Quantum Noise In Laser

Is there a relation between quantum mechanics and electrical engineering?

Tx for A2a: Yes! [My PhD is in EE, I am an EE fellow, and I taught EE at grad/undergrad level at Cornell.] EE depends on Quantum at three levels of increasing intensity:1) Many EE formulas are based on Quantum. (Thermal noise, transistor band gaps, laser cavity analysis, etc.) Quantum here is ontologically antecedent but you can memorize the formulas and get paid to use them without knowing Quantum yourself. 2) Many  EE systems directly use Quantum properties, formulas, and concepts. Examples include optical communications, radar, most remote sensor design, and nanotechnology. Here you need to understand Quantum, as a consumer of the theory.3) The third level, the most exciting to me, is Quantum Entanglement technology. This includes improved sensing, timing, navigation, simulation, and computing. Here you become an active research contributor not a mere consumer.

What is the equivalent of the transistor in a quantum computer?

That is an area of active research. If you had asked what is the equivalent of a bit, the basic abstract unit of information, in a quantum computer, the answer is a qubit.In a digital computer, people knew about bits and Boolean logic for a long time before having a concrete implementation. Inventors tried punched cards and mechanical wheels with holes in them, then electrical approaches: mercury switches, vacuum tubes, electromechanical relays. These were all large, error-prone, and consumed wasteful amounts of energy. The transistor was the winning implementation that was (relatively) small, mass-produceable, efficient, reliable, and self-correcting.So you are asking, what is the winning physical implementation of a qubit? Here are some popular approaches:Nuclear spins in a large molecular, used in NMR / MRI approaches, addresses by strong magnetic fields. This was an early approach that was hard to scale because it worked on bulk ensembles, and the signal Energy states of an electron in trapped ions, addressed by lasers tuned to the transition frequencies of that particular atomic species. Some states, like the hyperfine states, are extremely well-shielded from many kinds of noise, but to cool an ion to such a state requires a good vacuum, low noise coherent laser control, and a large mechanically stabilized table (usually in a dark room underground) full of optical equipment. Some experiments can trap tens of ions in a single trap, address them selectively, and allow arbitrary interactions.Polarization of photons, requiring single-photon sources, beamsplitters, and couplers. I know the least about this approach.The quantized flux, charge, or phase of a superconducting current through a loop. This is the general approach used by the D-Wave machine, and also the Martinis group, both being employed by Google's Quantum AI Lab. Again, it requires large refrigeration and cooling apparatuses to achieve superconductivity, but it uses many techniques from modern lithography to produce thousands of qubits on the same silicon wafer. In that sense, it is the closest, in the sense I described above, to a quantum transistor. However, it still has some scaling problems, such as an architecture that would allow arbitrary qubits to interact.

Why can't we use multiple photons stream for quantum cryptography instead of a single photon?

We can. It's called continuous variable quantum key distribution. See for example, Continuous variable quantum key distribution with modulated entangled statesIn general there is no restriction on the medium of information transfer as long as the information is encoded in a quantum degree of freedom.Take a laser for example. The laser output is a pure quantum state called a coherent state. Yet lasers are used as carriers for digital information through optical fibres. However, that information is classical. An example of classical information is the intensity of the beam. The intensity is related to the average photon number, which is a classical value.For quantum key distribution, you need to transmit quantum information. That is because quantum information cannot be cloned, according to the no cloning theorem. Therefore if you're going to use a laser, you need to rely on the key information embedded as quantum information. That's where things get tricky with lasers, but there are many articles written on continuous variable quantum information. For a laser, the continuous variables are called the phase quadratures. Measuring phase quadratures isn't as straight forward as single photon detection. Furthermore, phase quadrature measurements can also yield classical values. The quantum part of the phase quadratures correspond to quantum noise.From this point it would be best to jump into the literature to really immerse yourself in the details. Just be aware that such details are only for experts. If you're not aware of the ball and stick representation of a coherent state and you haven't studied advanced quantum optics, you'll be immersed in a world of jargon and detail way beyond layperson level.However if your interest has been piqued, then maybe take a look at this quantum computing startup that is based on continuous variable quantum optics. Xanadu - Quantum Computing Powered by LightThere are a few good links for further reading here:Is "probabilitistic,universal, fault tolerant quantum computation" possible with continuous values?

If man portable laser weapons existed, would firing them make a noise?

Yes, depending on size.Firing a gun causes noise in three ways. some of which would apply to laser weapons.First is the mechanical “click “ of the action parts of the fire arm. The firing pin, the cycling etc.Second is the muzzle blast, which is just the expanding gasses of the powder burning and expanding. That's the “boom”Third is the supersonic crack of the projectile passing the sound barrier.In a laser weapon the supersonic crack would be missing, but the other two might be present.Now, the physics people out there can correct me, but as I understand it one of the problems with large laser is the heating of atmospheric molecules along the path. As particals of water vapor, heated expanding nitrogen, and ignited oxygen rapidly expand this causes the same “woosh" noise of other gas expanding reactions such as burning gun powder.But the actual laser likely would not make to much of the mechanical noise, though some is probable. all of this of course depends on the size, power output and simular factors. As well as the environmental factors of the discharge location. High altitude low mostiure and low oxygen atmosphere would create less noise, were as a discharge on a humid day in a swamp would create much more noise.

What is the definition of continuous variables quantum cryptography?

Continuous Variable Quantum Cryptography is a quantum cryptographic scheme in which small phase and amplitude modulations of continuous light beams carry the key information. The scheme relies on quantum correlations of the modulated photons comprising the light beam to detect any evesdropping.Understanding the concept requires acquantance with four ideas.https://arxiv.org/pdf/quant-ph/9...There are some disadvantages in working with single photons, particularly in free-space where scattered light levels can be high. This motivates a consideration of quantum cryptography using multi-photon light modes. In particular we consider encoding key information as small signals carried on the amplitude and and phase quadrature amplitudes of the beam.http://www.squeezed-light.de/Squeezed light (a squeezed state of light) is a special form of light that is researched in the field of quantum optics. The light's quantum noise is a direct consequence of the existence of photons, the smallest energy quanta of light. When light is detected with an ideal photo diode, every photon is translated into a photo-electron. For squeezed light the resulting photo-current exhibits surprisingly low noise.https://en.wikipedia.org/wiki/Ho...In optical interferometry, homodyne signifies that the reference radiation (i.e. the local oscillator) is derived from the same source as the signal before the modulating process. For example, in a laser scattering measurement, the laser beam is split into two parts. One is the local oscillator and the other is sent to the system to be probed. The scattered light is then mixed with the local oscillator on the detector.https://en.wikipedia.org/wiki/EP...The essence of the paradox is that particles can interact in such a way that it is possible to measure both their position and their momentum more accurately than Heisenberg's uncertainty principle allows, unless measuring one particle instantaneously affects the other to prevent this accuracy, which would involve information being transmitted faster than light as forbidden by the theory of relativity ("spooky action at a distance")

What would happen if I fire a powerful laser beam into a mirrored torus, will the photons circle around indefinitely? If not, how long before they lose energy?

This is something that is done quite a lot. In fact there are many ways to achieve this, such as making a loop of optical fibre.The problem with any resonator is that if you can couple light into it, then it can also leak out. However, this can actually be inhibited using interference, in a regime known as critical coupling, allowing direct injection into the resonator.Once in the resonator, the light will remain there for some lifetime limited by internal losses. This limits the Q factor of the cavity.In general, keeping light trapped in a mirrored cavity is not that difficult. In fact it forms the basis of a very sensitive spectroscopic technology, called cavity ringdown spectroscopy.To my knowledge, the highest reported optical resonator Q was about 10^13 for a hand polished millimetre sized MgF2 toroid. However, longer ringdown times can be achieved with larger resonators simply because there is a greater path length between mirrors.Light will eventually be lost from any cavity due to a number of factors. Any solid-state toroid will be limited by material losses. Otherwise there are mirror losses due to a finite probability of scattering out of the resonator.Ultimately, you could look at the effect of photon recoil or quantum noise on the linewidth that could be achieved. This would likely only be a hypothetical limit, as there are many other losses that limit a real device.