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First Observation Of A Macroscopic Quantum Jump

Physicists have watched an artificial atom jump from one state to another using a monitoring technique that could have important implications for quantum computing.

One of the defining characteristics of quantum objects is their ability to change from an excited state to a ground state without passing through any intermediate states.

The consequences of quantum jumps fill our world: chemistry, for example, is essentially the science of quantum jumps.

But while it’s easy to see the consequences of quantum jumps, it’s much harder to catch them in the act.

In recent years, physicists have worked hard to actually watch while various quantum objects make a jump. They’ve done it for photons, electrons, trapped ions and atoms, even some molecules. It’s not easy but it can be done

But they’ve never watched as a macroscopic object jumped from one energy level to another. That’s not for lack of macroscopic quantum phenomenon; there are plenty to choose from, such as lasing and superconductivity.

All that changes today with an announcement by Rajamani Vijayaraghavan and buddies at the University of California, Berkeley, that they’ve watched a macroscopic quantum object jump for the first time.

The object in question is a superconducting qubit, what physicists sometimes call an artificial atom. The atom is a superconducting circuit in which flow of charge in a particular direction can represent a 0 while the flow in the opposite direction represents 1, for example.

Physicists can watch a superconducting qubit by bathing it in microwave photons inside a cavity. the interaction between photon and qubit changes the properties of the photon, such as their phase, which can be measured as they come out of the cavity.

But to watch a qubit jump, the photons have to hang around for a fairly long time, about a microsecond or so. But photons being ephemeral things, they tend to wander off long before this.

The trick that Vijayaraghavan and buddies have perfected is to design a cavity that keeps the photons busy long enough to experience the jump. When that happens, it is straightforward to see. They say it is “the first observation of quantum jumps in a macroscopic quantum system.”

By macroscopic, they mean about 10 micrometres across, the size of their superconducting circuit. That’s about the size of a red blood cell.

That’s a novel result but it’s also a potentially useful one. The ability to monitor qubits jumping from one state to another is an enabling technology that could transform quantum computing. For example, error correcting codes, without which computers just don’t work, rely on this kind of control.

What’s more, Vijayaraghavan and pals say their ideas can easily be applied to other kinds of quantum systems. “Our technology can be readily integrated into hybrid circuits involving molecular magnets, nitrogen vacancies in diamond, or semiconductor quantum dots,” they say.

If that turns out to be true, this could be one of those engineering breakthroughs that can turn impractical demonstration devices into practical powerhouses capable of operating in the real world. Let’s wait and see.

Ref: arxiv.org/abs/1009.2969: Observation Of Quantum Jumps In A Superconducting Artificial Atom

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