We may be able to harvest the quantum property of entanglement from apparently empty space, according to a group of researchers. If their proposal can be realised, it has the potential to reveal new information about the fundamentally quantum nature of the vacuum.
According to quantum physics, what we consider empty space is not actually empty. Instead, nearly imperceptible flickers of quantum fields are always present. Over a large volume, these fluctuations of quantum fields cancel each other out to give the impression of emptiness. But for any given point in empty space – or “vacuum” – they are non-zero.
This idea has been repeatedly verified by experiments. But experimentally uncovering how the quantum fluctuations at different points in space correlate with one another has been more challenging.
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Last year, at the Federal Institute of Technology Zurich in Switzerland and his colleagues met that challenge. They used very short laser pulses to measure, for the first time, the correlations between two different points of vacuum. Now, they propose a way to go further and measure entanglement – the quantum property that can inextricably link objects across any distance – between the two points.
Based on their past experiments and a complex theoretical model, their proposal is to fire two laser pulses – each tens of trillionths of a second in length – into different parts of a special crystal. Inside the crystal, the pulses should interact with quantum fields, including the flickering quantum fields characteristic of empty space – so-called vacuum states.
These interactions should, in turn, create infrared light signals that propagate out of the crystal along with the light from the original laser pulses. They will then be collected by a sensitive detector. The researchers’ calculations showed that analysing these signals’ properties could show whether they had picked up, or harvested, entanglement from the vacuum states.
“This [would] show that the vacuum is really a quantum state… which you can harvest and really get some quantum resource out of,” says at the Albert Ludwig University of Freiburg in Germany who worked on the proposal.
at the University of Waterloo in Canada says that quantum correlations like entanglement aren’t just a property of vacuum, but a crucial feature of our best theories for how quantum mechanics and special relativity combine. In fact, theories predict that empty space would be very different if it didn’t have any quantum correlations. For instance, it could never cool to the low temperatures approaching absolute zero that are seen in most of outer space, he says.
Harvesting quantum entanglement in empty space could also help us answer questions about the quantum nature of the vacuum near exotic objects like black holes or during events like the big bang says at Dartmouth College in New Hampshire. Here, the vacuum is theorised to be teeming with particles popping in and out of existence, but it is unclear how to measure their quantum nature, he says. Using special interactions between light and crystals, like in the new proposal, may be a good candidate, he says.
Based on these ideas, and competing proposals that involve either quantum bits – similar to those used in quantum computers – or extremely cold atoms, Martin-Martinez is optimistic that vacuum entanglement will be directly measured within five years. This could lead to applications including sensing properties of very distant objects without interacting with them, by simply relying on the fact that everything in contact with empty space shares a quantum connection.
But Faist says that many experimental challenges remain. Signals stemming from vacuum fluctuations are inherently weak, and vacuum fluctuations can also never be turned off, so it can be difficult to establish a baseline against which to compare them during measurements, he says.
arXiv
