A lot of research on the fundamentals of quantum computing has been devoted to error correction. Part of the difficulty stems from another of the key properties of quantum systems: Superpositions can only be sustained as long as you dont measure the qubits value. If you make a measurement, the superposition collapses to a definite value: 1 or 0. So how can you find out if a qubit has an error if you dont know what state it is in?

One ingenious scheme involves looking indirectly, by coupling the qubit to another ancilla qubit that doesnt take part in the calculation but that can be probed without collapsing the state of the main qubit itself. Its complicated to implement, though. Such solutions mean that, to construct a genuine logical qubit on which computation with error correction can be performed, you need many physical qubits.

How many? Quantum theorist Aln Aspuru-Guzik of Harvard University estimates that around 10,000 of todays physical qubits would be needed to make a single logical qubit a totally impractical number. If the qubits get much better, he said, this number could come down to a few thousand or even hundreds. Eisert is less pessimistic, saying that on the order of 800 physical qubits might already be enough, but even so he agrees that the overhead is heavy, and for the moment we need to find ways of coping with error-prone qubits.

An alternative to correcting errors is avoiding them or canceling out their influence: so-called error mitigation. Researchers at IBM, for example, are developing schemes for figuring out mathematically how much error is likely to have been incurred in a computation and then extrapolating the output of a computation to the zero noise limit.

Some researchers think that the problem of error correction will prove intractable and will prevent quantum computers from achieving the grand goals predicted for them. The task of creating quantum error-correcting codes is harder than the task of demonstrating quantum supremacy, said mathematician Gil Kalai of the Hebrew University of Jerusalem in Israel. And he adds that devices without error correction are computationally very primitive, and primitive-based supremacy is not possible. In other words, youll never do better than classical computers while youve still got errors.

Others believe the problem will be cracked eventually. According to Jay Gambetta, a quantum information scientist at IBMs Thomas J. Watson Research Center, Our recent experiments at IBM have demonstrated the basicelementsof quantum error correction onsmalldevices, paving the way towards larger-scaledevices where qubits canreliablystorequantum informationfor a long period of time inthepresence of noise. Even so, he admits that a universal fault-tolerant quantum computer, which has to use logical qubits, is still along way off. Such developments make Childs cautiously optimistic. Im sure well see improved experimental demonstrations of [error correction], but I think it will be quite a while before we see it used for a real computation, he said.

For the time being, quantum computers are going to be error-prone, and the question is how to live with that. At IBM, researchers are talking about approximate quantum computing as the way the field will look in the near term: finding ways of accommodating the noise.

This calls for algorithms that tolerate errors, getting the correct result despite them. Its a bit like working out the outcome of an election regardless of a few wrongly counted ballot papers. A sufficiently large and high-fidelity quantum computation should have some advantage [over a classical computation] even if it is not fully fault-tolerant, said Gambetta.

One of the most immediate error-tolerant applications seems likely to be of more value to scientists than to the world at large: to simulate stuff at the atomic level. (This, in fact, was the motivation that led Feynman to propose quantum computing in the first place.) The equations of quantum mechanics prescribe a way to calculate the properties such as stability and chemical reactivity of a molecule such as a drug. But they cant be solved classically without making lots of simplifications.

In contrast, the quantum behavior of electrons and atoms, said Childs, is relatively close to the native behavior of a quantum computer. So one could then construct an exact computer model of such a molecule. Many in the community, including me, believe that quantum chemistry and materials science will be one of the first usefulapplications of such devices, said Aspuru-Guzik, who has been at the forefront of efforts to push quantum computing in this direction.

Quantum simulations are proving their worth even on the very small quantum computers available so far. A team of researchers including Aspuru-Guzik has developed an algorithm that they call the variational quantum eigensolver (VQE), which can efficiently find the lowest-energy states of molecules even with noisy qubits. So far it can only handle very small molecules with few electrons, which classical computers can already simulate accurately. But the capabilities are getting better, as Gambetta and coworkers showed last September when they used a 6-qubit device at IBM to calculate the electronic structures of molecules, including lithium hydride and beryllium hydride. The work was a significant leap forward for the quantum regime, according to physical chemist Markus Reiher of the Swiss Federal Institute of Technology in Zurich, Switzerland. The use of the VQE for the simulation of small molecules is a great example of the possibility of near-term heuristic algorithms, said Gambetta.

But even for this application, Aspuru-Guzik confesses that logical qubits with error correction will probably be needed before quantum computers truly begin to surpass classical devices. I would be really excited when error-corrected quantum computing begins to become a reality, he said.

If we had more than 200 logical qubits, we could do things in quantum chemistry beyond standard approaches, Reiher adds. And if we had about 5,000 such qubits, then the quantum computer would be transformative in this field.

Despite the challenges of reaching those goals, the fast growth of quantum computers from 5 to 50 qubits in barely more than a year has raised hopes. But we shouldnt get too fixated on these numbers, because they tell only part of the story. What matters is not just or even mainly how many qubits you have, but how good they are, and how efficient your algorithms are.

Any quantum computation has to be completed before decoherence kicks in and scrambles the qubits. Typically, the groups of qubits assembled so far have decoherence times of a few microseconds. The number of logic operations you can carry out during that fleeting moment depends on how quickly the quantum gates can be switched if this time is too slow, it really doesnt matter how many qubits you have at your disposal. The number of gate operations needed for a calculation is called its depth: Low-depth (shallow) algorithms are more feasible than high-depth ones, but the question is whether they can be used to perform useful calculations.

Whats more, not all qubits are equally noisy. In theory it should be possible to make very low-noise qubits from so-called topological electronic states of certain materials, in which the shape of the electron states used for encoding binary information confers a kind of protection against random noise. Researchers at Microsoft, most prominently, are seeking such topological states in exotic quantum materials, but theres no guarantee that theyll be found or will be controllable.

Researchers at IBM have suggested that the power of a quantum computation on a given device be expressed as a number called the quantum volume, which bundles up all the relevant factors: number and connectivity of qubits, depth of algorithm, and other measures of the gate quality, such as noisiness. Its really this quantum volume that characterizes the power of a quantum computation, and Gambetta said that the best way forward right now is to develop quantum-computational hardware that increases the available quantum volume.

This is one reason why the much vaunted notion of quantum supremacy is more slippery than it seems. The image of a 50-qubit (or so) quantum computer outperforming a state-of-the-art supercomputer sounds alluring, but it leaves a lot of questions hanging. Outperforming for which problem? How do you know the quantum computer has got the right answer if you cant check it with a tried-and-tested classical device? And how can you be sure that the classical machine wouldnt do better if you could find the right algorithm?

So quantum supremacy is a concept to handle with care. Some researchers prefer now to talk about quantum advantage, which refers to the speedup that quantum devices offer without making definitive claims about what is best. An aversion to the word supremacy has also arisen because of the racial and political implications.

Whatever you choose to call it, a demonstration that quantum computers can do things beyond current classical means would be psychologically significant for the field. Demonstrating an unambiguous quantum advantage will be an important milestone, said Eisert it would prove that quantum computers really can extend what is technologically possible.

That might still be more of a symbolic gesture than a transformation in useful computing resources. But such things may matter, because if quantum computing is going to succeed, it wont be simply by the likes of IBM and Google suddenly offering their classy new machines for sale. Rather, itll happen through an interactive and perhaps messy collaboration between developers and users, and the skill set will evolve in the latter only if they have sufficient faith that the effort is worth it. This is why both IBM and Google are keen to make their devices available as soon as theyre ready. As well as a 16-qubit IBM Q experience offered to anyone who registers online, IBM now has a 20-qubit version for corporate clients, including JP Morgan Chase, Daimler, Honda, Samsung and the University of Oxford. Not only will that help clients discover whats in it for them; it should create a quantum-literate community of programmers who will devise resources and solve problems beyond what any individual company could muster.

For quantum computing to take traction and blossom, we must enable the world to use and to learn it, said Gambetta. This period is for the world of scientists and industry to focus on getting quantum-ready.

See the rest here:

The Era of Quantum Computing Is Here. Outlook: Cloudy …

- Quantum computing could change everything, and IBM is ... - May 15th, 2019
- Quantum Computing - Intel - April 29th, 2019
- IBM expands universities in its quantum computing research ... - April 25th, 2019
- Quantum computing is a marathon not a sprint | VentureBeat - April 22nd, 2019
- The CIO's Guide to Quantum Computing - Smarter With Gartner - April 19th, 2019
- This Startup Just Raised $21 Million To Bring Quantum ... - April 18th, 2019
- What is Quantum Computing ? Top 18 Quantum Computing ... - April 6th, 2019
- The promise of quantum computing - businessinsider.com - March 27th, 2019
- Quantum computing is coming: Heres why we need to get our ... - March 23rd, 2019
- Quantum computing will break your encryption in a few ... - March 21st, 2019
- Microsoft has formed a coalition to promote quantum computing ... - March 19th, 2019
- Quantum computing for everyone | Michael Nielsen - March 12th, 2019
- Ask a Techspert: What is quantum computing? - blog.google - March 6th, 2019
- IBM hits quantum computing milestone, may see 'Quantum ... - March 6th, 2019
- Its Time You Learned About Quantum Computing | WIRED - March 6th, 2019
- Microsofts quantum computing network takes a giant leap ... - March 2nd, 2019
- When Will Quantum Computing Have Real Commercial Value ... - February 25th, 2019
- The Case Against Quantum Computing - IEEE Spectrum - February 22nd, 2019
- How Does Quantum Computing Work? - ExtremeTech - January 31st, 2019
- Quantum technology - Wikipedia - January 23rd, 2019
- CES 2019: IBM's Q System One Is the Rock Star Quantum ... - January 13th, 2019
- Quantum Computing | The MIT Press - January 11th, 2019
- IBM thinks outside of the lab, puts quantum computer in a box - January 11th, 2019
- IBM unveils its first commercial quantum computer - January 9th, 2019
- A new type of quantum computer has smashed every record ... - December 21st, 2018
- China bet big on quantum computing. Now the US races to ... - October 26th, 2018
- US takes first step toward a quantum computing workforce ... - September 17th, 2018
- China bet big on quantum computing. Now the ... - money.cnn.com - September 17th, 2018
- The reality of quantum computing could be just three years ... - September 12th, 2018
- The quantum computing race the US cant afford to lose - September 3rd, 2018
- Quantum Computing | USRA - August 30th, 2018
- What Is Quantum Computing? The Complete WIRED Guide | WIRED - August 22nd, 2018
- Quantum Computing Market Research Report- Forecast 2022 | MRFR - August 1st, 2018
- Two Quantum Computing Bills Are Coming To Congress - July 5th, 2018
- Senate bills would make quantum computing a priority - June 10th, 2018
- What is quantum computing? - Definition from WhatIs.com - February 5th, 2018
- IBM puts its quantum computer to work in relaxing, nerdy ASMR ... - January 8th, 2018
- Quantum computing is going to change the world. Here's what ... - January 8th, 2018
- Is Quantum Computing an Existential Threat to Blockchain ... - December 25th, 2017
- What is Quantum Computing? | SAP News Center - December 23rd, 2017
- Quantum Computing Explained | What is Quantum Computing? - December 21st, 2017
- New silicon structure opens the gate to quantum computers - December 14th, 2017
- Microsoft offers developers a preview of its quantum ... - December 12th, 2017
- Quantum Computing Is the Next Big Security Risk | WIRED - December 8th, 2017
- Yale Professors Race Google and IBM to the First Quantum ... - November 16th, 2017
- IBM's processor pushes quantum computing ... - engadget.com - November 16th, 2017
- Quantum computing - news.microsoft.com - November 1st, 2017
- Intel Takes First Steps To Universal Quantum Computing - October 13th, 2017
- Qudits: The Real Future of Quantum Computing? - IEEE Spectrum - October 13th, 2017
- quantum computing - engadget.com - October 13th, 2017
- Quantum Computing | Intel Newsroom - October 13th, 2017
- What will you actually use quantum computing for? | ZDNet - October 11th, 2017
- Here's what quantum computing is and why it matters - October 6th, 2017
- Microsoft just upped its multi-million bet on quantum computing - ZDNet - September 7th, 2017
- Microsoft's Aussie quantum computing lab set to scale up next-gen ... - ARNnet - September 7th, 2017
- An Entirely New Type of Quantum Computing Has Just Been Invented - Futurism - September 7th, 2017
- Quantum computing event explores the implications for business - Cambridge Network - August 30th, 2017
- Quantum Computing Is Coming at Us Fast, So Here's Everything You Need to Know - ScienceAlert - August 27th, 2017
- How quantum mechanics can change computing - San Francisco ... - San Francisco Chronicle - August 25th, 2017
- Commonwealth Bank investing in Australia's first quantum computer company - Which-50 (blog) - August 25th, 2017
- How quantum mechanics can change computing - The Conversation US - August 23rd, 2017
- Introducing Australia's first quantum computing hardware company - Computerworld Australia - August 23rd, 2017
- IEEE Approves Standards Project for Quantum Computing ... - insideHPC - August 23rd, 2017
- $495.3 Million Quantum Computing Market 2017 by Revenue Source, Application, Industry, and Geography - Global ... - PR Newswire (press release) - August 18th, 2017
- Physicists Have Made Exotic Quantum States From Light - Futurism - August 16th, 2017
- Machine learning tackles quantum error correction - Phys.Org - August 15th, 2017
- Quantum Internet Is 13 Years Away. Wait, What's Quantum Internet? - WIRED - August 15th, 2017
- Blind quantum computing for everyone - Phys.org - Phys.Org - August 12th, 2017
- Quantum Computing Is Real, and D-Wave Just Open ... - WIRED - August 12th, 2017
- Quantum Computing Market Worth 495.3 Million USD by 2023 | 08 ... - Markets Insider - August 10th, 2017
- China uses a quantum satellite to transmit potentially unhackable data - CNBC - August 10th, 2017
- Physicists Take Big Step Towards Quantum Computing and ... - Universe Today - August 1st, 2017
- Why you might trust a quantum computer with secretseven over ... - Phys.Org - July 12th, 2017
- Quantum-computer node uses two different ion species - physicsworld.com - July 10th, 2017
- Quantum Computers vs Bitcoin How Worried Should We Be? - The Merkle - July 10th, 2017
- Quantum cheques could be a forgery-free way to move money - New Scientist - July 10th, 2017
- Technique for measuring and controlling electron state is a ... - UCLA Newsroom - July 9th, 2017
- Quantum Computers Made Even More Powerful with New microchip generating 'Qudits' - TrendinTech - July 8th, 2017
- Quantum Computing Record Broken - Wall Street Pit - July 8th, 2017
- Alkermes and IBM's quantum computing. Who'll be the big winner? Malcolm Berko - Durham Herald Sun - July 6th, 2017

## Recent Comments