Quantum state and collective behaviour

The Nobel Prize for Physics recognises efforts that push boundaries of quantum mechanics to macroscopic scales.

Quantum mechanics, first formulated in the early 1900s, has never failed a test. An early success was understanding the behaviour of a single hydrogen atom in terms of its quantum states. But can a large number of particles display any kind of quantum mechanical behaviour that isn’t just a feature of its units, taken one by one?

To get large numbers of particles to collectively display quantum mechanical behaviour requires that they be cooled to very low temperatures. In the right conditions, these particles can condense into a single quantum state. For collections of electrons in metals, such a state is a superconductor, in which a current can flow endlessly.

Superconductors (also superfluids) are examples of quantum mechanics operating at scales much larger than an atom – there’s no size limit to the number of electrons in that single quantum state. The property of superposition is uniquely quantum mechanical. The quantum mechanical analogue of a ‘bit’, a quantity which takes only the basic values 0 and 1, is the ‘qubit’, any intermediate combination of these two basic states.

Superposition leads to apparent paradoxes when we consider large objects. Consider Schrodinger’s cat. Could one come up with real experiments in which a cat – or the same number of atoms that might go into a cat – could exist in a superposition state of alive and dead?

Another quantum mechanical feature is that of tunnelling. A quantum particle can cross a nominally insurmountable barrier, ‘tunnelling’ through, simply because quantum mechanics allows quantum states to leak into such forbidden regions.

Specifying a quantum mechanical state of a collective of particles involves assigning an angle to that state, most straightforwardly thought of as a point along the circumference of a circle, moving around the circle and returning to its original position after a full rotation.

To understand the Nobel Prize-winning experiments, consider two superconductors separated from each other. Each has its own, independent ‘quantum’ angle. Now bring them together, with a tiny sliver of non-superconducting material between them. This is called a Josephson junction.

In the case of superconductors, electrons pair up to form Cooper pairs. These pairs condense into one quantum state, where they behave as if they were one single unit. While Cooper pairs live only within superconductors, tunnelling implies that superconductivity can also leak into regions that are not
superconducting. As a result, a current can flow effortlessly across the Josephson junction, even without a voltage to drive particles across
the junction.

When a voltage source is introduced into the circuit, the difference in angles on both sides of the junction changes steadily with time, driven by this voltage and described by the point going around the circle. A steady voltage leads to a current that goes up
and down in time at a regular rate, predicted by quantum mechanics.

Testing at scale

The experiments that won the Nobel Prize were designed by UK-born John Clarke, then a young faculty member at the University of California at Berkeley, together with Michel Devoret, a French postdoc, and American PhD student John Martinis. It relies on applying a fixed current to a Josephson junction.

They started with the system in a zero-voltage state, but with current flowing through. This ‘metastable state’ is not what the system prefers to be in, but a barrier must be surmounted first. Quantum tunnelling enables the crossing of this barrier.

Clarke and his collaborators went to great lengths to isolate the purely quantum mechanical parts of the experiment. They measured how long the system stays in the zero-voltage state. They shone microwaves of different frequencies on their apparatus, exploring its quantised energies, providing precise tests of quantum mechanics at a scale much larger than that of atoms.

Josephson junctions, and devices built with them, allow for many tests of quantum mechanics. In particular, they allow us to test the robustness of the quantum state.

Making quantum computers and related devices requires that we understand quantum mechanical behaviour, and its robustness at a scale much larger than that of a single atom or molecule. The experiments of Clarke, Devoret and Martinis show precisely how quantum mechanics continues to work even at these scales. Because of them, we know that quantum mechanical states of multiple atoms large enough to constitute even a cat can exist. Everything we know about quantum mechanics applies here, too.

All this confirms the primacy of quantum mechanics as the defining theory of the physical world.

(The writer is Dean, Research, and Professor of Physics and Biology, Ashoka University)

Disclaimer: The views expressed above are the author's own. They do not necessarily reflect the views of DH.