**I am a Quantum Computing Sceptic**.

**But last week at Dalhousie** I met Jordan Kyriakidis who explained one feature of Quantum Computing that I had not appreciated. That even if a quantum computer only gave the right answer one time in a million operations, it might still be useful.

**His insight made me believe** that Quantum Computing just *might* be possible.

[**Please be aware** that I am not an expert in this field. And I am aware that experts are less sceptical than I am. Indeed many consider that the power of quantum computing has already been demonstrated. Additionally Scott Arronson argues persuasively (in his point 7) that my insight is wrong.]

**Background**

**Conventional digital computers** solve problems using mathematics. They have been engineered to perform electronic operations on representations of numbers which closely mimic equivalent mathematical operations.

**Quantum computers are different**. They work by creating a physical analogue of the problem which requires solving.

**An initial state is created** and then the computational ‘engine’ is allowed to evolve using basic physical laws and hopefully arrive at a state which represents a solution to the problem at hand.

**My problem**

**There are many conceivable implementations** of a quantum computer and I am sceptical about them all!

**My scepticism** arises from the analogue nature of the computation. At some point the essential elements of the quantum computer (called ‘Qubits‘ and pronounced Q-bits) can be considered as some kind of oscillator.

**The output of the computer** – the answer – depends on interference between the Qubits being orchestrated in a precise manner. And this interference between the Qubits is completely analogue.

**Analogue versus digital**

**Physics is fundamentally analogue. **So, for example, the voltages present throughout a digital computer vary between 0 volts and 5 volts. However the engineering genius of digital electronics is that it produces voltages that are either relatively close to 0 volts, or relatively close to 5 volts. This allows the voltages to be interpreted unambiguously as representing either a binary ‘1’ or ‘0’. This is why digital computers produce exactly the same output every time they run.

**Quantum Computing **has *outputs* that can be interpreted unambiguously as representing either a binary ‘1’ or ‘0’. However the *operation* of the machine is intrinsically analogue. So tiny perturbations that accumulate between the thousands of operations on the Qubits in a useful machine will result in different outputs each time the machine is run.

**Jordan’s Insight**

**To my surprise **Jordan acknowledged my analysis was kind-of-not-wrong. But he said it didn’t matter for the kinds of problems quantum computers might be good at solving. The classic problem is factoring of large numbers.

**For example** working out that the number 1379 is the result of multiplying 7 × 197 will take you a little work. But if I gave you the numbers 7 and 197 and asked you to multiply them, you could do that straightforwardly.

**Finding the factors of large numbers** (100 digits or more) is hard and slow – potentially taking the most powerful computers on Earth hundreds of years to determine. But multiplying two numbers – even very large numbers – together is easy and quick on even a small computer.

**So even if a quantum computer** attempting to find factors of a large number were only right one time in a million operations – that would be really useful! Since the answers are are easy to check, we can sort through them and get rid of the wrong answers easily.

**So a quantum computer could reduce the time to factor large numbers** even though it was wrong 99.9999% of the time!

**I can easily imagine quantum computers being mostly wrong** and I had thought that would be fatal. But Jordan made me realise that might still be very useful.

Thanks Jordan 🙂

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**By the way,** you might like to check out this web site which will factor large numbers for you. I learned that the number derived from birth date (29/12/1959>>>29121959) is prime!

Tags: Quantum Computing

February 2, 2016 at 11:45 am |

All you would need is a co-processor next to the quantum processor to automatically verify whether the q’s calculations held up.

February 2, 2016 at 1:04 pm |

Exactly