Quantum Physicists Turn To The “Dark State”

Physicists have tamed the so-called “dark state” in a superconducting qubit – an important advance for the realization of quantum computers.

A superconducting qubit is an artificial atom used as an electrical circuit that’s fabricated on a silicon chip. Scientists succeeded in taming a “dark state” where the qubit no longer absorbs or emits electromagnetic waves of certain frequency, as if it were operating under an invisible cloak – hence dark state. The findings were published in the journal Nature Communications.


Conceptual illustration of the internals of a qubit. Image credit: M. Johansen

Here’s where is gets tricky. In the experiment, after inducing a dark state in a three-level qubit, scientists used a sequence of carefully crafted microwave pulses to realize a transfer of a single photons worth of energy, from the ground energy level to the second energy level, without populating the first energy level. This strange sequence was verified using a method called quantum tomography. The behavior appears to be consistent with dark state theory in qubits.

“The matching between the experimental data and the theoretical model is quite remarkable, and this gives us confidence that we understand what is happening and we can control this quantum system. This demonstrates that three-level systems (also called qutrits) can be used in quantum processors instead of the standard two-level qubits,” said team member Antti Vepsäläinen at Aalto University.

Team member Sorin Paraoanu explains the mind-boggling sequence scientists observed: “Suppose you want to travel from Helsinki to New York and you have to change your flight in London. Normally you would first fly on a plane from Helsinki to London, then wait for some time in the airport in London, then board the flight London-New York. But in the quantum world, you would be better off boarding a plane from Helsinki to London sometime after the flight London-New York took off. You will not spend any time in London and you will arrive in New York right at the time when the plane from Helsinki lands in London.”

The results not only have implications for quantum computing, but they also support the strangeness of quantum physics. You would think real objects don’t just appear somewhere from nothing, but the experiment seems to defy this. Things seemingly appear out of nowhere.

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