Happy World Quantum Day! We are a group of quantum science researchers at the University of Maryland (UMD), and we’re back again this year to answer more of your burning quantum queries. Ask us anything!

World Quantum Day promotes the public understanding of quantum science and technology. At UMD, hundreds of faculty members, postdocs, and students are working on a variety of quantum research topics, from quantum computing and quantum algorithms to quantum many-body physics and the technology behind new quantum sensors. Feel free to ask us about research, academic life, career tips, and anything else you think we might know!

For more information about all the quantum research happening at UMD, check out the Joint Quantum Institute (JQI), the Joint Center for Quantum Information and Computer Science (QuICS), the Condensed Matter Theory Center (CMTC), the Quantum Materials Center (QMC), the Quantum Technology Center (QTC), the NSF Quantum Leap Challenge Institute for Robust Quantum Simulation (RQS), and the Maryland Quantum Thermodynamics Hub.

Our schedule for the day is (in EDT):

10 a.m.-12 p.m.: Alan Migdall (experimental quantum optics, JQI) and Jay Sau (theoretical many-body physics, CMTC, JQI)

12-1 p.m.: Lunch 😊

1-3 p.m.: Charles Clark (theoretical atomic, molecular, and optical physics, JQI), Nathan Schine (experimental quantum simulation and information with atoms and optics, JQI, RQS), and Alicia Kollár (experimental quantum simulation and information with optical waveguides, graph theory, JQI, RQS)

3-5ish: UMD graduate student and postdoc takeover

For a beginner-friendly intro to the quantum world, check out The Quantum Atlas.

And, check out today's iAMA by Princeton professor Andrew Houck, a physicist known for developing superconducting qubits and studying quantum systems.

Here's our proof!

Comments: 540 • Responses: 61  • Date: 

DigiMagic157 karma

There have been some news in recent years that scientists have managed to separate entangled particles for hundreds of meters or even kilometers. How do they know that they are still entangled after separating them to desired distance?

jqi_news222 karma

AM: Entanglement means that there are correlations (that can not possibly happen in a classical world) between measurements made at the two ends. It's like rolling two dice, and when you roll them they always match. How is that possible without cheating? Entanglement is like there's some cheating going on to correlate the outcomes. To combat the chance that that's happening, scientists separate the entangled particles by long distances so that the cheating would have to happen faster than the speed of light (which isn't possible). Entanglement is a joint property -- a property of a system that contains at least two objects. It's only when you make a measurement on one object or the other that you collapse this jointness to properties of the individual objects. The individual objects aren't 1 or 0 or up or down before you make the measurement. Weirder still is that in some sense you can't say which of the two sides actually made the measurement! But after many repeated measurements at either end, the non-classical correlations survive.

zephyrus196852 karma


How do we know that "The individual objects aren't 1 or 0 or up or down before you make the measurement."

jqi_news71 karma

AM: If you created a system in which the individual objects were predetermined, then you cannot get the results seen in actual measurements of entangled particles. The fact that actual experiments give results that cannot happen if things are predetermined means that the individual objects are not a 1 or 0 or up or down before you make the measurement. We want to assume that when you make a measurement in quantum physics and get a result, that the object had that value before you made the measurements. Again and again, quantum measurement results tell us that's not the case. My son implemented an idea from Howard Wiseman in an Android app that tries to get some of the intuition behind this across.

r00s31419 karma

Is there any way to influence one of the entangled particles to spin up for example (or is it always random) or would that violate some physics law since that would allow faster than light communication?

jqi_news20 karma

AM: It's always random.

Emily Townsend: You can't send information or make the other particle do something that you want it to do.

PoopIsAlwaysSunny12 karma

So how is this actually done? You separate the particles somehow, put one in a truck and drive it a few hundred miles, then at a scheduled time both parties measure both?

jqi_news14 karma

AM: One experiment was done by creating a pair of entangled photons, sending one to a receiving telescope 100km away and keeping one with you.

nickstroller92 karma

Hi, I'm uncertain about the uncertainty principle. More specifically 'Observing it changes it's state'. Surely observing is a human construct, what does observing actually mean? Looking at it? Detecting it in some way? Could it have been 'observed' before animals existed?

Total layman here ...

jqi_news104 karma

AM: Observing means taking information about the thing you're trying to observe and putting it in something else (or the environment). So it has nothing to do with humans or animals. It's whatever mechanism carries the information away from the thing being observed into the rest of the universe!

JS: In principle observations in quantum mechanics amounts to making a copy of the information in one system on another system, and this involves no conscious observers. This is a reflection of the no-cloning theorem in quantum mechanics, which forbids making such a copy without interfering with the system.

Smeegs66656 karma

For someone who has a next to nothing understanding on the matter, is there a book you would recommend that wouldn't be too hard to comprehend what is being said?

jqi_news87 karma

JQI: One of us can recommend What Is Real?: The Unfinished Quest for the Meaning of Quantum Physics. We also recommend our own Quantum Atlas! (And we'll write back with more ideas later as new people filter in.)

AM: Umesh Vazirani taught a class on Coursera, but it seems to be gone now. It looks like this YouTube playlist has the videos from the course.

PeterPolymer32 karma

What happens at scales smaller than Planck Length?

jqi_news65 karma

JS: The short answer is we don't know. The quantum mechanical theory of light tells us that the vacuum is filled with so-called virtual photons, popping in and out with energies that are higher and higher at shorter and shorter wavelengths. When this is combined with Einstein's general theory of relativity (if we were to naively combine with QM), this suggests that the gravitational pull from these virtual photons would be strong enough to trap the photon as if it were in a black hole (the Schwarzchild radius for the experts). This is clearly nonsense.

AM: Or as Han Solo would say, "I have a bad feeling about this."

JS: What this means is that we do not have a consistent way of combining Einstein's general theory of relativity with quantum mechanics. This does not effect physics in the quantum world, which is small enough to evade the effects of gravity. This reflects in the fact that the Planck length is 10-35 m, many orders of magnitude smaller than any length scale we access. For reference, the size of a nucleus of an atom is 10-18 m, which is a billion billion times larger than the Planck length. So while we do not know what happens below the Planck length, we are very far from encountering this physics in real life.

AM: Of course, any time a physicist says it's impossible to do X, there's significant potential for embarrassment in the future.

YESthisisnttaken25 karma

So basically, at a wavelength of the Planck length, a vmvirtual photon would yield so much energy that it would be trapped in its own Schwarzchild radius?

jqi_news16 karma

AM: Yeah, that's right.

Fit-Reveal897731 karma

Hi. I had a query, (might be a foolish one) as the size of the chipsets keep shrinking nm by nm, what is the point at which heisenbergs uncertainty principal will affect the function of chipsets?

jqi_news57 karma

AM: Not a foolish question at all! This is the whole concern behind the idea of Moore's Law ending. It's all quantum from here on down. Really, it's been quantum, but now it will become more and more quantum as the sizes continue to shrink.

JS: The fact that we have transistors, which are the basic ingredients of chipsets, was already quantum mechanical. But electrical engineers at this point can basically forget the microscopic understanding. As chips shrink, the barriers to control electrons become weaker partly because of Heisenberg's uncertainty principle. Material scientists can still change the materials of these barriers to make some progress, keeping Moore's Law alive for a little bit longer. However, we are likely not far from where these tunnel barriers cannot be shrunk further. It's not like transistors will become quantum mechanical at that point, so rethinking the quantum mechanics of transistors may be the path to reducing chip sizes further. But the path here is far from clear.

charatatata16 karma

If you had to explain quantum physics to a 5 year old, how would you?

jqi_news47 karma

AK: Quantum physics takes a lot of math and it's not something you can see or touch every day. If I were talking physics to an interested child, that's not the area I would start with. I'd start with the physics of slingshots, bouncy balls, electronics and cars.

CC: I would demonstrate properties of light using a flat panel display and two polarized sunglasses. This is visually appealing. My boss was once amazed by it. And, it is an example of projective measurement that is the basis of quantum measurement. Here's a video of what I mean.

thorunnr15 karma

Hi there, thank you for this AMA and your time. What is your favorite QM interpretation? Do you believe the universe could still be deterministic or does QM make it intrinsically indeterministic?

jqi_news35 karma

AM: Bell tests sort of show that there is inherent randomness of the kind that made Einstein unhappy

JS: Although he didn't have to live to see it be experimentally demonstrated.

AM: So it seems that it is intrinsically random or indeterministic as you say.

JS: Ultimately, Bell tests do not rule out a non-local deterministic world, so it is still possible that the universe is deterministic in some strange way where actions at one point in space can have an effect over the entire universe (faster than light). There is of course no evidence for any such theory and the easiest way to think of quantum mechanics is as fundamentally indeterministic.

CriticismOwn531213 karma

I'm wondering what kind of math do you use on a daily basis? What kind of hand calculation you guys do? How can I work in Quantum Computing? How soon you reckon it will take for an ecosystem to evolve around it and what kind of services it needs? Im currently doing undergrad in physics from BGU, does undergrad grades matter? Like, you reckon I can apply with mediocre grades to Quantum Computer masters degree? Thanks,


jqi_news21 karma

CC: Linear algebra is probably the most important thing you need to understand the basic elements of quantum.

AK: Linear algebra is usually the way in to quantum mechanics if you don't already know it from the physics side.

CC: I've been teaching a quantum bootcamp, and I find that the main challenges are basic linear algebra of simple vectors and matrices

AK: Quantum is such an interdisciplinary field that many quantum masters' problems are looking to unify interested people from very different backgrounds. They're looking for a non-traditional student rather than a pure physics major, so they are a little more open to someone trying to re-invent themselves.

Belzebutt13 karma

There seems to be some computing problems for which quantum computers hold a future promise if the current trend holds, for example breaking RSA encryption, to the point where governments are storing data/conversations today hoping that they will break them in a few years.

There are computing problems for which quantum computers seem to provide roughly the same performance as classic computers, i.e. they don't provide a tangible benefit (I don't recall an example of that though).

What are some computing problems TODAY for which quantum computers are already used to considerably speed up the solution, or which a classical computer cannot solve?

jqi_news17 karma

CC: In my opinion, today there are no such problems. But, we see people doing quantum chemistry calculations using quantum computers. Those are worth doing. I don't think they yet show that they are competitive with standard techniques running on some of the world's supercomputers. But there's hope that some day there might be.

AK: The challenge is that classical computers are extremely powerful, and quantum computers are in their infancy. So, there is no small task which a quantum computer can do better today.

PessimisticChap10 karma

My brother posed me an interesting thought experiment:

Suppose there are two identical (down to the every last particle) rooms containing a cat, Room A and Room B. We accelerate Room A very close to the speed of light, such that time slows down in that particular room. We do this for a duration until the perceived time difference between both rooms is five years.

My brother thinks that the cat in Room B that has aged five years (in the room that didn’t accelerate) relative to the cat in the Room A, is basically the exact same cat but 5 years older, and is basically the future version of the cat in Room A. Everything the cat in Room B has experienced in five years, cat A will experience.

What do quantum physicists think of this? (Thanks in advance, just thought it was incredibly apt for me to ask this in this particular AMA)

jqi_news17 karma

CC: It depends upon whether the cat is dead or alive :)

AK: I'll give the practical answer, which is that it's impossible to get systems that large identical in every way so that over the course of five years no fly has hit one room and not the other or anything like that. So, their experiences will inevitably diverge.

keilwerth10 karma


jqi_news34 karma

CC: I'm uncertain. I 'shut up and calculate', but I also wonder sometimes.

AK: This is religion. We don't talk about it to try to stay friendly.

sproglet_918 karma

What's your favourite physics theory that you wish was more widely studied?

jqi_news7 karma

AM: (not necessarily a physics theory) Benford's law.

Bobagabush426 karma

Quantum computing is an area that shows extrodinary promise, but is also extremely complex and difficult to learn. Also, while there have been massive private and public investments in quantum computing, there are no publicly announced quantum computing projects that are being commercially used in production. If I were a student trying to decide if I wanted to study quantum computing, how should I evaluate the risk that quantum computing will succeed?

jqi_news18 karma

AM: There are emerging commercial quantum technologies (single photon detectors, single photon sources, quantum communication, quantum sensing), so there is an emerging quantum widget industry, which allows people to get into the business without having to make everything themselves. Quantum information is a field that covers many things. Information? That's the entire universe! There's also the idea of quantum adjacent fields or quantum aware fields that will be critical to making quantum computers work. Single photon detectors are basically used in cell phones to measure distances when focusing a camera in low-light levels. It's also used for an automatic soap dish dispensers.

JS: Quantum mechanics is only a small part of the expertise that goes into quantum computing as an industry. So someone can participate in the quantum industry even understanding very little of quantum mechanics. For example, microwave circuitry and optics are critical components of quantum computers. Basically, quantum mechanics, which is the science behind quantum computing, is already finding its applications in many places, and will continue to find new applications.

AM: So, if you study any of these fields that feed into quantum computing, you won't be affected too much depending on how quantum computing turns out. You get into physics because the whole universe is something to try and explain, and you can do anything physics.

arkhammer6 karma

When a magnet is attracted to something, what is actually happening, at the quantum level, to cause that?

jqi_news5 karma

Michael Winer: I think there are two questions. Why does ferromagnetism exist, and why is there magnetic attraction? In some way, the first one is sort of due to the Pauli exclusion principle and the exchange interaction (don't ask me to explain off the top of my head). The second one is sort of due to the exchange of virtual particles (which mediate all forces), just like electrostatics is.

Divayth--Fyr6 karma

What is your current position and velocity?

jqi_news17 karma

AM: We are...uncertain.

jqi_news6 karma

AK: I fidget at 1 Hz. Maybe 2

sleepyshortcake6 karma

What do you think about the various articles claiming to have entangled macroscopic particles? And that one article about entangling tardigrades??

jqi_news19 karma

AM: That's cool! Scientists are slowly pushing limits to entangling bigger and bigger things. That paper has not yet been peer reviewed yet but we have no reason to doubt the work.

JS: To be clear, they did not entangle two tardigrades but one tardigrade with superconducting qubits. In fact, a superconducting qubit itself is several microns in size, which is actually bigger than the tardigrade. Despite this, superconducting qubits have been entangled and generally been key participants in quantum computers. This is because the key challenge of entanglement is to entangle and verify entanglement for a large number of degrees of freedom (example: ways it can move or vibrate). A superconducting qubit at milikelvin temperatures, despite being macroscopic has very few degrees of freedom. A tardigrade at ten milikelvin might also have very few degrees of freedom. It is impressive that it comes back to life after that though.

AM: And there is a lot of effort directed at non-tardigrade macroscopic entanglement.

Unobtanium695 karma

1 - how can atoms exist in multiple places at the same time? or they don't and we detect them like that because of the deficiencies of our detecting devices?

2- the double slit experiment? is observing the atom actually collapsing it into a wave function? ot is it the effect of our measuring devices?

jqi_news11 karma

AM: For 1), a single atom can have the potential to exist in multiple places at the same time, and it is the measurement that collapses it into one of those potential places. So it's not a detector deficiency issue, if you will. Detecting the output of a double slit experiment is an example of making a measurement that collapses an atom's potential place into an actual place.

JS: One can check that there is never more than one atom at a time in a double slit experiment (hypothetically, for example, by weighing the mass of the setup, although measuring the mass of an atom is not easy).

Bigbird_Elephant5 karma

How realistic is time travel?

jqi_news57 karma

AM: Very, in the forward direction.

UltimaGreen5 karma

Do we have a better understanding yet of the double slit experiment? Why does observation (of a camera) change reality? Or why does reality change based on perception?


Why are intricate computer programs embedded into reality?

jqi_news4 karma

CC: There is a theory of the double slit experiment called the Englert-Greenberger-Yasin duality relation that describes the balance between partial information of which slit the particle went through with the sharpness of the contrast of the interference fringes.

NS: This is a smooth transition from not measuring and getting interference to measuring and getting no interference.

AK: It's usually simpler to talk about these measurements in terms of a perfect measurement (which slit did the particle go through), but you can treat things in between.

Maury_poopins5 karma

The most recent physics book I’ve read was Brian Greene’s The Elegant Universe.

Do you have any recommendations for newer books on string theory or any other advances in physics in the last 13 years?

I’d love to catch up, but there’s so much out there I have no idea where to start.

jqi_news12 karma

AK: One of the books that I really like is by George Gamow, and it's called Mr. Tompkins in Paperback. Mr. Tompkins goes to popular physics lectures and falls asleep in the back and dreams of worlds where the physics is different, like the speed of light is 100 miles an hour. At the end, the janitor wakes him up and kicks him out.

CC: I like Quantum Steampunk by Nicole Yunger Halpern, which combines serious science with a coming-of-age novel with a Victorian era flavor.

witthatbit4 karma

Have any quantum physics jokes?

jqi_news23 karma

AK: In my lab, sometimes we find things that we call 'Heisenbugs'. These are bugs that change when you try to figure out what they are.

Mu_Fanchu4 karma

Thanks for doing this AMA, y'all! My question is: what are the leading countries for quantum physics? Leading organizations?

jqi_news7 karma

NS: Everyone in the room is a member of the American Physical Society and goes to their annual conferences.

AK: But there is lots of activity worldwide

CC: We're partial to the University of Maryland and the National Institute of Standards and Technology!

some_random_noob4 karma

what is quantum foam and why cant I get it in my starbucks late?

jqi_news6 karma

CC: It's a secret menu item.

AK: Available as long as no one anywhere ever reads the menu.

NOVA_stova4 karma

Hello, any advice for people trying to get into a masters of physics program? I finished undergrad a year ago with a degree in data science and did some research in the physics department on the Hubbard Model, but when I applied for grad school I got denied for a lack of undergrad physics courses. Now I’m trying to reevaluate my next steps but physics is still #1 on my list

jqi_news6 karma

AK: I would look at quantum science/engineering or quantum information programs. Many of these programs are looking to unify fields, and are looking for non-traditional students. It's very hard in a traditional physics graduate program to complete the coursework without an undergraduate degree in physics.

AM: Search 'quantum-adjacent' or 'quantum-aware' programs.

AK: There is another route, which is to find a faculty member in a computer science department that does quantum information related research. Something like the the Joint Center for Quantum Information and Computer Science at UMD is a good place to look!

STREXincEmployee4 karma

How do we know that nothing is faster than the speed of light? I have heard (and could be 100% wrong Im dumb as rocks) that there were experiments that were able to slow down light particles(?) doesn’t this break the rules? How does light function as a speed in science if it changes?

jqi_news8 karma

CC: The limitation is about the speed of light in a vacuum. The speed of light in glass is about 2/3 of the speed of light in vacuum.

NS: You're absolutely right that there are experiments that slow light down by sending it through a cloud of atoms. These slow light experiments can go as slow as a millimeter per second. You can slow light down to a crawl, which is really cool.

ackillesBAC3 karma

What are the chances that dark matter is gravity from a parallel universe? Could the many worlds concept support this idea?

I know you're quantum theorists not string theorists, I had a chance to ask Brian Greene this question and string theory does support the idea that dark matter could be gravity from other membranes, tho the math has not been worked out yet.

jqi_news7 karma

Michael Winer: I think we can conclusively say that dark matter is not gravity from a parallel universe, and we know this because Saturn has a moon called Hyperion whose orbit is so chaotic that quantum physics would cause it to be in a complete different place in a matter of months. The fact that we feel gravity where Hyperion is and not where it could have been means that gravity can't go from one branch to another. (I'm cribbing this from an article by Sean Carroll: https://www.discovermagazine.com/the-sciences/quantum-hyperion)

quantum_jim3 karma

Quantum computing is getting a huge amount of attention at the moment, and not just for the science behind it. There's lots of talk about applications and business use cases too.

Do you feel that this gets in the way of other quantum science, or is the QC hype positive for all of quantum?

jqi_news10 karma

AK: I think this hype is really dangerous. One of the problems that it creates is people having the perceptions that things are done when they aren't, which makes it really hard to do the actual R&D that's required.

NS: On the other hand, the attention helps attract people, resources and ideas to quantum science that is beneficial for quantum more generally.

TuvoksDoRag3 karma

If you hit someone with a severed foot, does it count as a kick?

jqi_news2 karma

AK: Find me some dead zombies to experiment with and I'll try it.

fleischio3 karma

Thanks for doing this AMA!

I have a few questions, answer as many or few as y’all have time for.

  1. Superposition: From what I’ve gathered, a particle is in a supposition before it is measured, meaning it exists in all possible states predicted by its wave function before it collapses. My questions are, are all unmeasured quantum systems in superposition before they are measured and how exactly does superposition manifest in the physical world? Is the particle really in multiple places or velocities at once, is it simply a consequence of the wave function, or we just don’t know? I guess the double slit experiment answers this question, but I’d like y’all’s interpretation.

  2. The Measurement Problem. Will we ever truly solve the measurement problem? I understand that The Copenhagen Interpretation is the most widely accepted theory, but I’d like to believe Everett was correct and there’s one universal wave function and an infinite number of universes. What do y’all think is the most probable answer, and what are the implications of Many Worlds, say, on future technology. Do you think we’ll ever be able to communicate with other universes if Everett was correct?

  3. Entanglement. We’ve proven that Bell was correct and “local hidden variables” do not exist. My question concerning entanglement is how we actually measure spin and how the filtering works; are they akin to polarized sunglasses or something to that effect?

Thanks again for doing this!

jqi_news7 karma

CC: For 3. Measurement of spins is indeed very much like polarized sunglasses. This was first done by Stern and Gerlach with silver atoms. There are modern measurements on neutron spins that use the same type of projective measurements using magnetic field gradients.

NS: And with cold atoms, one can use carefully tuned lasers to detect individual spin states.

AK: For 1. In practice, very few things are in superposition. The surroundings tend to make accidental 'measurements' of the system that collapse the superposition. The hard part with quantum computing is keeping the superposition safe from the surroundings.

NS: But if a particle is in a superposition, say of going with this velocity and that velocity, it actually is doing both. This may betray my allegiance to some particular interpretation, but if it looks like a duck and quacks like a duck it probably is a duck. All possible measurements agree with this explanation.

ShogunOfNY2 karma

Please share your thoughts on the approaches taken by top quantum computing companies such as Xanadu or Rigetti computing or the other big names. Any clear winning approaches? Is 10 years too soon to see commercially available use-cases?

jqi_news3 karma

NS: It is still early enough that it's good that there are many approaches on the table. It's not clear which platform will be the best.

Maksitaxi2 karma

What do you know about the quantum computers at IBM and will they get quantum supremacy soon?

jqi_news10 karma

JS: I think the main challenge of applying the idea of quantum supremacy as a benchmark for a quantum computer is that you're comparing the performance of a quantum computer on a problem that was chosen for the quantum computer to be fastest and was never previously attempted on a classical computer. Oftentimes the quantum computer is benchmarked against the performance of a classical computer running a hastily designed algorithm to solve a new problem. Given a new problem, computer scientists often take a significant amount of time to come up with a fast algorithm to solve that problem. The risk of a quantum supremacy benchmark is that the benchmark might be overturned a few years following the achievement when a smart computer scientist finds a way to solve that problem faster. This problem is less likely to arise if the problems chosen for quantum supremacy were more traditional, useful problems which had been attempted on a classical computer before, such as factoring, search or optimization.

LeatherWrangler33782 karma

Any advice at all for aspiring physicists?

jqi_news4 karma

NS: Stay in school kids ;)

AK: Try not to fall into the trap of 'real physicists are different from me and I could never do that'. When I was a first year graduate student, I remember looking at my older labmates and thinking and that there was just no way I could ever learn all the things they knew. And the only thing that would have made that true is if I'd let that thought stop me. They were super nice, they taught me a lot, and I learned it.

CC: I decided I really wanted to be a physicist when I actually worked in a physics lab as a student. Dealing with real scientists and doing really cool things was just wonderful.

Proactively2 karma

Does quantum entanglement fade away or deteriorate over time? Are there any properties that don't translate through this entanglement? What's the greatest number of entangled particles we've managed to maintain within a "local system" (apologies, idk terminologies well enough to make the question with good clarity)? If we did have a system of entangled particles, would the forces acting on one system effect their pairs? Can we entangle multiple particles?

Awesome AMA, and thanks in advance.

jqi_news2 karma

Emily Townsend: Quantum entanglement deteriorates (actually it leaks out) when our system interacts with the rest of the universe.

AM: Can we entangle multiple particles? Yes! Well, some of us can.

Emily Townsend: The more entangled particle A is with particle B, the less it's entangled with particle C.

JQI: In some systems, there is naturally occurring entanglement, but that's not the kind of entanglement that you can use for any quantum information tasks.

Cabbages69692 karma

If von Neumann and Einstein got into a fist fight, who would win?

jqi_news6 karma

AK: My heritage is Hungarian so I have to back von Neumann.

NS: Well how much do they bench?

MiroslavusMoravicus2 karma

Are there things that the general public is not aware of in your field that you wish they knew? Also thanks for doing what you are doing. Whatever it is!

jqi_news6 karma

AK: The word qubit can mean different things in different contexts. If you look in a textbook, it means a perfect qubit that lives forever. Creating an object like that requires perfect quantum error correction which no one has. The objects we use in the lab today are sometimes called 'physical' qubits, and they make a lot of errors.

NS: There seems to be a perception that quantum mechanics is poorly understood, magical behavior. It obeys different rules, but it is well understood and controllable.

CC: One example that occurs to me is quantum cryptography. Many seem to think that this involves the use of a magical quantum technique for sending messages. In fact, it's a way of using quantum mechanics for two individuals to obtain the same cryptographic key that can be used to encode/decode messages. The key itself does not result in an exchange of information, or faster than light communication.

spacemanaut2 karma

If you could get a true answer to a single question in your field, what would you ask?

jqi_news3 karma

AM: How and where does the Standard Model break down?

Steve Rolston: What is dark matter?

JQI: How do you show that P != NP? Or can you?

payne7472 karma

Do we know what is happening with entanglement yet? Is it action at a distance, hidden local variables etc?

jqi_news8 karma

CC: The Nobel prizes in physics in 2022 were awarded for definitive experimental tests of entanglement. It is impossible to reconcile it with hidden local variables.

CC: Einstein was awardee the Nobel prize in physics in 1922 and the specific work of his that was cited there was the discovery of the law of the photoelectric effect. Einstein's explanation of that effect was a foundational step in quantum physics. In the late 1930's, Einstein, Podolsky and Rosen published a paper that described a hypothetical experiment of detecting what we now call entangled particles. Einstein found the results that quantum mechanics predicted for such an experiment to be unacceptable. In fact, Einstein's analysis of what quantum mechanics predicted was accurate as shown in the experiments that got the 2022 Nobel prize. It's taken a long time to do the foundational experiments and Einstein wasn't wrong about his analysis he just found quantum mechanics philosophically objectionable.

fatbunyip2 karma

There seems to be a few organisations and companies that have quantum computers, as well as a bunch of entanglement experimental setups and other stuff.

However they seem to remain within the academic arena. Quantum technology seems to be in the same space as fusion power - perpetually "just a few years away".

Do you think commercial quantum devices (eg computers) or some kind of technology involving entanglement (eg communication) will be possible? What are the current limitations preventing this technology becoming commercialised? How long do you think it will take?

jqi_news3 karma

AK: The main limitation to quantum computers being commercialized is actually the power of classical computers. Supercomputers can do so much that it's hard to find something a quantum computer can do that a supercomputer can't. I think that quantum communication or quantum sensors that use entanglement will be possible more quickly.

NS: There are already entanglement-based sensing techniques that are improving measurement beyond what is possible classically. They are not commercial yet, so far they are being used for fundamental physics, such as in advanced LIGO's gravitational wave detector.

evlswn2 karma

Does gravity “move” (affect things) faster than light?

jqi_news8 karma

CC: A recent experimental test by LIGO, called GW 170817, showed that a gravitational wave travelled at the speed of light within experimental uncertainties. This is the most precise experimental test to date, but modern physics predicts that gravity travels at the speed of light.

agpc1 karma

The amount of energy that is released from splitting atoms is quite large. This is from breaking the strong force? I know quantum field theory involves waves coming into existence and going out of existence in a vacuum in a very short period of time. Is there an associated energy level involving this quantum behavior that could release energy at a similar scale as a nuke? Totally not trying to make a quantum bomb or anything.

jqi_news2 karma

CC: Nuclear reaction that release energy occur because of an encounter between two or more particles that are stable when isolated. You might think of a car driving up a hill and then rolling down into a deep valley and acquiring a high speed. An example from my own experience is a neutron colliding with a boron-10 nucleus and releasing about 2 million electronVolts of energy in the form of alpha and gamma radiation.

NS: The particles coming into and out of existence are 'virtual'. There are real effects due to these virtual particles (eg the Lamb shift). That said, barring a black hole or something else exotic, it is not possible to extract work from virtual particles themselves.

Emily Townsend (the ET): Splitting an atom is in itself a quantum process, so a nuclear bomb is already a quantum bomb.

chadi71 karma

I have heard that when you are holding a clear glass filled with water and you see your fingerprints on the inside of the glass, this occurs because of quantum tunneling.

Is this truly an example of real world quantum tunneling?

jqi_news1 karma

Michael Winer: This happens because of frustrated total internal reflection, which happens because of the wave nature of light. Whether you consider that quantum or not is kind of up to you.

[lots of discussion in the room]

Victor Gatlitski: If the question is "Is this truly an example of real world quantum tunneling?" then the answer is no.

redditorx135791 karma


jqi_news12 karma

AM: Data transmission is limited to the speed of light. It is randomness that can be made to appear to travel faster than the speed of light. That distinction is important, so we will not be able to communicate instantaneously with anyone. As a separate note, quantum key distribution requires a classical channel of communication with the usual speed limits.

Im_a_redditlurker1 karma

What is the best way of couch to marathon for learning Quantum Physics ?

jqi_news1 karma

JQI: We don't know the best way, but check out The Quantum Atlas!

supermoderators1 karma


jqi_news7 karma


JQI: Here's what Steve Rolston, JQI Fellow and Chair of the UMD Physics Department, had to say: https://today.umd.edu/quantumania-meets-quantum-reality

TheeSlyGuy1 karma

How are we building the coldest thing in the universe here on earth?

jqi_news1 karma

Steve Rolston: At JQI, we work with atoms at tens of nanokelvin above absolute zero .

Victor Galitski: And, counterintuitively, they use lasers to cool the atoms down. You might expect lasers would heat the atoms, but the opposite happens.

akairborne1 karma

When my kids were younger and would ask the series of never ending questions e.g. "Why is that car yellow?" I would get them to accept the answer "Because gravity". That would satisfy them until the next question arose.

For years I've recommended that new parents with inquisitive kids use the "Because gravity" to help satisfy the child thirst for knowledge.

Based on current trends, should I recommend parents now use "Because quantum"?

jqi_news4 karma

Emily Townsend: I think you should answer your kids the best you can! If you don't know the answer, tell them there are people in the world that try to answer these questions and they are called physicists. Encouraging your kids' curiosity is really important.

Nastypilot1 karma

Did you discover anything new recently?

jqi_news5 karma

AM: I fixed an oscilloscope!

AK: I discovered just how slow python for-loops are.

cabeshpash1 karma

What is your stance on the Many Worlds Interpretation of quantum mechanics? Also, do you think the differences between the different interpretations of quantum mechanics are actually testable (even if we don’t have the technology yet)?

jqi_news2 karma

Emily Townsend: I really like the relational interpretation of quantum mechanics. It's related to the many worlds interpretation, but you don't have to believe in a whole bunch of universes. You can just think of knowledge as relative, and how you would describe a system might be different than how another observer would, but anything you could measure would be consistent between the descriptions.

Michael Winer: (to the second question) For most interpretations, you can show that they are sort of equivalent, and there's no test that could distinguish between the different interpretations.

Emily Townsend: That's what we mean when we talk about different interpretations.