To catch a quantum jump

Albert Einstein once said, “God doesn’t play dice with the universe” (he called himself as a non-believer, so this was rhetorical).  Einstein was expressing his doubts about quantum physics, which suggests sub-atomic particles behave randomly and probabilistically.


Ironically, Einstein was a founder father of quantum theory. His 1905 paper on the photoelectric effect first proposed light consisted of packets of specific quanta of energy. Another physics legend, Erwin Schrödinger, who derived the equation for wave functions, also had doubts about quantum theory. While it yielded excellent experimental results, both these geniuses felt (or hoped) that the probabilistic behaviour of particles masked a deeper, deterministic principle.


Schrödinger satirised quantum superposition in a famous thought experiment. A cat is locked in a box with a Geiger counter connected to a lever that controls the flow of a deadly gas. A single atom of a radioactive element with a half-life of one hour, is in the box. The box is opened an hour later.


There’s a 50 per cent chance the atom has decayed, tripping the lever, and releasing the gas, which kills the cat. Or else, the cat is unharmed. The decay/non-decay are superposed wave-functions. When an observer opens the box, one wave-function collapses. Until then, the cat is both dead and alive!


This collapse of the wave function is an example of a quantum jump — a change from one energy state to another. It was always assumed that jumps were probabilistic and instantaneous and there was no way to determine how this experiment would turn out.


A new experiment from Yale indicates this understanding is due for substantial review. It seems quantum jumps are not instantaneous and there is a sign when one is about to occur. It may even be possible to stop or reverse jumps. The discovery has huge fundamental significance and it could mean a breakthrough in computing.


As described in “To Catch and Reverse a Quantum Jump in Mid-flight”, Yale Professor Michel Devoret, lead author Zlatko Minev and their team, examined quantum jumps using an “artificial atom”.


Minev is associated with IBM’s Thomas Research Center. The co-authors include Robert Schoelkopf, Shantanu Mundhada, Shyam Shankar and Philip Reinhold, of Yale, Ricardo Gutiérrez-Jáuregui of the University of Auckland; and Mazyar Mirrahimi, from the French Institute for Research in Computer Science and Automation.


The “artificial atom” is a superconducting circuit with an insulating junction (a Josephson junction, which is a very thin non-conducting material) placed in the middle. The superconducting electrons “tunnel” through the non-conductor.


In actual atoms, energy states are represented by the location of the electron around the nucleus. In this artificial model, the energy states are represented by changing values as electrons tunnel through the junction.


The “atom” was placed inside an aluminium box, and bombarded with microwaves. The microwaves cause photon emissions and quantum jumps. Given sensitive enough detectors, every photon emission is observed. The researchers discovered that each jump was preceded by an interesting “non-signal”.  Photons stopped being emitted just before there was a quantum jump.


This absence of photons is advance warning. What’s more, it was discovered that it was possible to reverse a jump state by hitting the “atom” at the right instant with the right microwave signal to trigger photon emission. While jumps start randomly and can be prematurely interrupted, the deterministic signal comes as a great surprise. Also jumps are not instantaneous, resolving one of the puzzles of quantum theory.


In a scientific analogy, this discovery was compared to our understanding of volcanic eruptions: we have no idea when a volcanic eruption is due but geologists do know the warnings signs that occur just before an eruption.


Coming back to quantum computing, this may have huge applications. Computing by quantum bits (qubits) may be much quicker than conventional computing since qubits store more information and calculate faster.


A conventional bit stores only two states (on/off, or one and zero) and can be in only one state at any given time. A qubit can be both states at the same time due to superposition. A three qubit system can have a superposition of eight states at the same time, where a three-bit system can only be in one of these eight states. The differential grows for larger quantum computing systems due to this quirk.   


But when qubit superposition collapses, it causes computing errors. However, this “artificial atom” is also technically a two-qubit quantum computer. So this result could yield vital clues as to how to prevent, or correct, wave-collapse errors. It would also suggest that Einstein was perhaps, correct, if there is indeed a deterministic principle that is deeper than current quantum theory.

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