Joe Fitzsimons

5,596 reputation

1237

bio	website	jfitzsimons.org
	location	Singapore, Singapore
	age	31
visits	member for	2 years, 6 months
	seen	May 18 at 22:33
stats	profile views	457

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.

	5,596 reputation	bio	website	jfitzsimons.org	visits	member for	2 years, 6 months
1237 badges		location	Singapore, Singapore		seen	May 18 at 22:33

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Apr 15	answered	Advice on doing physics under the umbrella of mathematics and the converse
Apr 15	comment	Advice on doing physics under the umbrella of mathematics and the converse Most string theorists I know are in Mathematics departments.
Apr 14	comment	What is the physical difference between states and unital completely positive maps? No problem. Hope it helps.
Apr 8	answered	What is the physical difference between states and unital completely positive maps?
Mar 26	comment	Multiqubit state tomography by performing measurement in the same basis @Alex: Ah I guess you mean in the correlations. Sorry I forgot to account for that. I'll fix it now.
Mar 26	answered	Multiqubit state tomography by performing measurement in the same basis
Mar 2	comment	Unknown quantum state with promise of classical data Collective measurements tend to do better. But as mentioned in the comments the original question asks about distinguishing states which are indistinguishable: they have the same reduced density matrix.
Feb 28	comment	Unknown quantum state with promise of classical data In that case, the Holevo quantity bounds the amount of information you can obtain, and hence can be used to upper bound the probability of correctly guessing the correct state. But be warned, it can be an extremely loose bound.
Feb 28	comment	Unknown quantum state with promise of classical data @PiotrMigdal: Looks like we were typing at the same time about the same thing!
Feb 28	comment	Unknown quantum state with promise of classical data I'm not quite sure I understand: If $H$ is chosen randomly then $Ha$ is uncorrelated with $a$. Averaging over $H$ you get $\rho = \sum_H p(H) (H a)^{\otimes K} = \sum_H p(H) (H b)^{\otimes K} = \sum_H p(H) (H c)^{\otimes K} = \sum_H p(H) (H d)^{\otimes K}$, so no matter how many copies you have you can't distinguish the initial state, since these states are related by unitary operators (and you've just applied a random unitary).
Feb 19	comment	Constructing a Hamiltonian (as a polynomial of $q_i$ and $p_i$) from its spectrum @PiotrMigdal: For a finite spectrum there is no unique solution. I suspect this is also true for the infinite case, but I am not sure.
Feb 18	answered	Constructing a Hamiltonian (as a polynomial of $q_i$ and $p_i$) from its spectrum
Jan 31	comment	Readable books on advanced topics @PratikDeoghare: No.
Jan 26	comment	POVMs that do not require enlargement of the Hilbert space @MateusAraújo: In your case the effective identity operator still collapses the state onto whatever basis you picked, so repeated measurements do nothing. However, my method does not collapse the state in the case of an identity term, which is why multiple rounds makes sense.
Jan 26	comment	Do any entanglement measures for mixed states exist that use only single site correlation functions? @Calvin: Exactly, although there is no need to restrict to just Pauli operators.
Jan 26	answered	Do any entanglement measures for mixed states exist that use only single site correlation functions?
Jan 26	comment	POVMs that do not require enlargement of the Hilbert space @MateusAraújo: I should have been more careful with my language. You do indeed simply add $pI$ to a particular round. However this is an important difference to your procedure, and allows you to construct a significantly wider range of POVMs: In each round you can choose to implement one POVM element + the scaled identity. Repeating as necessary you can obtain measurements consisting of many POVM elements.
Jan 25	comment	Solution to the Schrodinger equation for periodically time dependent Hamiltonians Do you want dynamics or average ground state?
Jan 18	answered	POVMs that do not require enlargement of the Hilbert space
Jan 17	comment	How can one build a multi-scale physics model of fluid flow phenomena? This is not my area at all, so this might be a particularly ignorant comment, but can you not simply simulate a small section at the microscopic level for a range of different conditions for the boundaries, and then piece together these into a solution for a larger volume, repeating several times if necessary? The basic idea being that there are many areas for which exactly the same calculation is necessary, and probably relatively few distinct such calculations, so better to build a lookup table than to calculate anew each time.