When I think of your economic and elegant experiment, I see two tiny pale clouds, glowing against grainy blackness – colder than anywhere else in our universe.
When you say ‘go’, they free fall, side by side, 1mm, only a small distance for us, but vast for the atoms. And when you shine light across these falling clouds we see the stripes, the beautiful and brilliant dashes of an interference pattern. Here, we are seeing the very heart of quantum mechanics - the interference of the wave functions.
Beneath the twelve floors of the physics department, in the vivid yellow corridored basement, you share your space with the air conditioning and electrical ducts. Like any optics lab, there is a table that looks like a bomb’s gone off – beam splitters, lenses, lasers everywhere. It may look like a bomb has exploded, but each item is carefully arranged and this has taken years. You and Ben love to hate the set up, for sure you would both set it up differently now...there has been so much learning.
On an adjoining table, shrouded in black cloth is the small metal chamber where the cold rubidium clouds are made and dropped.
Central to this is the little atom chip for holding and controlling the tiny clouds: An inch square, its gold surface glinting as you turn it around in your hand. We look closely and examine the fine lines etched on its surface. You run currents through the wires which create magnetic fields that hold the cold atoms in a potential well. Turn another switch and gently the well divides the little clutch of atoms into two and holds them at slightly different potentials.
You show me the very first atom chip. Considering this beautiful artefact is like going back in time from a silicon chip to a valve – I like its physicality – the clarity of the mechanism spells out its workings. A taught gold wire stretched across a gorgeous one inch gold disk.
Rubidium has a single outer electron, so behaves similarly to our simplest atom hydrogen, that’s why you use it. And you cool it to a remarkable 200 nano Kelvins (0.0000002K). There is nowhere in our universe this cold except for places like this, made by us. And you do this cooling using lasers and then evaporation. This brings all the atoms into the same state, so they are correlated to produce a beautiful, single resulting pattern.
Just like dropping two stones into a pond, when you switch off the golden atom chip to release the two cold rubidium clouds, the waves they comprise interact and make a new pattern....the wondrous interference stripes we see. * And, we are lead again to ask the question...what it is about the universe that means we can observe these patterns across such vastly different things?
You are revealing our mathematical understanding of nature in this elegant experiment.
These little lines can even give us a measure of our weakest force - gravity, as the cloud infinitesimally closest to earth is affected infinitesimally more by gravity and this in turn remarkably affects the interference pattern. Though much greater accuracy can be achieved you say, smiling, with the classic ball and string pendulum of our schooldays. The gravity measurement however helps you gauge accuracy.
Possibly one day, this experiment will help us discover other extremely small dimensions.
There is much more careful work to do.
*[Explanation: OK reader - this is going to be tricky to explain, it is quantum mechanics, but stay with me.....and consider a single atom. What we can say about its likely location in space is governed by the square of a wavy probability distribution we call the wave function. This is different to the large scale world you and I know where with fair accuracy we can say where things are for sure....where your cup is on the table, your car in the street. In the world of the very small we just have probabilities for the location of say an atom. Remember in our experiment – we aren’t considering just a single atom – it is a cloud of them all indistinguishable, and when we shine a light on them to take a look they take up positions somewhere within the form of the square of the wavy probability function, some occupy one part of the probability distribution and others elsewhere. When the two little clouds are overlapped, these waves of atoms produce the interference pattern.]
Image: interference fringes
Video: Cooling of rubidium atoms to 0.2 micro kelvins and below. The process begins with a tiny amount of atoms - ~20 million and this number is reduced to around 20,000 after cooling. The last flash shows the atoms being loaded into the atom chip.
Joe Cotter and Ben Yuen who are experimentalists working on the interference of Bose Einstein Condensates. They work within the laboratory of Ed Hinds.