15 Questions about Quantum

Fifteen questions (and answers) about quantum computing you always wanted but were afraid to ask.

1. What is quantum computing?

Quantum computing is an emerging technology powered by the quantum properties of matter and light that promises to solve problems that cannot be solved on supercomputers.

2. Why should I care?

What is impossible on supercomputers, may very well be possible on quantum computers, even as supercomputers become faster and larger.

We can use quantum computers as accelerators, where for certain algorithms quantum computers provide exponential speed-ups, or to simulate real quantum systems from physics, chemistry, biochemistry, and materials science efficiently.

3. When can I expect a significant advantage?

2030–2040 is when we expect quantum advantage, when quantum computers can outperform (super)computers at realistic problems.

4. Why should I bother with quantum computing now?

Because it will take time to be ready for the quantum revolution, especially since qualified personnel to take advantage of near-term quantum computers is scarce.

Many companies are already exploring quantum computers today. These organizations solve downscaled versions of realistic problems, as this is an essential step on the road to quantum advantage and a skilled quantum workforce.

5. What is preventing us from scaling up quantum computers?

There are many engineering challenges that must be overcome.

Today’s so-called noisy intermediate-scale quantum (NISQ) computers do not yet implement efficient error correction schemes. These must account for and mitigate all sources of errors, because quantum mechanical properties are very delicate.

Many types of quantum computers also require bulky equipment around the quantum chip (e.g. dilution refrigerators, vacuum chambers), which also poses a challenge when scaling up.

While some types of quantum chips rely on current semiconductor fabrication processes, others do not and cannot, because they are fundamentally different. There is no technology that is superior, nor is it clear whether there will be a single modality (e.g. superconductors, ion traps, photons, quantum dots, neutral atoms, nitrogen vacancies) suitable for all problems at any scale.

6. What can I do with a quantum computer today?

The top-3 use cases are optimization, simulation, and machine learning.

7. Who is already using quantum computers?

More than a quarter of Fortune Global 500 companies are working with quantum technologies in a variety of industries: aerospace and defence, pharmaceuticals, telecommunications, automotive, financial services, and manufacturing.

8. How are quantum computers different from regular computers?

Current computers comprise billions of transistors that are on/off switches that encode bits: zeros or ones.

Quantum computers rely on quantum bits, or qubits, which have a few remarkable properties:

• A single qubit can be in a combination of both on and off at the same time (superposition).
• Multiple qubits can be correlated, so that independent operations on one qubit affect the other(s) instantaneously (entanglement).
• Arbitrary qubits cannot be copied or deleted (no cloning).

Superposition enables quantum computers to process information in parallel (quantum parallelism), but it comes at the cost of probabilistic computation: each execution of a quantum algorithm only gives us a single possible outcome (measurement). However, for certain problems, we can surface the desired solution with high probability (interference). In such cases, a quantum computer acts as a sampler and multiple runs are needed.

9. Does that mean quantum computers are inexact?

There are quantum algorithms that provide an exact answer in a single execution. When used as a sampler, quantum computers sample a probability distribution and multiple executions are required to obtain a candidate solution.

10. If multiple runs are needed, what is the benefit?

With a 5-qubit machine, you can explore a solution space of 2⁵ = 32 (complex) numbers simultaneously. With 50 qubits, that figure becomes 2⁵⁰ = 𝒪(10¹⁵), and with 500 we end up with 2⁵⁰⁰ = 𝒪(10¹⁵⁰). For comparison, the number of atoms in the observable universe is ‘only’ on the order of 10⁸⁰.

As long as you do not need to scale the number of executions exponentially, quantum computers can provide a significant speed-up for complex problems in optimization, simulations (e.g. industrial chemistry, battery design, drug discovery), and machine learning.

11. Will quantum computers replace regular computers?

No. We still need classical computers to control and interface with quantum computers. As samplers, quantum computers need classical computers to aggregate and filter candidate solutions.

Not every algorithm runs more efficiently on a quantum computer. Quantum computers are useful for very specific types of number crunching, but definitely not all or even most. In that sense, quantum computers are a bit like GPUs and TPUs.

12. When will I have a quantum computer at home or in my pocket?

Not for the foreseeable future. While there are smaller room-temperature quantum computers, many large-scale quantum computers require cryogenic (e.g. superconductors) and/or ultra-high vacuum environments (e.g. ion traps). Shielding a quantum computer from its environment is paramount.

13. Where can I learn more?

Here are my suggestions for resources to learn more about quantum computing.

14. How can I try out a quantum computer?

You can try out various quantum libraries in your browser with Zoose Quantum, either on GitHub or with Gitpod. That way, you have immediate access to Braket, Cirq, cuQuantum, OpenFermion, PennyLane, pytket, Qiskit, QuTiP, Strawberry Fields, and many more packages.

Knowledge of the basics of quantum computing and Python suffice to get started. You need not be a quantum physicist.

15. Is quantum computing a hoax?

No. You can access and program real quantum computers in the cloud right now.

There are sceptics who claim we cannot construct universal fault-tolerant quantum computers because it is impossible. So far, no scientific proof exists that disproves the possibility of the existence of such machines. Each new generation of quantum computers brings us closer to quantum advantage and universal fault-tolerant quantum computers.

(Bonus) Have scientists recently created a wormhole with a quantum computer?

Absolutely not! Instead, they simulated a highly simplified model of information being sent through a wormhole on a quantum computer.