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# I'm Andrew Houck, a quantum engineering professor at Princeton. AMA for World Quantum Day!

**WE ARE DONE! Thanks everyone for all the great questions.**

Hello, Reddit! It’s World Quantum Day. I am an engineering professor at Princeton University who studies and builds quantum information devices. What does that mean?

In science, we used to say that quantum mechanics was the physics of atoms and molecules, because that’s the realm where it was first discovered and where its effects are most pronounced.

All of that is changing now.

Beyond the hype around a “second quantum revolution” lies an enormous effort by engineers, physicists, chemists and computer scientists to create fundamentally new applications in computing, communications and sensing. And together we are harnessing quantum mechanics into larger and larger systems — actual technologies — to solve some of the world’s most complex problems.

What do you want to know?

I'll be answering questions at r/IAmA from 1:30pm – 2:30pm ET. You can find out more about my work through my website and read my overview of the topic at Princeton Engineering. Here's my proof!

PrincetonEngineers40 karma

Quantum information is exceptionally fragile. Defects or impurities in materials can lead to decoherence, the process by which quantum information is destroyed. In order to make better qubits, we need to find better materials. This is a big part of my research at Princeton. Working with Nathalie de Leon and Bob Cava, we are trying to figure out what metals, substrates, oxides, etc are the leading causes of loss in superconducting qubits, and how to make them better. This is also a major research thrust as part of our DOE National Quantum Initiative Center (the Co-Design Center for Quantum Advantage).

Defects in diamond are a useful platform for quantum networking and quantum sensing, but there are an enormous number of defects (helpfully cataloged by De Beers). Trying to design and grow new diamonds for quantum purposes is another major effort here on campus, in conjunction with the Princeton Plasma Physics Lab.

There are other long term materials questions that could impact quantum as well. Scientists have speculated that an elusive state known as a Majorana fermion could encode quantum information in a way that is naturally robust against noise. Scientists like Ali Yazdani and Phuan Ong at Princeton are trying to find new materials that actually realize these particles as a new avenue for quantum computing.

PrincetonEngineers23 karma

Thanks for all the great questions!! I'm signing off now. This was great.

Used_Statistician31817 karma

What, for you, would be the coolest thing you'd like to see a quantum computer do in the next couple years? Often it seems like the uses are either very speculative (discover new medicines!) or very esoteric (give us insights into quantum systems themselves). Maybe there's very cool stuff in the latter ...

PrincetonEngineers28 karma

The two coolest outcomes would be to do literally anything practical on a quantum computer, or to discover some fundamental reason why quantum computers fail that completely changes our perspective on the world.

We don’t actually know what the first practically accessible quantum algorithm would be. I could probably speculate that it would be something related to simulating chemistry, possibly catalysis.

numberandphase10 karma

Dear Prof. Houck,

I am a big fan of your research work, especially your experiments on tunable couplers. I have some questions, specifically about tunable coupling architecture. You worked on suppressing photon shot noise to increase coherence times for a tunable coupler setup consisting of two transmons coupled to a superconducting island. Would it be possible to apply this idea to the currently implemented tunable coupler architecture proposed in 2018 by F. Yan?

Also, tunable coupler architecture has resulted in the implementation of high-fidelity two-qubit gates, which is not possible with static couplers. It has enabled realizing fidelity up to 99.85%, not counting the 99.9% fidelity gate result that was presented in the APS March meeting recently. Given this, how can we further improve on the coupling architecture? In what research directions do you see them being used in the future?

Finally, a general question pertaining to the field of superconducting circuits-based quantum computing. I would think one possibleresearch milestone achieved in the field in the coming years would be in Quantum Error Correction, given the recent paper by Google in regard to successfully implementing the five-distance surface code with their Sycamore qubit processing unit. What do you think are the big research goals in the field that would be achieved in the current decade and the next decade?

Looking forward to reading your replies, and thank you very much for the AMA.

Best regards

numberandphase

PrincetonEngineers10 karma

This is a great but detailed question. Feel free to send me an email.

SnooBooks467710 karma

What QC learning materials would you recommend for someone who has a solid background in math and physics, but not much particular exposure to QC itself?

PrincetonEngineers26 karma

Tom Wong (currently at White House Office of Science and Technology Policy) has a great free(!) book on quantum computing available on his website: https://www.thomaswong.net/

manuhe1010 karma

Will quantum computers ever replace the normal computers everyone uses? Will there come a day where we can watch Netflix, play video games and so on on a quantum computer?

PrincetonEngineers29 karma

Definitely not (that’s the kind of answer that could come back to haunt me in twenty years). Quantum computers can do anything that classical computers can do, but they’ll almost assuredly be slower and costlier than their classical counterparts at running applications/algorithms that don’t have a large amount of quantum speedup.

Also, we are in an era where you don’t actually see or touch most of the classical computing you use – your phone or laptop just needs to be fast enough to connect to all of the data centers around the world. Once we have quantum computers, those will be no different.

PrincetonEngineers21 karma

Not accurate. Plus, there was no Michael Peña in Quantumania, which is a total fail.

PrincetonEngineers35 karma

It’s hard to quantify, but on a scale of one to ten, I’d say at least a 007.

RobustQuantumSim7 karma

You and the University of Maryland researchers are all a part of the NSF Quantum Leap Challenge Institute for Robust Quantum Simulation. What is robust quantum simulation and why is it important to have quantum researchers from multiple universities collaborating on this topic?

PrincetonEngineers15 karma

Given your user name, I would think you could answer this one! Quantum simulation involves using the pieces designed for quantum computing to simulate other systems. For robust quantum simulation, you want to actually be able to verify that your simulation is correct, or at least bound to how well it can work. Collaboration – across universities, with national labs and industry, across disciplines – is the lifeblood of science. No one has all the answers, and new eyes bring fresh ideas to a field.

fritter_away4 karma

Has there been any measurable progress in quantum computing over the last few decades? Not just number of qubits, but also factoring in speed and error correction.

In classical computing, there’s Moore’s law. Is there anything roughly similar in quantum computing?

Is quantum volume per second going up over the years? If so, what is the rate of change of this metric? Is it doubling every year or two?

PrincetonEngineers8 karma

All of the progress in quantum computing has been made over the last few decades! The first algorithms came in the 80’s and 90’s, and the first superconducting qubit was only measured in 1999. Since then, we’ve seen a Moore’s-Law-like growth in the scale of experimental machines. At first, this showed up in the length of time that a qubit could store quantum information, which has risen from a nanosecond to hundreds of microseconds over the past two decades – that’s five orders of magnitude! We’ve seen a recent surge in the number of qubits per quantum processor. Quantum volume is a metric that tries to incorporate all of this in one number, and any plot of quantum volume over time shows the kind of exponential growth that you’d want to see.

plsthrowmeawayagain3 karma

To my (rudimentary) understanding, quantum computing is great because it can solve complex problems that classical computing cannot. But this doesn’t seem very relevant to a lot of people who use computers in their daily lives, like to surf the internet, play a video game, check email, etc. In that sense, would resources not be better spent developing classical computing? What are some benefits of quantum computers that you think will be relevant in our everyday lives?

Second, more technical, question: I’ve been told that quantum computing is inherently non-deterministic because of Heisenberg’s uncertainty principle, and therefore it would offer some interesting possibilities for predicting stochastic phenomena, for example turbulence. What are your thoughts on this?

PrincetonEngineers5 karma

Quantum computers aren’t going to replace classical computers, and we should of course keep putting a lot of resources into making classical computers better. But, for certain computing problems (including some involving quantum chemistry/physics), we know that classical computers aren’t going to be the solution, and these are the problems where quantum can shine.

For your second question: it’s true that the outcome of measurement in quantum computers is probabilistic. I don’t know of an algorithm that uses this to make them particularly useful for problems involving randomness.

sax_master2253 karma

I have two questions! First what got you into quantum research, and what keeps you motivated?

Second, more personally, what advice would you give someone starting graduate school at Princeton in the fall?

PrincetonEngineers5 karma

The variety of the job helps me stay motivated. Some days it’s cool science or actual progress on technology, but others it’s trying to find new ways to teach and inspire, or new ways to bring a large center together.

PrincetonEngineers6 karma

To your second question: Keep an open mind! Lots of people come into graduate school thinking that they have to keep doing what they did as an undergrad. You can change fields, and there might be fun stuff to do that you haven’t even considered!

JazibZahir2 karma

Are you seeing progress in introducing topics related to quantum science and engineering to the K-12 curriculum? Do you see value in this approach or how do you feel it should be done?

PrincetonEngineers8 karma

I am hearing more and more about introducing quantum in K-12 classes, and I’ve loved the curiosity and enthusiasm that I hear every time I get into a classroom like that. So much of early education is about inspiring kids to want to do science – space, dinosaurs, quantum!

Quantum is counterintuitive – it goes against what people expect based on their everyday experience of the world – and so it can inspire and delight. Practically, early quantum education also needs to go hand in hand with probability, which is often not covered enough in K-12.

Aergia-Dagodeiwos2 karma

So I have been reading a lot of papers on quantum research. Not a physicist btw... just interested. With the amount of energy required to entangle particles, is it possible that entanglement isn't what we think it is? So far, I believe the hypothesis is that it is creating miniature wormholes. Seems overly complicated when most everything is based on more simpler forces and interactions. I do believe there is something happening at a much smaller scale than we can detect currently.

PrincetonEngineers5 karma

It doesn’t actually take a lot of energy to entangle particles. There are ways that you can use quantum information as a lens to think about other parts of physics -- like black holes or worm holes -- but entanglement just means that you can’t describe the state of more than one particle by describing the individual states -- you have to describe the whole.

prest0G2 karma

What are some practical applications of quantum computing that is rarely discussed or acknowledged?

PrincetonEngineers11 karma

The least discussed application of quantum computers is to simulate a system of 2^N masses on springs. That’s because this class of applications was only discovered a few weeks ago, which just shows that this field is still figuring out what quantum computers can do! Here’s a link to the preprint: https://arxiv.org/abs/2303.13012

gazzza2 karma

In your opinion, what are the major obstacles currently facing the field of quantum computing, and what novel approaches or breakthroughs do you think are necessary to overcome these challenges and realize its full potential?

PrincetonEngineers3 karma

There are obstacles at every layer of the technological stack – that’s why this is such a fun interdisciplinary challenge! We need better materials to improve coherence times so quantum information can last longer; we might need better qubits that are immune to noise. Measurement is relatively accurate, but it’s hard to measure enough qubits fast enough to actually do error correction. We know error correction is possible, but we don’t know the best way to actually do it in a real system. And of course, we don’t actually even know all the things that a quantum computer can do once we have one.

Seeker_Of_Knowledge-1 karma

I didn't do any physics in high school and right now I'm doing a general CS degree. Is it manageable to specialize in Quantum Computing or do I need a background in physics and have physics as my minor?

PrincetonEngineers6 karma

You can definitely understand and contribute to quantum computing without much of a quantum background. We teach a course that requires no serious physics – all you need is linear algebra. Quantum physics gives us the rules for quantum computers, but if you accept those, you can do a lot!

Alphaq_asr1 karma

Do you think there will ever be a quantum advantage or a quantum speedup that can be achieved with today's Noisy intermediate-scale quantum devices? If so, which field is set to benefit the earliest? Is it in chemistry, machine learning, or cryptography? If not, when can one realistically hope that quantum computers/quantum sensors can do certain tasks (with real human value) better than classical ones? Is it possible in our lifetime? Can you give an example of a problem where quantum devices can out-perform current state-of-the-art methods?

As a beginning graduate student, are these exciting times or over-hyped times?

Thanks a ton.

ASR

PrincetonEngineers3 karma

They are both exciting and probably a bit overhyped. We are making real progress towards building the first type of computer that isn’t a Turing machine.

At the same time, I am somewhat NISQ averse. I don’t think there are any real advantages to be had, especially on practical problems, though I hope I am wrong. But, we are learning a lot about building scaled systems by building and using these machines, and I think that a steady focus on fault-tolerant systems will get us to something useful within my (expected, based on actuarial tables) lifetime.

IAmJonS1 karma

How would you compare (1) atom arrays, (2) ion traps, (3) superconducting circuits, and (4) prayer as approaches to scalable quantum computing?

PrincetonEngineers3 karma

We need them all! Neutral atoms, ions and superconductors all are making fantastic progress, and it’s way too early to pick a winner. At the same time, all of them have challenges on the horizon that will require real scientific breakthroughs. Perhaps that’s where (4) comes in?

HealthyAliens1 karma

How can entanglement not be related to just the smallest quantum particle? Why should it make sense that objects with more than one particle be entangled together ? At which point do multiple particles combine to become a seperate objects that can be entangled together?

PrincetonEngineers5 karma

Entanglement has more to do with how many degrees of freedom that a system has than it does with particle size. Superconducting qubits are enormous – some you can even see with your naked eye – but when cooled, they have a single collective degree of freedom and all of the atoms in the qubit act together. We can couple multiple qubits and show those degrees of freedom are entangled.

ITSMART11 karma

As an upcoming class of 2027 Princeton Undergraduate interested in Astrophysics, but also REALLY FASCINATED by Quantum Computing and Quantum Engineering, what are the possibilities to get involved in this amazing field, whether through classes or alternatives? How could a astrophysics career incorporate Quantum Computing?

PrincetonEngineers2 karma

We have a growing number of quantum courses at Princeton, including a course for people without a quantum physics background, a course on how to build quantum computers, and a new lab course where you can actually work on experimental quantum systems. There are also a lot of research opportunities to work closely with faculty – undergrads have made real contributions to some of my lab’s best work! https://quantum.princeton.edu/

DJlonghammer29 karma

What are the materials science challenges and opportunities for accelerating the development of quantum devices?

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