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Physicists Discover How to Entangle at High Temperatures

The long-standing belief that entanglement can only occur at low temperatures is finally put to rest.

Entanglement is the weird quantum process in which two objects share the same existence. So a measurement on one object immediately influences the other, not matter how far apart they may be.

Entanglement is a strange and fragile thing. Sneeze and it vanishes. The problem is that entanglement is destroyed by any interaction with the environment and these interactions are hard to prevent. So physicists have only ever been able to study and exploit entanglement in systems that do not interact easily with the environment, such as photons, or at temperatures close to absolute zero where the environment becomes more benign.

In fact, physicists believe that there is a fundamental limit to the thermal energies at which entanglement can be usefully exploited. And this limit is tiny, comparable to very lowest temperatures.

Today, Fernando Galve at the University of the Balearic Islands in Spain and a few buddies, show how this limit can be dramatically increased. The key behind their idea is the notion of a squeezed state.

In quantum mechanics, Heisenberg’s uncertainty principle places important limits on how well certain pairs of complementary properties can be observed. For example, the more accurately you measure position, the less well you can determine momentum. The same is true of energy and time and also of the phase and amplitude of a quantum state.

Physicists have learnt how to play around with these complementary observables to optimise the way they make measurements. They’ve discovered that they can trade their knowledge of one complementary observable for an improvement in the other.

This process is called squeezing and and it has had important implications for the way physicists control and interact with the quantum world.

Usually, physicists think about squeezing in steady state situations where everything is in equilibrium. What Galve and co have done is to ask how squeezing might be applied in a non-equilibrium situation where everything is changing.

The example they study is a pair of driven quantum oscillators that are entangled and which sit in a hot environment.

There are two important processes at work here. First is the way that the entanglement leaks into the hot environment, a process that depends on the temperature. Second is the way that the process of driving these oscillators also squeezes their state in a way that helps preserve entanglement.

Clearly these processes are competing against each other. The important discovery that Galve and co have made is that this competition dramatically increases the temperature at which entanglement can occur.

They’re saying that by constantly squeezing the system, you can preserve entanglement at high temperatures.

But how high? Galve and co say this depends on the coupling between the oscillators. But they calculate that entanglement between a pair of calcium ions in an rf trap–a standard set up in many labs–and show how it could be sustained at 50K. That’s significantly better than the sub-4K systems that experimenters have to manage with today. “We believe this to be a huge experimental step,” they say.

What about room temperature experiments? That would require a very strong coupling and may cause other problems. The squeezing causes the quantum states to become more delocalised, in other words they become smeared out in space. That could be a problem if the ions end up largely outside the trap in which they are supposed to be confined.

Galve and co carefully avoid any discussion of biological systems but there is growing evidence that quantum coherence and entanglement play a role in the wetware of life. We’ve talked about examples here and here.

This evidence has puzzled physicists because of what they thought was a fundamental limit to the temperature at which entanglement could survive. Now Galve and co have show that this only applies to steady state systems and their new approach applies to non-equilibrium systems.

The exciting implication is that this may provide the theoretical foundations to finally understand the role that quantum mechanics plays in living things.

Ref: Bringing Entanglement To The High Temperature Limit

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