Tuesday, February 28, 2012

IBM joins quest to build a qubit

A reader writes:
While it's refreshing to find someone as skeptical of scientific hype as you, I think you have done too far with the claim that "no one has a qubit yet".
So I report progress in efforts to prove me wrong. Today's NY Times reports:
I.B.M. is jumping into an area of computing that has, until now, been primarily the province of academia: the quest to build a quantum computer.

A computer that took advantage of the oddities of quantum physics could solve in seconds certain problems that would occupy present-day computers for billions of years. But for now, it is impossible to build such a computer because the bits of information it would need for the calculations fall apart before a calculation can be completed. The problem is, in essence, like trying to knit a sweater with yarn that unravels before the first purl.

On Tuesday, I.B.M. researchers will present experimental results that they say put them close to solving this problem, both by lengthening the lifetime of the quantum bits of information and by quickening the pace of computation. The presentation will take place at a meeting of the American Physical Society in Boston.

“In the past, people have said, maybe it’s 50 years away, it’s a dream, maybe it’ll happen sometime,” said Mark B. Ketchen, manager of the physics of information group at I.B.M.’s Thomas J. Watson Research Center in Yorktown Heights, N.Y. “I used to think it was 50. Now I’m thinking like it’s 15 or a little more. It’s within reach. It’s within our lifetime. It’s going to happen.”
This acknowledges that they cannot build a quantum computer because no one can make a qubit that is stable enough to be used in a computation.

The IBM method is commercially impractical because it requires a circuit that is a whole lot colder than liquid helium:
When cooled to a hundredth of a degree above absolute zero, the circuits act as qubits.

The problem is that a qubit becomes scrambled in short order, and the information it carries turns into gibberish. ...

Even though that is still not long enough for perfect calculations, it is almost good enough for error correction algorithms to detect and fix any mistakes. “We’re just crossing this threshold,” Dr. Ketchen said, “which is a big morale booster that says, gee, this is becoming doable.”

Below the threshold, generating reliable answers is impossible. “No matter how many qubits you had, you couldn’t even get one effectively good one because of the error rates being too high,” he said.
I read this as saying that IBM is claiming to make progress towards a qubit that is suitable for computation, but that no one has made such a qubit yet.

I get a lot of flack for being a quantum computer skeptic. But when I deny that anyone has made a true qubit, I am just saying that same thing that this NY Times article says, and what the experts quoted in it say. Where I disagree with them is where they express confidence in doing something that has never been done before.

To me, it seems much more likely that quantum computing is impossible. I say this because (1) it is contrary to conventional notions of computability, (2) it is not implied by quantum mechanics, (3) it is relentlessly overhyped, and (4) it has been pursued by a lot of smart men and money for decades, without much to show for the work.

Of course you have no more reason to believe my predictions than those of a big-shot MIT professor. Just ask yourself why you keep reading stories about quantum computers are doing complex computations with dozens of qubits, and yet no one has a convincing demonstration that anyone has even computed anything with a single qubit.

Update: Lubos Motl summarizes quantum computers and adds:
Errors in the calculations and decoherence make all existing prototypes of a quantum computer unusable. So far it is the case. Maybe one of you will help to solve the technological obstacles.
The obstacles may be more than technological; as a comment below points out, the laws of physics may require that calculating with qubits requires exponentially large amounts of energy.


  1. Perhaps this is a good reason to sell my IBM stock.

  2. I think the real reason why large-scale quantum computing cannot work is because the amount of energy required to keep n qubits entangled in nondegenerate states becomes exponential with n.