4 ms·

Here's something that has always bothered me about this topic: > That, Ritter says, could extend the application of the network even further: once two atoms ar

Here's something that has always bothered me about this topic: > That, Ritter says, could extend the application of the network even further: once two atoms are entangled, the quantum state of one depends on the quantum state of the other. And what exactly is the benefit from that? It sounds as if now you could manipulate one of the atoms, and the other would follow suite, allowing you to transmit information. But my understanding of quantum physics is that manipulating one atom immediately breaks up the entanglement, so no information transmitted. And this is not just a technical limitation, but a conceptual one. Am I wrong? And if no, how would a pair of entangled atoms help us in building a communication network?

boxein 15y ago
This is my understanding as well. I hope someone who knows definitively comes along.
redwood 15y ago
You're spot on, this is why I find "quantum computing" a bit less exciting than most (just like the "hydrogen economy" say). Both sound great to those who scratch the surface but both are really misnomers. Quantum networks presumably would have a security advantage, but not a speed advantage. Thus there is really not much to get excited about. Basically entanglement is always dealt with in a misleading way. It's not that one's state influences the other. It's that both are equivalently altered by a third entity limited to the speed of light (as far as we know so far anyway).
jeffwass 15y ago
Basically, entanglent of individual qubits is an important component of quantum computation. Namely, two coupled qubits have more 'information' and behave differently than two isolated qubits. Eg, a single qubit can be represented as a 2 element vector (spinor) with complex number elements. That is four Degrees of Freedom (DOF). However, the vector must be normalized, which eliminates one DOF, and the entire vector is phase invariant (can be multiplied by an arbitrary phase exp^(i gamma) for any phase gamma, without effect). So that really leaves two true DOF for a single qubit. These can be represented as angles on the Bloch sphere. See my other post on this thread for a link to my java app to visualize a qubit, and also show the relationship to the spinor vector. However, a system of 2 qubits can be represented as a 4 element complex vector. With the same constraints above for single qubits, this leaves 6 DOF for a two-qubit system. Each qubit individually has 2 DOF, but the entanglement itself represents another two DOF! Many of the quantum computational processes you may have heard about (quantum teleportation, quantum communication, Shor's algorithm) invariably make use of his extra information in multi-qubit systems. Eg, two spin-1/2 systems can couple together in the spin-0 singlet state, or as a spin-1 triplet state, or as a linear combination of both. This means that two fermions (subject to Pauli exclusion principle) can act like bosons when entangled, and do weird things in aggregate like Bose Einstein condensation. Very funky.

boxein 15y ago

This is my understanding as well. I hope someone who knows definitively comes along.

redwood 15y ago

You're spot on, this is why I find "quantum computing" a bit less exciting than most (just like the "hydrogen economy" say). Both sound great to those who scratch the surface but both are really misnomers. Quantum networks presumably would have a security advantage, but not a speed advantage. Thus there is really not much to get excited about. Basically entanglement is always dealt with in a misleading way. It's not that one's state influences the other. It's that both are equivalently altered by a third entity limited to the speed of light (as far as we know so far anyway).

jeffwass 15y ago

Basically, entanglent of individual qubits is an important component of quantum computation. Namely, two coupled qubits have more 'information' and behave differently than two isolated qubits. Eg, a single qubit can be represented as a 2 element vector (spinor) with complex number elements. That is four Degrees of Freedom (DOF). However, the vector must be normalized, which eliminates one DOF, and the entire vector is phase invariant (can be multiplied by an arbitrary phase exp^(i gamma) for any phase gamma, without effect). So that really leaves two true DOF for a single qubit. These can be represented as angles on the Bloch sphere. See my other post on this thread for a link to my java app to visualize a qubit, and also show the relationship to the spinor vector. However, a system of 2 qubits can be represented as a 4 element complex vector. With the same constraints above for single qubits, this leaves 6 DOF for a two-qubit system. Each qubit individually has 2 DOF, but the entanglement itself represents another two DOF! Many of the quantum computational processes you may have heard about (quantum teleportation, quantum communication, Shor's algorithm) invariably make use of his extra information in multi-qubit systems. Eg, two spin-1/2 systems can couple together in the spin-0 singlet state, or as a spin-1 triplet state, or as a linear combination of both. This means that two fermions (subject to Pauli exclusion principle) can act like bosons when entangled, and do weird things in aggregate like Bose Einstein condensation. Very funky.