By manipulating the quirky world of subatomic particles, physicists have moved a step closer to the possibility of converting one quantum resource to another. A team of researchers from India, Spain and the UK has mathematically proved that it is possible to convert an amount of ‘quantum coherence’ in a system into an equal amount of ‘quantum entanglement’1.
All subatomic particles have wave-like properties that give rise to the fundamental ‘superposition principle’ of quantum mechanics, which allows the state of a particle to be in a superposition of multiple states. Quantum coherence is a direct manifestation of this principle and is at the heart of all intriguing features of the quantum world. Quantum entanglement also arises from the superposition principle. However, the superposed quantum state of two entangled particles cannot be split into states of two individual particles.
Coherence and entanglement play a critical role in emergent fields such as nanoscale thermodynamics and quantum biology and in technologies such as quantum teleportation and quantum cryptography. However, our understanding and quantitative characterisation of coherence as an operational resource is still very limited.
The researchers (clockwise from top left): Himadri Shekhar Dhar, Uttam Singh, Manabendra Nath Bera, Alexander Streltsov and Gerardo Adesso . 
“The importance of this work is in the theoretical connection between coherence and entanglement, which in turn will be beneficial in unraveling quantum resources,” report researchers Uttam Singh, Himadri Shekhar Dhar and Manabendra Nath Bera, from the Harish-Chandra Research Institute (HRI), Allahabad. “This study brings us a step further in the quest to identify the most genuine ingredients for quantum computers and related technologies” Singh told Nature India.
It is well known that entanglement stems from the superposition principle, which is also the essence of coherence. Then how can one resource emerge quantitatively from the other?
The team, which included Alexander Streltsov from ICFO-The Institute of Photonic Sciences, Barcelona, Spain, and Gerardo Adesso from the University of Nottingham, provided a mathematically rigorous approach to resolve this question using a common frame to quantify quantumness in terms of coherence and entanglement. They show that any non-zero amount of coherence in a system could be converted to entanglement via incoherent operations.
“The results provide coherence measures with another operational interpretation related to its potential for conversion into entanglement,” Streltsov points out. Coherence, therefore, becomes indirectly useful for all tasks based on entanglement as well, such as quantum teleportation, Adesso says.
The physicists say that coherence has a significant role in all quantum protocols, where entanglement is a potential resource, such as one-way quantum computing and teleportation. Some of these protocols are experimentally possible and are at the heart of the quantum revolution.
