How does a quantum computer work? The simple guide.
Learn how qubits, superposition, and entanglement work, and why quantum computers could transform certain calculations.
COMPUTER SCIENCEAI
Lucas GRANDIER
10/21/20269 min read
If you want to better understand how a computer works overall, check out our article How does a computer work?
Table of Contents
1. Classic bit and qubit: what's the difference?
3. Why can qubits be useful for computation?
4. What happens when multiple qubits work together?
5. What are quantum computers used for?
6. Why is it so difficult to build a quantum computer?
7. Will a quantum computer replace classical computers?
FAQ: Frequently asked questions about quantum computers
How could we simulate a complex molecule, design new materials or solve some particularly difficult mathematical problems? To answer these questions, scientists are exploring a technology that is based on the laws of quantum physics: the quantum computer.
Unlike a classical computer, which processes information with bits worth 0 or 1, a quantum computer uses qubits. These units of information can be prepared in particular states, transformed by quantum operations, and combined to perform calculations.
But what makes a qubit different from a classical bit? And how can this difference help solve a problem?
To understand this, let's follow how a qubit works, and then find out how multiple qubits can work together.
1. Classic bit and qubit: what's the difference?
To understand quantum computing, let's start with what we already know.
In a classical computer, all information is represented using bits. Each bit takes a value from two possibilities: 0 or 1.
These bits are manipulated by electronic circuits that execute the instructions of the programs. By combining a very large number of bits, a computer can perform calculations, display images, or run an application.
A quantum computer also uses units of information based on two reference states, but these are called qubits, for quantum bits.
The essential difference is that a qubit can be prepared in a superposition of its 0 and 1 states.
In concrete terms, this means that its quantum state is not limited to state 0 or state 1 taken separately. It can combine the two possibilities in precise proportions.
This property is at the heart of quantum computing. But to understand its interest, we need to see what we can do with a qubit.
2. How does a qubit work?
Let's imagine a qubit that we initially prepare in state 0. We will apply different operations to him to observe how his condition evolves.
Step 1: Prepare the qubit
At the beginning of the calculation, the quantum computer prepares the qubit in a known state, in this case 0.
If we measure it immediately, we get 0 in the ideal case.
This step is comparable to the initialization of a variable in a classic program: we start from a known starting state before performing operations.
Step 2: Create an overlay
The computer then applies an operation called the Hadamard gate.
A quantum gate is an operation that transforms the state of a qubit, or several qubits. It plays a role comparable to a computer instruction, even if its operation is based on the rules of quantum physics.
In our example, the Hadamard gate transforms the qubit initially in state 0 into a balanced superposition of states 0 and 1.
What happens if we measure the qubit at that time?
In the ideal model, we have a 50% chance of getting 0 and a 50% chance of getting 1.
Be careful, however: this does not mean that we can read both results simultaneously. The measurement provides a single classic result. Superposition describes the quantum state that exists before this measurement.
So, why create an overlay if we can't recover all the possibilities at once?
This is where quantum operations come into their own.
Step 3: Transform the state of the qubit
After creating a superposition, the computer can apply other quantum gates.
These operations change the state of the qubit. In particular, they can change the probabilities associated with the results and the phase relationships between the components of its state.
Phase is a quantum property that does not directly correspond to a visible result such as 0 or 1. Yet, it plays a key role: it influences how the different components combine when other operations are applied.
Let's go back to our example. If we apply a second Hadamard gate to the superimposed qubit, we bring it back to state 0 in the ideal case.
If we then measure it, we get 0 with a probability of 100%.
So we started with a qubit in state 0, created an overlay, and then applied another operation to get back to state 0.
This simple example shows that quantum computing is not just about creating superpositions: it is about how operations transform quantum states.
Step 4: Measure the result
At the end of the calculation, the computer measures the qubits to obtain classical results, such as 0s and 1s.
The result depends on the quantum state obtained after all the operations performed.
For a useful calculation, the operations must be carefully chosen so that the desired results have a sufficiently high probability of appearing during the measurement.
The principle to remember: a qubit is not a box that contains two answers that can be read at will. It is a quantum system whose state can be prepared and transformed to perform a calculation.
3. Why can qubits be useful for computation?
We now know how a qubit evolves during a calculation. But this property alone is not enough to make a quantum computer more efficient.
Its interest comes in particular from two phenomena: superposition and quantum interference.
Layering: combining several possibilities
A qubit can be in a superposition of states 0 and 1. With multiple qubits, the system can combine an increasing number of reference states.
For example:
1 qubit corresponds to 2 reference states
2 qubits correspond to 4 reference states
3 qubits correspond to 8 reference states
10 qubits correspond to 1,024 reference states
More generally, a system of (n) qubits has a state space of dimension (2^n).
This growth is impressive. But it doesn't mean that the computer can recover all these possibilities in one measure. A measure provides a classic result, not the full list of states that make up the overlay.
The challenge is therefore to transform these states before the measurement, in order to obtain useful information.
Interference: Promoting Certain Outcomes
To understand this mechanism, let's take an analogy with waves.
When two waves meet, they can strengthen or diminish depending on how they combine.
In quantum mechanics, the different components of a state can also combine. This is called quantum interference.
Some contributions may be reinforced, while others may cancel each other out. A quantum algorithm exploits these effects to change the probabilities of the final results.
The goal is not to try all the answers and then read the right one directly. It is a question of designing a succession of operations that increases the probability of the desired results.
This mechanism is one of the reasons why some quantum algorithms can solve specific problems more efficiently than the best known classical methods.
4. What happens when multiple qubits work together?
So far, we've tracked only one qubit. However, quantum computers use several of them to perform more complex calculations.
With two classical bits, we can represent four possible combinations: 00, 01, 10, and 11.
Two qubits can also be in a superposition of these four reference states. Quantum operations can transform them together and create relationships between them that do not exist in a system of independent bits.
Quantum entanglement
One of these phenomena is called quantum entanglement.
When multiple qubits are entangled, their overall state cannot be described as a simple combination of independent states for each qubit.
Imagine two qubits in an entangled state that, when measured in a given database, give the results 00 or 11. In this ideal example, both outcomes are equiprobable.
We then observe correlated results: when the first qubit gives 0, the second also gives 0 and when it gives 1, the second also gives 1.
This correlation does not mean that each qubit simply has a hidden classical value that we do not know. It results from the quantum state shared by the two qubits.
Entanglement is an important resource for some quantum calculations. However, it does not automatically guarantee that an issue will be resolved more quickly.
It is the combination of quantum operations, superposition, interference and, depending on the algorithms, entanglement that can lead to an advantage.
5. What are quantum computers used for?
Now that we understand the principle, a question arises: what problems can really benefit from this technology?
Quantum computers are not more efficient for all tasks. Their interest is focused on certain categories of problems.
Simulating molecules and discovering new materials
Atoms and molecules themselves obey the laws of quantum physics.
Simulating them accurately can require considerable resources from a classical computer, especially when interactions become complex.
A quantum computer could make it possible to better model certain molecules, study chemical reactions or explore materials with particular properties.
These applications are particularly promising for scientific research, but their potential still depends on the performance of the machines and algorithms available.
Advancing pharmaceutical research
One of the things that drug design requires an understanding of the interactions between molecules.
Quantum calculations could contribute to some difficult chemical simulations. However, they would not replace laboratory experiments or the tests needed to verify the efficacy and safety of a treatment.
Modifying some cryptography methods
Some quantum algorithms can solve precise mathematical problems more efficiently than known classical methods.
The most famous example is Shor's algorithm, which could allow a sufficiently powerful quantum computer to factor large integers and threaten some current cryptographic systems.
The quantum computers available today cannot instantly break all computer protections. Much more reliable and powerful machines would be needed for some large-scale attacks.
This is why post-quantum cryptography methods are being developed to withstand known quantum attacks.
Fix some optimization issues
Researchers are also studying applications in logistics, finance and other fields that involve many possibilities.
However, we must remain cautious: a complex problem is not automatically easier to solve with a quantum computer. A real benefit must be demonstrated for each application.
6. Why is it so difficult to build a quantum computer?
While the principles of quantum computing can be explained simply, their implementation is extremely delicate.
A qubit must retain its quantum state and be manipulated with great precision. However, it is sensitive to involuntary interactions with its environment.
These interactions can disrupt the computation and degrade the necessary quantum properties: this is the phenomenon of decoherence.
Highly demanding technologies
Several approaches are being explored to make qubits, including:
Superconducting circuits
Ions trapped by electromagnetic fields
neutral atoms
photons
Each technology has its own advantages and constraints.
Some quantum computers with superconducting circuits need to be cooled to extremely low temperatures, close to absolute zero. Other systems use different physical devices, but all of them must control the qubits with high precision.
Correcting errors
Quantum operations and measurements are not perfect. Errors may appear during the calculation.
To limit them, researchers are developing methods for correcting quantum errors. They usually rely on several physical qubits to protect a unit of information called a logical qubit.
This requires a lot of material resources and is one of the major challenges in the field. Building a truly useful quantum computer is therefore not simply a matter of increasing the number of qubits: it is also necessary to be able to control them and correct their errors.
7. Will a quantum computer replace classical computers?
No, because the two technologies meet different needs.
Conventional computers are very good at everyday tasks: browsing the Internet, running software, playing video games, or processing documents.
Quantum computers are designed for certain categories of computations that can take advantage of quantum properties.
In the future, they could work as a complement to conventional supercomputers. A classical computer would handle the usual tasks and prepare the data, while a quantum processor would perform some specialized calculations when a relevant advantage is possible.
This complementarity will depend on advances in hardware, error correction and the development of really useful algorithms.
FAQ: Frequently asked questions about quantum computers
Is a qubit worth 0 and 1 at the same time?
A qubit can be in a superposition of states 0 and 1. But when measured in the usual base, only one classic result is obtained. Superposition describes the quantum state before measurement; it does not allow two answers to be read simultaneously.
Is a quantum computer faster than a classical computer?
Not systematically. Its advantage depends on the problem and the algorithm used. For many everyday tasks, a classic computer remains the most suitable solution.
Why do you sometimes need to cool a quantum computer?
Some qubits, especially those made with superconducting circuits, require extremely low temperatures to maintain the physical properties essential for their operation.
Can you use a quantum computer at home?
It is not realistic to make a general-purpose quantum computer at home comparable to laboratory machines. On the other hand, simulators and online platforms make it possible to learn how to program quantum circuits and, in some cases, to run them on real machines that can be accessed remotely.
Conclusion
A quantum computer doesn't work like a classical computer, which is simply faster. It uses qubits whose states can be superimposed, transformed by quantum operations, and combined through phenomena such as interference and entanglement.
The measurement finally provides a classic result. The challenge is to prepare and transform quantum states in such a way as to make the desired results sufficiently probable.
This approach could advance the simulation of molecules, the search for new materials and some mathematical calculations. But quantum computers still face significant technical challenges, including qubit fragility and error correction.
Quantum computing will therefore not replace classical computing as a whole. Rather, it could become a complementary tool, particularly valuable in solving some difficult problems.
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