Quantum Dots for Wavelength-tunable Entangled Photon Sources(Invited)
The quantum internet is a network that can transmit and store quantum information and connect multiple quantum processors.Such quantum networks are the basis of quantum information technologies such as quantum communication,distributed quantum computing,and quantum sensing.Due to photons having low environmental interaction and strong resistance to interference,mature optical fiber networks can provide a solid foundation for the transmission of optical quantum states,serving as an excellent carrier for long-distance transmission.Although the loss of photon transmission in optical fibers will limit the communication distance,the generated quantum state can still be successfully distributed to any remote destination through quantum repeaters.Solid-state sources that emit pairs of entangled photons are an important part of optical quantum networks.In the past,spontaneous parametric down conversion has been used in most photon-based entanglement experiments.However,the generation of photons in this way is a probabilistic process following Poisson statistics.Hence,the probability of generating multiple pairs of photons within an excitation cycle is non-zero,necessitating a balance between the source brightness and the multi-photon emission probability.This would limit applications in complex quantum protocols.Self-assembled semiconductor quantum dots are one of the primary candidates for the deterministic generation of entangled photon pairs,possessing excellent properties of high efficiency,high purity,high indistinguishability,and high entanglement fidelity.Moreover,it can be compatible with semiconductor micro-nano processing technology widely used in the industry and integrated into photonic chips.Among various systems suitable for quantum information research,quantum dots are one of the most attractive choices.However,establishing quantum relays based on quantum dot entangled photon sources still faces some challenges.On the one hand,due to the stochastic nature of quantum dot growth,different quantum dots exhibit variations in size,shape,and composition.It is difficult to find multiple quantum dots within the same quantum chip with identical biexciton or exciton energies,directly limiting the Bell state measurement and thereby preventing entanglement transmission between quantum network nodes via quantum interference.On the other hand,anisotropies in strain,shape,and composition reduce the symmetry of quantum dots,the electron-hole exchange interaction leads to fine-structure splitting between two bright exciton states.This causes the entanglement fidelity to exhibit oscillating behavior over the time span t between biexciton and exciton photon emission,resulting in lower long-term integrated entanglement fidelity in experimental measurements.When fine-structure splitting is present,a well-entangled photon source can be obtained through temporal post-selection.However,this will lose most of the entangled photon pairs and reduce the effective brightness of the source.Alternatively,using high-resolution single-photon detectors or external optical techniques to'compensate'for the evolution characteristics of entangled states.Yet,these technical requirements diminish the appeal of quantum dots in scalable quantum technologies.The most direct and effective approach is the elimination of fine-structure splitting itself.In the past few decades,it has been confirmed that the post-growth tuning of quantum dots through thermal annealing,magnetic fields,lateral electric fields,vertical electric fields,optical fields,and strain fields successfully eliminates fine-structure splitting and generates entangled photon pairs.Despite the help of these tuning techniques,there is still a problem that the two factors of emission wavelength and fine-structure splitting cannot be tuned independently.When one factor is tuned to the target position,it often leads to the deviation of the other factor from its target value,making simultaneous improvements challenging.Wavelength-tunable entangled photon sources are a fundamental requirement for transmitting entanglement between quantum network nodes via quantum interference.This promotes the development of the field from single-degree-of-freedom tuning to multi-degree-of-freedom tuning.This multi-degree-of-freedom joint tuning paves the way for wavelength-tunable quantum dot entangled photon sources.In this article,the causes behind the non-uniform wavelength and fine-structure splitting in quantum dots are elucidated,providing an overview of recent years that use multi-degree-of-freedom tuning to achieve the tuning of both emission wavelength and fine-structure splitting.According to the different combined approaches,the combined tuning mechanisms are divided into four types:combined stress and electric field,combined magnetic field and electric field,anisotropic strain,combined light field and electric field.The tuning methods and current research status of these distinct mechanisms are reviewed.Finally,apart from the two important aspects of tunable emission wavelength and high entanglement fidelity,applying quantum dot entangled photon sources effectively in quantum relays necessitates enhancing the collection efficiency and indistinguishability of the entangled photon source.It is highlighted that these aspects could potentially be improved by coupling quantum dots into optical microcavities and using the Purcell effect,offering a perspective on the future development of wavelength-tunable quantum dot entangled photon sources.
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