Introduction
Quantum Physics (also called Quantum Mechanics) is the fundamental theory in physics that describes the behavior of matter and energy at the smallest scales. It was developed in the mid-1920s to explain phenomena that could not be accounted for by classical physics, such as the photoelectric effect and the atomic spectra. Quantum Physics reveals that the physical world is not deterministic, but probabilistic and uncertain. It also introduces concepts such as wave-particle duality, superposition, and entanglement, which challenge our common sense and intuition.
Quantum Computing is a new field of study that applies the principles of Quantum Physics to create a new paradigm of computation. Unlike classical computers, which use bits that can only be in one of two states (0 or 1), quantum computers use qubits that can be in a superposition of both states (0 and 1) at the same time. This allows quantum computers to explore multiple solutions or paths simultaneously, potentially solving problems that are intractable for classical computers. Quantum Computing also exploits the phenomenon of entanglement, which enables two qubits to share a quantum state and influence each other, even when they are far apart.
In this article, I will briefly explain some of the key concepts of Quantum Physics and Quantum Computing, and show how they are related to each other. I will also provide some examples of how quantum computers can be built and programmed, and what kind of problems they can solve.
Double Slit Experiment
One of the most famous experiments in Quantum Physics is the double slit experiment, which demonstrates the wave-particle duality of light and matter. The experiment consists of a source of light (or other particles) and two slits, as shown below:
When the light passes through the slits, it behaves like a wave and creates an interference pattern on the screen behind them. The interference pattern is the result of the constructive and destructive interference of the light waves that emerge from the two slits. When the light waves are in phase, they add up and create bright spots. When they are out of phase, they cancel out and create dark spots:
However, when the light source is dimmed so that it emits one photon at a time, the interference pattern still appears, even though there is no other photon to interfere with. This implies that each photon acts as a wave and interferes with itself, passing through both slits at the same time.
Moreover, when a detector is placed near the slits to measure which slit each photon passes through, the interference pattern disappears and only two bright spots are seen on the screen. This means that the act of observation collapses the wave function of the photon and forces it to choose one slit or the other, behaving like a particle.
The double slit experiment shows that light and matter can exhibit both wave-like and particle-like properties, depending on how they are measured. It also shows that the quantum state of a system is not fixed until it is observed, and that observation affects the outcome of the experiment.
Superposition and Entanglement
Superposition and entanglement are two of the most important and counterintuitive features of Quantum Physics. They are also the basis of Quantum Computing.
Superposition is the ability of a quantum system to exist in a combination of two or more states at the same time, until an observation is made. For example, a qubit can be in a superposition of 0 and 1, denoted as \(\ket{0}\) + \(\ket{1}\), where \(\ket{0}\) and \(\ket{1}\) are the basis states and the plus sign indicates a linear combination. The coefficients of the linear combination, called amplitudes, determine the probability of finding the qubit in each state when measured. For example, if the amplitude of \(\ket{0}\) is 0.6 and the amplitude of \(\ket{1}\) is 0.8, then the probability of finding the qubit in \(\ket{0}\) is 0.6^2 = 0.36 and the probability of finding the qubit in \(\ket{1}\) is 0.8^2 = 0.64. The sum of the probabilities must always be 1.
Entanglement is the phenomenon in which two or more quantum systems, such as qubits, share a quantum state and become correlated, even when they are physically separated. For example, two qubits can be entangled in a state called a Bell state, denoted as \(\ket{00}\) + \(\ket{11}\), where \(\ket{00}\) and \(\ket{11}\) are the basis states of the two-qubit system. This means that the two qubits are in a superposition of being both 0 and both 1 at the same time. When one of the qubits is measured, the other qubit instantly collapses to the same state, regardless of the distance between them. This is called quantum nonlocality.
Quantum Computing
Quantum Computing is the use of quantum features, to perform computation. Quantum computers can manipulate qubits using quantum gates, which are analogous to logic gates in classical computers. Quantum gates can perform operations such as flipping, rotating, swapping, and entangling qubits. Quantum gates can also be combined to form quantum circuits, which are sequences of quantum gates that perform a specific function.
Quantum Computing is still in its infancy, as the number of qubits and quantum gates that can be reliably controlled is limited. The largest quantum computer to date (2023), built by Google, has 72 qubits and can perform a specific quantum circuit in 200 seconds, while a classical supercomputer would take 10,000 years to do the same. This is called quantum supremacy, the point at which a quantum computer can outperform a classical computer on a certain task. However, this does not mean that quantum computers can solve any problem faster than classical computers, as the quantum circuit used by Google was designed to be hard for classical computers and has no practical use. Moreover, the quantum computer still has a high error rate and cannot run more complex or useful quantum algorithms.
Quantum Computing is a promising and exciting field that has the potential to revolutionize many areas of science, technology, and society. However, it also poses many challenges and limitations that need to be overcome before it can become widely available and applicable. Quantum Computing is not a magic bullet that can solve any problem, but a new tool that can offer new perspectives and possibilities.
Conclusion
Quantum physics and quantum computing are two fascinating and interconnected fields of study that challenge our classical understanding of nature and computation. Quantum physics reveals the strange and counterintuitive phenomena that occur at the smallest scales of reality, such as wave-particle duality, superposition, and entanglement. Quantum computing harnesses these phenomena to create a new paradigm of computation that can potentially solve problems that are intractable for classical computers. However, quantum computing also faces many challenges, such as building and controlling qubits, simulating quantum systems, and developing quantum algorithms. Microsoft is one of the leading companies in the field of quantum computing, offering various tools and platforms for quantum research and development. Quantum computing is not just a theoretical possibility, but a practical reality that is closer than many people think.