Scientists view a quantum jump in real time

Apr 11, 2011 by Miranda Marquit feature

(PhysOrg.com) -- For more than two decades, scientists have been "watching" electrons in atoms make the jump between energy levels in real time. "Atoms have energy levels, and when electrons 'jump' from one level to another, you can detect this optically. You can encode information in real atoms to make a quantum bit, or qubit," Irfan Siddiqi tells PhysOrg.com.

While the formation of a qubit from real atoms has the advantage of long coherence times, Siddiqi points out that by the same token single atoms are difficult to couple to each other and have fixed parameters. Qubits made from ‘artificial atoms’ have the advantage of tunability and can, in principle, be produced en masse. The use of superconducting electrical circuits, engineered to have discrete energy levels, is a way to develop artificial atoms.

“These artificial atoms make use of circuitry so that we can control the parameters. We are able to realize analogues of experiments performed with real atoms, and are even able to access different parameter regimes but at the expense of short-lived quantum ,” Siddiqi says. “A problem with these superconducting circuits, though, is that it has not been possible to continuously monitor their state with high fidelity,” he continues. This lack of measurement sensitivity has limited the possibility of real time quantum feedback. Until now.

Siddiqi, a professor at the University of California, Berkeley, along with Dr. Rajamani Vijay and Daniel Slichter, created an experiment that allowed them to watch the quantum state of a superconducting qubit acting as an artificial atom. Their work is described in : "Observation of Quantum Jumps in a Superconducting Artificial Atom."

“In real atoms, the jump between energy levels typically happens on the timescale of seconds and changes another optical process which can be readily detected,” Siddiqi points out. “Artificial typically undergo jumps in less than a microsecond. Furthermore, they are typically probed with weak microwave frequency signals and are much harder to detect.”

In order to get around this problem, the team constructed a superconducting amplifier of their own design, made with thin aluminum films and Josephson junctions. The qubit was also constructed from the same basic building blocks and cooled down to 30 mK inside a cryogenic dilution refrigerator.

“When the jump is made, it modifies the resonant frequency of an adjacent circuit which we continuously probe with on average a few microwave photons,” Siddiqi explains. “The emission is spontaneous, so you don’t know when it will happen. We now had the ability to detect an individual jump. By tallying the jump times, we reproduced the average ensemble behavior that we were all familiar with..”

Hopefully, this will lead us one step closer to realizing a quantum computer. “In order to correct errors which will occur in any realistic quantum computer, we need to detect them quickly and efficiently,” Siddiqi points out. “Now that we have shown that it is possible to track changes in the quantum state in real time, it should be possible to apply this functionality to quantum information processing.”

Next, Siddiqi and his group want to work on error correction. “It should be possible now that we have this ability. Implementing this would be a major step for solid-state quantum computing.”

Explore further: Deceptive-looking vortex line in superfluid led to twice-mistaken identity

More information: R. Vijay, D.H. Slichter, and I. Siddiqi, "Observation of Quantum Jumps in a Superconducting Artificial Atom," Physical Review Letters (2011). Available online: link.aps.org/doi/10.1103/PhysRevLett.106.110502

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ennui27
5 / 5 (1) Apr 18, 2011
In real atoms, the jump between energy levels typically happens on the timescale of seconds and changes another optical process which can be readily detected,

Seconds? This needs explaination (to me at least). Given the distances involved - this is slower than walking.
Avitar
not rated yet Apr 21, 2011
The statement is wrong for most atoms are much faster than that but there a classes of material that the electrons lock into their excited stater for long periods. For example the material that TLD radiation badges are made from the electrons are kicked into a higher state and stay there.
When the badge is heated the electrons drop back in state and emit a photon as they do so. Detecting the susntity of photons producd allows the radiation that the badge was exposed to be determined many hours after exposer. Over extended time the badge material would return to base state after in some cases many years.
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not rated yet Apr 21, 2011

For an example the long delay material in the material that the familar TLD (Thermal Luminescent Dosimeter) radiation badges are made from the electrons are kicked into a higher state by the radiation that they are exposed to and stay there. When the badge is heated the electrons drop back in state and emit a photon as they do so. Detecting the density of photons produced allows the radiation that the badge was exposed to be determined many hours after exposer. Over extended time the badge material would return to base state after, in some cases, many years.