6,262 reputation
1943
bio website jfitzsimons.org
location Singapore, Singapore
age 32
visits member for 4 years, 1 month
seen Nov 17 at 5:14

I have just moved to the Center for Quantum Technologies in Singapore, after spending the last 3 years as a Merton College JRF in Theoretical Physics and a Senior Research Fellow in Oxford University Department of Materials. My research focuses largely on theoretical aspects of quantum information processing. In particular I am interested in spin networks, measurement based computation, cryptography and computational complexity.


Sep
13
comment What can the D-Wave quantum computer do?
@Jus12: Yes, I know. I just thought I should point out that the statement needed is slightly stronger.
Sep
13
comment Size of a quantum computer to effectively calculate macroscopic reality from quantum mechanics
This answer is incorrect and tries to violate the Holevo bound. From an $n$-qubit system, you can at best extract $n$ bits.
Sep
13
comment What can the D-Wave quantum computer do?
@Jus12: Solving an NP-complete problem would mean that it could solve any problem in NP (drop the -complete), and as factoring is in NP, you are correct that it could solve it. However, you will notice that nowhere in my answer do I say it could sole NP-complete problems in polynomial time, and DWave has backed away from making such claims. Thus, even if it works as advertised, there is no reason to believe it would be could at factoring. There is a generic polynomial speed-up from quantum mechanics, and that is what they are counting on, even for exponential time algorithms.
Sep
13
comment What can the D-Wave quantum computer do?
@Lagerbaer: Factoring is not known to be NP-complete, but this can't be proven without first proving that P$\neq$NP.
Sep
12
comment Theoretical Physics - How to?
I disagree with the emphasis you are putting on having an experimental background. In many of the top universities theoretical and experimental physics are different departments, and there is a reason for this. Theoretical physicists need a very different set of skills than experimental physicists. Their ability to do path integrals, for example, tends to be of much greater importance than their ability to align optical elements. Both fields require a huge amount of background knowledge, and it is very rare to be successful at both (I can think of examples, but they are few and far between).
Mar
16
comment Home experiments to derive the speed of light?
@Nic: yes, you are right of course.
Feb
19
comment Why can't the outcome of a QM measurement be calculated a-priori?
@Sklivvz: There has been plenty of work on this basis issue and there are fairly good reasons why you don't see superposed states. Decoherence, for example, means that the states become non-local so asking why we do not observe them as a superposition is a simple case of the fact that we are making a local observation on a non-local state and so cannot observe all coherences.
Feb
19
comment Why can't the outcome of a QM measurement be calculated a-priori?
@Sklivvz: I don't understand. Are you dismissing the Everett interpetation out of hand? It is certainly the most widely believed interpretation among quantum physicists, followed closely by the "shut up and calculate" school. Penrose has some weird and very non-standard views about quantum measurements, so his book may not be the best primer on the subject.
Feb
18
comment Why can't the outcome of a QM measurement be calculated a-priori?
@Sklivvz: That's totally incorrect. In the Everett interpretation there is no instantanous measurement event, it can all be treated as unitary evolution.
Feb
18
comment Why can't the outcome of a QM measurement be calculated a-priori?
@Sklivvz: Only in the Copenhagen interpretation is there any discontinuity. In the Everett interpretation there is no such issue. However there is no experimental way to distinguish the two.
Jan
25
comment Does decoherence need non-determinism?
@WIMP: I know you didn't ask whether it leads to signalling. I am saying that it is impossible (even for the kinda sorta collapse you talk about) because it allows it implies signalling. That is a reason to discard it as a theory immediately.
Jan
23
comment Can gravitational potential energy be released in a fire?
@Noldorin: Yes, but there is an ambiguity in the question. As Peter mentioned, if there is no (or little) gas emitted then the situation is different.
Jan
23
comment Is it possible to separate the poles of a magnet?
Seriously? A down vote with no comment? What exactly is incorrect with this answer?
Jan
23
comment Can gravitational potential energy be released in a fire?
@Sklivvz: I guess so.
Jan
23
comment Does decoherence need non-determinism?
I've posted an updated answer to answer the question as I now understand it.
Jan
23
comment Does decoherence need non-determinism?
@WIMP: Can you please revise the question to make it clear exactly what you are asking? It's currently not clear either from the question or subsequent comments.
Jan
23
comment Does decoherence need non-determinism?
You need to rephrase your question if both myself and Matt have misunderstood your intention. Are you now asking if you can deterministically collapse to a particular eigenstate? The answer to that is no, because it violates linearity.
Jan
23
comment Does decoherence need non-determinism?
@WIMP: The first line of my answer reads: "if by deterministic you mean possessing a local hidden variable interpretation". Certainly global hidden variables can be made to work, but then they always can.
Jan
23
comment Can gravitational potential energy be released in a fire?
@Sklivvz: If the entire mass of the fuel did remain at the top, then you would be correct. However, this is not presumably not the case. When you burn something, in most cases you are converting a significant proportion of the mass to gas.
Jan
23
comment Why does a ballerina speed up when she pulls in her arms?
@Sklivvz: I have no objections to the new form, and have removed my downvote.