An important step for photonic quantum technologies is the demonstration of a quantum advantage through boson sampling. In order to prevent classical simulability of boson sampling, the photons need to be almost perfectly identical and almost without losses. These two requirements are connected through spectral filtering: improving one leads to a decrease of the other. A proven method of generating single photons is spontaneous parametric down-conversion (SPDC). We explore the fundamental limits of several important SPDC sources and show that an optimal tradeoff between indistinguishability and losses can always be found. We conclude that a 50-photon scattershot boson-sampling experiment using SPDC sources is possible from a computational complexity point of view. To this end, we numerically optimize SPDC sources under the regime of weak pumping and with a single spatial mode.
An important step for photonic quantum technologies is the demonstration of a quantum advantage through boson sampling. In order to prevent classical simulability of boson sampling, the photons need to be almost perfectly identical and almost without losses. These two requirements are connected through spectral filtering: improving one leads to a decrease of the other. A proven method of generating single photons is spontaneous parametric down-conversion (SPDC). We explore the fundamental limits of several important SPDC sources and show that an optimal tradeoff between indistinguishability and losses can always be found. We conclude that a 50-photon scattershot boson-sampling experiment using SPDC sources is possible from a computational complexity point of view. To this end, we numerically optimize SPDC sources under the regime of weak pumping and with a single spatial mode.
An important open challenge in quantum information processing is to demonstrate a quantum device outperforming a classical computer. One of the most promising ways is by sending single photons through a passive linear-optical network, and sampling their output distribution [1]. However, this requires the nontrivial generation of many spectrally pure (indistinguishable) single photons. Most importantly, -spectral distinguishability reduces the visibility of quantum interference of the photons, but can be mitigated by filtering at the cost of optical losses. Unfortunately, losses are also detrimental to quantum interference. We recently quantified the combined role of spectral imperfections and loss in a multiphoton interference experiment [2]. We now apply these results to the problem of constructing spontaneous parametric down-conversion sources. In particular, we investigate the optimal filter bandwidth for different sources and optimize the down-conversion crystal parameters. Our results show that demonstrating a quantum advantage using photonics is difficult, but possible
Entangled-photon sources with simultaneously near-unity heralding efficiency and indistinguishability are the fundamental elements for scalable photonic quantum technologies. We design and realize a degenerate telecommunication wavelength entangled-photon source from an ultrafast pulsed laser pumped spontaneous parametric down-conversion (SPDC), which shows simultaneously 97% heralding efficiency and 96% indistinguishability between independent single photons without narrow-band filtering. Such a beamlike and frequency-uncorrelated SPDC source allows generation of the first 12-photon genuine entanglement with a state fidelity of 0.572±0.024. We further demonstrate a blueprint of scalable scattershot boson sampling using 12 SPDC sources and a 12×12 mode interferometer for three-, four-, and five-boson sampling, which yields count rates more than 4 orders of magnitude higher than all previous SPDC experiments.
Boson sampling is considered as a strong candidate to demonstrate the “quantum advantage / supremacy” over classical computers. However, previous proof-of-principle experiments suffered from small photon number and low sampling rates owing to the inefficiencies of the single-photon sources and multi-port optical interferometers. In this talk, I will report two routes towards building Boson Sampling machines with many photons. In the first path, we developed SPDC two-photon source with simultaneously a collection efficiency of ~70% and an indistinguishability of ~91% between independent photons. With this, we demonstrate genuine entanglement of ten photons. Very recently, we managed to observe 12-photon entanglement using a novel SPDC source. Such a platform will provide enabling technologies for teleportation of multiple properties of photons and efficient scattershot boson sampling. In the second path, using a QD-micropillar, we produced single photons with high purity (>99%), near-unity indistinguishability for >1000 photons, and high extraction efficiency—all combined in a single device compatibly and simultaneously. We build 3-, 4-, and 5-bosonsampling machines which runs >24,000 times faster than all the previous experiments, and for the first time reaches a complexity about 100 times faster than the first electronic computer (ENIAC) and transistorized computer (TRADIC) in the human history. We are currently increasing the rate by optimizing the single-photon extraction efficiency to near unity, background-free resonance fluorescence, and using improved schemes such as boson sampling with photon loss, with the hope of achieving 20-photon boson sampling this year.
Optimizing the brightness of a spontaneous parametric down-conversion (SPDC) source is an important task for many quantum information applications. We investigate the optimal focusing conditions to maximize the number of photons produced in an SPDC process and coupled with single-mode fibers. We provide a general expression for the two-photon wave function, generalizing previous known results, by considering collinear and noncollinear emission. We present analytical expressions for our results in the thin-crystal limit and clarify the relation between different focusing conditions already existing in the literature. Differently from what was previously reported, we show that the optimal ratio between the pump waist and the generated photons waist depends on the emission angle: It is 1/2 for collinear degenerate emission and approaches 1/2 for larger collection angles. The role of spectral filters is also analyzed. We support and enrich our discussion with numerical simulations, performed for type-I SPDC in a $\ensuremath{\beta}$ barium borate crystal. For this type of emission, we also investigate the role of the transverse walk-off outside the thin-crystal regime.
Abstract Entangled photon sources are desirable and key elements for photonic quantum information processing. The spontaneous parametric down‐conversion (SPDC) process is a convenient way to produce entangled photon pairs. It has been widely used and has already made great contributions to quantum information science. SPDC sources have been thoroughly studied, and a detailed review, previously lacking, is now appropriate. In this article, different types of SPDC sources suitable for multiphoton experiments are reviewed. A comparison of their respective performances is presented, focusing on the parameters of indistinguishability, collection efficiency, and brightness, from which it is possible to see how the spatial and spectral properties of the SPDC process can be tailored, step by step, to achieve near‐perfect photon pairs. Recent progress on multiplexed heralded single‐photon sources, which effectively mitigate the nondeterministic nature of the SPDC process, is also described.
Sampling the distribution of bosons that have undergone a random unitary evolution is strongly believed to be a computationally hard problem. Key to outperforming classical simulations of this task is to increase both the number of input photons and the size of the network. We propose driven boson sampling, in which photons are input within the network itself, as a means to approach this goal. We show that the mean number of photons entering a boson sampling experiment can exceed one photon per input mode, while maintaining the required complexity, potentially leading to less stringent requirements on the input states for such experiments. When using heralded single-photon sources based on parametric down-conversion, this approach offers an ∼e-fold enhancement in the input state generation rate over scattershot boson sampling, reaching the scaling limit for such sources. This approach also offers a dramatic increase in the signal-to-noise ratio with respect to higher-order photon generation from such probabilistic sources, which removes the need for photon number resolution during the heralding process as the size of the system increases.