Qubit questions: Understanding quantum computing

New technologies are often surrounded by promise. That’s certainly true of quantum computers, the next-generation computing tools that are expected to grow to an $8.6 billion USD market by 2027. Scientists believe they could help design lifesaving drug compounds, better investment strategies and faster AI systems. But what exactly are quantum computers, and what can they do?

“We are now very much in the age of quantum discovery, where we can use these quantum machines for real scientific applications to basically do scientific computations which are not otherwise possible.”

Mikhail Lukin, Professor of Physics, Harvard University

For several decades, scientists have been interested in exploiting the properties of quantum mechanics, the science of atoms and subatomic particles, as a platform for a new kind of computer. Part of their reasoning was that traditional computers, like the laptops and smartphones of today, are limited because they rely on physical transistors for calculations involving on-or-off binary states. As the size of transistors decreases, quantum mechanical phenomena are inevitably coming into play.

Building blocks of the future

Mikhail Lukin is the Co-Director of the Harvard Quantum Initiative in Science and Engineering at Harvard University.
Professor Mikhail Lukin is the Co-Director of the Harvard Quantum Initiative in Science and Engineering at Harvard University.

Over the past several decades, quantum phenomena have already been used for the basis of technologies such as GPS atomic clocks, MRI scanners and electron microscopes. About four decades ago, scientists pointed out that these effects can be used as a basis for building a new kind of machines called quantum computers.

Unlike traditional computers, quantum computers are based not on physical bits, but quantum bits, or “qubits.” These qubits should be encoded into quantum degrees of freedom, such as an electron spin or a photon polarization, or engineered systems like superconducting circuits.

The idea behind using these qubits is to take advantage of the quantum mechanical properties of superposition and entanglement — that is, particles existing in several quantum states simultaneously and being connected to or entangled with other particles. In doing so, qubits can represent different states simultaneously. As a result, quantum computers can, in principle, explore all possible solutions to a problem at the same time, like trying all the routes in a maze instantly to find the right path. 

“If you had an array of just 300 atoms, in this 300 quantum computer, you could potentially simultaneously store more solutions than the number of particles in the universe,” Mikhail Lukin, a Harvard University physicist who specializes in quantum computing, said in a lecture at the 2022 Rakuten New Year Seminar. “This is obviously something that no conventional computer can possibly do.”

Real solutions to quantum problems

Quantum computers can outshine traditional computers when it comes to problems such as determining the best routes for deliveries, or factoring the primes of very large numbers, a fundamental part of RSA encryption.

At the same time, quantum computers are challenging to build, since qubits are inherently fragile and subject to errors as they interact with electromagnetic fields, or other atoms or molecules. Quantum error correction is a major challenge in the field, and scientists have been advancing research with strategies including storing information redundantly on arrays of qubits called logical qubits.  

If these challenges can be overcome, quantum computers could become important tools to tackle the knottiest problems, working in tandem with, and not replacing, conventional machines.

“If you had an array of just 300 atoms, in this 300 quantum computer, you could potentially simultaneously store more solutions than the number of particles in the universe … something that no conventional computer can possibly do.”

Given the robust investment buoying the field, quantum computer research is accelerating. In his lecture, Lukin presented a real-time demonstration of a quantum processor unit of about 200 qubits with geometry and connectivity that can be reconfigured in real time. The demonstration involved a machine operated by Rakuten-backed U.S. startup QuEra Computing, that commercializes the research from Lukin’s lab. With its 256-qubit machine announced last year, QuEra is already ahead of rivals in a quantum computing race that includes giants like IBM, Google, Intel and Microsoft.

“We are now very much in the age of quantum discovery, where we can use these quantum machines for real scientific applications to basically do scientific computations which are not otherwise possible,” said Lukin. “But we are also on the cusp of this quantum industry where new, potentially revolutionary applications and products can be foreseen.”

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