The great promise of quantum computing is the ability to perform calculations of mind-boggling complexity that are far beyond the capabilities of conventional computers. Physicists have long known that a quantum computer with just 50 qubits could outperform even the world’s most powerful supercomputers.
But surpassing the limits of conventional computing—achieving quantum supremacy, as physicists call it—has turned out to be more difficult than everyone expected. Quantum states are delicate objects—sneeze and they vanish. Because of this, physicists have become bogged down with the practical challenge of isolating quantum computers and their quantum processing machinery from the outside world. As a result, quantum supremacy seems as far off as ever.
But there may be another way to demonstrate quantum supremacy that does not require a general-purpose quantum computer that can run various quantum algorithms. Instead, physicists have begun playing with quantum systems that can do just one thing. If they can show that this one thing is beyond the capability of any conventional computer, then they will have demonstrated quantum supremacy for the first time. But just how this can be done is far from clear.
Today, Charles Neill at the University of California Santa Barbara and Pedram Roushan at Google say they know how quantum supremacy can be achieved, and have successfully demonstrated a proof-of-principle version of the machine for the first time. The work raises the prospect that the first demonstration of quantum supremacy could be just months way.
First some background. The big advantage of qubits over ordinary bits is that they can exist in a superposition of states. So while an ordinary bit can be either a 1 or a 0, a qubit can be both a 1 and 0 at the same time.
That means two qubits can represent four numbers at the same time, three qubits can represent eight numbers, and nine qubits, 512 numbers simultaneously. In other words, their capability increases exponentially.
That’s why it doesn’t take many qubits to outperform conventional computers. Just 50 qubits can represent 10,000,000,000,000,000 numbers. A classical computer would require a petabyte-scale memory to store that number.
So one way to achieve quantum supremacy is to create a system that can support 49 qubits in a superposition of states. This system doesn’t need to perform any complex calculations—it just needs to be able to explore the entire space of a 49-qubit superposition reliably. So Neill and Roushan’s goal is to create a 49-qubit superposition.
That’s easier said than done. But the work they announce today is a proof-of-principle demonstration.
Their approach is straightforward. Qubits are quantum objects that can exist in two states at the same time, and there are lots of ways of making them. For example, photons can be polarized both vertically and horizontally at the same time, atomic nuclei can spin with their axis both up and down at the same time, electrons can travel along two paths at the same time. Physicists are experimenting with all these systems for quantum computing.
Neill and Roushan have chosen different route, however. Their quantum system is a superconducting qubit. This is essentially a loop of metal cooled to low temperature. Set a current flowing through this loop and it will flow forever—a quantum phenomenon known as superconductivity.
But this quantum nature leads to a neat trick: the current can flow in one direction and in the other at the same time. And this is what allows it to act as a qubit that can simultaneously represent both a 0 and 1.
The big advantage of superconducting qubits is that they are relatively easy to control and measure. They can also be linked to each other when several loops sit next to each other on a chip. This linking of neighbors is harder and requires another trick.
The flow of current in one direction or the other is just a low-energy configuration. Put in more energy and other states are possible, too. It is these higher energy states that can interact with each other, creating bigger superpositions. In this way, neighboring loops can share the same, much more complex state.
The proof-of-principle experiment that Neill and Roushan and co have pulled off is to make a chip with nine neighboring loops and show that the superconducting qubits they support can represent 512 numbers simultaneously.
That is nowhere near the number of qubits required for quantum supremacy, but the experiment offers encouraging hints that this will be possible.
The big fear among physicists is that it is not only the numbers but also errors that increase exponentially in these quantum systems. If the errors increase too quickly, they will swamp the system, making quantum supremacy impossible.
The key result from this experiment is to show that the errors do not scale rapidly in these superconducting chips. Instead, the team showed that the errors increase slowly in a way that should allow the meaningful superposition of up to 60 qubits. “These results provide promising evidence that quantum supremacy may be achievable using existing technology,” say the researchers.
That’s interesting work. It clearly suggests that quantum supremacy should be possible with a chip that holds 50 superconducting loops instead of just nine. Making such a chip should be straightforward—indeed, it’s hard to imagine that the team isn’t forging one right now.
But there is an important caveat. A 50-qubit chip will only be possible if the errors continue to scale in the way the team has shown. And that raises an important question. The team has shown how errors scale as the number of qubits increase from five to nine. But will the errors scale in the same way as the qubits increase from nine to 50?
If not, quantum supremacy is still a long way off. But if they do, this team will be hoping to claim quantum supremacy in the coming months.
So that’s a question that Neill, Roushan and co will be working hard to answer right now. We’ll look forward to reporting their results.
Ref: arxiv.org/abs/1709.06678: A Blueprint for Demonstrating Quantum Supremacy with Superconducting Qubits
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