6,069 reputation
1642
bio website jfitzsimons.org
location Singapore, Singapore
age 32
visits member for 3 years, 8 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.


Jan
22
comment Temperature of a System of molecules
Actually temperature is usually defined as $T = (\frac{\partial S}{\partial E})^{-1}$, where $S$ is the entropy of the system.
Jan
22
answered To calculate the correlation functions of an XX spin chain, Wick's theorem is used. But is it valid for a chain of any size?
Jan
22
answered Is it possible to separate the poles of a magnet?
Jan
22
comment Is it possible to separate the poles of a magnet?
Monopoles are prohibited by Maxwell's equations, specifically $\nabla \cdot B = 0$. If you want monopoles you need to modify this particular equation.
Jan
22
comment Double slit experiment near event horizon
@kakemosteret: You won't make it across the event horizon anyway. The gradient of the gravitational field will rip you apart first.
Jan
22
comment Does decoherence need non-determinism?
Good answer!
Jan
22
comment Does decoherence need non-determinism?
This isn't technically correct. Decoherence can be the result of an entirely deterministic (and unitary process), since you only care about the reduced density matrix for the system in question. Open quantum systems -do- decohere, even though the entire process is unitary. The larger wavefunction of the entire system is still pure, but the state of the local system becomes mixed.
Jan
21
revised Does decoherence need non-determinism?
added 807 characters in body
Jan
21
answered Does decoherence need non-determinism?
Jan
21
answered If a 1kg mass was accelerated close to the speed of light would it turn into a black hole?
Jan
20
revised Using delayed choice interference experiments as a computing device
deleted 2 characters in body
Jan
20
comment Where's the best place to add weight to a Pinewood Derby car?
@Mark: Sure, but since the question presumably relates to a real car, it it would be more reliable to simply measure them for the car in question.
Jan
20
comment Where's the best place to add weight to a Pinewood Derby car?
@Mark This isn't possible without knowing more about the car. The change in potential energy is $m \Delta H$. To calculate the value of $\Delta H$ we need to know the position of the front axle as well as how low down the weight can practically be placed (the lower the better), we also need $m$ the mass added. Even then, although we can calculate the potential energy, this doesn't translate into a specific speed increase (even if it is all converted to kinetic energy). This is because the kinetic energy scales quadratically in velocity, so we need to know the velocity without the extra weight.
Jan
20
comment What is the most efficient way to destroy the universe?
Or simply refuse to accept the axioms of logic. Then you can insist whatever you want is true.
Jan
20
comment What is the most efficient way to destroy the universe?
This makes no sense. What does it mean to make the "strong force weak"? There are different mechanisms that give rise to either force and you can't simply invert their relative strengths at every point in space.
Jan
20
answered What is the most efficient way to destroy the universe?
Jan
20
comment Using delayed choice interference experiments as a computing device
Note: In the case of these examples there is always interference, and it is simply measuring which pattern you get that gives the answer. However it is possible to break the schemes so that you get interference for the answer 0, and so the photon always comes in a particular mode, where as for 1 the coherence is deliberately destroyed, so you get the photon randomly in one of several modes. This is no longer deterministic computation, but can be made arbitrarily close to deterministic. It is worth noting,however,that theres no advantage to this and its more sensible to keep everything coherent
Jan
20
comment Using delayed choice interference experiments as a computing device
@vonjd: You won't see the pattern instantly, you are limited by the finite speed of light which means it takes a finite amount of time for the light to pass through the network, which will be proportional to the depth of the network.
Jan
20
answered Using delayed choice interference experiments as a computing device
Jan
20
comment Can anybody provide a simple example of a quantum computer algorithm?
"Now, a quantum computer isn't just a parallel processor, where you can give it a superposition of the configurations and get back f evaluated on all of them." Actually, it isn't -even- a parallel processor. Quantum parallelism is not the same as having an exponentially parallel classical computer, although an exponentially parallel classical computer can simulate a quantum computer efficiently.