6,079 reputation
1642
bio website jfitzsimons.org
location Singapore, Singapore
age 32
visits member for 3 years, 9 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
comment Causality and operationalism: from sets and functions to monads
"Of course, states don't exist, only processes do." - That's one hell of a statement. Perhaps we would be better sticking to physics than philosophy.
Nov
26
answered Quantum causal structure
Nov
25
comment Quantum causal structure
@Peter: Sorry Peter, I was referring to the original question, not your comment. I don't dispute that the quantum gravity case is open, but we know so little about that area that it is hardly surprising that it is open. I know some of the results the question refers to, but can't make much sense of what the poster has in mind. I'll cast a virtual vote to close, but since my vote is binding, I won't actually close the question yet. I'd like to give the OP time to actually explain what they mean. If nothing happens in a few days, I'll kill the question.
Nov
25
comment Quantum causal structure
I don't understand what you mean by quantum causal structure. Quantum mechanics is non-signalling so causal structure is the same as in the classical case.
Nov
25
comment Hilbert-Schmidt basis for many qubits - reference
+1 from me. I use it a lot too, but couldn't think of anything interesting to say!
Nov
21
comment CHSH violation and entanglement of quantum states
@PiotrMigdal: Perhaps you should post that as an answer.
Nov
16
comment Accurate quantum state estimation via “Keeping the experimentalist honest”
@ChrisFerrie: Sorry, I meant if the measurements can depend on $\sigma$ not $\rho$. I've edited my above comments to reflect this.
Nov
15
comment Accurate quantum state estimation via “Keeping the experimentalist honest”
And hence you need only measure in the $X$ basis, even though this does not have sufficiently many linearly independent outcomes to uniquely identify an arbitrary density matrix.
Nov
15
comment Accurate quantum state estimation via “Keeping the experimentalist honest”
This argument is for the case where the set of measurements is fixed and independent of $\sigma$. If the scheme need only work for certain classes of $\sigma$ then this imposes correlations between entries in the density matrix which reduces the number of linearly independent measurement operators required to uniquely identify it. An example of this is where $\sigma = |+\rangle\langle +|$, where purity implies that the state has expectation value 0 for $Z$ and $Y$ measurements, and if $\mbox{Tr}(\rho X) = \mbox{Tr}(\sigma X)$ then $\mbox{Tr}(\rho Y) = \mbox{Tr}(\sigma Y)$, etc.
Nov
15
comment Accurate quantum state estimation via “Keeping the experimentalist honest”
Also, if you take any complete or over-complete basis for tomography you can make the measurements and make them arbitrarily weak, you still satisfy the criterion (though Alice's expected loss tends towards zero as the measurement tends towards the identity).
Nov
15
comment Accurate quantum state estimation via “Keeping the experimentalist honest”
@ChrisFerrie: No, that is not the same as the sufficient condition I gave since I only required linear independence and not orthogonality and only for a sufficiently large subset of $\{E_i\}$, so it includes a bunch of weak measurements. However that was only an example. The necessary and sufficient condition is that all expectation values are the same. This includes a huge range of measurements, such as measuring a randomly chosen Pauli operator, which yields only a single bit of information.
Nov
14
answered Accurate quantum state estimation via “Keeping the experimentalist honest”
Nov
12
comment Length of publication cycle for peer-reviewed journals
that's the time I meant. It really depends on the referees you get.
Nov
11
comment Length of publication cycle for peer-reviewed journals
I get the impression that the variance is huge. I'm not sure an average will be very meaningful.
Nov
10
comment What Shannon channel capacity bound is associated to two coupled spins?
@AramHarrow: This update sounds an awful lot like metrology. It seems like the Heisenberg limit may apply.
Nov
10
comment What Shannon channel capacity bound is associated to two coupled spins?
@JohnSidles: To be honest, I am far from expert in this myself. Aram can probably give you far more reliable answers on this than I can.
Nov
9
comment What Shannon channel capacity bound is associated to two coupled spins?
@AramHarrow: To explain what I mean: Imagine you have two spin-1/2 systems. Then your coupling induces at most 4 distinct eigenspaces which pick up relative phase, independent of the number of ancillae, or how they are prepared prior to coupling. Given that the information transfer in periods of free evolution is determined entirely by the relative phases between these 4 spaces, this is identical to what can be achieved with two spins. The only subtlety then comes from how you can combine multiple uses of such a channel.
Nov
9
comment What Shannon channel capacity bound is associated to two coupled spins?
@AramHarrow: Good point. However, it seems like you can bound the effective dimensionality via the number of splittings in the eigenvalues of local Hamiltonians.
Nov
8
comment Explicit construction for unitary extensions of CPTP maps?
@Marcin: I deleted it because there was an error in the proof.
Nov
7
answered What Shannon channel capacity bound is associated to two coupled spins?