2026年天津（第二次） Passage B

Secret codes once thought impossible to crack now face a challenge from quantum (量子) computers. Luckily, to protect information, physicists have developed a theoretically reliable system called device-independent quantum key distribution (DI-QKD), which uses entangled (纠缠的) quantum particles (粒子) to fix weak points in existing methods. Now, a team in China has demonstrated the technique in real city conditions, an advance that could clear the way for a secure quantum internet in which trust is taken for granted.

Most traditional cryptography (密码学) works by encrypting (加密) messages with a secret code that requires a special key to decode. The challenge lies in making sure the key can’t be decoded when sent. For instance, the keys in one popular encryption scheme are based on the main factors of large numbers, which take enormous effort to crack. But such calculations could be quick work for the large quantum computers. QKD offers a solution, but it has a weakness: hardware must be perfect.

DI-QKD removes the need for trust in the hardware by employing another feature of quantum mechanics — entanglement — which closely links properties of widely separated quantum particles. If the sender and receiver entangle (纠缠) a pair of particles across the network, they can each perform tests that confirm the particles’ properties are strongly linked, well beyond chance. After this “handshake”, they can be sure they’re the only ones on the channel. Then, other measurements on the entangled particles can establish a key, which can be shared with confidence that nobody can decode it.

In 2022, a UK-based team managed to create and share a DI-QKD key for only about 2 meters. Now, the Chinese team led by Jianwei Pan have extended this to practically useful distances. After collecting data for 26 days, they showed they had the statistics to establish and share a key across 11 kilometers. The researchers also showed that in theory — and given about 23 years for data collection — they could have sent a key across 100 kilometers. Pan’s group now plans to explore ways to perform DI-QKD using satellites in space.