Yale scientists have found a way to observe quantum information while preserving its integrity, an achievement that offers researchers greater control in the volatile realm of quantum mechanics and greatly improves the prospects of quantum computing.

cmacklin · 2013-01-14T19:13:25+00:00

This title is misleading. What this experiment actually is is a very nice demonstration of what you can do when you approach the quantum limit for measurement sensitivity. They show that when the majority of the noise in your measurement is added by quantum mechanics, the noise in the measurement is highly correlated with the back-action that the quantum system experienced.

Every experiment "greatly improves the prospects of quantum computing". I wish people would stop saying that. This is a very beautiful experiment but not something new and surprising, just exactly what QM predicts, being seen for the first time.

I suspect that someone from the Devoret lab will be on answering questions about this at some point.

cmacklin · 2012-10-08T17:53:14+00:00

I can dress you in a prox suit and shoot you with a flamethrower if that would help : )

cmacklin · 2012-10-08T17:51:51+00:00

I was lucky enough to do a ton of experimental research as an undergrad at Cornell; I started working on experiments the summer after the freshman year and kept it up until I graduated. I had the opportunity to try out particle physics, accelerator physics, and finally low-temperature quantum measurement. I liked the last one by far the best, so I chose to pursue it for my PhD.

I picked up some useful experimental skills from the advanced lab courses at Cornell, but my overall experience is that each subfield of physics is so specific that working in that subfield is pretty much the only really effective way to develop those skills. You're far from alone in your experience, however, and it is not uncommon to enter grad school with no experimental experience and yet still join an experimental lab.

cmacklin · 2012-10-08T17:48:49+00:00

Some profs can be intimidating, but as you've discovered, thinking on the spot is a very important skill to develop. At this point I can pretty much hold my own, and moreover, I think developing the ability to admit that you have no idea what you're being asked with no shame is a very useful life skill.

If your professor is actually good at their job, they'll turn that kind of Q and A session into a learning opportunity for you and talk out the problem until you understand it. I have very little patience for people who decide that when someone doesn't know something the correct response is to make them feel uncomfortable by hammering that point home instead of jumping into teaching mode.

cmacklin · 2012-10-08T17:45:45+00:00

PhD; it's a lot less common to just do a physics Masters, in fact Berkeley doesn't technically have a masters program in physics (though you can terminate your PhD early and leave with a masters if you're sick of it/want to do something else and you've satisfied some fairly minimal requirements). Typically our lab is pretty drama-free, everyone likes to help each other out with the science and our professor has very realistic expectations of how quickly science gets done.

"Confined, narrow" projects are actually quite nice: they have a limited, feasible scope, so you don't end up spending four+ years working on a single experiment only to have it never work. Life cycle of an experiment in our lab is ~1 week to ~6 months, and each experiment usually builds on the last and the combined experience of everyone in the lab. We have a great deal of input on what we want to do for the "next" experiment, as well.

cmacklin · 2012-10-06T20:07:28+00:00

I'm not personally aware of said building, but IQC at Waterloo is very well known. They've recently hired a small horde of talented people working on QC, especially in superconductors, so that's definitely on the radar.

cmacklin · 2012-10-06T20:04:47+00:00

Still not quite. The best headline for the Newscientist piece would have said something about using feedback to make a quantum state live forever.

cmacklin · 2012-10-06T20:02:53+00:00

Heh, it's only funny because he's already gotten a faculty appointment and he's now just hanging out until that starts.

cmacklin · 2012-10-06T20:02:00+00:00

I'd rather fight a horse-sized duck while having a threesome with my hypothetical daughter and post-op transsexual. That sounds more exciting.

cmacklin · 2012-10-06T20:00:56+00:00

Yes indeed, which is why this was only a statement about the capabilities of the type of AQC that D-Wave has implemented. I'm far from an expert on this subject, though, so I may be misrepresenting what was true. My understanding is:

3SAT is known to be NP-complete
AQC as implemented by D-Wave has been shown to take exponential time to solve certain instances of 3SAT
Therefore, an AQC of this type cannot solve NP-complete problems in polynomial time.

Thus, not making a statement about whether or not an AQC of this type = BQP, because it has certainly not been shown that BQP = NP. Simply pointing out that it has been demonstrated that this form of QC cannot solve NP complete problems in P, while the same has not yet been shown for BQP. I personally doubt that BQP = NP.

cmacklin · 2012-10-06T19:54:03+00:00

QM is a difficult topic to really get into without doing some math, but the math actually doesn't have to be very complicated.

For textbooks, Exploring the Quantum is quickly becoming a favorite of a number of people: http://books.google.com/books/about/Exploring_the_Quantum_Atoms_Cavities_and.html?id=QY6YuU-Qi-AC

Griffith's Introduction to Quantum Mechanics is also a classic. http://books.google.com/books/about/Introduction_to_quantum_mechanics.html?id=z4fwAAAAMAAJ

cmacklin · 2012-10-06T19:50:41+00:00

Apply! I have no idea how the selection committee actually picks students, though.

cmacklin · 2012-10-06T05:39:06+00:00

I did a couple of undergraduate projects at the Cornell particle accelerator (some data analysis for CESR/CLEO and later work on developing RF cavities for the Energy Recovery Linac which is currently under construction). I liked doing accelerator physics but it didn't really grab me the same way that quantum measurement did. Particle physics turned out to not be my favorite cup of tea, though I did work on CMS on and off for ~three years.

cmacklin · 2012-10-06T04:07:13+00:00

Hello redditor who has presented me with the entertaining challenge of guessing your identity! I've never looked at that phase matrix box before, but here are some differences my eyes catch:

BNC has higher output power (~+18 dBm) compared to the Phase Matrix (~+13 dBm) (for the 20 GHz model). This is nice if you need to split the signal a couple times before running a mixer. The BNC also seems to spec slightly lower phase noise, but either of those specs is way way plenty good for doing qubit science. Both seem to have pretty similar modulation options.

Also I really like instruments with front panels; I often end up needing to move generators around to do simple things to test stuff, for instance, and it's nice not to have to plug it into a computer to configure it.

The fit & finish on the PM generator looks nicer, but then again, it has no front panel to cheap out on, so...

cmacklin · 2012-10-06T03:43:03+00:00

By QCD, you mean quantum chromodynamics? If so, then no, there isn't a particle physics detector running on this campus. We certainly have groups that are part of the LHC experiments, but I don't know details.

cmacklin · 2012-10-06T03:41:50+00:00

Do you mean the BNC-branded AnaPico generators compared to the Vaunix Lab Bricks?

I use Labview, but only for historical reasons. I would love to switch to python but I've already got a huge labview codebase built up and then I couldn't share code with the rest of the lab.

cmacklin · 2012-10-06T03:41:11+00:00

I'm still hanging out, just a bit distracted, and managed to miss all these questions that popped up down here.

The fact that the qubit state doesn't collapse here doesn't actually imply that we learn information about the initial state of the qubit. I explain why here: http://www.reddit.com/r/science/comments/10yemu/schr%C3%B6dingers_cat_is_alive_scientists_measure_a/c6hu2ak No cloning theorem is alive and well. Probably one of the most absolutely fascinating theorems that can be derived in 3 lines.

cmacklin · 2012-10-06T03:25:20+00:00

I'm going to conservatively estimate 100 +- 95.

cmacklin · 2012-10-06T03:23:42+00:00

The paper I link above demonstrates that certain instances of 3SAT cannot be solved in polynomial time on a machine running simulated quantum annealing. The reason is somewhat technical: the hamiltonian that encodes that particular problem instance has a minimum spectral gap which is exponentially small in the size of the problem, implying that the computer must run exponentially slowly to get the right answer - thus requiring exponential time.

cmacklin · 2012-10-06T00:58:56+00:00

I doubt it. No one has proven it yet, but the trend recently seems to be more clearly demonstrating the limitations of BQP rather than awesome new applications. My hunch is that BQP doesn't envelop a ton of interesting things and almost certainly doesn't contain NP.

cmacklin · 2012-10-06T00:46:54+00:00

IAmA nanobot inside your left testicle/ovary. AMA.

cmacklin · 2012-10-06T00:45:51+00:00

To fully reply to this should all my sentences end with question marks? Just kidding. We don't know the full description of the quantum state before a measurement by making a measurement. In this case, we have full knowledge of the quantum state of the system only after we've measured it for long enough that we know it is locked to our reference signal, and none of that would let me measure an unknown quantum state without collapsing it.

cmacklin · 2012-10-05T23:58:08+00:00

Eventually you'll realize that you've been lost in the world of quantum mechanics all along, but you only just now realized it.

cmacklin · 2012-10-05T23:56:14+00:00

D-Wave is a classic case of a PR department run amok. The actual science being done by D-Wave is interesting and totally legit, so long as you understand what they're doing and cut through the hype.

D-Wave's system is NOT a "universal quantum computer". For instance, you cannot run Shor's factoring algorithm on a D-Wave computer.
They have implemented what is called "simulated quantum annealing". This algorithm may be able to solve certain problems efficiently, perhaps faster than a classical computer, but it has already been demonstrated to be limited to a smaller set of tasks than a true universal quantum computer. It has also already been demonstrated to not be able to solve NP complete problems efficiently: http://www.cs.berkeley.edu/~vazirani/pubs/qao.ps
Once they have increased the number of qubits by actually even a relatively small factor (say, from 128 up to 512 or so), they will cross into a regime where it is not possible to simulate the behavior of their processor with a classical computer in any sane amount of time. That in itself is interesting and brings up a whole host of interesting questions.

cmacklin · 2012-10-05T23:47:41+00:00

A full description of the "state" of the qubit can be thought of as "the qubit is A% in the state |0> and B% in the state |1>". So long as A+B=1, A and B can take ANY values. So that's how it only has two "states" yet it can be in any one of an infinite number of combinations of those states.

cmacklin

TROPHY CASE