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Wednesday, 1 September 2021

Why it might be impossible to build a practical quantum computer

In recent years, quantum computing has grown considerably and is a very active field of research. Although the prototypes currently available are not yet really practical, some institutions have already started to demonstrate the computational potential of quantum computers. However, researchers still face a major pitfall: stabilizing qubits in the face of background noise. And for some experts, this problem might be impossible to solve.

It has been 40 years since physicist Richard Feynman identified that quantum methods ought to be ready to perform a completely new type of computation that outperforms even probably the most highly effective standard computer systems. “Feynman argued that quantum computing should offer an exponential speed-up for many classical computations,” says Cristian Calude at the University of Auckland in New Zealand. And with a slew of breakthroughs, quantum computer systems appear like they might now be hitting the large time. Perhaps.

Because they’ve properties that simply don’t exist within the classical world, quantum entities resembling atoms, photons, electrons and the like have entry to a completely different set of routines for info processing if used to make quantum bits, or qubits – a probably way more highly effective set.

The computing power offered by qubits

Part of that’s down to quantum superposition, which implies a qubit can be used to symbolize a complicated mixture of the 0 and 1 binary states utilized in regular computing. That doesn’t imply it is 0 and 1 on the identical time. A greater means to put it is that might prove to be 0 or 1.

Quantum algorithms use a course of known as “interference” to skew these undefined properties and bias the interactions of a number of qubits in a means that will increase the chance they’ll arrive at a ultimate state that incorporates a resolution to the issue they’re making an attempt to resolve.

That’s the place entanglement comes into the combination. The spooky connections between qubits it generates by some means enable for a sample of interference the place the paths main to every incorrect answer destroy each other and cancel out, whereas the paths main to the precise answer are strengthened. In 2019, Google's quantum computing team announced that it had achieved “quantum supremacy ”; that is, when a quantum processor performs tasks that a conventional computer cannot.

Its 54-qubit Sycamore processor took just 3 minutes and 20 seconds to solve a problem that would take 10,000 years to solve on the world's most powerful conventional computer, the researchers said. That's not to say that Google's quantum computer, or anyone else that has achieved quantum supremacy since, is about to do anything useful. The problem Google solved was highly esoteric. In May, Isaac Chuang of the Massachusetts Institute of Technology (MIT), one of the world's leading authorities on quantum computing, described the current state of the technology as a simple ad-generating system.

Stabilizing qubits: an extremely complex challenge

This brings us to the long journey ahead of a practical and useful machine. The inconvenient truth is that in quantum computing, size matters. Qubits containing data must retain their delicate quantum states for a long time and not succumb to environmental influences such as heat and vibration that can cause them to decoher, creating miscalculations.

This is a problem that can only be overcome by scaling. Current estimates suggest that in large programmable quantum computers, most qubits - perhaps as many as 5 out of 6 - will perform error correction, not a calculation. This means that we'll need a million qubits before we can do anything really useful. Keeping so many qubits cold enough or maintaining all of their quantum states long enough to perform a computation is a monumental engineering challenge.

It could take decades, but researchers are at least taking a few steps in the right direction… IBM aims to build an 1,121-qubit machine by 2023, and the company envisioned a colossal helium-cooled refrigerator to contain it. Others, including Winfried Hensinger of the University of Sussex, UK, want to avoid the complications of cooling: they are stepping up operations with qubits of trapped ions that shuttle around a large circuit to perform calculations.

Noise and qubits: an impossible problem to solve?

Still others perform calculations by sending qubits of photons around a silicon nitride chip that can be fabricated on a large scale using processes already proven in the semiconductor industry. However, Gil Kalai, a mathematician at the Hebrew University of Jerusalem in Israel, argued that the basic noise level in a quantum computer will always be too high, no matter how many qubits are available. "My analysis says that correcting quality errors will not be possible."

Sabrina Maniscalco from the University of Helsinki in Finland is also skeptical. “Finding a cure for the effect of environmental noise is not only, in my opinion, a technological problem, but more conceptual and fundamental. I would say that I am optimistic, rather than confident”.

Via: NewScientist

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