6,099 reputation
1643
bio website jfitzsimons.org
location Singapore, Singapore
age 32
visits member for 3 years, 10 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.


Oct
6
comment What is the use of a Universal-NOT gate?
Yes, dynamical decoupling is one reason you may want a universal-NOT. However, given that these don't exist, you are likely far better with WaHuHa pulses and refinements there of, than using approximate universal-NOTs. After all, you only need the effective change to add up to zero, and having more than two types of evolution involved doesn't change that end result (see my comment on Marco's answer above). +1 anyway though, as it is indeed a reason for wanting a universal-NOT.
Oct
6
comment Does entropy measure extractable work?
@Matt: Good answer, and you get +1 from me, but I do not understand the claim that thermodynamic entropy is not well defined for non-equilibrium systems. It's given by $-k_B \sum_i p_i\log p_i$ where $p_i$ is the probability of the $i^{th}$ microstate. This is obviously defined for non-equilibrium systems and equilibrium systems alike.
Oct
6
comment Does entropy measure extractable work?
@Aaron: Actually, the reconciliation is that I was answering yes to whether or not the two entropy definitions agree. I didn't mean that entropy measures extractable work.
Oct
5
comment What is the use of a Universal-NOT gate?
You can simply edit this answer if you want.
Oct
5
comment What is the use of a Universal-NOT gate?
The universal-NOT isn't just a quantum bit flip operator (the Pauli $\sigma_X$ gate fills this role). It maps every state to its antipodal point on the block sphere, and hence anti-commutes with all Pauli operators. I'm not clear on why you bring up feed forward corrections in MBQC, as these all correspond to Pauli operators, not universal-NOTs.
Oct
3
comment Which symmetric pure qudit states can be reached within local operations?
@Piotr: I posted a new answer which hopefully addresses your question as I now understand it. I'll leave this answer, as I think it is interesting in its own right, but is entirely different from the new answer.
Oct
3
answered Which symmetric pure qudit states can be reached within local operations?
Oct
3
comment Bell polytopes with nontrivial symmetries
Apparently I need to wait 24 hours to award the bounty, but I'll do so then.
Oct
3
comment Bell polytopes with nontrivial symmetries
Welcome to TP.SE. It's good to see you here. Sorry to have stolen your reference! You deserve the rep from my answer too, so I'll use a bounty to transfer the rep.
Oct
3
comment Stabilizer formalism for symmetric spin-states?
@Earl: No, I meant you do the multiplication term by term. There are polynomially many terms, which get multiplied pairwise, so you only need a poly sized look-up table.
Oct
3
comment Stabilizer formalism for symmetric spin-states?
@Earl: Re (i), yes, that would be nice (but wasn't part of the question). Symmetric measurements should be trivial to encorporate, but non-symmetric ones might be hard. If you increase the local dimension you essentially have an optical QC, but for fixed dimension I guess it might be efficiently simulable for any non-entangling measurements. Re (ii): Yes, all symmetric operations, encluding entangling ones. Re (iii) it is entirely possible that there are tricks I have failed to exploit here.
Oct
3
comment Stabilizer formalism for symmetric spin-states?
@Earl: $(X_1 + X_2)(Y_1 + Y_2) = (X_1Y_1 + X_2Y_2) + (X_1Y_2 + Y_1X_2) = i(Z_1 + Z_2) + (X_1Y_2 Y_1X_2)$. Note that both quantities in brackets are $\gamma$ terms.
Oct
3
comment Stabilizer formalism for symmetric spin-states?
@Earl: Even with a look-up table you get efficient multiplication, since the table contains only a polynomial number of entries.
Oct
2
answered Stabilizer formalism for symmetric spin-states?
Oct
1
comment Which symmetric pure qudit states can be reached within local operations?
@NieldeBeaudrap: Yes. In fact the basis of product states is exactly the reason why there cannot be some observable which has a different expectation for some entangled state on this space if it's expectation value is constant across all product states.
Oct
1
comment An entropy of the Wigner function
Welcome to TP.SE, Earl. Good to see you here.
Oct
1
comment Which symmetric pure qudit states can be reached within local operations?
Welcome to TP.SE! I wasn't aware of that paper, but it seems an interesting observation, and looks like it might generalize beyond qubits.
Sep
30
answered Which symmetric pure qudit states can be reached within local operations?
Sep
29
awarded  Nice Answer
Sep
29
comment What is the use of a Universal-NOT gate?
The first answer you give is exactly the justification they give in the paper, but it's not a reason why you would actually want to be able to do it. As regards the second, you don't actually need a universal NOT for that, you simply need an operator that anti-commutes with the Hamiltonian, and there are potentially far better time reversal techniques (i.e. WaHuHa etc.). The partial transpose test does seem to be one reason to want it though.