How a Quantum Computer Works

 

Introduction

Quantum computers represent a revolutionary leap from traditional digital technology, which relies on bits being either 0 or 1. Instead, quantum computers harness the principles of physics, specifically three fundamental elements: superposition, measurement, and entanglement. More about this is discussed in this blog about Quantum Computing Fundamentals.

As William Philips (1997) eloquently put it, the difference between a quantum computer and a classical computer is greater than that between a classical computer and a wooden abacus. This highlights how fundamentally distinct quantum computers are from their classical counterparts. They operate differently, look different, and perform tasks that classical computers cannot.

In a classical computer, a bit is either a 0 or a 1. However, in a quantum computer, a qubit can exist in a state of superposition, meaning it can be both 0 and 1 simultaneously, as well as any state in between. When a qubit is measured, it collapses to either |0⟩ or |1⟩. Entanglement, another key feature, allows qubits to be interconnected regardless of distance; if one qubit is measured as |0⟩, its entangled partner will instantaneously be |1⟩.

Superposition is crucial for both scaling and simultaneous processing. For scaling, a single qubit can represent more information than a classical bit. For example, two qubits can represent four classical bits (2^2), and ten qubits can represent 1024 classical bits (2^10). For simultaneous processing, superposition enables a quantum computer to evaluate all possible outcomes at once and present the optimal solution, whereas a classical computer must evaluate each outcome sequentially, which is far more time-consuming.

Currently, achieving commercial viability for quantum computers requires reaching one million qubits. It is estimated that 100 million qubits would be needed to break most encryption methods used on the internet within a day.

1. How qubits are created

Qubits, or quantum bits, are the fundamental units of quantum computers. They can be made from various quantum materials such as electrons, photons, ions, and atoms. These materials exhibit quantum properties and have distinct energy levels, typically referred to as the ground state (|0>) and an excited state (|1>). The energy levels of these materials can be manipulated using different methods, such as laser light or magnetic fields.

Each quantum material behaves like a wave, with properties such as amplitude and phase. When waves interact, they can either reinforce each other (constructive interference) or cancel each other out (destructive interference).

In quantum mechanics, qubits can exist in a superposition, meaning they can be in a state that represents both |0> and |1> simultaneously, or a qubit is in the middle of  |0> and |1>. When a qubit is measured, it collapses from this superposition into one of the definite states, either |0> or |1>, based on its energy level. This measurement process is delicate, as it can change the state of the qubit.

Qubits have a property called coherence time, which is the duration they can maintain their quantum state before decaying to the ground state (|0>). This time is typically very short, often in the microsecond range, requiring quick and precise readout methods. Environmental factors, such as temperature changes and noise from the quantum computer's hardware, can affect the qubit's energy levels and coherence time.

To manipulate qubits, quantum gates are used. These gates control the interference patterns of the quantum waves, altering the probabilities of measuring a |0> or |1>. The choice of gate depends on the specific hardware and the qubits involved.

Qubit states in different energy levels, by IBM (2021)

More about this is discussed in this blog about Quantum Computing Fundamentals.

2. Types of Quantum Computers

At this moment, there are applications for each of these approaches, and one is not necessarily better than the other. It could be possible that one type is better for one scenario and another type is better for another scenario. It also depends on how these types evolve; solving scalability issues in ion computers or control issues in photon computers would be a massive improvement for these types.

An important limitation is that the software is specifically written for a type of quantum computer. Selecting a vendor should be outcome-based because you buy the hardware and the software from one vendor. It is not possible to run software from one vendor on the computer of another vendor.

The definition of a quantum computer is a system that has these five characteristics, called the DiVincenzo Criteria:

1. A quantum computer must be a scalable system with characterized qubits.

2. A quantum computer must be able to bring a qubit to a stable starting position. The operations on a qubit may not depend on the starting position of a qubit.

3. The qubit needs to be measured to read out the value.

4. The coherence time must be long enough for a reliable readout.

5. A quantum computer needs quantum gates to perform operations on qubits. The most important ones need to be supported.

Read more about quantum gates in this blog about Quantum Computing Fundamentals.

2.1 Quantum Computers Based on Electrons

The most well-known quantum computers are based on electrons as qubits. Qubits made of electrons are closest to a classical computer because they also use electronic waves, and existing technology can be used. There are a lot of investments in these quantum computers, and great advances have been made, such as a high number of qubits and high scalability (IBM, 2021).

Drawbacks include sensitivity to errors, a refrigerator that cools the computer down to a lower temperature than found in outer space, difficult control of qubits, and qubits can only connect to their neighbors and not to other qubits.

2.2 Quantum Computers Based on Trapped Ions

Ions are created by removing one electron from the atom, making them charged. Ions can be controlled in a magnetic field to create qubits. Qubits made of ions are less complex to control, more stable with fewer errors, have a long coherence time, can connect to all other qubits, and entanglement is easier (IonQ, 2025).

Drawbacks compared to electron-based quantum computers: Ion-based quantum computers are less scalable, and gate operations are slower.

2.3 Quantum Computers Based on Photons

Photons are widely used for various purposes, including reading out qubits. However, it is also possible to use photons to create qubits. Quantum computers based on photons can work at room temperature and use existing optical technology.

Drawbacks compared to electron-based quantum computers: Photon-based quantum computers are less scalable, have limited quality of qubits, and are difficult to control.

3. How to build a Quantum Computer

3.1 Electron based Quantum Computer

When we think of a quantum computer, we think of an futuristic chandelier as shown in the introduction of this blog. In reality, this quantum computer must be cooled down in a refrigerator, which looks like a tube. The chandelier hangs in the tube.

Quantum computers at IBM Quantum as of 2023 (Researchgate, 2025)

This type of quantum computer has several important parts: the processor, the refrigerator, microwave electronics, cryogenic isolators and amplifiers, and a classical computer. These components control qubits, cool the quantum computer, and reduce interferences. A quantum computer does not have storage or memory at this moment; these are all part of the classical computer.

The most important part of the quantum computer is the quantum processor. The quantum processor looks like a classical processor—a square, flat chip with small lines on it. The more qubits it has, the larger the processor is. As discussed, the qubits need to be cooled to -273 degrees Celsius (-459.67°F), which is near absolute zero (Exoswan Insights, 2025). The quantum processor is located at the bottom of the chandelier.

To cool the quantum processor, it is placed in the dilution refrigerator. The quantum processor is in one of the coldest places in the universe (Exoswan Insights, 2025). The dilution refrigerator has a cylindrical form to cool evenly. Quantum computers must be that cold to enable superconductivity, allowing qubits to remain longer in their state, preventing errors due to thermal changes, and making it easier to control qubits. The components higher in the chandelier require less cooling, while those lower in the chandelier require more cooling.

To interact between the program and the processor, microwave electronics are used. They convert algorithms to qubit instructions and are connected to the dilution refrigerator (Exoswan Insights, 2025).

On the outside of the quantum computer, cryogenic isolators and amplifiers are used to shield from outside interferences (Exoswan Insights, 2025).

A classical computer interface is used to program the quantum computer, send tasks, and receive results (Exoswan Insights, 2025).

3.2 Trapped Ion based Quantum Computer

The trapped ion-based quantum computer does not need cooling but does require a cleanroom.

To manipulate specific atoms, a vacuum is created because air contains many different atoms. The vacuum is created with an ultrahigh vacuum system, constructed of metal and glass. The ultrahigh vacuum needs 2-4 weeks to reach the desired vacuum level at temperatures of 200 degrees Celsius (392°F). This process takes time because not only the air needs to be pumped out, but also all other atoms that have entered the system. To strip an electron from an atom, lasers are used. The atom needs to have certain characteristics that make this process easier; therefore, Ytterbium (Yb) is used. Several lasers with different colors, such as ultraviolet, blue, visible light, and infrared, are needed (UPM Quantum Computing Technology, 2021).
To hold ions in place, a magnetic field is used. The ions are trapped in the magnetic field, which is part of the quantum processor used to change the states of trapped ions. To change the states of trapped ions, laser pulses are used. To measure the qubit state, a laser is used, and the photons that bounce off are counted. A high count results in |1>, and a low count results in |0>. Trapped ion computers can measure several qubits at the same time (UPM Quantum Computing Technology, 2021).

After measurement, all qubits are reset to the ground state, which is |0>, and the processor is ready for the next computation (UPM Quantum Computing Technology, 2021).

The software stack interfaces with a system controller, which is connected to the quantum processor (UPM Quantum Computing Technology, 2021).

Trapped Ion Quantum Computer (IonQ, 2023)

3.3 Photonics based Quantum Computer

A photonics-based quantum computer is able to work at room temperature. There are three main parts: a light source, a processing unit, and a detection syste. Only the detector needs to superconduct and be cooled down, but this is a small part of the computer. The photon-based qubits work with a method called "squeezed light," where a reduction of uncertainty in one aspect of the wave increases the uncertainty in another aspect of the wave. The quantum state can have different numbers of photons, each with its own probability. When those photons are sent through an interferometer, those probabilities can be calculated. The quantum states are generated with a phase-stabilized dual-frequency pulsed laser (Thoss et al., 2024).

The quantum gates manipulate photons with beam splitters, mirrors, and phase shifters.

Photonics-based quantum computers can have a modular setup, where you start with one quantum computer with a number of qubits and add another server when you need more computational power. This is comparable to adding classical servers in a cluster. Another setup is where every part of a quantum computer is housed in its own box, which are connected together with optical fibers (Yirka, 2025).

Xanadu Aurora System (Yirka, 2025)

4. Conclusion

In conclusion, quantum computers represent a revolutionary leap from classical computing, leveraging the principles of quantum mechanics to perform complex calculations at unprecedented speeds. Unlike classical computers, which rely on binary bits, quantum computers use qubits that can exist in multiple states simultaneously due to superposition. This, combined with entanglement and quantum gates, allows quantum computers to process vast amounts of data more efficiently.

The development of quantum computers involves various approaches, including electron-based, ion-based, and photon-based systems, each with its own advantages and challenges. While electron-based quantum computers are currently the most advanced, ion-based and photon-based systems offer promising alternatives with unique benefits. 

As the technology continues to evolve, the potential applications of quantum computing are vast, ranging from breaking encryption to solving complex optimization problems. However, significant challenges remain, particularly in achieving scalability and error correction. The future of quantum computing holds immense promise, and ongoing research and development will be crucial in unlocking its full potential.

Sources

ETH Zurich (n.d.). Sources of Decoherence. https://qudev.phys.ethz.ch/static/content/courses/QSIT11/QSIT11_V08_slides.pdf

Exoswan Insights. (2025, 24 January). Inside a Quantum Computer: A Visual Guide. https://exoswan.com/quantum-computer-visual-guide

IBM (2021, 5 October). Lecture 9.1 - Introduction to Quantum Hardware [Video]. YouTube. https://www.youtube.com/watch?v=PI8PqARCwWo

IonQ Forte: the first Software-Configurable Quantum computer. (2023, 3 March). IonQ. https://ionq.com/resources/ionq-forte-first-configurable-quantum-computer

IonQ (2025). IonQ Harmony. https://ionq.com/quantum-systems/harmony

MIT (2023, 25 September).New qubit circuit enables quantum operations with higher accuracy.  https://news.mit.edu/2023/new-qubit-circuit-enables-quantum-operations-higher-accuracy-0925

Researchgate (2025). Quantum Computers at IBM Quantum as of 2023 available on the cloud. https://www.researchgate.net/figure/Quantum-computers-at-IBM-Quantum-as-of-2023-available-on-the-cloud-The-actual-quantum_fig2_382228776

Thoss, A., Raue, H., Mitn, D., Sevilla,C. (2024, 6 August).How to Build a Photonic Quantum Computer. https://www.photonics.com/Articles/How_to_Build_a_Photonic_Quantum_Computer/a70182

UPM Quantum Computing Technology. (2021, 18 July). Building a Quantum Computer with Trapped Ions [QCT20/21, Seminar #09] [Video]. YouTube. https://www.youtube.com/watch?v=GkAHI_OeJkE

Yirka, B. (2025, 31 January). World’s first scalable, connected, photonic quantum computer prototype developed. Phys.org. https://phys.org/news/2025-01-world-scalable-photonic-quantum-prototype.html