Exploring Quantum Computers: Understanding Qubits and Entangled Particles

[LaserMind]

Quantum Computers use Qubit with Up Down spin to holds bits |0> and |1> for entangled particles in a Bell state. Is this the only way of doing it?

 

[f95toli]

Well, yes and no.
In theory ANY two-level system can be used as a qubit although in reality there are of course many practical problems. When dealing with macroscopic systems (e.g solid state qubits) some of the requirements are

*All relevant energy scales should be much larger than kBT
*The levels used to implement the qubit should as far as possible be decoupled from the environment and have a level splitting that differs from the splitting to the next level (otherwise the probability of leaving the 2-level "subspace" become significant), this means that harmonic potentials can't be used since all levels are equidistant.
This is incidentally why Josephson junctions are used to make superconducting qubits, they are very non-linear devices and can be used to create various anharmonic potentials.

*You also need to be able to the interaction on/off so that you can both manipulate the system when needed AND let it evolve freely; this is usually done using electric/magnetic fields (even when microscopic qubits are used, e.g. electron or atomic spins).

The point here is that you don't need to use "particles" at all. The fact that we talk about up/down spins, use the Palis matrices etc for all types of qubits is simply due to the fact that it is convenient and the math is the same; it does not imply that there are real particles involved, and the "spin" can be a circulating current, the state of a Josephson junction, the number of electrons on an island etc.

Note that some people are even trying to create qubits using micro-mechanical resonators where the two states would simply correspond to different vibrational modes (although so far no one has even been able to get them into the quantum regime, but this is just a matter of time).

 

[LaserMind]

Well, yes and no.
In theory ANY two-level system can be used as a qubit although in reality there are of course many practical problems. When dealing with macroscopic systems (e.g solid state qubits) some of the requirements are

*All relevant energy scales should be much larger than kBT
*The levels used to implement the qubit should as far as possible be decoupled from the environment and have a level splitting that differs from the splitting to the next level (otherwise the probability of leaving the 2-level "subspace" become significant), this means that harmonic potentials can't be used since all levels are equidistant.
This is incidentally why Josephson junctions are used to make superconducting qubits, they are very non-linear devices and can be used to create various anharmonic potentials.

*You also need to be able to the interaction on/off so that you can both manipulate the system when needed AND let it evolve freely; this is usually done using electric/magnetic fields (even when microscopic qubits are used, e.g. electron or atomic spins).

The point here is that you don't need to use "particles" at all. The fact that we talk about up/down spins, use the Palis matrices etc for all types of qubits is simply due to the fact that it is convenient and the math is the same; it does not imply that there are real particles involved, and the "spin" can be a circulating current, the state of a Josephson junction, the number of electrons on an island etc.

Note that some people are even trying to create qubits using micro-mechanical resonators where the two states would simply correspond to different vibrational modes (although so far no one has even been able to get them into the quantum regime, but this is just a matter of time).

Say a sound wave where Bit 1 = 180 degrees phase and Bit 0 = 0 degrees phase -
hmmmm... that's a revelation to me if its true!