6,182 reputation
1943
bio website jfitzsimons.org
location Singapore, Singapore
age 32
visits member for 4 years
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.


Nov
3
comment Does entropy measure extractable work?
@WetSavannaAnimalakaRodVance Most of the work on this topic postdates Jayne's papers, so they are far from the last word on the subject. I'm not really sure what you mean by the "ensemble" being well defined. For any quantum system $p_i$ is well defined, since they are simply the eigenvalues of the density matrix.
Nov
3
comment Does entropy measure extractable work?
@WetSavannaAnimalakaRodVance: I'm afraid I don't see how this is related to the questions or answers. I don't believe any of us are discussing the ability to accurately infer entropy from measurements, but rather whether three concepts (work extraction, information theoretic entropy and physical entropy) are equivalent.
Nov
3
comment Does entropy measure extractable work?
@WetSavannaAnimalakaRodVance Sure, but something like the von Neumann entropy of a state is always defined. It doesn't depend on the system being in a Gibb's state, and hence doesn't require temperature to be defined.
Oct
9
comment What can the D-Wave quantum computer do?
@Blaisorblade: I meant that the converse can't be shown easily (i.e. that it is not NP-complete). Regarding noise, I use "noise" as a synonym of "decoherence".
Aug
17
comment The Many Body problem
@BenCrowell: Even for single atoms, I believe thulium is an example of a system where the ground state of the electrons is hard to find computationally.
Aug
17
comment The Many Body problem
@BenCrowell: None of what you have just said is an accurate description of the true situation. Being chaotic does not in any way mean a system is computationally difficult to simulate. Rather it means that the outcome of a simulation for one initial condition may diverge rapidly from a similar initial condition. That's what makes weather hard to predict, not that we can't numerically solve the equations. Secondly the bit about ground states is also inaccurate. The ground state of the transverse Ising Hamiltonian, for example, is computationally hard to solve.
May
31
comment What can the D-Wave quantum computer do?
@PeterShor: Indeed. I didn't mean to suggest that it was the only algorithm that works. It seems even trial division is possible with a few months and a few dozen GPUs on your hands.
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).