The quantum computer is today one of the most discussed and best-funded technologies in the world, with governments, research centres and companies investing billions to build machines capable of solving problems that no classical supercomputer will ever be able to tackle in useful time. But behind the term lie concepts that are often misunderstood, because popular science tends to compress them into slogans, while those who have to make decisions need to understand where physics ends and where marketing begins. This guide answers the most searched questions about quantum computing, explaining what a quantum computer really is, how it works, what qubits are, what problems it solves, what its current limits are and – above all – how an organisation can start working with it today, through quantum emulation. The aim is to provide a solid and verifiable foundation, with institutional sources, for those who have to assess the impact of this technology on their sector, from managers to researchers, to public decision-makers and IT professionals who want to move from curiosity to strategy.
What is a quantum computer?
A quantum computer is a processing system that uses the principles of quantum mechanics – superposition, entanglement and interference – to represent and manipulate information. Whereas a traditional computer encodes data in bits that are either 0 or 1, a quantum computer uses qubits, physical objects (atoms, ions, superconducting circuits, photons) that can exist in combinations of both states. This property makes it possible to explore a huge number of configurations in parallel and makes some classes of problems, intractable today, potentially solvable. The idea was formalised by Richard Feynman in 1982, when he observed that simulating nature – which is quantum – requires a quantum machine. Since then research has moved from academic laboratories to the industrial programmes of IBM, Google, IonQ and to the strategic plans of the United States, China and the European Union. The term therefore does not denote a single device, but an entire computational paradigm that includes hardware, algorithms, programming languages and – increasingly often – emulation platforms that make quantum computing accessible before the hardware is mature. An introductory overview is also available on Wikipedia.
How does a quantum computer work?
The operation unfolds in three steps. First the qubits are prepared in an initial state, then they are manipulated through quantum logic gates that modify their probability amplitudes, finally a measurement ‘collapses’ the system into a readable classical result. The skill of the algorithm lies in using interference to amplify the correct answers and cancel the wrong ones. It is a radically different paradigm from the sequential execution of instructions typical of CPUs. The best-known gates – Hadamard, CNOT, Pauli – are the building blocks with which quantum circuits are built, just as AND and OR are for digital circuits. The main enemy is decoherence, that is, interaction with the environment, which destroys the quantum state in fractions of a second, and it is the reason why many processors operate at temperatures close to absolute zero, where thermal noise is reduced enough for the state to survive for the time needed to complete the computation. For a step-by-step in-depth look, read the dedicated guide on how a quantum computer works.
What is a qubit and why is it different from a bit?
The qubit (quantum bit) is the fundamental unit of quantum information. Unlike the classical bit, which takes a single value at a time, the qubit can be in a superposition of 0 and 1, described by complex amplitudes. With n qubits one represents a space of 2n simultaneous states, so much so that 300 qubits are enough to describe more configurations than there are atoms in the observable universe. It is this exponential growth of the computational space that gives quantum computing its potential. The physical realisations are many and range from superconducting circuits (IBM, Google) to trapped ions (IonQ, Quantinuum), passing through neutral atoms, photons and spins in semiconductors. Each technology offers different trade-offs between fidelity, speed and scalability, and none has yet won the race. You will find definitions, examples and physical implementations on the page dedicated to the qubit.
Scientific in-depth section: the formalism of the qubit
For readers with a scientific background, in Dirac notation the pure state of a qubit is the vector |ψ⟩ = α|0⟩ + β|1⟩, with α, β ∈ ℂ complex amplitudes subject to the normalisation constraint |α|² + |β|² = 1. Measurement in the computational basis returns 0 with probability |α|² and 1 with probability |β|² (Born rule), and the state can be represented geometrically as a point on the Bloch sphere. The maximal entanglement of two qubits is exemplified by the Bell state |Φ⁺⟩ = (|00⟩ + |11⟩)/√2, which cannot be factorised into the tensor product of single-qubit states. It is precisely this non-separability, quantified by the entanglement entropy, that is the resource distinguishing quantum computing from any classical model of computation.
What are superposition and quantum entanglement?
Quantum superposition is the ability of a system to exist in several states at once until it is measured. Quantum entanglement is instead a non-classical correlation between two or more qubits, whereby measuring one of the two instantly determines the state of the other, regardless of the distance. Together, these two phenomena constitute the computational resource that no classical architecture possesses, and they are studied experimentally by institutions such as INFN, the National Institute for Nuclear Physics. Their physical reality is no longer in question, given that the 2022 Nobel Prize in Physics was awarded to Aspect, Clauser and Zeilinger precisely for those experiments on entanglement that closed the debate opened by Einstein, paving the way for the modern science of quantum information.
What is the difference between a quantum computer and a classical computer?
The differences are structural and concern the unit of information (qubit versus bit), the processing logic (reversible quantum gates versus Boolean gates), the nature of the result (probabilistic versus deterministic) and the operating conditions (cryogenics and isolation versus room-temperature electronics). A quantum computer is not a ‘faster’ version of a PC, but a complementary tool, effective only on specific classes of problems. In practice the two architectures will work together, and hybrid quantum-classical workflows, in which a CPU orchestrates the work and delegates only the critical computational core to the quantum processor, are already the standard operating model today. The detailed comparison, with tables and use cases, is in the article quantum computer vs classical computer.
Which problems can a quantum computer solve?
The classes of problems with demonstrated or expected quantum advantage include the factoring of large numbers (Shor's algorithm, with direct impacts on post-quantum cryptography), unstructured search (Grover), combinatorial optimisation for logistics and finance, the simulation of molecules for pharmaceuticals and materials science, and quantum machine learning. It is important to be rigorous, because while for many everyday problems, such as spreadsheets, management software and the web, the quantum computer offers no advantage whatsoever and will never replace traditional computing, its value emerges instead where the complexity of the problem grows exponentially with its size. The complete overview of quantum algorithms explains for each one the type of speed-up that can be obtained.
What are the current limits of quantum computers?
We are in the NISQ era (Noisy Intermediate-Scale Quantum), in which the available processors have from tens to a few thousand physical qubits, noisy and subject to decoherence. Quantum error correction requires many physical qubits to obtain a single reliable logical qubit, and large-scale fault-tolerant machines are expected no earlier than the next decade. To this are added high access costs, limited availability and scarce skills, factors that make it strategic to be able to develop algorithms without depending on the hardware. Anyone who gives firm deadlines for generalised ‘quantum advantage’ is doing marketing and not science, because the trajectory is clear but the timing is not. It is precisely this uncertainty that makes it rational to invest now in skills and methods, which mature more slowly than the hardware.
What is the difference between quantum simulation and quantum emulation?
Quantum simulation mathematically reproduces the behaviour of a quantum physical system; quantum emulation instead replicates the logical operation of a quantum processor on high-performance classical infrastructure. Emulation makes it possible to design, test and validate quantum algorithms today in a deterministic, repeatable and noise-free environment, with controlled costs and times. In an emulator every state is inspectable, every execution is reproducible and noise can be activated in a controlled manner to estimate behaviour on real hardware, ideal conditions for research, training and industrial prototyping. It is the approach on which the EQM platform and the Italian Quantum Emulation Centre (Centro Italiano di Emulazione Quantistica) are founded.
What are the applications of quantum computing in Italy and Europe?
Europe has launched the Quantum Flagship, a ten-year programme worth over a billion euros, and Italy takes part with research centres, the National Recovery and Resilience Plan (PNRR) and industrial initiatives in defence, healthcare, mobility, energy and finance. Quantum technologies are also classified among the critical technologies subject to Golden Power, confirming their national strategic relevance. Sector use cases are explored in depth on the pages dedicated to quantum computing applications, healthcare and finance. For the Italian ecosystem the stakes are twofold, because industrial competitiveness is joined by technological sovereignty, since depending on foreign quantum infrastructure would mean repeating the fragility that the continent has already experienced with semiconductors.
How can a company prepare for the quantum era today?
Preparation does not require buying a quantum computer, but a quantum readiness path. It means identifying the business processes with computationally critical problems, assessing cryptographic exposure to future quantum attacks – NIST has already standardised the first post-quantum algorithms – and experimenting with quantum algorithms in an emulated environment, building internal skills. The typical path lasts a few months, not years, and is structured as an assessment of use cases, proof of concept in emulation, team training and an adoption roadmap. Those who start today accumulate an advantage that is hard to recover for those who wait for hardware maturity, because quantum skills take a long time to build and the labour market is already under strain.
Start today with HumanQ's quantum emulation
HumanQ, with the EQM platform, allows businesses and institutions to develop and test quantum algorithms right away, on real use cases in defence, healthcare and mobility. Discover how quantum emulation works or contact the HumanQ team to assess your organisation's quantum readiness.