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Quantum computing is the field of computer science and technology which uses the strange and powerful rules of quantum mechanics to build computers that can solve problems too complex and time consuming for today’s regular computers.

Basic knowledge of quantum mechanics is necessary for understanding quantum computing. Quantum mechanics is a field of physics that studies how things behave at an atomic or sub-atomic level.  At this tiny scale the universe behaves very differently from what we are used to in our day-to-day lives.

Quantum computers use special particles called qubits instead of the regular bits found in a traditional computer. These qubits behave in unique ways that are described by the principles of quantum mechanics. Quantum computers use these rules to solve complex math problems that traditional computers can’t handle.

Strange rules of quantum mechanics

Four key principles of quantum mechanics are

• Superposition
• Entanglement
• Decoherence
• Interference

Superposition

Superposition is the principle of quantum mechanics that states a quantum particle can exist in multiple possible states at the same time. This is a very counter-intuitive concept because in our everyday world, an object can only be in one state at a time. For example, a light switch is either on or off, and a coin is either heads or tails. In classical computing a bit is either 1 or 0. But a qubit can be a 0, a 1, or a combination of both at the same time.

Quantum entanglement

Quantum entanglement is a strange and fascinating phenomenon where two or more particles become linked in such a way that they are no longer independent, even if they are separated by vast distances. Their states are so deeply connected that they must be described as a single, unified system. In an entangled system ,changing state of one particle automatically effects state of other particle.This principle enables qubits to correlate their state with other qubits. By  measuring a single entangled qubit, we can immediately determine information about other qubits in the entangled system.

Interference

In quantum mechanics, interference refers to how a quantum particle’s “probability waves” interact with each other.

In the classical world, interference is what happens when two waves—like sound or water waves—meet. When the crests(max) of two waves meet, they add up to create a bigger wave (constructive interference). When a crest(max) meets a trough(min), they cancel each other out (destructive interference).

In the quantum world, particles like electrons or photons also have wave-like properties. Their waves aren’t physical waves in the traditional sense, but “probability waves,” which describe all the possible paths a particle could take. When a particle is in a superposition of states (meaning it’s taking multiple paths at once), its probability waves can interfere with each other.

The famous Double-Slit Experiment:The double-slit experiment is the best example of quantum interference. In this experiment, electrons are fired one at a time at a wall with two slits. You would expect the electrons, acting as particles, to go through one slit or the other and create two simple bands on a screen behind the wall.

However, the electrons, acting as waves, pass through both slits at the same time and then interfere with themselves. This interference leads to a pattern of light and dark bands on the screen, known as an interference pattern, which is a clear sign of wave behaviour. This happens even when a single electron is fired at a time, proving that the particle is interfering with itself.

Once we try to observe which slit the electron goes through, the interference pattern disappears. Instead of behaving like a wave, the electrons act like particles, producing two distinct bands on the screen. It’s as if the act of measurement collapses the quantum possibilities into a single reality.

This is one of the most profound insights of quantum mechanics:observation affects outcome.

Decoherence

Decoherence refers to the phenomenon by which a quantum system collapses from a quantum state to a classical state. This process may occur deliberately through the measurement of the system or inadvertently as a result of environmental influences. In quantum computing, it is essential to minimise decoherence to maintain quantum coherence for reliable computation.

As we have seen in the double-slit experiment, when a measurement is made using a detector, electrons lose their quantum behaviour, collapse into a non-quantum state, and behave like classical particles.

How do quantum computers work?

Classical computers use bits to encode data, while quantum computers use qubits (quantum bits) as their basic units of information.

Qubits are generally created by manipulating and measuring quantum particles, such as photons, electrons, trapped ions, superconducting circuits and atoms.

Quantum computers uses hardware and algorithms that take advantage of the principles of quantum mechanics. A computation on a quantum computer works by preparing a superposition of computational states. A quantum circuit perform operations on entangled qubits and generate interference patterns, as governed by a quantum algorithm. Many possible outcomes are cancelled out through interference, while others are amplified. The amplified outcomes are the solutions to the computation.

In other words, a group of entangled qubits controlled by a quantum circuit performs computation by receiving inputs and producing outputs. Here outputs are generated when measurement is done on qubits. When measured, qubits collapse to a classical bit giving an output(decoherence).

Classical Bit vs. Quantum Qubit

A classical bit is like a light switch: it can be either on (represented as a 1) or off (represented as a 0). It can only be in one of these two states at any given time.

A qubit is different. Thanks to the principles of quantum mechanics, it can be in a state of superposition, meaning it can be a 0, a 1, or both at the same time! Think of it like a spinning coin. While it’s spinning, it’s not strictly heads or tails; it’s a combination of both. When you stop the coin (which is the equivalent of “measuring” the qubit), it will land on either heads or tails. Similarly, when a qubit is measured, its superposition collapses, and it will be read as either a 0 or a 1.

This “in-between” state allows a single qubit to hold significantly more information than a classical bit. For example, while two classical bits can represent only one of four states at a time (00, 01, 10, or 11), two qubits in superposition can represent all four states simultaneously. As you add more qubits, the number of states they can represent grows exponentially, which is what gives quantum computers their immense power.

In addition to superposition, qubits also make use of entanglement. This is a bizarre quantum phenomenon where the state of one qubit becomes linked to the state of another, even if they are far apart. When you measure one of the entangled qubits, you instantly know the state of its partner. This property allows quantum computers to perform complex calculations much faster than classical computers.

How qubits represent and process more data than classical bits?

In traditional computing, a  bit can be thought of as a piece of binary information,  as either a 0 or a 1. Modern computers typically represent bits using electrical voltage or current pulse  or by the electrical state of a flip-flop circuit.

Think of a binary processing system  which process two bits and produces an out put for a specific set of input .

Truth Table

Input A	0	0	1	1
Input B	0	1	0	1
Output C	0	0	0	1

In the above system there are four sets of inputs. 00,01,10 and 11. Output is obtained when both inputs are one. For completing the computing, we must feed one input at a time. Suppose, processing unit takes 1ms to process one set of inputs, it takes 4ms to complete the processing.

A qubit encodes and process data differently from traditional bits. Since two qubits in superposition can represent all four states simultaneously, a 2 qubit system can process all four inputs simultaneously. This is called parallelism. This is possible because of the quantum principle of superposition.

When a measurement is done its quantum state collapses and gives desired output.  It takes only 1ms to process the all set of inputs.   This explanation is theoretical; practical implementation of quantum systems involves greater complexity.

When you have two qubits, their combined state isn’t just one of the four possible classical combinations. Instead, their state is a superposition of all four possibilities simultaneously. This means the system exists in all four states (00, 01, 10, and 11) at once, each with a specific probability amplitude. When you measure the two qubits, the superposition collapses, and you will get one of the four classical outcomes.

This ability to simultaneously process multiple states is why quantum computers can solve certain problems exponentially faster than classical computers. The power of a quantum computer scales with the number of qubits: each additional qubit doubles the number of states that can be in superposition.

Exponential effect of Qubits

Two qubits can be in a superposition of the four possible 2-bit strings, three qubits can be in a superposition of the eight possible 3-bit strings, and so on. With 100 qubits, the range of possibilities is astronomical.

Total no of superpositions in 2 qubits system=4= 2²

Total no of superpositions in  3 qubits system=8=2³

Total no of superpositions in 100 qubits system=2¹⁰⁰ =1267650600228229401496703205376

Total no of superpositions in N qubits system=2^N

What If we manage to control 100 qubits ?  Then total no of superposition in that system will be 1267650600228229401496703205376.

Which means it can handle 1267650600228229401496703205376  ×100 bits of data simultaneously(2¹⁰⁰  no of 100 bits data  ) .Meanwhile 100 bit binary system can process only 100 bits of data at a time. That means 100qubit system is  1267650600228229401496703205376  times faster than classical 100 bits system.

The real power comes from cleverly designing a quantum algorithm that manipulates the probabilities of the different states. The algorithm is designed so that the correct answer has a much higher probability of being measured, while the incorrect answers are “cancelled out” through a process called quantum interference. This allows the quantum computer to efficiently find the right answer without having to test every possibility, which is how it achieves its speed advantage for certain types of problems.

Types of qubits

1.Superconducting Qubits

These are currently the most common type of qubit used in commercial quantum computers. They are built from superconducting circuits, which are materials that conduct electricity with zero resistance when cooled to extremely low temperatures. A Josephson junction, two superconducting units separated by an insulating layer, is the key component of superconducting qubits. The qubit state is encoded in the properties of the circuit, such as the charge, magnetic flux, or a combination of both.

They are highly scalable and can be manufactured using existing semiconductor fabrication techniques. They require extremely low temperatures (near absolute zero) to maintain their superconducting state, making them expensive and difficult to operate.

IBM is a major player in superconducting qubit technology. Their quantum processors—like Falcon, Hummingbird, and Eagle—are all built on transmon superconducting qubits, which are a type of qubit formed using Josephson junctions in superconducting circuit.

2.Trapped-Ion Qubits

Trapped-ion qubits use charged atoms (ions) that are suspended in a vacuum by electromagnetic fields. Lasers are used to manipulate the ions’ electronic states, which serve as the 0 and 1 states of the qubit. The ions can be arranged in a line or a 2D array, and their collective motion allows for entanglement between them

The quantum state is stored in the electron’s energy levels. Lasers are used to “pump” the electron into a specific state, perform logic operations, and measure the final state. It’s difficult to scale up to a large number of qubits due to the complexity of controlling many individual ions with a multitude of lasers.

3.Photonic Qubits

Photonic qubits use individual particles of light, or photons, to encode information. The quantum state can be encoded in properties like the photon’s polarization (e.g., vertical or horizontal polarization for 0 and 1, respectively) or its path of travel.

Photons are sent through a circuit of optical components (like beam splitters and mirrors) to perform quantum logic operations. Photons interact weakly with their environment, giving them long coherence times. They can also operate at room temperature and are easily transmitted over long distances using fiber optic cables, making them ideal for quantum communication and networking.

4. Quantum-Dot Qubits

Also known as spin qubits, these qubits use the spin of an electron or a hole (the absence of an electron) confined within a semiconductor material. A quantum dot is essentially an “artificial atom” that can trap a single electron. The electron’s spin, either “spin-up” or “spin-down,” represents the 0 and 1 states.

The spin state is manipulated by external magnetic fields or microwaves. Two-qubit gates are performed by controlling the interaction between the spins of neighbouring quantum dots.

They leverage existing semiconductor manufacturing technology, offering a potential path to scalable and compact quantum processors. They are susceptible to noise from the surrounding solid-state environment, leading to shorter coherence times

5.Neutral-Atom Qubits

Similar to trapped ions but use neutral atoms held in place by focused laser beams known as optical tweezers. They offer good scalability and long coherence times.

6.Nitrogen-Vacancy (NV) Centre Qubits

These are naturally occurring defects in a diamond crystal lattice where a carbon atom is replaced by a nitrogen atom, with an empty space (vacancy) next to it. The spin of the electrons at this defect site serves as the qubit. They can operate at room temperature.

7.Topological Qubits

A theoretical type of qubit that stores quantum information in the topological properties of a system, making it inherently resistant to environmental noise and decoherence. While promising, their physical implementation is still a major research challenge.

Microsoft has claimed their recent launch of quantum computing processor uses topological Qubits based on Majorana particles. A Majorana fermions, or Majorana particles,  are basically quasiparticles that are their own antiparticles. Unlike current qubit implementations, which are often based on elementary particles such as ions, electrons or photons, topological qubits are based on a topological state — or phase — of matter. Topological qubits are theorized to be more stable than any other qubit engineered to date because quantum information is stored in the topological properties of a physical system rather than in the properties of individual particles or atoms.

Nokia bell labs is also building their own  Qubits with a goal of demonstrating a stable topological qubit by 2026.

Applications of quantum computing

Quantum computing promises transformative applications across a variety of fields.

•  In cryptography, quantum algorithms have the potential to break widely used encryption methods, prompting the development of new quantum-safe protocols.
• In drug discovery and materials science, quantum computers can simulate complex molecular interactions and quantum systems with unprecedented accuracy, accelerating the design of new compounds and materials.
• Optimization problems—found in logistics, finance, and engineering—benefit from quantum approaches that can process vast solution spaces far more efficiently than classical methods.
• Machine learning and artificial intelligence may also advance through quantum-enhanced algorithms that extract patterns and insights from large datasets much faster.
• Quantum computing could revolutionize weather forecasting, climate modeling, and even fundamental research in physics by allowing scientists to model phenomena previously out of reach for classical supercomputers.

As quantum hardware matures, these applications will shift from theory to practice, driving innovation in technology, industry, and scientific exploration.

Will Quantum Computers Replace Classical Computers?

Quantum computing has attracted significant attention among researchers and technologists due to its potential to address problems that are currently unsolvable by classical computers. Progress across diverse qubit technologies—including trapped ions, neutral atoms, and topological qubits—signals the advent of a new era in computation. Nonetheless, important questions persist:

Will quantum computers replace classical computers?

Despite their power, quantum computers are not general-purpose replacements for classical computers. Some of the main limitations include:

• Specialized Problems: Quantum computers excel only at specific tasks; classical computers remain more efficient for most everyday operations such as word processing, browsing, and basic calculations.
• Error Rates and Stability: Today’s quantum hardware is error-prone and requires extremely controlled environments, whereas classical computers are robust and highly reliable.
• Development Stage: Quantum computers are still in their infancy; many architectures, such as topological qubits, remain experimental.
• Resource Requirements: Quantum systems often require cryogenic temperatures or sophisticated isolation from environmental noise, making them impractical for routine use at present.

The most realistic vision is a hybrid computing future. Quantum computers will work alongside classical computers, each handling the tasks they are best suited for.Classical computers will continue to serve as the backbone for most applications, while quantum processors will be invoked as specialized accelerators for certain algorithms or simulations. Cloud-based quantum computing services will allow widespread access without replacing personal and enterprise classical computers.

Quantum computers will not replace classical computers. Rather, they will complement them, opening new possibilities in science and industry.