Will Quantum Computers Replace Classical Computers Altogether?
By: Frank Zickert, Quantum Machine Learning
Tags: Hardware, Computer Science, Quantum Computing, Computers, Quantum Physics

Will Quantum Computers Replace Classical Computers Altogether?

Do you want to get started with Quantum Machine Learning? Have a look at Hands-On Quantum Machine Learning With Python.

Would you replace your fridge with a freezer? Probably not. Even though your freezer can do everything that your fridge can do. Just let the door open a little bit so that the outside heat compensates for the higher cooling power of the freezer.

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Do you think this is inefficient? Then, exponentiate this inefficiency, and you end up with what replacing classical computers with quantum computers would mean.

If you think that this analogy is inappropriate, let me add a brief disclaimer: This is not an analogy at all.

A classical computer is nothing but a fridge. The fans inside cool down the CPU (central processing unit) to not exceed 100°C. From a physical perspective, the main output of a CPU is not the computation. It is heat. The computation is only a by-product.

In that notion, a quantum computer is nothing but a freezer. The apparatus surrounding the QPU (quantum processing unit) cools it down to -273°C. Pretty freezing. It doesn’t do it because a QPU produces so much heat. On the contrary, a QPU doesn’t produce much heat at all. But a quantum computation is extremely sensitive to any heat at all — if it is appropriate to speak of heat when we operate close to the absolute minimum.

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So, it doesn’t seem efficient to replace a classical computer with a quantum computer as long as the classical computer gets the job done.

But, we have to keep in mind that the development of quantum hardware is still in its infancy. If we look back at the early days of classical computers, nobody predicted the capabilities and the miniaturization we could reach one day. The early computers used electromechanical relays instead of transistors. They were inefficient and error-prone. But they served their purpose. They proved that building computers was possible.

The underlying technology changed completely pretty fast. Transistors replaced relays. And in 1965, Moore postulated that the number of transistors we can pack into a given unit of space would double about every two years. This trajectory has been true for the last 50 years. Yet, we currently witness Moore’s Law dramatically slowing.

But the demand for computing performance continues to increase. New technologies, such as deep neural networks, Blockchain, and huge amounts of data we collect every day depend on the underlying hardware to improve.

Of course, quantum computers won’t replace classical computers for as long as they are as inefficient and error-prone as the early classical computers. But what happens once we find a way to build quantum computers efficiently? What happens if we enter an era where Moore’s law holds for quantum devices?

Generally, we can run any computation on a QPU that we can run classically. The quantum bit (qubit) is a generalization of the classical bit. While the classical bit is either 0 or 1, the qubit is in a complex linear combination of 0 and 1. We call this state superposition. But this state of superposition comprises the special case of 0 and 1. So, we could use a qubit just like an ordinary bit. Of course, it would be a huge waste of capabilities. Yet, it would be possible.

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But, qubits are interesting because they can do things classical bits can’t. We can entangle qubits so that they form a shared state of superposition. Then, the number of concurrent states we can represent by qubits increases exponentially. This ability lets us solve problems we can’t solve classically (in any reasonable time).

However, even if we enter the era of “quantum Moore,” it will take quite a while for QPUs to become as big (in terms of the number of qubits) as classical computers. This is because we don’t need so many qubits to do things classical computers can’t do.

Whenever QPUs become affordable hardware, we will likely see them complement our computers rather than replacing the underlying technology altogether. Even though the QPU could do any task, it is not reasonable to use it all the time. If a problem is solvable classically, we will continue to solve it classically. We will only use the QPU to solve the problems that are intractable classically.

We are already witnessing a similar development. Not long ago, the CPU was the processor that mattered the most when we looked at the performance of a computer. First, we looked at clock speed. Later, we looked at the number of cores.

But the requirements mentioned above of modern technologies already exceed what the best CPU can do today. As a result, we equip our computers with GPUs (graphics processing units) and even TPUs (tensor processing units) to do the jobs that the classical CPU struggles with. Yet, we do not replace the CPU because it remains the best processor for most tasks.

GPUs, TPUs, and the future QPUs are special-purpose processors that solve particular kinds of problems. They complement the CPU but do not replace it.

However, technically speaking, isn’t a computer that has a QPU a quantum computer? So in that notion, even if QPUs don’t replace CPUs, we will see quantum computers replace classical computers.

Do you want to get started with Quantum Machine Learning? Have a look at Hands-On Quantum Machine Learning With Python.

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