Quantum entanglement at room temperature???
Thirty years ago I worked with MeV proton beams and microscopic samples at Oxford. Today physicists are entangling the motion of a levitated glass nanoparticle with light. The physics has changed dramatically. Some of the experimental problems have not.
I am naturally suspicious of quantum headlines (check out my previous articles).
“Quantum entanglement at room temperature” certainly qualifies as one.
But a recent experiment from Quentin Deplano, Antonio Pontin, Francesco Marino and Francesco Marin made me pause. The researchers report entanglement between the centre-of-mass motion of an optically levitated silica nanosphere and an electromagnetic field, with the correlations detected in a propagating optical mode. The sphere is approximately 100 nm in diameter.
What caught my attention wasn’t simply the quantum physics. It was how strangely familiar the experimental philosophy felt.
Back to Oxford
In the 1990s I worked on the Oxford Scanning Proton Microprobe at the Nuclear Physics Laboratory at the University of Oxford. Our 1994 paper describes an instrument that focused MeV protons and light ions onto microscopic samples and extracted analytical information from the resulting interactions.
The techniques included particle-induced X-ray emission (PIXE), Rutherford backscattering (RBS) and scanning transmission ion microscopy (STIM). Collectively they allowed us to identify and map elemental composition and obtain structural information on extremely small spatial scales.
PIXE was particularly powerful. Bombard an atom with a sufficiently energetic charged particle and you can eject an inner-shell electron. When an electron from a higher energy level fills the vacancy, an X-ray is emitted whose energy is characteristic of the element concerned. Detect the X-rays and a microscopic specimen begins to reveal its elemental composition.
There is a pleasing simplicity underneath all the engineering:
prepare a beam then focus it then interact it with matter then detect what comes out then infer what happened.
Thirty years later, I see something philosophically similar in Florence, although what is being extracted from the experiment is considerably more quantum.
A glass particle floating in light
The Florence experiment replaces the proton beam and specimen stage with something rather extraordinary.
A silica nanosphere about 100 nm across is optically trapped in a tweezer and positioned inside an optical cavity. Its motion couples to the cavity field through coherent scattering.
Two optical fields then perform complementary roles. The reported experimental configuration uses a red-detuned field to cool and stabilise the mechanical motion and a blue-detuned field to generate the interaction that entangles the mechanical oscillator with the optical field.
The team then uses phase-sensitive heterodyne detection to reconstruct the optical-mechanical correlations. They report violation of separability bounds between the mechanical degrees of freedom and a propagating optical mode.
That last word matters.
Propagating.
The quantum correlation isn’t described simply as something confined to the interaction region. It appears in the outgoing optical field. The authors therefore describe the experiment as demonstrating the distribution of non-classical correlations beyond the interaction region.
That, to me, is considerably more interesting than the headline about a levitating speck of glass.
But is it really “room-temperature quantum”?
Yes, with a substantial asterisk.
The researchers explicitly report that their results are obtained at room temperature.
But we shouldn’t translate that into:
A warm lump of glass containing millions of atoms has casually become quantum-entangled.
The quantity being entangled is specifically the centre-of-mass mechanical motion of the nanosphere with the electromagnetic field.
Nor does “room temperature” mean that thermal motion has magically ceased to matter. The experimental system uses optical cooling and stabilisation of the relevant mechanical motion.
That distinction is important because quantum experiments are ultimately battles against unwanted degrees of freedom.
And that is where I found myself transported back to those lab days of my past.
NOTE 1: Cooling an instrument’s detector is not the same thing as cooling the physical degree of freedom whose quantum behaviour we are attempting to preserve. Technology continually changes where the experimental difficulty lives. In the proton microprobe, the problem was making an extraordinarily small, well-defined beam, controlling interactions within a vacuum environment and recovering weak analytical signals from radiation and particles leaving the specimen. In levitated optomechanics, the challenge has shifted. Now the object itself can be held without mechanical contact, its motion controlled optically and its correlations with an electromagnetic field measured.
Cooling an instrument’s detector is not the same thing as cooling the physical degree of freedom whose quantum behaviour you are attempting to preserve.And therein lies something rather profound about experimental physics.
NOTE 2: The Florence experiment isn’t providing us with a photograph of entanglement. Measurements of the outgoing cavity field are used to reconstruct optical-mechanical correlations, and the researchers test those correlations against separability criteria. That is worth remembering whenever quantum experimental results reach the popular press.
NOTE 3: How big is macroscopic? This experiment will inevitably be presented as quantum mechanics entering the macroscopic world. A 100 nm sphere is enormous compared with an atom, yet still exceedingly small by everyday standards. More importantly, the experiment concerns a collective mechanical degree of freedom rather than demonstrating that every constituent atom has somehow been independently placed into a spectacular macroscopic superposition.
The engineering question therefore becomes increasingly inseparable from the foundational one:
How effectively can we isolate a progressively larger physical system from everything else?
Photo 1: Schematic comparison of the two systems.
Photo 2: The Denis Wilkinson Building on Keble Road, where the Oxford Scanning Proton Microprobe was housed.
