6,127 reputation
1743
bio website jfitzsimons.org
location Singapore, Singapore
age 32
visits member for 3 years, 11 months
seen Oct 18 at 6:46

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
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
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
3
comment Decoherence and measurement in NMR
@JohnSidles: Standard quantum limit often refers to separable states only, whereas the Heisenberg limit is in metrology the Heisenberg limit is the "performance that can't be bettered supposing that standard QM is true".
Nov
2
comment Decoherence and measurement in NMR
@JohnSidles: As it happens I have a paper with some others on exactly the question of the extraction of information from a spectrometer beyond the standard quantum limit (arXiv:1007.2491).
Nov
2
comment A resource theory of quantum discord?
+1 from me. Welcome to the site Animesh.
Nov
2
comment Decoherence and measurement in NMR
@Kaveh_kh: Yes I know, but I think this is a much stronger effect than the weak measurements actually being made on the system, and I suspect is actually coherent, which would affect the time dynamics (which are important since the spectrometer is essentially measuring change with respect to time).
Nov
2
comment Decoherence and measurement in NMR
I took his concern to be about the current creating a field which effects the spectrum, rather than about the fact that you are making weak measurements.
Nov
1
comment Decoherence and measurement in NMR
I don't quite follow. How would a low quality coil prevent back action?
Oct
24
comment Linearizing Quantum Operators
@Qmechanic: Argh! I wish people wouldn't do that. It makes it impossible to keep track of whether it has been answered or not.
Oct
24
comment How to write a paper in physics
You also want to give your readers a hint, since if it is an important result you get a wider range of readers, and many may not necessarily be familiar with all of the tools you use.