Time crystals. Microwaves. Diamonds. What do these 3 disparate things have in popular?
Quantum computing. Unlike classic pcs that use bits, quantum computer systems use qubits to encode facts as zeros or ones, or the two at the exact same time. Coupled with a cocktail of forces from quantum physics, these fridge-sized machines can method a complete large amount of information — but they are much from flawless. Just like our standard desktops, we need to have to have the appropriate programming languages to correctly compute on quantum computers.
Programming quantum personal computers demands recognition of a thing identified as “entanglement,” a computational multiplier for qubits of sorts, which interprets to a lot of electricity. When two qubits are entangled, actions on one qubit can transform the benefit of the other, even when they are physically divided, providing increase to Einstein’s characterization of “spooky motion at a length.” But that efficiency is equal pieces a resource of weakness. When programming, discarding 1 qubit without having remaining mindful of its entanglement with yet another qubit can ruin the information stored in the other, jeopardizing the correctness of the system.
Researchers from quantum computing called Twist. Twist can describe and verify which pieces of data are entangled in a quantum program, through a language a classical programmer can understand. The language uses a concept called purity, which enforces the absence of entanglement and results in more intuitive programs, with ideally fewer bugs. For example, a programmer can use Twist to say that the temporary data generated as garbage by a program is not entangled with the program’s answer, making it safe to throw away.
While the nascent field can feel a little flashy and futuristic, with images of mammoth wiry gold machines coming to mind, quantum computers have potential for computational breakthroughs in classically unsolvable tasks, like cryptographic and communication protocols, search, and computational physics and chemistry. One of the key challenges in computational sciences is dealing with the complexity of the problem and the amount of computation needed. Whereas a classical digital computer would need a very large exponential number of bits to be able to process such a simulation, a quantum computer could do it, potentially, using a very small number of qubits — if the right programs are there.
“Our language Twist allows a developer to write safer quantum programs by explicitly stating when a qubit must not be entangled with another,” says Charles Yuan, an MIT PhD student in electrical engineering and computer science and the lead author on a new paper about Twist. “Because understanding quantum programs requires understanding entanglement, we hope that Twist paves the way to languages that make the unique challenges of quantum computing more accessible to programmers.”
Yuan wrote the paper alongside Chris McNally, a PhD student in electrical engineering and computer science who is affiliated with the MIT Research Laboratory of Electronics, as well as MIT Assistant Professor Michael Carbin. They presented the research at last week’s 2022 Symposium on Principles of Programming conference in Philadelphia.
Untangling quantum entanglement
Imagine a wooden box that has a thousand cables protruding out from one side. You can pull any cable all the way out of the box, or push it all the way in.
After you do this for a while, the cables form a pattern of bits — zeros and ones — depending on whether they’re in or out. This box represents the memory of a classical computer. A program for this computer is a sequence of instructions for when and how to pull on the cables.
Now imagine a second, identical-looking box. This time, you tug on a cable, and see that as it emerges, a couple of other cables are pulled back inside. Clearly, inside the box, these cables are somehow entangled with each other.
The second box is an analogy for a quantum computer, and understanding the meaning of a quantum program requires understanding the entanglement present in its data. But detecting entanglement is not straightforward. You can’t see into the wooden box, so the best you can do is try pulling on cables and carefully reason about which are entangled. In the same way, quantum programmers today have to reason about entanglement by hand. This is where the design of Twist helps massage some of those interlaced pieces.
The scientists designed Twist to be expressive enough to write out programs for well-known quantum algorithms and identify bugs in their implementations. To evaluate Twist’s design, they modified the programs to introduce some kind of bug that would be relatively subtle for a human programmer to detect, and showed that Twist could automatically identify the bugs and reject the programs.
They also measured how well the programs performed in practice in terms of runtime, which had less than 4 percent overhead over existing quantum programming techniques.
For those wary of quantum’s “seedy” reputation in its potential to break encryption systems, Yuan says it’s still not very well known to what extent quantum computers will actually be able to reach their performance promises in practice. “There’s a lot of research that’s going on in post-quantum cryptography, which exists because even quantum computing is not all-powerful. So far, there’s a very specific set of applications in which people have developed algorithms and techniques where a quantum computer can outperform classical computers.”
An important next step is using Twist to create higher-level quantum programming languages. Most quantum programming languages today still resemble assembly language, stringing together low-level operations, without mindfulness towards things like data types and functions, and what’s typical in classical software engineering.
“Quantum computers are error-prone and difficult to program. By introducing and reasoning about the ‘purity’ of program code, Twist takes a big step towards making quantum programming easier by guaranteeing that the quantum bits in a pure piece of code cannot be altered by bits not in that code,” says Fred Chong, the Seymour Goodman Professor of Computer Science at the University of Chicago and chief scientist at Super.tech.
Reference: “Twist: Sound Reasoning for Purity and Entanglement in Quantum Programs” by Charles Yuan, Christopher McNally and Michael Carbin.
The work was supported, in part, by the MIT-IBM Watson AI Lab, the National Science Foundation, and the Office of Naval Research.