Research from UC Berkeley's Kater Murch and team has allowed fine observation of a quantum waveform collapse. Observing single quantum trajectories of a superconducting quantum bit, published in Nature, describes the experiment, which used indirect observations of microwaves that had passed through a box containing a circuit where a particle was in a state of superposition, allowing the researchers to view the collapse in slow-motion.
I finally came to have some (admittedly crude) understanding of what all this means in 1992, thanks to Greg Egan's novel Quarantine, which is one of the best — and most exciting and comprehensible — explanations of superposition and uncertainty I've ever encountered.
Atomic and solid-state physicist Kater Murch of the University of California, Berkeley, and his colleagues performed a series of weak measurements on a superconducting circuit that was in a superposition — a combination of two quantum states. They did this by monitoring microwaves that had passed through a box containing the circuit, based on the fact that the circuit's electrical oscillations alter the state of the microwaves as they pass through the box. Over a couple of microseconds, those weak measurements captured snapshots of the state of the circuit as it gradually changed from a superposition to just one of the states within that superposition — as if charting the collapse of a quantum wavefunction in slow motion.
Although equivalent experiments have been done on the quantum states of photons of light, this is the first time such work has been done in a typically noisier solid-state system. "It demonstrates how much progress we've made in the solid state in the past 10 years," says Murch. "Finally, systems are so pure that we can rival experiments in photons."
The team also found that decoherence, the process by which noise in the environment causes quantum states to decay, can be minimized by repeated weak measurements. Murch says that the microwaves used to probe the superconducting circuit can be thought of as its environment because they are the predominant thing interacting with it. By monitoring the environment, the fluctuations in the microwaves become a known quantity rather than a source of unknown noise.
Physicists snatch a peep into quantum paradox [Eugenie Samuel Reich/Nature]
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