Quantum particles have the unique property of being able to exist in multiple states at the same time. Measuring a quantum system however typically steers it towards a classical state. Delft researchers have found a way to circumvent this phenomenon.

The researchers of the Kavli Institute of Nanoscience performed a so called parity measurement, which projects the state of multiple qubits (quantum bits) to a state with an even or odd number of excited qubits. The method is described this week in Nature. The findings are important for the development of quantum computers, which could solve complex problems much faster than supercomputers.

The fact that quantum particles lose their quantum-mechanical properties as soon as they are measured is known to us mainly through ‘Schrödinger’s Cat’. The famous founder of quantum mechanics performed the following thought experiment: put a cat in a box with a flask of poison and a switch based on a quantum particle. This imaginary switch thus has the property of being ‘on’ and ‘off’ simultaneously; therefore the cat in the box is both dead and alive at the same time. This state persists until the box is opened, when nature is forced to make a decision as the cat will be either dead or alive; the outcome is random.

The article titled ‘Deterministic entanglement of superconducting qubits by parity measurement and feedback’ presents a new way to look inside Schrödinger’s box and still maintain a quantum superposition, or so they explain in this video (in Dutch).

In their research, the Delft team used two quantum bits (qubits), the building blocks of the quantum computer. They consisted of two tiny superconducting electric circuits placed in an extremely cold little metal box. In Schrödinger’s analogy, each qubit represents a cat, which means that they put not one but two cats in the sealed box.

Leonardo DiCarlo, the leader of the research team, explains in a TU Delft press release what happens next. DiCarlo: “We’ll call the cats Erwin and Niels for convenience. According to quantum theory, four possible states exist at the same time: Erwin and Niels are dead, Erwin and Niels are alive, Erwin is dead but Niels is alive, and vice versa. Normally, as soon as you look inside the box, the probability of each outcome is 25%. Now, however, we can look inside the box and determine whether the cats share the same fate: both dead or alive, or one dead and the other alive. In all cases, Erwin and Niels are still simultaneously dead and alive after the observation, and quantum superposition is therefore maintained. However, now the possible number of outcomes is no longer four, but two.”

The outcome of the measurement is still random, entirely in accordance with laws of quantum mechanics, but the scientists have gone a step further. DiCarlo: “To stay with the example of the cats, if the measurement shows that one of the cats is dead and the other one is alive, we can alter the state of one cat so that they are both dead or both alive. The quantum state is still maintained: they are still dead and alive at the same time, but we can influence the outcome so that their fate is the same.”

For this purpose the researchers developed a method that uses feedback control. Two years ago, this was almost unthinkable for qubits. Until recently, these circuits retained their quantum behaviour for barely a millionth of a second. “Advances in superconducting qubits have increased this time by a factor of ten to a hundred, so that we were finally able to close the feedback loop quickly enough”, explains the first author of the Nature article, Diego Ristè.

This method is important for the development of the quantum computer, pursued by a large team of Delft researchers. Theoretical physicist Yaroslav Blanter: “The main problem with quantum bits is that they lose their quantum state after a time. There is a method that can maintain this state, using quantum error correction. Our experiment demonstrates the two steps that are crucial in carrying out quantum error correction and preserving quantum states longer. The next step for the team will be to develop within five years a self-correcting quantum memory using twenty qubits.”

*D. Ristè, et., al. ‘Deterministic entanglement of superconducting qubits by parity measurement and feedback’, Nature, 17 October 2013*

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