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9. 3. 2026

Security Cryptography

Comparing Digital and Quantum Computers, Part 5: How Quantum Computer Works

Everyone in the IT world is wondering how big a threat quantum computers pose to cryptography and how to deal with the problem. This series of articles tries to explain the problem in a popular way. After explanation of quantum computer attack it is appropriate to understand how quantum computers actually work.

required to attack current asymmetric algorithms using quantum computersu

These are sophisticated devices that usually use extremely low temperatures to achieve the necessary state of matter, with the exception of photonic quantum computers. In order to work with it, it is necessary to perform seemingly contradictory steps, to limit its energy and at the same time bring it into an excited state. The goal is to create isolated islands in which three basic properties of the quantum world can manifest themselves. These are superposition, entanglement and interference. Such an island (qbit) can achieve all possible states in the case of superposition, i.e. logical 1, logical 0 and everything in between. If there are more qbits, these two points can simultaneously contain the values 00, 01, 10 and 11. In addition to these states, specific states can of course also contain indeterminate states, i.e., in simple terms, everything between 00 and 11. It is possible to continue in the same way. The states are probabilistic, they can interfere with each other, i.e., act on each other. The result of the calculation can only be determined by measurement.

Unlike a classical computer, where each bit contains one value, a qbit contains both. As a result, to store 256 bits during a calculation, we need 256 digital bits on a digital computer, but only 8 qbits. It is this principle, going through all the possibilities, that enables the extreme computing power provided by quantum computers. At the same time, it is a certain disadvantage, because due to the probabilistic nature of the calculation, it is necessary to repeat it several times.

Although we currently call it a quantum computer, this is not entirely accurate. These computers are more like field-programmable gate arrays (FPGAs) that are programmed for specific requirements. At the moment, it is not possible to enter parameters directly as part of the program and it is necessary to provide this data by setting the circuit structure. This allows us to imagine the calculation as a series of waves, more precisely a probability wave, that passes through the circuit. The circuit will allow some waves to pass through and amplify their result, and will not allow others to pass through.

How do we actually imagine the behavior of an individual qubit? From computers, we are used to a bit that has specific values of 0 or 1, nothing in between. On or off. But when working with qubits, this does not apply. The state of a qubit can be imagined as the surface of a globe. There is a one at the south pole, a zero at the north pole. During the calculation, the value can be anywhere on the surface, but when measured, it "falls" to the north or south pole. Similarly, particles can have an orientation in one of the x, y or z axes, which again points to the surface of the globe, i.e. any place. However, the descriptions given are just different views of the same matter from a computer science, mathematical and physical point of view.>

Types of gates and theirspeed

In quantum computers, it is possible to encounter single-qubit gates, such as Hadamard (H), Pauli gates (X, Y, Z), qubit rotation. Then there are two-qubit gates, mainly CNOT, CZ, iSWAP and others. Their speeds depend on the technology used and they can be roughly distinguished as follows.

Technology	Single-qubit	Two-qubit	Comment
Superconducting qubits	10–50 ns	100–400 ns	Very fast, shorter coherence
Ion traps	1–10 µs	50–300 µs	Slower, extremely accurate
Neutral atoms	0.5–5 µs	1–10 µs	Good scalability
Photonic qubits	~ps–ns	probability	Fast, heavy interaction
Topological qubits (theoretical)	ON	ON	Experimental

The theoretical upper limit of the switching speed of quantum gates (QSL – Quantum Speed Limit) should be in the order of picoseconds to tens of femtoseconds, i.e. in the range of 10^-12 s to 10^-14 s. Current technologies are approximately 4-8 orders of magnitude slower. Furthermore, there is a difference between the calculation speed in a quantum processor and the speed of information propagation (the interaction time, for example, the propagation of a microwave signal ensuring this interaction). In our universe, this cannot exceed the speed of light. It sounds a bit absurd, but the propagation of information ensuring setting the qubits to the desired state is limited. The next time is taken by the change (changing the orientation in Hilbert space), this time is again minimal. But what takes up the most time is the control electronics (1-10µs) and quantum correction codes. But the vast cooperation on the probabilistic calculation takes place (with the exception of the mentioned delays) thanks to the laws of the quantum world practically instantly.

Explanation of the delay on QEC and control electronics:
If several physical qubits form one logical one, it is necessary to be aware of the dependencies between the QEC cycles (Quantum Error Correction, 0.5-2 µs) and the corresponding error corrections at the logical qubit level. Measuring and decoding errors takes approximately 1-50 µs. Only then is it possible to process the logical qubit (built thanks to QEC over several physical ones) on the gate, where the resulting logical gate has a speed of roughly 10-200 µs. Note that the logical, not the physical, qubit is processed. The gate can therefore have a "speed" of around 100 µs. The electronics themselves create additional delays. First, it is a control, currently usually on FPGA (< 10 µs). In addition, signal generation, when using microwaves, requires another ~1 µs and a readout (decision) of approximately 10 µs. The speed of the gates is therefore greatly simplified in the example.

To be continued in the next section Shor's Algorithm (March 16th 2026)

References:

Suppressing quantum errors by scaling a surface code logical qubit
Source: https://www.nature.com/
Limitations in Quantum Computing from Resource Constraints
Source: https://journals.aps.org/
Exponentially tighter bounds on limitations of quantum error mitigation
Zdroj: https://www.nature.com/
Limitations of noisy quantum devices in computing and entangling power
Source: https://www.nature.com/
Practitioners' Rule of Thumb for Quantum Volume
Source: https://www.mdpi.com/
Quantum circuits — single-qubit and two-qubit gates
Zdroj: https://academic.oup.com/
Design and integration of single-qubit rotations and two-qubit gates in silicon above one Kelvin
Zdroj: https://www.nature.com/
Lieb–Robinson bounds — speed of information propagations
Zdroj: https://link.aps.org/
Speed limits and locality in many-body quantum dynamics
Zdroj: https://arxiv.org/
On propagation of information in quantum many-body systems
Zdroj: https://www.sciencedirect.com/

Autor článku:

Jan Dušátko

Jan Dušátko has been working with computers and computer security for almost a quarter of a century. In the field of cryptography, he has cooperated with leading experts such as Vlastimil Klíma or Tomáš Rosa. Currently he works as a security consultant, his main focus is on topics related to cryptography, security, e-mail communication and Linux systems.

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Blog

Comparing Digital and Quantum Computers, Part 5: How Quantum Computer Works

required to attack current asymmetric algorithms using quantum computersu

Types of gates and theirspeed

References:

Autor článku:

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