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How Quantum Computers Work

Author: Maria Flores

Editors: Hwi-On Lee and Kevy Chen

Artist: Esther Chen

In recent years, quantum computers have garnered significant attention due to their prospective contributions in revolutionizing the field — contributions significant enough to be compared to that of mobile devices. Google, IBM, Intel, and Microsoft have all invested billions of dollars to develop the current most powerful quantum computers. Today’s skilled engineers are realizing the dreams of the centuries of deceased quantum physicists around the globe. 

Quantum physicists study subatomic particles such as electrons and photons. Their notoriety rose after classical physics failed to adequately explain the universe, as highlighted by the history of the electron. In 1887, physicist J.J. Thomson was the first to discover negatively charged electrons through his work with a cathode ray. He explained his interpretation of the structure of an atom by likening it to plum pudding, where the negative charges are distributed throughout a positively charged sphere. Later, his student Ernest Rutherford disproved this theory in 1910. Rutherford located the positively charged nucleus and gathered that an atom is mostly filled with space. Building on Rutherford's experiment, Niel Bohr theorized that electrons, similar to particles, moved to and from fixed orbits in response to gained and lost energy. 

The notion that these subatomic “particles” behaved strictly as particles was quickly disproved during the double-slit experiment conducted by Thomas Young. Young proposed that electrons moving through two slits would manifest as two distinct, straight lines on the receiving wall. However, what was observed were multiple stripes residing far from the location slits. This phenomenon, known as an interference pattern, forms from the collision of two waveforms. The overlapping of two peaks and two troughs of the waves intensify the effect on the detector screen, also known as constructive superposition interference. Similarly, destructive interference refers to the overlapping of a peak and a trough, which counteracts any impact. Through this experiment, Young depicted the wave-particle duality of quantum mechanics, where subatomic particles exhibit both particle and wave-like qualities. This revelation shattered the conventional beliefs of a photon, paving the way for future insights into the unknown nature of subatomic particles. 

The result of Young’s work helped physicists understand that subatomic particles operated within the realm of probability waves. Using calculus, Erwin Schrodinger introduced a wave equation that indicates the behavior of a particle in a field of force. Thanks to the Schrodinger equation, Bohr’s model of an atom was challenged. With the help of Schrodinger and several other physicists, Bohr revolutionarily theorized the Copenhagen Interpretation. The theory states that a quantum particle can simultaneously exist and not exist in all possible states at once. It is only when we observe the object that the superposition collapses, causing it to fall into one of the states of the wave function. Schrodinger explained this complex concept through his famous thought experiment: Schrodinger’s cat. Schrodinger posited that a cat in a box with hazardous material can be thought of as both alive and dead. The only way to be certain of the outcome is to open the box. However, we influence the outcome of the event, as stated by the Heisenberg Uncertainty Principle. The innate randomness and probability of these subatomic particles drive the qubits, the basic unit for quantum information, to perform a multitude of calculations more efficiently than a classical computer. 

Quantum computers differ from classical computers because quantum computers are not confined to the binary system. Classical computers associate 1s with a certain action and 0s with the opposite action. Qubits, the basic unit for quantum information, are undefined — only measured through probability. When two qubits are in the state of superposition, the particles will become entangled and their information will hold complementary properties. For instance, when observed, one electron from a pair will have a clockwise spin while the other will have an anti-clockwise spin. Entanglement has some interesting applications to the nature of transmitting information, particularly cryptography. Since the two parties are correlated until interference, if unknowingly intercepted by a third party, the quantum state will change, alerting the two parties of the third party. Vehicular companies such as Volkswagen are closely watching the developments of quantum computers as a means to improve battery efficiency in electric vehicles.

While these applications are certainly enticing to all fields, the challenges of these abstract and barely grasped concepts lead to many questions about the feasibility of quantum computers. Any stray heat, noise, or other environmental factors can lead to decoherence, where the powerful superposition disappears. The delicate conditions, the large size, and the error-prone nature of quantum computers hinder physicists and engineers from unleashing their full potential. For the foreseeable future, classical computers will continue to dominate the world.



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