从爱因斯坦的思想实验到量子网络研究 白皮书

From Einstein’sGedankenexperiment to White paper By Peter Vetter, President of Bell Labs Core Research, Nokia Entanglement and teleportation explained — how quantum physics provides new opportunitiesfor realizing quantum networks and enhancing network security, and the research challenges Acknowledgement:A special thank you to my colleagues Rene Essiambre, Ayed Sayem, Tzu-YungHuang, Jeremie Renaudier, Amir Ghazisaeidi, and Ludovic Noirie for insightful discussions, input Contents Introduction3From Einstein’s thought experiment to Alain Aspect’s experimental proof3A new way to communicate information7Quantum key distribution8Quantum networks9Quantum teleportation: transferring a quantum state through a network repeater10Exploration research on future quantum networks11References13 Introduction The present paper explains how the concept of quantum entanglement can be used to transfer a quantumstate across a future quantum network to realize end-to-end quantum security, distributed quantumcomputing and distributed sensing. Previous articles by Nokia Bell Labs have discussed how quantum In order to grasp the spooky capabilities of quantum technology, we will start with a short history ofexperimental quantum mechanics, beginning with the Gedankenexperiment of Albert Einstein in 1935 and This paper will then elaborate on how entanglement can be used to transfer a quantum state across alink and even a network. This transfer is sometimes called quantum teleportation, though it has nothingto do with the means of transportation known from the popular science fiction series Star Trek. Thefundamental principle of quantum entanglement provides a means to detect eavesdropping over a The research on quantum networks is still in its infancy. The paper thus concludes with the researchchallenges we are overcoming and the building blocks we are exploring at Nokia Bell Labs. From Einstein’s thought experiment to AlainAspect’s experimental proof It is well known that Albert Einstein struggled to accept the weird uncertainty of quantum mechanics andthe idea that quantum theory could not predict the outcome of an experiment in a deterministic manner.He famously said, “God didn’t play with dice.” He and Niels Bohr would have endless debates on whetherthe quantum state is unknown until an experiment is performed (Bohr’s view) or the state is already One of the later thought experiments developed by Einstein with Boris Podolsky and Nathan Rosen in 1935was pivotal for the concept of quantum entanglement and the notion of spooky action at a distance that Figure 1 briefly explains the idea. Assume two particles are created in the same quantum process andtherefore their quantum states are entangled. After a certain time, the particles respectively move toposition x’1with speed v’1and position x’2with speed v’2. According to Heisenberg’s uncertainty principle, An experiment on particle 1 will yield a particular measured value for position x’₁ (and because of thequantum uncertainty, this value may be different for each subsequent measurement with a statisticaldistribution). With the outcome of one experiment for particle 1, it is possible to exactly calculate therespective position of particle 2, because they were created in the same quantum process and there Einstein believed that the outcome of a physical experiment must be entirely determined by localconditions and with 100% predictability. Einstein argued that quantum mechanics is incomplete becausethe position of particle 2 cannot be predicted by the theory in a deterministic way (there is the uncertaintyof particle 1) and is non-local (the outcome of 2 is determined by the measurement of 1). Accordingto Einstein, there must be another theory, yet to be discovered, that explains this “spooky action at adistance”. The particles would carry so-called hidden variables determined at the time of creation, which A few scientists remained intrigued by the EPR paradox and came up with simplified variants of thethought experiment that ultimately resulted in realizable experiments. In 1951, David Bohm reduced theproblem to a thought experiment of entangled particles that have two discrete states (spin up and spindown), and he even formulated a hidden-variable theory for it. This inspired John Bell to come up witha mathematical proof called Bell’s Theorem. According to Bell, if hidden variables exist for a process ofentangled particles with two possible states, the statistical distribution of independent measurementsmust show an inequality, which came to be known as the Bell Inequality[4]. If on the other hand, an Alain Aspect took up the challenge and built on Clauser’s work using the setup in Figure 2. It involves anentangled photon source S and two polarizing beam splitters that separate light into two orthogonal It is worthwhile explaining this in a bit more detail because it helps to develop an intuitive understanding ofhow future quantum networks could work. Figure 3 s