6,069 reputation
1642
bio website jfitzsimons.org
location Singapore, Singapore
age 32
visits member for 3 years, 8 months
seen Dec 6 '13 at 16:11

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.


Nov
27
awarded  Caucus
Nov
5
awarded  Yearling
Sep
9
awarded  Nice Answer
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
revised Reversing gravitational decoherence
added 64 characters in body
Aug
29
revised Reversing gravitational decoherence
added 64 characters in body
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.
Aug
27
answered Reversing gravitational decoherence
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awarded  Talkative
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awarded  Mortarboard
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4
awarded  Enlightened
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awarded  Enlightened