While an ordinary computer operates using bits, where each bit could be represented by, say, a switch which is either on or off, a quantum computer requires quantum bits, or qubits for short.

The fundamental information-carrying components must be qubits, since we want to simulate highly complex entangled quantum systems that no ordinary computer on Earth will be able to. Entanglement is probably the most defining feature of quantum mechanics. An entangled state cannot be factored as a product of its local constituent quantum states, and so, it can only be described as an inseparable whole.

Attempting to write a quantum description of a few hundred qubits, in such a complex entangled state, may require more bits than the number of atoms in the visible universe. While quantum computers may be the next revolutionary technology, the astounding complexity brought about by the quantum realm may not always surpass the capabilities of classical computers. However, we have at least three good reasons for thinking that quantum computers in the future may become more powerful than classical ones.

Attempting to write a quantum description of a few hundred qubits, in such a complex entangled state, may require more bits than the number of atoms in the visible universe

First, we are aware of problems that are believed to be extremely hard to solve for classical computers, but for which quantum algorithms have been devised that could easily solve them. The best-known example is finding the prime factors of a large composite integer. Apart from having numerous applications in the field of mathematics, these large numbers made up of two or more primes are utilised in the Rivest–Shamir–Adleman (RSA) algorithm, which is the most widely-employed cryptographic technique used to encrypt data transmission, such as in web browsers, emails, and other communication channels.

Second, computer scientists have also argued that easy-to-prepare quantum states on a quantum computer have properties which can be considered superclassical, such as entanglement. This means that if we measure all the qubits in our quantum state, we are sampling from a correlated probability distribution that cannot be efficiently sampled from by any classical means. This implies that quantum computers may be able to always perform better than classical computers.

Finally, and perhaps the most compelling reason why we think quantum computers are powerful, is that we do not know of any way to simulate a quantum computer using a classical one!

*Mirko Consiglio is physicist, programmer and a PhD student. He is carrying out research on quantum computers at the University of Malta within the QVAQT (Quantum Variational Algorithms for Quantum Thermodynamics) project, funded by the Malta Council for Science and Technology.*

**Sound Bites**

• In 2016, IBM put up the first superconducting quantum processor with open access on the cloud. Since then, various different models have been made available for potential users, with an open plan covering five and seven quantum processing units. Apart from real devices, IBM quantum cloud possesses a diverse range of different quantum simulators and programs that anyone can use.

To sign up and use one yourself, visit: quantum-computing.ibm.com.

• Albert Einstein is synonymous with the world’s most famous equation E = mc^{2}, having developed the theories of general and special relativity. However, in 1921 he won the Nobel prize “for his services to theoretical physics, and especially for his discovery of the law of the photoelectric effect”, a cornerstone in the development of quantum mechanics. Nonetheless, despite his contributions to quantum physics, he later started to oppose what it had evolved into, objecting that “God does not play dice”.

*For more science news, listen to Radio Mocha on www.fb.com/RadioMochaMalta/.*

**DID YOU KNOW? **

• The first results on entanglement, a fundamental resource in quantum computing, were presented in a paper in 1935 by Albert Einstein, Boris Podolsky, and Nathan Rosen.

• Peter Shor devised the quantum algorithm for breaking the widely-used RSA encryption protocol in 1994.

• In 1996, Grover’s algorithm established a quantum speedup for the widely-applicable unstructured search problem.

• In January 2020, an international team of researchers from the Austrian Academy of Sciences, University of Vienna and University of Malta established a quantum encrypted connection between Sicily and Malta.

• Qalypso, a summer school on quantum computing and open quantum systems was held in Gozo for a week in September, where graduate and post-graduate students, as well as various invited speakers attended.

*For more trivia see: www.um.edu.mt/think.*