Joe Fitzsimons

5,596 reputation

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bio	website	jfitzsimons.org
	location	Singapore, Singapore
	age	31
visits	member for	2 years, 6 months
	seen	2 days ago
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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.

	5,596 reputation	bio	website	jfitzsimons.org	visits	member for	2 years, 6 months
1237 badges		location	Singapore, Singapore		seen	2 days ago

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Aug 29	comment	Reversing gravitational decoherence @ScottAaronson: In case of discrepancy I defer to John!
Aug 29	comment	Reversing gravitational decoherence @RonMaimon: No SQUID or other device can do that. It would violate linearity. I don't think Scott is confused at all. His paraphrasing of John's answer seems pretty spot on to me.
Aug 29	comment	Reversing gravitational decoherence contd. It most certainly does not depend on actual measurement results obtained, as you seem to suggest, but rather only on the probability of being able to distinguish the field states.
Aug 29	comment	Reversing gravitational decoherence contd. The amount and nature of the decoherence incurred will depend on the exact nature of the entanglement. If the particle in Scott's example becomes maximally entangled with the field then there will be complete depolarization. However, as John explains and Scott recaps above, the state is only weakly entangled because the two states of the field are very similar (having high overlap), and hence the decoherence is weak. This is indeed related to whether or not you can distinguish between states with a measurement, but it is not the binary picture you present.
Aug 29	comment	Reversing gravitational decoherence @RonMaimon: In your above argument with Scott you are making an error. Is not directly related to measuring a which way information at some other point in space. Rather, since you care only about the reduced density matrix for the system in question, you get this by taking the partial trace over the environment. In a sense this incapsulates all possible measurements, but no measurement need ever be made. If the field and the particle are even weakly entangled, then this reduces the purity of the local system, so if you consider only the state of the particle it appears to decohere.
Aug 28	comment	Reversing gravitational decoherence @RonMaimon: You seem to think something different to what I am actually saying. My point was that in coupling to a field you can excite the field. This is pretty much what you say in you comment, but you phrase it in a weirdly adversarial way. My comments were aimed at pointing out that you can't simply assert that the field is static, but rather that you need to take account also of the effect the particle has on the field.
Aug 28	comment	Reversing gravitational decoherence +1 from me too.
Aug 28	comment	Reversing gravitational decoherence @RonMaimon: Your comments contain a flase premise, namely that a field will remain static if you introduce a particle in a superposition of states which couple differently to the field. That is not true in general, though it can be true in specific instances. Obvious examples of fields being effected by particles in them include Bremsstralung radiation and the Jaynes-Cummings model.
Aug 27	comment	Reversing gravitational decoherence This delocalisation appears locally as decoherence (even though it is not true decoherence).
Aug 27	comment	Reversing gravitational decoherence @RonMaimon, I mean that gravity couples the system to its environment, and so you get a change in the environment over time depending on the state of the system, and hence the reduced density matrix becomes mixed. As regards unitaries, I was refering to the general case. Even if you think about a static EM field, this induces a unitary transformation on the system (no decoherence, but it is altered) and to recover the initial state you need to apply another unitary. In the general case, states can become delocalised, mixing the reduced density matrix, even if the global state remains pure.
May 1	comment	What can the D-Wave quantum computer do? @episanty: That is true of every problem in PSPACE. It might be that P=PSPACE, but we're pretty sure it's not true. But again, the only real evidence is that we can't prove equality.
Apr 18	comment	Higgs mass and the hierarchy problem I fixed your latex. You can include it yourself by enclosing it in dollar signs (like an inline equation in latex).
Apr 18	comment	partial trace with sparse matrices In general, a partial trace of a sparse matrix does not yield a sparse matrix, so you will get only limited improvement by using the fact that the matrix is sparse (maybe a factor of d/s).
Apr 17	comment	Advice on doing physics under the umbrella of mathematics and the converse @Moshe: Maybe. I've no idea what the true distribution is like. My sample size is relatively small (10 or 12) and is not all that geographically diverse.
Apr 15	comment	Advice on doing physics under the umbrella of mathematics and the converse Most string theorists I know are in Mathematics departments.
Apr 14	comment	What is the physical difference between states and unital completely positive maps? No problem. Hope it helps.
Mar 26	comment	Multiqubit state tomography by performing measurement in the same basis @Alex: Ah I guess you mean in the correlations. Sorry I forgot to account for that. I'll fix it now.
Mar 2	comment	Unknown quantum state with promise of classical data Collective measurements tend to do better. But as mentioned in the comments the original question asks about distinguishing states which are indistinguishable: they have the same reduced density matrix.
Feb 28	comment	Unknown quantum state with promise of classical data In that case, the Holevo quantity bounds the amount of information you can obtain, and hence can be used to upper bound the probability of correctly guessing the correct state. But be warned, it can be an extremely loose bound.
Feb 28	comment	Unknown quantum state with promise of classical data @PiotrMigdal: Looks like we were typing at the same time about the same thing!