Classical computers (laptops, mobiles phones, gaming consoles…) are built around the concept of “bits”. Bits are binary units of information, meaning that a bit can take the values “0” or “1”. This binary information is for instance stored in the Random-Access Memory (RAM) of classical computers using microscopic capacitors that can be either charged (“1”) or discharged (“0”).
While there are high-performance classical computing centers capable of tackling computationally intensive tasks such as fluid simulations and artificial intelligence training, some problems remain intractable in a reasonable amount of time. For instance, complex optimization problems, like train schedule programming, where many alternatives must be evaluated one-by-one, are very time consuming. Simulating quantum systems is also challenging for classical machines, due to the memory requirements scaling exponentially with the system.
This is where quantum computers come into play. Quantum computers are hardware devices that use the laws of quantum mechanics to perform computational tasks by executing quantum algorithms. Quantum computers physically store information as “quantum bits”, so-called qubits, and apply quantum operations on the qubits to process this information. Quantum computers use physical phenomena such as quantum superposition, quantum measurements and quantum entanglement to perform certain computations more efficiently than classical computers.
Quantum computing platforms
Promising candidates for quantum computing platforms include
- Superconducting qubits :
- Trapped ion qubits
- Neutral atoms
- Photonic qubits (continuous- and discrete-variable frameworks)
- Spin qubits
- Cold atoms in lattices
- Topological qubits.
These platforms aim to satisfy the DiVincenzo criteria (Link to entry “DiVicenzo criteria”), a set of conditions necessary for a practical quantum computer.
Challenges in building a quantum computer
Constructing a scalable quantum computer is highly challenging due to hardware imperfections and environmental noise. Quantum systems are fragile, and uncontrolled interactions with their surroundings introduce errors in the computations. Consequently, reducing the rate of these error via techniques for error mitigation and quantum error correction is an essential component of fully-fledged quantum computers.
Frequently asked questions about quantum computers
- How is a quantum computer different from a classical computer?
A classical computer processes information using bits (0s and 1s) and classical gates. A quantum computer on the other hand operates on qubits, whose state can be in quantum superpositions of 0 and 1. Quantum computers act on qubits using quantum gates. This allows quantum computers to solve some problems faster than classical machines.
- What problems can quantum computers solve?
Quantum computers have promising applications in cryptography, materials science, optimization, and drug discovery.
- Are quantum computers available today?
Early-stage quantum computers exist (https://cloud.quandela.com/) but they are limited in size and capability. Researchers and companies are working toward transitioning from noisy intermediate-scale quantum (NISQ) devices to more powerful, fault-tolerant quantum computers.
- Will quantum computers replace classical computers?
No, quantum computers are not expected to replace classical computers entirely. Instead, they will complement them by solving specific problems that are difficult or impossible for classical computers to handle efficiently. Some problems may be tackled by combining a classical computer with a quantum device (Link to entry “Variational quantum algorithms”).
- What is quantum advantage?
Quantum advantage refers to the point at which a quantum computer can perform a computation that is practically impossible for any classical computer to execute within a reasonable timeframe.