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Chinese Physics B

Improving the purity of heralded single-photon sources through spontaneous parametric down-conversion process*

Jing Wang, Chunhui Zhang, Jing-Yang Liu, Xue-Rui Qian, Jian Li, Qin Wang

Published Apr 26, 2021• 3 citations
Publisher

Abstract

The high-purity single-photon source plays an important role in the field of quantum information. Usually, it is generated through spontaneous parametric down-conversion process. In this paper, we investigate and summarize a few approaches on obtaining single-photon sources with a high purity using either PPKTP or PPLN nonlinear crystals. Moreover, we present improved schemes to increase the purity based on existing work, corresponding applicable conditions and procedures are discussed and analyzed. Besides, we carry out numerical simulations and show that nearly perfect purity can be reached even without using any filters. Therefore, this work might provide valuable references for the generation and application of high purity single-photon sources.

DOI
10.1088/1674-1056/abfb5c

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The high-purity single-photon source plays an important role in the field of quantum information. Usually, it is generated through spontaneous parametric down-conversion process. In this paper, we investigate and summarize a few approaches on obtaining single-photon sources with a high purity using either PPKTP or PPLN nonlinear crystals. Moreover, we present improved schemes to increase the purity based on existing work, corresponding applicable conditions and procedures are discussed and analyzed. Besides, we carry out numerical simulations and show that nearly perfect purity can be reached even without using any filters. Therefore, this work might provide valuable references for the generation and application of high purity single-photon sources.
The generation of high purity single photons and entangled photon pairs is a crucial first step in many quantum optics experiments, including those that require quantum teleportation [1], entanglement swapping [2], and the generation of multi-photon entanglement [3]. The most widely used and established approach for creating photon pairs is spontaneous parametric down conversion (SPDC) - where pump photons undergo a non-linear optical process to create a down-converted photon pair [4]. Traditionally, the generation of high-purity heralded single photons required severe spectral filtering, introducing additional loss. However, through careful engineering of spectral correlations of the down-converted photon pairs one can circumvent the need for filtering and avoid the accompanying reduction in heralding efficiency.
Single-photon sources that emit only one photon at a time with high yield are important for security and efficiency in quantum communication and computation, respectively. One method to develop the desired single-photon source is through entangled single-photon pairs from spontaneous parametric down-conversion (SPDC) crystals where detection of idler photons “heralds” the presence of signal photons. If the SPDC crystal is pumped with weak intensity laser, high purity heralded single photons can be produced at the cost of low yield. Increasing the yield by using stronger laser intensity inevitably decreases the purity due to the presence of multiple photons. Here, we propose a conceptual method to overcome this trade-off between single-photon purity and single-photon yield. Our setup consists of a weakly pumped SPDC crystal seeded with strong coherent states (CS) at the idler mode. Clicks on a click detector at the idler mode conditionally prepare a superposition state of zero-, one-, and two-photon Fock states (FS) at the signal mode. By employing repeated processes of infinitesimal displacement (ID) and two-photon elimination (PE), we can obtain a final state of one-photon FS. Based on the heralding probability (single-photon yield) and single-photon fidelity (single-photon purity), we found that our method transforms the initial state with high heralding probability but low single-photon fidelity (near minimum) to a final state with high heralding probability and high single-photon fidelity (maximum). Furthermore, we discussed the conditions required for the experimental implementations of the processes. Thus, our work paves the way to producing heralded single photons with high purity and yield in SPDC sources.
Sources of single photons in pure quantum states are essential to many quantum optics applications. We explore a technique, known as group-velocity matching, that enables the heralded generation of pure single photons through nonlinear optical processes. It reduces or eliminates the requirement of spectral filtering that conventional sources rely on to achieve high purity, therefore increasing both heralding efficiency and photon flux. Implementation in both spontaneous parametric downconversion and spontaneous four-wave mixing is studied as we discuss several recent experimental realizations demonstrating purity of more than 0.84 for the heralded photons.
Many quantum information protocols require the high rate production of individual photons with near unit purity. The most mature platform for producing these photons is nonlinear optical sources, which exploit parametric processes to produce photon pairs, one of which is subsequently detected to “herald” the presence of its partner. The photon pairs produced in such sources often have strong frequency correlations, which adversely affect the purity of the heralded single photons, and so such sources are spectrally filtered to restore the purity. Because this process throws away photons, the rate of photon production is reduced - filtering increases purity at the cost of efficiency. This relationship has been explored extensively numerically [1-3], but has not been well understood analytically. As the demand for multi-photon experiments increases, it is becoming necessary to precisely quantify the trade-off between purity and efficiency. We believe our work enables this.
Single photons generated by spontaneous parametric down conversion are often utilized in heralded schemes, in which case the heralded photon purity must be maximized to preserve the indistinguishability of the photons that carry out quantum computing. In this work, we present software designed for accurate calculation of such purity, which was optimized to minimize computational resources and time, allowing for maximum purity searches in broad wavelength ranges. Lithium niobate and potassium titanyl phosphate (KTP) were studied in three phase-matching conditions, and an ultra-high purity configuration was found. The configuration was experimentally realized through an ion-exchanged KTP waveguide, exploiting flexibility in pump and phase matching temperature to compensate fabrication deviations, and purities as high as 0.923 were found, with very high generation rate of 40.7 MHz/mW. The experimental purity is significantly degraded by the multimode guiding at the desired wavelengths, which we propose to address by engineering with femtosecond laser writing, of which preliminary results are presented.
Multi-photon state generation is of great interest for near-future quantum simulation and quantum computation experiments. To-date spontaneous parametric down-conversion is still the most promising process, even though two major impediments still exist: accidental photon noise (caused by the probabilistic non-linear process) and imperfect single-photon purity (arising from spectral entanglement between the photon pairs). In this work, we overcome both of these difficulties by (1) exploiting a passive temporal multiplexing scheme and (2) carefully optimizing the spectral properties of the down-converted photons using periodically-poled KTP crystals. We construct two down-conversion sources in the telecom wavelength regime, finding spectral purities of > 91%, while maintaining high four-photon count rates. We use single-photon grating spectrometers together with superconducting nanowire single-photon detectors to perform a detailed characterization of our multi-photon source. Our methods provide practical solutions to produce high-quality multi-photon states, which are in demand for many quantum photonics applications.