediction procedure; the blue diamonds are triple coincidences measured with the prediction procedure. Both double and triple coincidences are fitted by Gaussian functions, and the visibility is the result of 1000 Gaussian fittings of the Monte Carlo method. The red and blue bands represent the fitted variance for the two- and three-fold HOM effects, respectively.

In this experiment, the team proposed an on-chip telecom band spectrum multiplexing HSPS based on LNOI, demonstrated its principle verification experiment and significantly improved performance; at the same time, the multiplexed source and weakly coherent single photon source For the first time, HOM interference experiments show evidence of non-classical properties.

The results show that large-scale integration of photonic chip HSPS has great potential in realizing high-performance single photon sources. In addition, if a larger frequency shift is applied, this integration scheme also has scalable multiplexing prospects, which is of great significance for the development of high-quality single-photon sources.

Reference link:

[1]https://opg.optica.org/DirectPDFAccess/F2582531-61A3-4B96-9D2BCAD86D0C28B5_473206/prj-10-6-1417.pdf?da=1id=473206seq=0mobile=no