THE spooky link that can exist between quantum particles even when they are far apart could provide an unexpected way to detect the ripples in space-time known as gravitational waves.
Gravitational waves are set off by extreme events, such as supernova explosions, but they are weak and notoriously difficult to detect. Now a group of physicists is suggesting that the waves could leave their signature on 鈥渆ntangled鈥� quantum particles lying in their path.
Entanglement is a weird quantum effect through which particles become intimately linked, so that measuring a property on one instantaneously affects the others, no matter how far apart they are. It is a fragile property that is hard to produce and manipulate, but this fragility may make entangled states sensitive enough to pick up the weak gravitational waves that have so far eluded other methods of detection.
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Two or more properties of one particle, for example, an electron鈥檚 spin and its momentum, can also become entangled, so that changing one will alter the other. Ye Yeo and his colleagues at the National University of Singapore propose that a set of such entangled particles, with precisely defined initial spins, could be used to detect gravity waves ().
Their calculations show that as the gravitational force from a passing wave slightly changes the momentum of the entangled particles, it should knock them out of their pristine spin state. In principle, that effect could be detected, but it is so small that no one has found a way to pick it up, explains Yeo. He and his team suggest that the effect could be amplified using a process called 鈥渆ntanglement swapping鈥�, which allows pairs of particles that have never been in contact to become entangled. 鈥淪pin and momentum become entangled to a higher degree so that changing one produces an even larger change in the other,鈥� says quantum physicist Chris Adami at the Jet Propulsion Laboratory in Pasadena, California.
Seth Lloyd, an expert in quantum computing at the Massachusetts Institute of Technology, is impressed by the group鈥檚 theoretical work. 鈥淛ust getting this connection between gravitational waves and quantum entanglement is quite an achievement,鈥� he says.
鈥淎 set of such entangled particles, with precisely defined spins, could be used to detect gravity waves鈥�
However, Lloyd does not think that a gravitational detector based on entanglement is a realistic prospect, even after amplification. 鈥淚t鈥檚 a real effect, but an unbelievably small one,鈥� he says. 鈥淭o get something observable you鈥檇 need a gravity field so large it would rip your lab apart.鈥�
