Quantum Supremacy Uber Alles

Why Quantum Computers Will Be Super Awesome, Someday
By Jeremy Kahn and Amy Thomson (Bloomberg)

Nobel Prize-winning physicist Richard Feynman was the first to suggest that the mind-bending properties of quantum mechanics could be harnessed to make a new kind of computer. Almost 40 years later and after a decade of significant progress -- and after a claim by Google that its computer had reached a milestone known as “quantum supremacy” -- it’s still easier to describe the approach’s potential importance than to describe how it works. Understanding quantum mechanics, whose principles underpin quantum computing, involves a lot of mental mountain climbing — like something about a cat in a box that might or might not be dead?

1. What’s the appeal of quantum computers?

Satya Nadella, the chief executive officer of Microsoft Corp., calls quantum computing one of three emerging technologies that will radically reshape the world, along with artificial intelligence and augmented reality. In the long run, quantum computers might make today’s fastest supercomputer look like an abacus. Now, however, your laptop can solve problems pretty much just as quickly. Tasks that seem more within reach include speeding up chemical reactions by creating more efficient catalysts, helping discover new drugs and improving the algorithms that shape industrial logistics and supply chains.


2. Who’s building them?

D-Wave Systems Inc., a Canadian company, became the first to sell quantum computers in 2011, although their usefulness is limited to certain kinds of math problems. International Business Machines, Alphabet Inc.’s Google, Intel and Rigetti Computing, a startup in Berkeley, California, have all created working quantum computers. Intel has started shipping a superconducting quantum chip to researchers. Microsoft has a well-funded program to build a quantum computer using an unusual design that might make it more practical for commercial applications. Meanwhile, China is building a $10 billion National Laboratory for Quantum Information Sciences as part of a big push in the field.


3. How do quantum computers work?

They use tiny circuits to perform calculations, as do traditional computers. But they also use two quantum phenomena called superposition and entanglement. Regular computers process information in units called bits, which can represent one of two possible states — 0 or 1 — that correspond to whether a portion of the computer chip called a logic gate is open or closed. By contrast, quantum computers use quantum bits, or qubits. Qubits can represent both a 0 and 1 at the same time. So two qubits can represent four numbers simultaneously, three qubits can represent eight numbers, and so on. That’s superposition.


4. OK, what’s entanglement?

In designing a standard computer, engineers spend a lot of time trying to make sure the status of each bit is independent from that of all the other bits. But in a quantum computer, each qubit influences the other qubits around it, working together to arrive at a solution. Superposition and entanglement are what give quantum computers the ability to process so much more information so much faster.

5. How do you make a qubit?

In theory, anything exhibiting quantum mechanical properties that can be controlled could be used to make qubits. IBM, D-Wave and Google use tiny loops of superconducting wire, others use semiconductors, and some use a combination of both. Some scientists have created qubits by manipulating trapped ions, pulses of photons or the spin of electrons. Microsoft is taking yet another tack, trying to twist elusive subatomic particles called Majorana fermions into a braided shape that would keep qubits in a quantum state longer. Many of these approaches require very specialized conditions, such as temperatures 180 times colder than those found in deep space.

6. When do I get my quantum computer?

Not anytime in the near future, and for two reasons, one of which is computing power. But in October, Google announced that it had used its machine to do something that would be impossible for a classical computer, a milestone known as “quantum supremacy.” Google said its computer had performed a calculation in 200 seconds that would take the fastest supercomputers about 10,000 years. That claim was met with skepticism by IBM researchers, who said a classical computer that had a big enough hard drive could do it in 2.5 days. Even when that milestone is reached, the first quantum applications may be very specialized — that is, useful in certain kinds of mathematical problems that are important in chemistry or physics but little else. A so-called “universal” quantum computer is further off.

7. What’s the other reason?

Errors, lots of them. Scientists have only been able to keep qubits in a quantum state for fractions of a second — in many cases, too short a period of time to run an entire algorithm. And as qubits fall out of a quantum state, errors creep into their calculations. These have to be corrected with the addition of yet more qubits, but this can consume so much computing power that it negates the advantage of using a quantum computer in the first place. In theory, Microsoft’s design should be more accurate — but so far it hasn’t succeeded in producing even a single working qubit.