Computer Science Professor Daniel Gottesman Answers Questions About Quantum Computing
The College of Computer, Mathematical, and Natural Sciences hosted a Reddit Ask-Me-Anything spotlighting quantum computing research.
University of Maryland Computer Science Professor Daniel Gottesman participated in an Ask-Me-Anything (AMA) user-led discussion on Reddit to answer questions about quantum computing on January 15, 2025.
Gottesman is the Brin Family Endowed Professor in Theoretical Computer Science and a Co-Director of QuICS. He also has an appointment in the University of Maryland Institute for Advanced Computer Studies (UMIACS). He came to UMD from the Perimeter Institute in Waterloo, Canada.
His research focuses on quantum computation and quantum information. He works in the sub-fields of quantum error correction, fault-tolerant quantum computation, quantum cryptography and quantum complexity. He is best known for developing the stabilizer code formalism for creating and describing a large class of quantum codes and for work on performing quantum gates using quantum teleportation.
This Reddit AMA has been edited for length and clarity.
Does quantum computer math involve the base 3 number system to account for the third quantum state, the way binary computing technology applies binary logic to account for "on/off" states?
Quantum computers don't have three states in a quantum bit, they have infinitely many. And so the math that they use is linear algebra over complex vector spaces.
Could you expand upon whatever algebra you're using in quantum systems?
A computer using base three arithmetic would not be fundamentally different from a regular classical computer, but a quantum computer can solve some problems much faster than any classical computer could (including one using base three).
I've been told that we are "a few Nobel Prizes" away from using quantum computers to break RSA and Diffie-Hellman. What's your take on this stance?
There have already been a couple of Nobel prizes in the field of quantum information. There may well be a few more, although probably for work that's already been done. Certainly, we're a number of years away from being able to use quantum computers to break classical cryptosystems. However, there may not be any more major fundamental obstacles to scaling up to big enough quantum computers to do that. It may be mostly a matter of difficult engineering work, which while important is not usually the kind of thing the Nobel committee likes to give prizes to.
One issue with using RSA or Diffie-Hellman today is that there's a risk that an eavesdropper could copy your encrypted messages and wait until a big enough quantum computer is available to decrypt at that future time. So anything you want to remain secret for a long time, you should definitely not be using RSA for. The other issue is that cryptographic protocols tend to hang around for a long time, sometimes even after they've been broken. So it's important to start the transition to more secure protocols well before the existing ones can actually be broken.
When quantum computers reach maturity, what real-life benefits will they provide or what practical problems will they solve?
The things that we know that quantum computers are good for are simulating quantum systems and breaking some cryptographic protocols. Most people don't want to break cryptographic protocols, but there are certain government agencies that definitely do. Most people don't want to simulate quantum systems either—but many physicists and chemists do, and by helping those physicists and chemists, quantum computers can help design new materials, drugs and other things. Once upon a time, people designed bridges and cars and whatnot without the aid of computers, but now an early step is always a computer simulation. When future scientists are making things at the atomic scale, they will use quantum computers to simulate beforehand.
There are a number of other applications that have been proposed for quantum computers. People have investigated the use of quantum computers for a variety of optimization problems (figuring out the best way to allocate resources, for instance). There have also been a number of proposals for using quantum computers to help machine learning. We're not certain that quantum computers will actually be much better at these things than classical computers, but if there are some specific cases where a quantum computer could help, that would have a wide range of applications.
What open problem in quantum complexity is most exciting to you right now?
I think this might be the right time to tackle quantum PCP again. There has been recent progress on some of the warmup problems to quantum PCP—including the resolution of the NLTS conjecture and quantum locally testable codes.
Which of the major open theoretical problems in quantum or classical complexity looks most amenable to being solved in the next 5 years, and why?
For non-technical readers, PCP stands for probabilistically testable proofs, which is a method of writing very long proofs that can be verified by only looking at a few bits. The classical PCP theorem shows that there exist (subject to some standard assumptions) problems where it is hard to even approximate the best solution. Quantum PCP basically tries to do the same thing for quantum problems.
Do you believe we live in a fundamentally mathematical/logical/algorithmic universe and if so, how close do you think we are to a grand unified theory of everything? Do you think such a theory will come out of experimental physics, theoretical physics, computing, philosophy, or some blend of all of them?
Yes, I do believe we live in a fundamentally mathematically describable universe. I think there needs to be experimental input before we can hope to figure out a grand unified theory. Right now, we have no real way of doing experiments at the Planck scale.
Will it be possible to break Bitcoin with a quantum computer?
I am not a Bitcoin expert, but my understanding is that Bitcoin uses digital signatures with protocols that are not secure against quantum attacks. If the digital signatures are changed to what's called a post-quantum system, meaning secure against quantum attack, then as far as I know, Bitcoin could still be secure. However, that doesn't exclude someone discovering a different attack using quantum computers.
Will there ever be a practical need for a personal quantum computer or will its applications always be more scientific/business oriented?
Hard to say—some applications of quantum computers are above. The applications breaking cryptosystems and simulating quantum systems are probably not for personal quantum computers. Some of the other more speculative ones might be.
Can you explain to my 10-year-old daughter what it is that you do and why it is important?
I do research on quantum computers, which are a new type of computer that can solve some kinds of problems much faster than existing computers. People are still trying to build large enough quantum computers to be really useful, and one of the things that I do is study the best ways to correct errors on those quantum computers so that they give the right answers. Another thing I am interested in is understanding the limits of quantum computers so that we know which problems quantum computers will help with and which problems quantum computers are no better at solving than existing computers. I've listed some potential applications of quantum computers here.
What is your take on quantum photonics for quantum computation? Do you believe it will scale up to reach a relevant number of qubits?
I would like for people to work on many different approaches to building quantum computers for as long as possible (which means for as long as funding is available). The technical barriers in different systems are pretty different, so it's good to have choices available.
What are you afraid of missing out on in your lifetime? Will it still take decades and decades to reach a quantum computing empire the same way MOSFETs have?
I've always been a relative optimist on quantum computers. While it will still take decades for quantum computers to reach their full potential, I think there is a good chance we will see lots of interesting applications within the next 20 years—and I hope to live that long.
What year do you expect practical quantum computers
Let's define a practical quantum computer as one that can solve a problem that someone cares about for non-quantum computing reasons. If we're lucky, the year could be 2025. Maybe 2035 is more likely. But it could certainly be longer than that.
Are there any controversial ideas in quantum computing that are challenging our conventional understanding of the field (or any broader field)?
A better question is if there are any phenomena in quantum computing that are not weird. Quantum information is constantly surprising, and if you work in the field for a long time, you just get used to that. One thing that I should clarify is that quantum computing is based on the mathematics of quantum physics, and we frequently rely on rigorous mathematical arguments. When you do that, there is nothing that's really inexplicable, just unintuitive things.
Are there any phenomena in quantum computing that are very weird and/or inexplicable? Perhaps some things are being ignored or circumvented but have evaded explanation.
Quantum information ideas have found their way into condensed matter and high-energy physics. I think it is fair to say that some of those quantum information ideas have challenged the conventional understanding of string theory and quantum gravity, which are fields that have no shortage of controversial ideas.
You've been in this field for over 20 years. Have you noticed a trend in the public interest for quantum computing/communications? Has there been a breakthrough that has pushed the entire community forward?
Quantum computing has always been a fairly hot topic in popular science media. There was a sea change maybe five years or so ago on the business side. Previously, it was viewed as a long-term thing that companies didn't need to worry about. Then all of a sudden, there were a lot of startups in the field and more big companies wanted to get into quantum computing. I don't think it was a single breakthrough that catalyzed this, more a slow cumulative experimental progress that made quantum computers seem more plausible. Then, once a few companies got involved, lots of others didn't want to be left out.
How will quantum computing actually affect us in the next 30 years or so? Game-changing revolution, barely noticeable for the average person, or something in between?
My expectation is that a lot of the effects of quantum computers will be invisible to the average person. It will happen behind the scenes in helping scientists. The average person will see the effects of scientific and technological progress enabled by quantum computers, but won't necessarily know that quantum computers were involved.
