Qubit

Quantum ‘world record’ smashed

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An artistic rendition of a 'bound exciton' quantum state used to prepare and read out the state of the qubits
Quantum systems are notoriously fickle to measure and manipulate

A fragile quantum memory state has been held stable at room temperature for a “world record” 39 minutes – overcoming a key barrier to ultrafast computers.

“Qubits” of information encoded in a silicon system persisted for almost 100 times longer than ever before.

Quantum systems are notoriously fickle to measure and manipulate, but if harnessed could transform computing.

The new benchmark was set by an international team led by Mike Thewalt of Simon Fraser University, Canada.

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“39 minutes may not seem very long. But these lifetimes are many times longer than previous experiments”

Stephanie Simmons Oxford University

“This opens the possibility of truly long-term storage of quantum information at room temperature,” said Prof Thewalt, whose achievement is detailed in the journal Science.

In conventional computers, “bits” of data are stored as a string of 1s and 0s.

But in a quantum system, “qubits” are stored in a so-called “superposition state” in which they can be both 1s and 0 at the same time – enabling them to perform multiple calculations simultaneously.

The trouble with qubits is their instability – typical devices “forget” their memories in less than a second.

There is no Guinness Book of quantum records. But unofficially, the previous best for a solid state system was 25 seconds at room temperature, or three minutes under cryogenic conditions.

In this new experiment, scientists encoded information into the nuclei of phosphorus atoms held in a sliver of purified silicon.

Magnetic field pulses were used to tilt the spin of the nuclei and create superposition states – the qubits of memory.

The team prepared the sample at -269C, close to absolute zero – the lowest temperature possible.

Artist's impression of a phosphorus atom qubit in silicon, showing a ticking clock

When they raised the system to room temperature (just above 25C) the superposition states survived for 39 minutes.

What’s more, they found they could manipulate the qubits as the temperature of the system rose and fell back towards absolute zero.

At cryogenic temperatures, their quantum memory system remained coherent for three hours.

“Having such robust, as well as long-lived, qubits could prove very helpful for anyone trying to build a quantum computer,” said co-author Stephanie Simmons of Oxford University’s department of materials.

“39 minutes may not seem very long. But these lifetimes are many times longer than previous experiments.

“We’ve managed to identify a system that seems to have basically no noise.”

However she cautions there are still many hurdles to overcome before large-scale quantum computations can be performed.

For one thing, their memory device was built with a highly purified form of silicon – free from the magnetic isotopes which interfere with the spin of nuclei.

For another, the spins of the 10 billion or so phosphorus ions used in this experiment were all placed in the same quantum state.

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“What’s most important is this is silicon. The global investment in this material means it has a lot of potential for engineering”

Dr Thaddeus Ladd HRL Laboratories

Whereas to run calculations, physicists will need to place different qubits in different states – and control how they couple and interact.

“To have them controllably talking to one another – that would address the last big remaining challenge,” said Dr Simmons.

Independent experts in the quantum field said the new record was an “exciting breakthrough” that had long been predicted.

“This result represents an important step towards realising quantum devices,” said David Awschalom, professor in Spintronics and Quantum Information, at the University of Chicago.

“However, a number of intriguing challenges still remain. For instance – will it be possible to precisely control the local electron-nuclear interaction to enable initialisation, storage, and readout of the nuclear spin states?”

The previous “world record” for a solid state quantum system at room temperature – 25 seconds – was held by Dr Thaddeus Ladd, formerly of Stanford University‘s Quantum Information Science unit, now working for HRL Laboratories.

“It’s remarkable that these coherence states could be held for so long in a measurable system – as measurement normally introduces noise,” he told BBC News.

“It’s also a nice surprise that nothing goes wrong warming up and cooling the sample again – from an experimental point of view that’s pretty remarkable.

“What is perhaps most important is that this is silicon. The global investment in this particular material means that it has a lot of potential for engineering.”

Common Blue Pigment Could Help Make A Quantum Computer.

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Sometimes you just have to look around. A new analysis of a common blue pigment—it’s used in the British five-pound note—found it has some unusual properties that make it a candidate semiconductor for quantum computers.

Researchers from the U.K. and Canada found molecules of copper phthalocyanine are able to hold the superimposed state of a quantum bit for as long as, or longer than, other materials being studied for quantum computers. Unlike ordinary bits, which must take on one of two states—for example, 0 or 1—quantum bits must hold two states at once. If a material is able to hold quantum states long enough, engineers could get them to store and pass on information.

Researchers are interested in building computers with quantum bits because such machines could work much faster than computers today. Some quantum computers already exist, but they’re still experimental and often aren’t able to solve practical problems.

Copper phthalocyanine has one other property that makes it a good prospect for a quantum semiconductor, the researchers wrote in a paper they published yesterday in the journal Nature. The researchers were able to produce it as a thin film, which is convenient for putting into electronic devices.

Quantum Computers Check Each Other’s Work.

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Image courtesy of Equinox Graphics

Check it twice. Quantum computers rely on these clusters of entangled qubits—units of data that embody many states at once—to achieve superspeedy processing. New research shows one such computer can verify the solutions of another.

Quantum computers can solve problems far too complex for normal computers, at least in theory. That’s why research teams around the globe have strived to build them for decades. But this extraordinary power raises a troubling question: How will we know whether a quantum computer’s results are true if there is no way to check them? The answer, scientists now reveal, is that a simple quantum computer—whose results humans can verify—can in turn check the results of other dramatically more powerful quantum machines.

Quantum computers rely on odd behavior of quantum mechanics in which atoms and other particles can seemingly exist in two or more places at once, or become “entangled” with partners, meaning they can instantaneously influence each other regardless of distance. Whereas classical computers symbolize data as bits—a series of ones and zeroes that they express by flicking switchlike transistors either on or off—quantum computers use quantum bits (qubits) that can essentially be on and off at the same time, or in any on/off combination, such as 32% on and 68% off.

Because each qubit can embody so many different states, quantum computers could compute certain classes of problems dramatically faster than regular computers by running through every combination of possibilities at once. For instance, a quantum computer with 300 qubits could perform more calculations in an instant than there are atoms in the universe.

Currently, all quantum computers involve only a few qubits “and thus can be easily verified by a classical computer, or on a piece of paper,” says quantum physicist Philip Walther of the University of Vienna. But their capabilities could outstrip conventional computers “in the not-so-far future,” he warns, which raises the verification problem.

Scientists have suggested a few ways out of this conundrum that would involve computers with large numbers of qubits or two entangled quantum computers. But these still lie outside the reach of present technology.

Now, quantum physicist Stefanie Barz at the University of Vienna, along with Walther and their colleagues, has a new strategy for verification. It relies on a technique known as blind quantum computing, an idea which they first demonstrated in a 2012 Science paper. A quantum computer receives qubits and completes a task with them, but it remains blind to what the input and output were, and even what computation it performed.

To test a machine’s accuracy, the researchers peppered a computing task with “traps”—short intermediate calculations to which the user knows the result in advance. “In case the quantum computer does not do its job properly, the trap delivers a result that differs from the expected one,” Walther explains. These traps allow the user to recognize when the quantum computer is inaccurate, the researchers report online today in Nature Physics. The results show experimentally that one quantum computer can verify the results of another, and that theoretically any size of quantum computer can verify any other, Walther says.

The existence of undetectable errors will depend on the particular quantum computer and the computation it carries out. Still, the more traps users build into the tasks, the better they can ensure the quantum computer they test is computing accurately. “The test is designed in such a way that the quantum computer cannot distinguish the trap from its normal tasks,” Walther says.

The researchers used a 4-qubit quantum computer as the verifier, but any size will do, and the more qubits the better, Walther notes. The technique is scalable, so it could be used even on computers with hundreds of qubits, he says, and it can be applied to any of the many existing quantum computing platforms.

“Like almost all current quantum computing experiments, this currently has the status of a fun demonstration proof of concept, rather than anything that’s directly useful yet,” says theoretical computer scientist Scott Aaronson at the Massachusetts Institute of Technology in Cambridge. But that doesn’t detract from the importance of these demonstrations, he adds. “I’m very happy that they’re done, as they’re necessary first steps if we’re ever going to have useful quantum computers.”