Ice Ice Baby — Why Quantum Computers have to be cold

It’s a cold, cold world out there if you’re a qubit.

Quantum computers are cold. In fact, many quantum computers have to be kept to near absolute zero temperatures.

For the non-chemists in the room, absolute zero means 0 Kelvin.

0 Kelvin = -273.15 Celsius = -459.67 Farenheit!

Almost -460 degrees Fahrenheit!! Think about it, coldest temperature on record was in Antartica at -128.6 Fahrenheit. The world literally cannot be cold enough for these computers.

Ok ok, so let’s address two questions: the why and the how.

Why: Why do quantum computers have to be that cold?

Alright, so we all know that quantum computers have quantum states, which are created from its two basis states. If you did not know that, why don’t you head over here for a quick refresher.

If you’re still with us, then let’s continue forward. As we said, quantum computers are typically created from 2 basis states, usually thought of as “0” and “1”. Now, pretend for a moment that these were the only two possible states (aka we were using a classic computer architecture). It’s not too hard to imagine that it takes some energy to change from 0 to 1 or from 1 to 0. Right? In fact, usually 0 and 1 are implemented by defining ranges of electrical voltages, where one range is classified as 0, and the other as 1. So for example, 0–2 volts could be considered a “0”, while as 5–7 could be considered a “1”. In that case, we literally need energy to move from one state to another.

Ok, cool. So let’s go back to a case where we can have quantum states in addition to our two basis states. Going back to our voltage range example, we now need to figure out how we can define ranges somewhere in there for the quantum states.

Most computers use ranges with extremely small gaps in between the ranges to represent the quantum states. So for example, if we had three contiguous quantum states (which we will call A, B, and C), their voltage ranges might be 0.5–0.75 V for quantum state A, 0.751–1.01 for quantum state B, and 1.02–1.27 for quantum state C respectively. That’s not a lot of room for error to begin with, and the gaps between these ranges are usually much smaller.

So think about it, if you get a tiny amount of error in your voltages, you could easily accidentally push a qubit from quantum state A to quantum state B. Or from B to C. Or from B to A. You get the idea.

This leads to the idea of why we need our quantum computers to be so cold. At extremely cold temperatures, atoms and molecules simply move around less. The lower a temperature is, generally speaking, the more stable a molecule becomes. Less movement means less energy being expelled.

At a molecular level, that means that less energy is flying around, and consequently (since voltage and energy are directly related) less volatility in the voltage. That means that there’s less of a chance that something outside of a human’s control will cause a qubit’s voltage to spike, causing the qubit to flip from one quantum state to another.

And that’s it. By keeping the computer cold, we are introducing less energy into the system, and thus minimizing the chances of qubits incorrectly flipping in between quantum states.

Now we get to the second part of our query…

How: How do we get temperatures that are this cold??

Let’s recap something real quick: we cannot reach absolute zero.

Seriously, there is no place, no technology, nothing that has in practice reached absolute zero.

Idk, I’m sure in the far far future they’ll figure it out. But for now, in the primitive 21st century, we can’t do it.

But we can get really close. The record lowest temperature was actually set by a group of MIT researchers, who cooled a molecule down to one-half-billionth of a degree above absolute zero.

That’s cold!

Current companies are keeping quantum computers a few degrees above absolute zero, so around -450 Fahrenheit. Here’s one way to go about it.

D-Wave uses liquid helium as a coolant for their refrigeration. The D-Wave refrigerators are “dry dilution” which means that the liquid helium is in a “closed cycle system”. It uses a pulse tube technology to recycle and recondense the liquid helium.

As quantum computers popularize, I am sure that the technology will become more innovative and cheaper.

In the meantime, all of the budding entrepreneurs out there who want to go make a quantum computer, you’d better go buy some freezers.