In 2012, physicists at the University of California, Berkeley achieved something long considered theoretically possible but practically elusive: they measured a quantum system without destroying its delicate superposition state. The breakthrough offered a potential path toward more stable quantum computers and represented a significant advance in the ability to observe quantum phenomena without collapsing them.

The Measurement Problem in Quantum Mechanics

Quantum objects possess the peculiar ability to exist in multiple states simultaneously, a phenomenon known as superposition. Physicist Erwin Schrodinger illustrated this principle through his famous thought experiment involving a cat whose fate is linked to the quantum state of a radioactive atom. Because the atom exists in superposition until measured, the cat is theoretically both alive and dead until the box is opened. This same principle applies to quantum bits, or qubits, which can hold values of both 1 and 0 simultaneously rather than being locked into one or the other. The problem is that any direct measurement forces the qubit to collapse into a single definite state, destroying the superposition that makes quantum computing powerful.

The Gentle Measurement Approach

Researchers had long theorized that it might be possible to make measurements soft enough to preserve superposition. The concept was analogous to looking at Schrodinger’s cat through blurry glasses, gathering indirect information without forcing the system to resolve into a definitive state. A team led by R. Vijay at UC Berkeley succeeded in building a working version of this approach. Using a tiny superconducting circuit commonly employed as a qubit in quantum computers, they placed the system in superposition by cycling its state between 0 and 1, causing it to pass through all possible mixtures of states. They then measured the frequency of this oscillation rather than attempting to determine whether the bit was a 1 or a 0 at any given moment.

The Feedback Correction Mechanism

While the gentle measurement preserved the superposition, it introduced a new complication: the measurement randomly altered the oscillation rate of the qubit. The team addressed this by making measurements extremely rapidly, allowing them to detect the drift and inject an equal but opposite correction that restored the qubit’s frequency to its undisturbed value. This feedback mechanism operated on the same principle as a cardiac pacemaker, detecting deviations from the desired state and nudging the system back on course. Previous attempts at this technique had been stymied by a fundamental tradeoff. Measurements gentle enough to preserve superposition produced signals too faint to detect, while stronger measurements introduced uncontrollable noise.

A New Amplifier Changes the Game

The key technical innovation was a new type of amplifier that could boost the measurement signal without contaminating it with additional noise. With this tool, the team demonstrated that their qubit maintained its oscillating superposition state throughout the entire experimental run, approximately one hundredth of a second. While brief in absolute terms, this duration was orders of magnitude longer than the microsecond-scale collapse that occurred without feedback, representing a substantial improvement in qubit stability.

Implications for Quantum Computing

Howard Wiseman of Griffith University in Brisbane, Australia, noted in a commentary accompanying the research that while the result was not yet perfect, the observed stabilization represented a significant step forward in individual qubit feedback control. Vijay suggested that the technique could be developed into quantum error correction protocols, automatically detecting and correcting qubits that were drifting toward collapse. Such capabilities would be essential for building practical quantum computers, which require superposition states to persist long enough to complete meaningful calculations. The research was published in the journal Nature.