In 1981, Richard Feynman suggested that a quantum computer might be able to simulate the evolution of quantum systems much better than classical computers. Except for several proof-of-principle experiments, no working quantum computer has yet been built.

While researchers have succeeded in creating qubits that survive long enough to take part in computations, entangling qubits so that they can form quantum registers large enough for any practical purposes has eluded experimenters up to now. Registers temporarily hold data within a processor during a computation.

Results published last week in* Physical Review X* by teams of researchers from Germany and Austria have rekindled optimism in the pursuit of a working quantum computer. They report a quantum register of 20 qubits that, when entangled, can store more than a million quantum states.

To create individual qubits, the researchers trapped 20 calcium ions in an electrostatic trap. One electric field forces these ions into a single line. Another, lateral electric field pushes them together, so that they occupy positions 5 micrometers apart.

These calcium ions have an outer valence electron whose spin can occupy two states, up or down. These are the two quantum states, 0 or 1, of the qubit. But a qubit can also occupy a state where both values are in superposition.

When two qubits in superposition are also entangled, they together can store all the possible combinations of the quantum states of the qubits, resulting in four values. Adding another qubit to the entangled pair will double the number of combinations and thus the values that can be stored, and so on. After 20 such doublings, 20 entangled qubits can store 2^{20}, or 1,048,576 values.

Although this sounds impressive in terms of classical computing, that number is too small to execute a quantum computation. Unlike a classical computer, which processes computations following a large number of sequential steps dictated by the program, the qubit register receives the entire instruction for a computation in one go, and spits out the result almost instantaneously, in a single process. Therefore, the quantum register has to contain sufficient qubits, at least several thousand, to absorb the instruction for the computation.

To entangle the individual calcium ions, the researchers used laser beams to bring the ions into alternating quantum states (1,0,1,0,1…). They did this with a very narrow laser beam, one wavelength wide, and with precision optics to hit alternating ions in the lineup.

“We used an acoustic device that deflected the laser beam over very small distances,” says Martin Plenio, a physicist at the University of Ulm, in Germany. Then a new laser beam, two wavelengths wide, was directed at the manipulated lineup. This induced qubit-qubit interactions that led to entanglement.

Initially, close neighbors in the string will become entangled, but quickly the entangled pairs turn into threesomes, then four, then five, and so on, until the entire chain is entangled, explains Plenio. “It spreads like a wave through the system,” he says.

As soon as the entangling of the entire chain was deemed complete, the state of individual ions could be identified with the laser, whereby the scattered light allowed the research team to determine the quantum state of the ion. “You can actually see the individual ions in an optical microscope,” remarks Plenio.

However, quantum mechanics allows you to look just once. “Every time you make the measurement, you destroy the entangled state. So measurements were made shortly after entanglement, and this was repeated thousands of times. This allowed us to build up statistics, and with this, we learned more about the quantum states,” explains Plenio.

The next step is to repeat the experiment with a lineup of 50 ions, says Plenio: “We have now lineups with 50 ions, but we have to learn how to control them.” And interpreting the results will not be easy, either. “Systems of quantum states with many entangled qubits become very complicated,” he says.

This will require a lot of fine-tuning and new ways for investigating large numbers of entangled qubits. “As we are getting bigger, we are realizing that we have to check what we are doing,” says Ben Lanyon, now with the Institute for Quantum Optics and Quantum Information, in Austria.

Roger Melko of the University of Waterloo, in Canada, is optimistic, however. “This demonstration of such exquisite control over the quantum properties of individual qubits bodes well for the continuing march towards the holy grail—a true quantum computer."

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