A crucial active ingredient that makes quantum technologies so effective is quantum entanglement, a state in which two or more quanta of light (or, photons) share in between them stronger correlations than is possible for classical light. In a nutshell, quantum repeaters store the delicate knotted state and change it into another quantum state that shares entanglement with the next node down the line. At the heart of such quantum-repeater networks are quantum memories in which quantum states of light can be stored. Practical executions of erbium-ion-based quantum memories showed relatively inefficient so far, hindering further development towards quantum repeaters.
In addition, the scientists integrated their quantum memory with a novel source of knotted photons on an incorporated chip.
The group of Prof. Xiao-Song Ma at Nanjing University has shown the faithful storage of quantum-entangled photons at telecom wavelengths for a record-long time of nearly 2 μs. Secret elements for this accomplishment were the combination of efficient generation of knotted photons (blue spheres) with an integrated microring resonator (bottom right) and long storage time in an ensemble of 167Er3+ ions doped in a Y2SiO5 crystal (cube) using atomic frequency combs (lower-left). Credit: Group of Prof. Xiao-Song Ma at Nanjing University
Physicists have achieved a major breakthrough in quantum technology by significantly extending quantum storage times at telecom wavelengths. This advancement is crucial for establishing useful quantum networks and incorporating them into existing fiber optic infrastructures.
Quantum technologies are currently developing at a breathtaking pace. These innovations exploit concepts of quantum mechanics in suitably engineered systems, with brilliant potential customers such as enhancing computational performances or interaction security well beyond what is possible with devices based on todays classical technologies.
Similar to classical devices, however, to realize their complete capacity, quantum devices will require to be networked. In concept, this can be done using the fiber optic networks employed for classical telecommunications. Practical implementation needs that the information encoded in quantum systems can be reliably kept at the frequencies used in telecom networks– an ability that has actually not yet been fully demonstrated.
Composing in Nature Communications, the group of Prof. Xiao-Song Ma at Nanjing University reports record-long quantum storage at telecom wavelengths on a platform that can be released in extended networks, paving the method for useful large-scale quantum networks.
Optical Fibers and Quantum Challenges
The glass fibers that make up these large networks are famously pure. Some losses are unavoidable, and the optical signals that take a trip through telecoms networks require to be revitalized at regular intervals when distances surpass a couple of hundred kilometers. For quantum states of light, however, these routinely used approaches are regrettably not suitable.
Why is quantum light various? A crucial ingredient that makes quantum technologies so powerful is quantum entanglement, a state in which two or more quanta of light (or, photons) share between them more powerful correlations than is possible for classical light. In standard optical signal regeneration, the optical signal is transformed into an electrical signal, which is enhanced before being converted back into light pulses. However, in such a procedure entangled photons would lose their necessary quantum connections. The exact same issue accompanies other standard techniques.
In a nutshell, quantum repeaters keep the fragile entangled state and change it into another quantum state that shares entanglement with the next node down the line. At the heart of such quantum-repeater networks are quantum memories in which quantum states of light can be saved.
Breakthrough in Quantum Storage
The excitement as Ming-Hao Jiang, Wenyi Xue and associates in the group of Xiao-Song Ma now report storage and retrieval of the knotted state of 2 telecom photons with a storage time of close to 2 split seconds. This is practically 400 times longer than what had been demonstrated before in this field and for that reason is a definitive step towards useful gadgets.
The memories established by Jiang, Xue et al. are based upon yttrium orthosilicate (Y2SiO5) crystals doped with ions of the rare-earth component erbium. These ions have optical homes that are practically best for use in existing fiber networks, matching the wavelength of around 1.5 μm. The viability of erbium ions for quantum storage has been known for some years, and the fact that they are embedded in a crystal makes them especially attractive with a view to large-scale applications. However, useful executions of erbium-ion-based quantum memories proved relatively inefficient so far, hindering additional development towards quantum repeaters.
Mas group has actually now made significant advances in refining the methods and has actually shown that even after keeping the photon for 1936 nanoseconds, the entanglement of the photon set is preserved. This indicates that the quantum state can be controlled during this time, as is needed in a quantum repeater. In addition, the researchers combined their quantum memory with a novel source of entangled photons on an incorporated chip.
This showed capability to both generate top quality knotted photons at telecom frequencies and store the knotted state, all on a solid-state platform ideal for low-priced mass production, is exciting as it develops an appealing structure block that might be integrated with existing large-scale fiber networks– thereby making it possible for a future quantum web.
Reference: “Quantum storage of entangled photons at telecom wavelengths in a crystal” by Ming-Hao Jiang, Wenyi Xue, Qian He, Yu-Yang An, Xiaodong Zheng, Wen-Jie Xu, Yu-Bo Xie, Yanqing Lu, Shining Zhu and Xiao-Song Ma, 1 November 2023, Nature Communications.DOI: 10.1038/ s41467-023-42741-1.