It was an Albert Einstein prediction – the general theory of relativity – that led to the first, much-anticipated detection of gravitational waves (GWs) by a laser detector part-built by scientists at Birmingham. Now, two years later, it’s another of Einstein’s predictions, quantum theory (ironically, one he tried to discredit) that has been harnessed by a group of physicists, including one from the University, in a bid to make future GW detectors more sensitive to the tiny ripples in the fabric of space-time.
It is the first time these two predictions have been used in tandem to probe yet further into the origins and workings of the universe.
Dr Haixing Miao, a Lecturer and Birmingham Fellow in the School of Physics and Astronomy, and seven colleagues from around the globe have used what Einstein referred to as ‘spooky action at a distance’ – quantum entanglement – to show how it is possible make future versions of the Advanced Laser Interferometer Gravitational-wave Observatory (LIGO) more effective. So effective, in fact, that scientists could ‘hear’ what happened further into the distant universe.
The new design, led by Dr Yiqiu Ma, Haixing and their team, which has already attracted considerable publicity, is outlined in a paper entitled ‘Proposal for gravitational-wave detection beyond the standard quantum limit through EPR entanglement.’ Published in Nature Physics earlier this year, it was subsequently awarded the College of Engineering and Physical Science’s Paper of the Month.
GWs carry unique information about our Universe: the first detection, in late 2015, revealed a population of previously unknown heavy binary black holes. However, they interact very weakly with particles, so detecting them requires incredibly sensitive instruments.
GW detectors use laser light to pick up tiny vibrations of space created when these black holes collide, creating huge gravitational explosions.
The reason this new design is so exciting is that it can measure signals below a limit – the standard quantum limit – that was previously thought to be impossible to surpass.
It all stems from when Einstein teamed up with Boris Podolsky and Nathan Rosen in 1935 to try to disprove quantum mechanics with their ‘EPR paradox’ (named after their initials), which described seemingly absurd correlations between widely spaced particles. Strangely, however far apart the objects moved, the entanglement did not diminish.
In the last decade, scientists have proved time and again that quantum entanglement is real – and Haixing and his team have used this knowledge to create a new GW detector design – which, in effect, uses the current detector twice.
The first time, photons in the detector are altered by the GWs so as to pick them up; the second time, the detector is used to change the quantum entanglement of photons in such a way that the quantum noise is reduced – thus making the GW signals stronger.
To do this requires a device called a quantum squeezer, which creates entangled pairs of photons. A miniscule amount of squeezed light is injected into the detector, whereupon it pushes down the level of quantum noise after harnessing the quantum entanglement.
‘It’s the first time EPR has been paired with the detection of GWs,’ explains Haixing, a member of Birmingham’s newly established Institute for Gravitational Wave Astronomy, whose scientists have been involved in the instrumentation of Advanced LIGO and in the data analysis. ‘There were several steps in our thinking: firstly, advanced GW detectors are limited by quantum noise. In quantum mechanics, this comes from quantum fluctuations. No matter how good your laser is, there will still be this quantum fluctuation.
‘One approach to reduce this quantum fluctuation is to use squeezed light. However, it turns out that you have to change the property of the squeezed light, and to do this usually requires an additional quantum filter. This new idea takes advantage of EPR entanglement and eliminates this complexity by having the detector itself as the quantum filter.’
Haixing says he and his team ‘know that this works’, but concedes there are technical difficulties still to be overcome.
‘Because the design uses entanglement, it’s fragile, and any loss destroys entanglement.’
‘At the moment, colleagues in our GW community are in the process of carrying out two table-top experiments: one by Australian National University, and the other by University of Hamburg. After these demonstrations, we eventually can get this idea implemented into the big detector.’
He adds: ‘The interesting thing is that both predictions link to Einstein. Scientifically, it’s a new idea, so we’re very excited about that.’