Happy World Quantum Day! We are a group of quantum science researchers at the University of Maryland. Ask us anything!

What is the most misunderstood part of quantum science?

Also, what is the coolest thing about quantum science that most people don't know?

Edit: what's your favorite movie that uses the word quantum?

What is the most misunderstood part of quantum science?

NL: Some people think that no one really understands it, but in reality we understand it enough to do quantum engineering now. We understand it as much as we understand other areas of science.

Do you guys just put the word quantum in front of everything?

Do you guys just put the word quantum in front of everything?

NL: I don't quantum think so.

[From the room]: You haven't seen Ant-Man?

What is something that blew your mind as a quantum science researcher ?

What is something that blew your mind as a quantum science researcher ?

SR: Personally, that would be the violation of Bell's inequality, which forces you to either give up on locality (causality) or objective reality. We tend to give up on the latter because we don't know what the world would be like otherwise.

NL: I was trying to cool down a single atom when I learned that the concept of temperature does not apply to a single particle.

What’s the most contentious theory in your field presently?

What’s the most contentious theory in your field presently?

NL: It's a bit technical, but in my field there's an argument over whether variational quantum algorithms (a quantum computer running an application that is controlled by a classical optimizer) can be used to solve problems of practical value.

AK: As far as I can tell, the biggest fight is over whose qubit is best.

What’s the easiest way to understand quantum science?

SR: Check out our Quantum Atlas!

AK: There's a book I like by George Gamow called Mr. Tompkins in Paperback. Mr. Tompkins goes to physics lectures and falls asleep and dreams about physics. Like the speed of light being 100 miles per hour, or being an electron -- things like this. And then the janitor wakes him up and makes him leave at the end of every chapter.

Are we in a simulation?

Are we in a simulation?

NL: Does it matter?

SR: I don't think there's any way to tell.

Do you think we will have quantum computers commercially in the next 50 years or is that too soon?

Do you think we will have quantum computers commercially in the next 50 years or is that too soon?

NL: We already have commercially available quantum computers now, but they cannot currently beat classical computers. But you can spend money on using them to run your algorithms. There's even competition from various companies.

SR: Quantum computers that will have significant economic impact ("change the world") are still many decades away.

AK: We have quantum computers now, but you can dream about a different machine that just works like our current computers do: Put an algorithm on it and let it run to the end and you know it'll be correct. We're stuck on something like 10 quantum bits now, so that kind of machine doesn't exist yet.

Do things still exist outside of direct observation? If there is no observer to cause the wave function to collapse can we be sure that things exist outside of direct observation at a macro scale?

Do things still exist outside of direct observation? If there is no observer to cause the wave function to collapse can we be sure that things exist outside of direct observation at a macro scale?

SR: This is a fun one to argue about. The classic question is "Is the moon there if no one is looking?"

AK: One of the things I find confusing about this quantum observer stuff is that if someone or something observes something and you don't know about it, it's still been observed.

SR: In the end it still comes back to probability.

NL: It doesn't have to be a human observer.

AK: Right, the trick is that direct observation doesn't need to mean that you looked at it.

NL: For example, the water in the sea will still be able to tell that the moon exists if no one looks at the moon. If an atom decays and sends out a photon, that atom will decay whether or not someone observes the photon. What matters is that information about the atom is lost.

Q1: I've read a few things indicating that if quantum computing becomes scalable, it would quickly make contemporary cryptography obsolete.

How accurate do you think that idea is, and if that idea is realistic how soon do you think it's possible?

Q2: Current computers use a chemical printing model for placing their transistors, what is the baseline process of making a quantum computer, are we at the equivalent stage of using quantum punch cards at the moment?

[for Q1]

SR: There's a whole world of post-quantum crypto that is being developed rapidly to take care of that problem.

NL: Once quantum computers are good enough to beat current encryption, there will be new forms of encryption that are safe from quantum computers.

[for Q2]

NL: There are different technology platforms for making qubits. Some are based on trapped atoms or ions suspended in free space and addressed by lasers. Others rely on printed superconducting circuits addressed by wires.

SR: Which platform wins out in the end is still a completely open question.

AK: And maybe different platforms will win out for different applications.

How can you observe quantum particles if they are a wave?

NL: The wave describes the probability of finding the particle at a certain place.

AK: How do you measure a wave? By putting something in its path and seeing what it does.

Will we ever beat the speed of light for communication and perhaps travel?

Will we ever beat the speed of light for communication and perhaps travel?

SR, NL, AK: No.

Do the double slit experiment and variations of it like the “quantum eraser” keep you up at night or do you just get over the strangeness of it?

Do the double slit experiment and variations of it like the “quantum eraser” keep you up at night or do you just get over the strangeness of it?

SR: Quantum mechanics is a description of our knowledge of nature, not a description of nature. Once you accept that, none of these things are weird. Quantum mechanics is fundamentally a probabilistic theory, and probability is not something you can hold in your hands.

AK: Here's an analogy I like: Learning to program, you have to learn a different set of rules and a different way of thinking of things. Quantum mechanics is kind of the same. It has rules you don't encounter in everyday life, but once you become familiar with them they make sense, the same way that once you understand how to program, code does what it's supposed to.

NL: In Newtonian mechanics, people tend not to ask what's "really" going on and are happy with a mathematical description that explains the observations.

We hear a lot about quantum computing in the news, but are there other practical applications of quantum? How might quantum technology improve motorsports, for instance?

We hear a lot about quantum computing in the news, but are there other practical applications of quantum? How might quantum technology improve motorsports, for instance?

AK: Sure, you just have to tunnel through the corners.

I could be misunderstanding, but I’ve heard about sending information by the uncertainty principle (or quantum tunneling?), ie knowing an electron’s speed, & so not knowing it’s location, allowing the electron to ‘jump’ through something it should not be able to - a) do I have the basic principle right?, b) what has been produced by that? Effective testing, I mean

What role, if any, does the underlying quantum nature of the world affect us? Anything we take for granted, but couldn’t exist if quantum mechanics wasn’t real?

I’ve heard a theory that all matter is (or can be expressed as?) an energy waveform (higher & lower energy levels) - do I have that concept correct, & if so how does that interact with qm?

I’ve heard that due to qm, infrared radiation is leaked at the edge of a black hole - if my understanding is correct, what prevents that energy from immediately falling back into the black hole?

What role, if any, does the underlying quantum nature of the world affect us? Anything we take for granted, but couldn’t exist if quantum mechanics wasn’t real?

AK: All of the building blocks that go into everyday things, deep down they are the way they are because of quantum mechanics. Rocks, pieces of metal, biological stuff -- they are all made up of stuff, and the building blocks the universe has to put together is determined by quantum mechanics (like the elements in the periodic table that are used to make molecules).

SR: For instance, we needed to understand quantum mechanics to be able to build a transistor and the computer we're typing this on.

When it comes to the question about lost information in the context of black holes, what is the prevailing wisdom? Is locality violated or is information really lost ect

NL: Under the dynamics of a closed quantum system (called unitary
dynamics), information can never be lost. So all a black hole can do is
distribute information widely amongst all its constituent particles.
This process is called "scrambling". There are theoretical protocols to
retrieve information from black holes in principle (....though not in
practice of course). Some of these have been demonstrated on a small
quantum computer (where the action of the black hole on information is
simulated): https://www.technologyreview.com/2019/11/28/102434/how-a-tabletop-experiment-could-test-the-bedrock-of-reality/

What keeps you up at night and why is it DiVincenzo carrying a jackhammer outside of your lab?

SR: Way back in 2000, a theoretical physicist David DiVincenzo, came up with a list of things needed to build a quantum computer (the DiVincenzo criteria) - and the list is still a good place to look at the state of the field. 1. Scalable qubits - scaling up to many (>>100) is still a BIG challenge for all systems 2. Initializing - the easiest one - most systems can do this well 3. Long coherence times - still a challenge but getting better for all systems - being able to tightly control qubits tends to interfere with isolation from the environment leading to decoherence 4. A universal set of gates - these exist but quite often are not perfect - leading to the daunting requirement of error correction (which also is relevant to 3.) 5. Measurement - most systems can do good measurements, although errors here are still something that needs improvement

So David is still at it with his jackhammer, but much progress has been made.

Question about quantum entanglement.

I see somebody put a red ball and a blue ball in separate boxes, then I take one of the boxes a light-year away from the other. I don't yet know which ball is in my box. I open the box and see that the ball is blue. I instantly know that the other ball back on earth is red, yet no information has actually been transferred between the boxes.

How is quantum entanglement different and can it really be used to transmit information faster than light?

AK: The loophole in your example is that you already knew that there was one red and one blue ball.

SR, NL: For entangled quantum particles, the difference is that there isn't just one property (color) that's correlated. There can be another property, like shape, the corresponds to a different measurement. Entangled particles remain entangled regardless of which of these quantum measurements you perform, which indicate that there are more correlations than in the box. To see these correlations at two different spots, you need to measure in the same basis, but that requires some classical communication, which is bound by the speed of light.

Please explain the idea of quantum algorithmss/software using the cloud (i read about some Israeli company focusing on this) and more about the software side of quantum and will the industry look like a bunch of cell phone companies with many brands or like the cpu industry Intel vs amd , a few players dominating the market. Basically if the hardware is so complicated to produce will it lead to only two or three competitive manufacturers with most competition being in software and components?

NL: Quantum computers are currently very large devices that need a lot of specialist care. As a result, access is facilitated via the internet. Either in an automated way, by companies, or in a personalized way (email) by academic institutions. There are dozens of startup companies for hardware and software and large corporations working on quantum computers, and it's not yet clear what the market will look like in the future.

Took a grad CS class on quantum computing a year or so ago. The professor mentioned he didn’t believe individuals would ever have the need to have quantum computers at home.

Do you agree?

NL: It's unclear. People said that the first classical computers would only be something that companies and big institutions would have. Currently our home computing needs are well met by a classical computer. In principle I agree with your professor. You can also access quantum resources remotely via the internet without needing a quantum machine of your own.

SR: It's a long way off.

1. What do you think is the best implementation for scalable, fault-tolerant quantum computers, and why?
2. What are the best quantum computers/systems right now at your lab(s)?

What do you think is the best implementation for scalable, fault-tolerant quantum computers, and why?

NL: Trapped ions. Perfectly repeatable qubits made by nature with strong, controllable interactions.

SR: Neutral atom arrays share similar features and are about decade behind trapped ions at the moment but coming on fast. Large-scale entanglement seems potentially easier in this system.

AK: Superconducting qubits never wander off. You make it, and it stays there!

So Quantum algorithms are approached quite differently from what I understand. Can you give a really basic example of what that might look like with a non-Quantum/simplified version to compare it to?

What are your thoughts on Dark Matter possibly being large masses of Bose-Einstein condensates?

So Quantum algorithms are approached quite differently from what I understand. Can you give a really basic example of what that might look like with a non-Quantum/simplified version to compare it to?

SR: I don't think of quantum algorithms as being all that different from classical algorithms. The gates are different.

NL: A simple example is an oracle problem, where you're trying to find a secret number. You can choose inputs and measure outputs to figure out the secret number. With a quantum algorithm you can try a superposition of many inputs and make the many outputs interfere to give you the answer with only one oracle query. A classical algorithm would require many queries.

SR: But keep in mind that a quantum computer is not just a large parallel processor.

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What do you think of the Everett many-worlds interpretation of quantum mechanics? David Deutsch, one of the founders of the field of quantum computing, has said that he sees the fact that it works as proof that many-worlds is the way to go- do you agree?

AK: My understanding is that many worlds was created because people didn't like the probabilistic aspects of quantum mechanics. I find infinitely many universes nucleating every split second to be significantly more disturbing than probability. Every time an atom decays, every time something is touched, there's a new universe. That bugs me.

SR: Any interpretation that doesn't make testable predictions distinct from what we already have is philosophy not physics. (Shut up and calculate.)