What Is Quantum Mechanics and Why Do Its Laws Contradict Conventional Logic?

Quantum mechanics is a fundamental theory in physics that studies the laws of motion, interaction, and behavior of microscopic particles (atoms, molecules, electrons). In this microcosm, energy is emitted or absorbed not in a continuous flow but in discrete packets—quanta.

Although the entire universe ultimately adheres to quantum laws (which apply at any scale), understanding this "microcosm" from a conventional perspective is nearly impossible. At the atomic and subatomic levels, classical physics becomes inapplicable, and reality conflicts with everyday logic, reflected in several principles.

Quantum Superposition

In the familiar macroworld, objects always have a definite state: a transistor switch is either on or off—"0" or "1". A quantum system, while isolated, exists in superposition—a mathematical combination of all its possible states. This means that until measurement occurs, the reality of an electron does not have a single clear value and is described only by a spectrum of probabilities.

Wave-Particle Duality and Heisenberg's Uncertainty Principle

Micro-objects are neither classical solid particles nor classical waves, but can exhibit properties of both depending on how they are observed. According to Heisenberg's uncertainty principle, the inability to simultaneously determine a particle's position and momentum precisely is a consequence of the fundamental quantum nature, not a limitation of measuring instruments.

The Observer Effect

At the quantum level, "observation" is a physical interaction with the system. While isolated, superposition is maintained. Measurements force particles to transition to one of the possible states—manifesting decoherence.

Visualization Through Probabilities

Quantum objects cannot be observed in the conventional optical range; however, scientists have learned to "see" them using scanning tunneling microscopes. These visualize not the particle itself but rather a "map" of where it is most likely to be found, represented as electron clouds, confirming the wave nature of matter.

What Is Quantum Computing?

In the 1920s, physicists Werner Heisenberg, Erwin Schrödinger, and Paul Dirac developed a theory that explained and connected the observable effects of the micro-world with mathematical laws. The main tool was the wave function, which defines the amplitude of states. Based on this, scientists were able to calculate probabilities and use equations to represent their changes over time.

Decades later, researchers learned to apply quantum mechanics to data processing. Quantum computing systems (QCS) operate using qubits (quantum bits).

Classical computers are based on regular bits—categorical values that always have a strict value: either "0" or "1". The key difference with qubits lies in the phenomenon of superposition, which removes the classical rigidity of the system. Until a physical measurement is made, the element is not fixed in any of the basic states. Instead, it represents a complex spectrum where the amplitudes of probabilities mathematically intertwine, ultimately yielding a result of either zero or one.

To combine qubits in QCS, quantum entanglement is used—a strict mathematical correlation between particles. Measuring one qubit correlates with the result of measuring another if the system was entangled. This does not allow information to be transmitted faster than the speed of light, but it exponentially increases the computational space of the system: each new qubit doubles the dimensionality of the possible states.

Scientists have learned to manipulate qubits using microwave pulses and targeted laser shots. Today, several parallel hardware platforms are developing in the industry:

  • Superconducting circuits (IBM, Google)—microscopic circuits made from special metals, cooled to nearly absolute zero;
  • Ions in traps (Quantinuum, IonQ)—charged atoms "suspended" in a vacuum by electromagnetic fields. Data is read using lasers;
  • Photons (PsiQuantum, Xanadu)—computations using particles of light on optical chips, resistant to interference and capable of operating at room temperature;
  • Spin qubits (Intel, Diraq)—using spin—the internal quantum magnetic property of an individual electron trapped in a silicon structure;
  • Neutral atoms (QuEra, Pasqal)—uncharged atoms (rubidium) captured by lasers ("optical tweezers"). This architecture allows for efficient scaling of logical qubits.

What Is a Quantum Computer and What Are Its Advantages?

Quantum supremacy is a milestone in technology development where a quantum computer solves a problem in minutes that would take the most powerful classical supercomputer thousands of years. Achieving this does not mean a victory in the speed of computations for every task, but rather pertains to specific implementations where the technology excels.

A classical computer is a fast but straightforward calculator. When solving a complex problem, such as cracking a cryptographic key, it must iterate through possible options one by one or use deterministic algorithms. In QCS, a fundamentally different mathematics is employed to achieve supremacy: they do not work with ready-made numbers but manipulate the probabilities themselves.

To manage this spectrum of probabilities, low-level programming of particles (ions, photons) is applied using quantum gates. In a conventional processor, a gate is a logical operation implemented by a silicon transistor that either allows or blocks current. In a quantum computer, it represents a physical interaction. This is a precisely calculated microwave or laser pulse that "rotates" the qubit, changing its state.

IBM Quantum Nighthawk quantum processor with 120 qubits. Source: IBM.

By arranging a sequence of such pulses, engineers develop an algorithm based on two principles:

  • Colossal Space for Maneuvering. A conventional processor works only with specific bit values at any given moment. In a quantum computer, adding each new qubit doubles the volume of the space. A processor with 300 logical qubits operates with a number of options exceeding the number of atoms in the entire visible universe. The machine does not "store" all this data but gains a vast field for complex algorithmic maneuvers;
  • Wave Control (Interference). Quantum algorithms configure the system for the mutual cancellation of all incorrect answers (destructive interference). To visualize, one can imagine stones thrown into water: the waves generated can collide and cancel each other out or resonate and merge into one large wave. By the time data is read, the probability of the correct answer mathematically increases.

This synergy makes quantum computing unparalleled in a narrow but extremely complex class of tasks: from accurately modeling chemical molecules to factoring large numbers using Shor's algorithm. On a sufficiently powerful logical quantum computer, this algorithm could theoretically break asymmetric cryptography—specifically, the ECDSA algorithm and the elliptic curve secp256k1, which underpins transaction signatures in the Bitcoin network.

How Is a Quantum Computer Physically Structured?

The appearance of a quantum machine entirely depends on the physical principles governing its qubits. Currently, two architectural approaches dominate the industry:

  • Cryostats ("chandeliers"). This design is used for superconducting qubits (IBM, Google). The "chandelier" is a dilution refrigerator where the quantum chip is located on the lowest tier, cooled to nearly absolute zero by a mixture of isotopes of liquid helium. During operation, the entire structure is hidden in a vacuum chamber that shields it from electromagnetic radiation and external noise;
  • Optical and Ion Systems. QCS using trapped ions or photons look different. Their main tools are laser systems, optical fibers, and vacuum chambers. Photonic processors typically do not require extreme cold, allowing their control electronics to be mounted in standard server racks. However, ion traps require maintaining a deep vacuum, and superconducting single-photon detectors still need to be cooled to cryogenic temperatures.
IBM Quantum Computer. Photo: Charlotte Hu/Popular Science.

Due to the complexity of installation and maintenance, cloud access remains the primary means of interacting with quantum computers.

Given the rapid development of quantum computing, the industry has moved beyond creating individual machines. Cisco has begun developing quantum infrastructure: specialized routers, optical channels, and communication systems. Their goal is to eventually connect disparate QCS into a global network and ensure secure data transmission using quantum key distribution (QKD).

QKD renders networks immune to interception, as any attempt to copy a key immediately destroys its state and alerts the parties to the interference due to the observer effect.

How Do Logical Qubits Address the Problem of Decoherence?

The main barrier to the widespread adoption of QCS is the incredible fragility of quantum states. Any interaction with the environment, even the slightest, causes a qubit to rapidly lose its quantum properties. This fatal process for computations is called decoherence. During this process, the system loses its ability to interfere, making further execution of quantum algorithms impossible.

For a quantum system, any external background, generally referred to by physicists as "noise," is detrimental:

  • Thermal Energy. Heat is the chaotic motion of atoms. To prevent a superconducting qubit from being disturbed by its own vibrations, the chip is cooled;
  • Electromagnetic Radiation. A random cosmic ray, background from a Wi-Fi router, or changes in the Earth's magnetic field can instantly destroy superposition;
  • Mechanical Vibrations. Microseismic vibrations of a building can easily disrupt the precise alignment of mirrors for lasers in optical and ion computers.

Due to decoherence, physical qubits can only survive for fractions of a millisecond, accumulating errors faster than the algorithm can complete.

To address this issue, engineers are developing logical qubit systems—a software-hardware method for error correction. In this approach, quantum information is distributed across the system: to obtain one reliable (logical) qubit capable of performing long computations, hundreds to thousands of unstable units must be combined into a network.

The industry has transitioned from the NISQ era to the early fault-tolerant era (FTQC), where software error correction has become the main metric of progress, replacing the simple race for the number of "raw" physical qubits.

Creating a machine with a sufficient number of logical elements is the industry's primary goal for the next decade.