Focus: Entanglement Warms Up
Entanglement is the strange quantum mechanical link between two separated objects that allows a measurement of one to apparently influence the other. Normally, entangling objects or particles other than photons requires temperatures as low as fractions of a degree Kelvin. But in the 29 October Physical Review Letters, theorists propose a technique for entangling two oscillators–which could be atoms or vibrating pieces of silicon–at higher temperatures. For atoms, the environment could be as warm as 50 degrees Kelvin, they say. By coupling the two oscillators together with the analog of a spring and then causing the strength of the spring to oscillate in time, the entanglement can be made to survive, despite interactions with a high-temperature environment. The team thinks their technique could ease the experimental burden on scientists currently studying entanglement at difficult-to-reach temperatures.
At its simplest, an oscillator is a spring with one end attached to a wall and the other to a block that is free to slide back and forth at the natural frequency of the system. The oscillating block could stand in for an atom in an electromagnetic trap; a tiny, vibrating strip of silicon; or light waves resonating inside a microscopic cavity.
Now imagine a second identical oscillator whose spring is attached to a wall opposite the first. If the two are put into an entangled state, a measurement of one oscillator causes the corresponding quantity for the other oscillator to be known immediately, with complete certainty, according to quantum mechanics. However, experimenters have not yet entangled a pair of macroscopic vibrating objects, in part because of the low temperatures required.
Very low temperatures are needed because even small amounts of thermal energy from the environment can destroy the carefully prepared quantum states. In their new paper, Fernando Galve, of the Institute for Cross-Disciplinary Physics and Complex Systems located at the University of the Balearic Islands in Spain, and his colleagues, suggest a way to overcome this environmental disturbance, or “decoherence,” which threatens entangled states. They imagine the two oscillator blocks connected to each other with a third spring whose stiffness oscillates with time and thereby drives oscillations in the blocks. The team calculated that this driving force would continually put the two oscillators into just the right combination of states to generate entanglement and to compensate for decoherence.
The key is that the oscillators would be driven into a so-called squeezed state. Heisenberg’s uncertainty principle says that the product of the uncertainties in two complementary quantities (like position and momentum) must exceed a certain amount. In a squeezed state, the uncertainty in one of the quantities is squeezed down to a very small value, leaving all the uncertainty in the other quantity. For the coupled oscillators, the two quantities are the sum of and the difference between the two block positions. The squeezed state has most of the uncertainty in the sum, so the difference is known very precisely. If the position of one block is measured, the position of the other is instantly known to high precision–the signature of entanglement. The driving spring would keep pushing the oscillators into this entangled, squeezed state and counter the tendency for thermal energy to destroy it.
Galve and his colleagues think that if two ions placed in electromagnetic traps are coupled together with a capacitor connecting the traps, and the voltage in the traps is allowed to oscillate, entanglement could be achieved in an environment up to 50 degrees Kelvin. Nanomechanical resonators, which resemble small diving boards or vibrating drums, could also be coupled capacitively. The team believes that they could be entangled at about one degree Kelvin, rather than thousandths of a degree Kelvin, as others have assumed.
Howard Carmichael from the University of Auckland in New Zealand says the proposal is “very interesting and even provocative.” However, Dagmar Bruss of the University of Dusseldorf in Germany cautions that “it will be very challenging to realize” experimentally.
Michelangelo D’Agostino is a physicist and freelance science writer in Berkeley, California.