Opinion |Dance of the synchronized quantum particles7 min read . Updated: 17 Jul 2020, 07:00 AM IST
Synchronizing spin-1 systems can provide insights in quantum networks
Synchronizing spin-1 systems can provide insights in quantum networks
Years ago, I spent a happy long weekend in New York with a gang of good friends. We took in a boat ride around Manhattan, a band in a tiny smoke-filled bar in Greenwich Village and plenty of New York’s famous pizza. And we spent time discussing, of all things, the McClintock Effect.
Three of our gang, you see, were women. On our second morning, all three found their periods had kicked in. They were so charmed and amused by this that they forgot any possible cramps or migraines. This was, they told us ignorant men, “menstrual synchrony" — the tendency for women who live together to begin menstruating on the same day every month. In 1971, a psychologist called Martha McClintock studied 180 women in a college dormitory. Menstrual synchrony, she concluded then, was real.
Now, this really didn’t apply that weekend in NYC, because these ladies had only spent one day together. Besides, more recent research has questioned McClintock’s findings. Even so, those long-ago NYC days came back to me after reading about some even more recent research, at IIT Kanpur. Not about menstruation, but about synchronization, and in the quantum world.
What’s synchronization? Imagine an individual — a bird, a pendulum — doing a particular motion over and over again. The bird is flapping its wings as it flies, the pendulum is swinging back and forth. Imagine several such individuals near each other, all doing the same motion — several birds flying together in a flock, several pendulums swinging while hanging from a beam. When they start out, the birds are flapping to their own individual rhythms, the pendulums going in different directions. But then something beautiful happens: these individual motions synchronize. The birds flap in perfect coordination, so the flock moves as one marvellous whole. The pendulums swing in harmony.
In fact, synchronization was first observed in pendulums. In 1665, the great Dutch scientist Christiaan Huygens attached two pendulum clocks to a heavy beam. Soon after, the two pendulums were in lockstep.
Similarly, fireflies are known to break into spontaneous synchrony. When there are just one or a few, they light up at different times—a pleasant enough sight, but nothing to write home about. But there are spots in the coastal mangroves of Malaysia and Indonesia where whole hosts of the little insects congregate every evening and suddenly, synchrony happens. They switch on and off in perfect unison, putting on a light show like none you’ve seen.
There are, yes, other examples. At a concert, the audience will tend to applaud in sync. The reason we only ever see one side of the Moon is that the orbital and rotational periods of the Moon have, over time, synchronized with the rotation of our Earth. Your heart beats because the thousands of “pacemaker" cells it contains pulse in synchrony. Some years ago, a bridge of a new and radical design was built over the Thames in London. When it was opened, people swarmed onto it on foot. It quickly started swaying disconcertingly from side to side — enough, in turn, to force the pedestrians to walk in a certain awkward way just to keep their footing. On video, you’ll see hundreds of people on the bridge, all walking awkwardly but in step.
In his book Sync: The Emerging Science of Spontaneous Order, the mathematician Steven Strogatz writes: “At the heart of the universe is a steady, insistent beat: the sound of cycles in sync. It pervades nature at every scale from the nucleus to the cosmos." He goes on to observe that this tendency for synchronization “does not depend on intelligence, or life, or natural selection. It springs from the deepest source of all: the laws of physics". And that’s where IIT Kanpur comes in.
In 2018, a team of Swiss researchers looked at the possibility of synchronization at the lower end of that scale that Strogatz mentions, or in some ways even off that end of the scale. Do the most elementary, fundamental particles known to physicists exhibit the same tendency to synchronize as somewhat larger objects such as starlings and pendulums and the moon? We’re talking about electrons and neutrons, particles that occupy the so-called “quantum" world. Can we get them to synchronize?
They concluded that the smallest quantum particles actually cannot be synchronized. These exhibit a “spin"—a form of angular momentum, in a sense the degree to which the particle is rotating — of 1/2 (half). But there are ways in which such “spin-half" particles can combine to form a “spin-1" system, and the Swiss team predicted that these combinations are the smallest quantum systems that can be synchronized.
So, a physics research group at IIT Kanpur decided to test this prediction. These are guys, I should tell you, who are thoroughly accustomed to working with atoms: One day in 2016, their professor, Dr Saikat Ghosh, took me into their darkened lab and pointed to a small red glow visible in the middle of their apparatus. “That’s a group of atoms," he said with a grin, and then tweaked some settings and the glow dropped out of sight. The point? They are able to manipulate atoms. On another visit, they underlined this particular skill by showing me their work with graphene, a sheet of carbon that is — get this — one atom thick.
So, after the Swiss prediction, Ghosh and his students took a million atoms of rubidium—a soft, silvery metal — and cooled them nearly to what’s known as “absolute zero", or -273° Celsius. Could they get these atoms to show synchrony?
Let’s be clear about what they were dealing with, though. The usual objects that synchronize — pendulums, birds — are called “oscillators" because they are in some regular, rhythmic motion. Strictly, it is that motion of the oscillators that synchronizes. But we’re dealing here with objects we can see, which means the rules of “classical" physics apply. Quantum objects like atoms behave differently. In fact, Ghosh told me that spin-1 atoms are not really oscillating in the same sense as pendulums and starlings in flight. Still, with that caveat in place, there are ways in which we can abstract their motion and treat them as oscillators.
In their experiment, the IIT team shot pulses of light at the group of rubidium atoms. Light is made up of photons, which are like minuscule bundles of energy. When they hit an atom, they “flip" its spin. Embodied in that flip is the photons’ quantum information; in a real way, the photons are actually stored in these flipped atoms. This happens with such precision that you can later flip the atoms back and release the photons, thus “retrieving" the stored light. In fact, with this storage and retrieval behaviour, the atoms are like memory cells, and this is part of the mechanism of quantum computing. (See my column from October 2018, Catch a quantum computer and pin it down).
But when the atoms are flipped and they store these photons, something else happens to them. When the light is retrieved, the IIT team found it displays “interference fringes" — a characteristic pattern of light and shadow (similar in concept to what causes stripes on tigers and zebras, or patterns in the sand on a beach). From this fringe pattern, the scientists can reconstruct the quantum state the atoms were in—and voilà, there’s synchrony.
Did each individual atom synchronize to the light — and since all one million atoms did so, is that how they are synchronized with each other as well? That’s to be tested still, but it’s a good way to think of what happened. Again, take fireflies. In one experiment, a single flashing LED bulb was placed in a forest. When the fireflies appeared, they quickly synchronized to the flashing bulb, and therefore to each other. As Dr Ghosh commented: “two fireflies synchronizing is interesting, but an entire forest filled with fireflies lighting up in sync reveals new emergent patterns."
There are implications in all this for, among other things, quantum computing. The IIT team’s paper remarks; “[The] synchronization of spin-1 systems … can provide insights in open quantum systems and find applications in synchronized quantum networks." (Observation of quantum phase synchronization in spin-1 atoms, by Arif Warsi Laskar, Pratik Adhikary, Suprodip Mondal, Parag Katiyar, Sai Vinjanampathy and Saikat Ghosh, published 3 June 2020).
There will be other applications too. But over 350 years after Christiaan Huygens stumbled on “classical" synchronization, the IIT team has shown for the first time that this strangely satisfying behaviour happens in the quantum world too. No wonder their paper was chosen recently for special mention in the premier physics journal, Physical Review Letters.
A round of applause for the IIT folks, please. I know it will happen in synchrony.
Once a computer scientist, Dilip D’Souza now lives in Mumbai and writes for his dinners. His Twitter handle is @DeathEndsFun