What Is Quantum Computing? A Simple Explanation

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What is quantum computing in simple terms?

Quantum computing is how quantum computers perform calculations. So a quantum computer is the device, while quantum computing refers to the technology and computing method behind it.

A quantum computer works differently from a classical computer. Where a regular computer works with bits that are 0 or 1, a quantum computer uses qubits.

Those qubits can process information in a different way. Because of that, quantum computers can approach some very specific problems differently than regular computers. Think, for example, of calculating molecules for research into new medicines and materials, or certain math problems that require a lot of computing power for classical computers.

That does not mean a quantum computer is automatically faster. For everyday tasks like browsing the internet, video calling, using spreadsheets, or running servers, a classical computer is still much better.

Quantum computers will probably not replace regular computers. In practice, both types of computers will more likely be used side by side, with a quantum computer only being used for problems where that approach can really offer an advantage.

Key Takeaways

Quantum computing uses qubits instead of only regular bits.
Qubits can show properties like superposition and quantum entanglement.
A quantum computer is not faster than a classical computer for every task.
The technology can be especially useful for specific complex problems.
A sufficiently powerful quantum computer can eventually create risks for cryptography and crypto.

How does quantum computing work?

A quantum computer performs a calculation with qubits. First, those qubits are put into a certain starting state. Then the computer performs operations on the qubits, and at the end they are measured.

Those operations are called quantum gates, or gates. You can think of them as instructions that change the state of a qubit. By carrying out several of these instructions one after another, a quantum algorithm is created.

During a calculation, qubits can also influence each other. Quantum algorithms use that to make the chance of certain outcomes bigger or smaller. The goal is for the desired outcome to appear more often when the qubits are measured.

After the measurement, you get regular values back, like 0 and 1. So a quantum computer does not automatically give all possible answers at once. The algorithm has to be built in a way that gives a useful outcome a high chance of being measured.

What are qubits?

A qubit is the basic unit a quantum computer uses to process information. You can compare a qubit to a bit in a regular computer.

A regular bit is either 0 or 1. A qubit works differently. Before you measure it, its state can be such that both 0 and 1 are possible outcomes. Only when you measure the qubit do you see one outcome: 0 or 1.

Qubits are not regular computer parts. Researchers can make them, for example, with very small electrical circuits, atoms, ions, or light particles.

The big problem is that qubits are very sensitive to their environment. Heat, vibrations, and electromagnetic noise can disrupt a calculation. That is why it is hard to build large quantum computers that can compute reliably long enough.

What is superposition?

Superposition is the name for the phenomenon where a qubit is not simply fixed at 0 or 1 before measurement.

You can think of it as multiple possible outcomes being part of the same quantum state. During measurement, you still only get one outcome in the end.

With two qubits, for example, four outcomes are possible:

A quantum computer can work during the calculation with a state in which all these possibilities play a role. That does not mean you can read out all four answers at the same time afterward. After a measurement, you get one outcome.

That is why a quantum algorithm has to guide the calculation so that a useful outcome has a greater chance of being measured.

Example: Suppose two qubits can eventually produce 00, 01, 10, or 11. A good quantum algorithm tries to make the chance of the desired outcome, for example 10, as large as possible.

What is quantum entanglement?

With quantum entanglement, two or more qubits become so connected that you can no longer fully describe their state separately.

For example, suppose two qubits are entangled in such a way that a measurement always gives 00 or 11. If you measure the first qubit as 0, then in this example the other one will also measure as 0. If you get 1, then the other one will also be 1.

Quantum computers can use these connections during calculations. Together with superposition and other quantum phenomena, this makes calculations possible that work differently from a classical computer.

By the way, quantum entanglement does not mean you can send information faster than light. You cannot choose which measurement outcome you get.

What is the difference between quantum computers and classical computers?

The main difference is that a classical computer works with bits, while a quantum computer uses qubits. A regular bit is 0 or 1. A qubit follows the rules of quantum mechanics and can therefore process information in a different way.

Quantum computers use phenomena like superposition and quantum entanglement. They also work with special operations called quantum gates.

That does not mean a quantum computer is automatically better or faster. For most everyday tasks, classical computers are much more suitable. Think of browsing the internet, processing files, video calling, or running servers.

Quantum computers are especially interesting for a limited number of problems for which special quantum algorithms exist.

Another difference is that qubits are much more sensitive to disturbances than regular bits. Heat, electromagnetic noise, and other environmental influences can cause errors. That is why it is difficult to build large quantum computers that keep computing reliably long enough.

Also, the number of qubits by itself does not say how much a quantum computer can do. It also matters, for example, how often errors happen, how well the qubits can work together, and how well those errors can be corrected.

In the future, quantum computers and classical computers will probably be used side by side. A classical computer can, for example, handle the regular processing and control, while a quantum computer only performs a specific part of a difficult calculation.

What can quantum computing be used for?

Quantum computing can be especially interesting for problems that are very difficult for classical computers.

One important example is simulating nature itself. Molecules and materials behave according to quantum mechanics on a very small scale. A quantum computer may therefore be suitable in theory for modeling such systems.

Possible applications include, for example:

research into new medicines;
discovering new materials;
simulating chemical reactions;
certain planning and optimization problems;
certain mathematical and cryptographic calculations.

In optimization, you can think of finding a good route, production planning, or logistics distribution. A lot of research is being done on this, but quantum computers do not yet have a proven practical advantage for every kind of optimization problem.

A well-known example from mathematics is Shor's algorithm. That is a quantum algorithm that a sufficiently powerful quantum computer could use to solve certain mathematical problems much more efficiently than known classical methods.

That is especially important for cryptography, because some security methods rely on the fact that such calculations are extremely difficult for regular computers.

Many quantum computing applications are still in the research phase. So an experiment in which a quantum computer performs one specific task better does not automatically mean the technology is already useful for businesses or consumers right away.

What impact does quantum computing have on cryptography?

Quantum computing can have consequences for some forms of cryptography in the future.

Public-key cryptography is especially important. In that system, you have a public key that you can share with others and a private key that must stay secret.

The security of these systems is based on mathematical problems that regular computers cannot solve in a useful amount of time.

A sufficiently powerful quantum computer could solve some of those problems much more efficiently with Shor's algorithm. That could make certain forms of encryption and digital signatures vulnerable.

That can have consequences in two ways.

With encryption, someone may be able to decrypt information that was supposed to stay secret.

With digital signatures, someone may be able to forge a valid signature without having the real private key.

Not all cryptography is affected in the same way. Symmetric encryption and hash functions are not attacked in the same fundamental way by Shor's algorithm.

For those systems, Grover's algorithm is especially relevant. It can theoretically speed up certain search problems, which may mean a larger security margin is needed.

Right now, there is still no quantum computer that can practically break widely used cryptography on a large scale.

Still, there is already a risk for information that needs to stay secret for years. An attacker can store encrypted data today and hope to decrypt it later with a more powerful quantum computer. This is called harvest now, decrypt later.

Which cryptographic methods can become vulnerable?

Cryptography that relies on factorization or the discrete logarithm problem can especially become vulnerable to a sufficiently powerful quantum computer.

RSA is a well-known example of that. The security of RSA relies on the fact that it is very difficult for regular computers to split very large numbers into their factors.

Diffie-Hellman, ECDH, DSA, ECDSA, and Schnorr signatures can also become vulnerable.

Many of these techniques use the discrete logarithm problem. That is a mathematical problem that is difficult for classical computers, but for which Shor's algorithm offers a much more efficient approach.

For crypto, elliptic-curve cryptography, or ECC, is especially important. Many blockchains use ECC to create digital signatures.

With such a signature, you prove that you are allowed to make a transaction without making your private key public.

If a future quantum computer could calculate the matching private key from a public key, an attacker might also be able to forge valid signatures.

Other techniques, such as AES and hash functions, are affected differently. Grover's algorithm can speed up certain searches, but it does not break these techniques in the same way Shor's algorithm can affect RSA and ECC.

What are the limitations of quantum computing?

Quantum computers still have major technical limitations.

One important problem is that qubits are very sensitive to disturbances. Heat, electromagnetic radiation, and other noise can change the quantum state.

That loss of the desired quantum state is called decoherence.

If too many errors happen during a calculation, the answer is no longer reliable. That is why quantum error correction is important.

In that process, multiple physical qubits are used together to form one more reliable logical qubit.

That means a quantum computer with many physical qubits may ultimately have far fewer usable logical qubits.

Scalability is also difficult. It is not enough to simply build more and more qubits. They also have to be controlled well, work together, and stay reliable long enough.

In addition, quantum computing does not offer an advantage for every problem. A quantum algorithm has to exist that can approach the problem in a useful way.

So quantum computers are not a general replacement for classical computers. They are mainly meant for specific types of calculations.

What risks does quantum computing bring for crypto?

For crypto, an important quantum risk lies in digital signatures.

Many blockchains use a public and a private key. You use the private key to sign transactions. The public key can be used to verify that signature.

A future powerful quantum computer may be able to calculate the matching private key from the public key for some cryptographic systems.

An attacker could then possibly create valid signatures themselves and move crypto without the real owner's permission.

How big that risk is exactly depends on the blockchain.

It depends, for example, on:

which cryptography the blockchain uses;
when a public key becomes visible;
how users use their crypto wallet;
how easily the protocol can be changed.

The transition to new security can also create risks.

Not only does the blockchain itself need to support new cryptography. Wallets, crypto exchanges, custody solutions, and other software also need to be updated in time.

New post-quantum signatures can also be larger than current signatures. That can affect transaction size, storage, bandwidth, processing time, and fees.

Systems outside the blockchain can also be affected. Think of encrypted communication, stored data, or other parts of the infrastructure around crypto.

There, harvest now, decrypt later can even become a problem sooner than on the blockchain itself.

Can quantum computing break Bitcoin?

Not with the quantum computers that exist now.

To attack Bitcoin practically with quantum computing, a much more powerful and fault-tolerant quantum computer is needed. Such a system does not exist right now, and it is not known when that will change.

Bitcoin uses elliptic-curve cryptography for digital signatures.

Traditional Bitcoin transactions use ECDSA with the secp256k1 curve. Since the Taproot upgrade, Bitcoin also supports Schnorr signatures.

Both techniques can theoretically become vulnerable to Shor's algorithm.

An important detail is when the public key becomes visible.

With certain Bitcoin addresses, only a hash of the public key is first stored on the blockchain. When you spend the bitcoin, the full public key becomes visible.

That means the risk does not work exactly the same way for every bitcoin.

So it is too simple to say that a quantum computer will automatically be able to "break Bitcoin" soon.

An attack depends, among other things, on how powerful the quantum computer is, which public keys are visible, how quickly such an attack can be carried out, and how the Bitcoin network responds.

Hash functions such as SHA-256 are also not affected in the same way as digital signatures.

Grover's algorithm can theoretically speed up certain attacks, but that is a different kind of risk than Shor's algorithm attacking elliptic-curve cryptography.

How can blockchains protect themselves against quantum computers?

The main solution is to replace vulnerable cryptography with cryptography that is supposed to resist quantum attacks.

That is called post-quantum cryptography.

For blockchains, this could mean adding new types of addresses and digital signatures.

Users can then move their crypto to addresses that use that new security.

An important concept here is crypto-agility. That means a system can replace its cryptographic security when needed.

That is not easy for a blockchain. The protocol has to be changed, and wallets, exchanges, and other software have to support the new rules.

Existing users also need to have the option to move their crypto safely.

For Bitcoin, it can also help not to reuse addresses and public keys unnecessarily. That keeps a public key visible for less time.

But that is not a complete solution. When a transaction is spent, the public key can still become visible.

There are also proposals to better prepare Bitcoin for a possible quantum threat.

One example is Pay-to-Merkle-Root, or P2MR. This proposal tries to limit the long-term exposure of certain public keys.

At the moment, it is only a proposal and not an active Bitcoin rule.

For full protection, digital signatures that are resistant to known quantum attacks would eventually also be needed.

What is post-quantum cryptography?

Post-quantum cryptography, often shortened to PQC, is cryptography designed to resist known attacks from both classical and quantum computers.

PQC runs on normal computers. So you do not need a quantum computer to use post-quantum cryptography.

That is different from quantum cryptography. That uses properties from quantum physics itself, for example to exchange keys.

Post-quantum cryptography, on the other hand, uses new math to better protect existing computers against future quantum attacks.

In 2024, the first major post-quantum standards were published.

These include:

ML-KEM for securely exchanging or agreeing on secret keys;
ML-DSA for digital signatures;
SLH-DSA for digital signatures based on hash functions.

These methods use different mathematical problems than RSA and elliptic-curve cryptography.

Post-quantum does not mean this kind of cryptography will be unbreakable forever. Cryptographic techniques will continue to be studied and tested.

For blockchains, the challenge is also not just choosing a new algorithm. Transaction size, verification costs, wallets, key management, and compatibility with existing systems also matter.

Conclusion

Quantum computing uses qubits to perform certain calculations differently from a classical computer. That can be especially useful for specific difficult problems, like simulating molecules and certain mathematical calculations. For everyday computer tasks, a classical computer is still much better.

For crypto, it is especially important that future quantum computers could weaken some forms of cryptography. That can affect the security of private keys and digital signatures. The quantum computers that exist today are still nowhere near powerful enough to practically break Bitcoin or widely used cryptography.

Still, work is already being done on protection against future quantum attacks. An important solution is post-quantum cryptography. For blockchains, such a transition means protocols, wallets, and other systems need to be updated in time and keep working well together.

What Is Quantum Computing? A Simple Explanation