Of course the thing about a 1.9 × 10^{6} joule laser is that you can’t go firing it off whenever you want. The repetition rates these lasers allow for are a few shots per hour at best. Forget wanting to actually build a reactor, actually being able to do fusion experiments at faster rate than that would be ideal and so compact high energy lasers with the potential to drive plasmas but which also have a high repetition rate are always being looked into.

This paper uses previous research which showed high intensity (which doesn’t necessarily mean high energy, remember when it comes to waves, intensity is amplitude while energy is frequency) of about 10^{18} W cm^{−2 }was able to produce fusion from a polyethylene ([C_{2}H_{4}]_{n}) target and expands upon the theory behind this event. It is shown that laser pulses of incredibly high intensity when aimed at deuterated nanostructures, in this case polyethylene nanowires functionalised with deuterium, can accelerate ions of the deuterium to have energies up into the mega electronvolt range. This can then in turn go on to drive the behaviour of fusion reactions and ultrafast neutron bursts. Through this process the researchers produced about 10^{6} neutrons (from the fusion) per joule. This is obviously less than the 10^{9} neutrons per joule from NIF except that while they required a 1.9 × 10^{6} joule laser to get that efficiency, this process only required a 1.6 joule laser. This is about 500 times greater than the standing record for the yield of a laser less than 10 joule, no doubt an incredible breakthrough.

Paper links: Micro-scale fusion in dense relativistic nanowire array plasmas

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If you know you have a complicated Hamiltonian whose ground state is the solution then what you need to do is set up a quantum computer mimicking the Hamiltonian and make sure its in the ground state and there the answer will be. Of course to actually achieve this what you do is set up the quantum computer so it’s modelling a simple problem with a simple Hamiltonian and so easily falls to the ground state. Then the adiabatic part comes in. Unlike in the thermodynamic change you do not want to rush this process. The aim is to evolve the simple Hamiltonian into the complex one without allowing the computer to excite to the first excited state. This means the work must be done slowly, the closer the gap between the first excited state and the ground state, the slower it must be. When the complicated Hamiltonian has been reached, so long as the computer has not left the ground state, you now have the ground state of the complex Hamiltonian and the solution to your problem.

This paper offers a method to test how “perfect” an adiabatic quantum computer is through a relatively simple method. A quantum quench is the rapid change between two Hamiltonians as opposed to the slow developed change talked about above. An ideal quench in this scenario would plunge the computer back to its ground state, breaking all symmetries and leaving perfectly spins all aligned. In reality though the quench will never be perfect due to floors in the final Hamiltonian, inherent noise even in the ground state, ect. As a result there will be spins in the opposite direction to the bulk, such defects can then be counted as a quantitative measure of how far the computer is from an ideal ground state and so creates an objective way to measure the imperfections of a quantum computer.

Paper links: Defects in Quantum Computers

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