Hi,
I have a question regarding the state of the atom in quantum mechanics. Is the atom in two different states or multiple states (same atom spinning in opposite directions? or …not sure). Also, since atoms cannot be seen (or can they?) how can data be stored on the atoms as I understand atomic computing can now do. I’d appreciate some feedback. Thanks.
davidmich
- Usually the atom is in a single state, the ground state, i.e. the energy state with the lowest energy.
2a. Atoms can be “seen” in various ways. One of the best is by scanning tunneling microscopy, in which a probe with a tiny tip probes the atom electronically.
2b. One way to store information with atoms is to have a two dimensional grid in which the presence or absence of an atom represents a zero or one.
To clarify one misconception, atoms apparentlycanbe seen.
An atom, or indeed any quantum mechanical system, can be in a superposition of seemingly-inconsistent states. It isn’t always in a superposition, though: Observing it will cause it to definitely be in some particular state.
To illustrate, let’s take a nice simple system, simpler even than an atom: A single electron. An electron has a property called “spin” (which is only vaguely similar to what you probably think of when you hear that term), which for any given measurement axis, can have a value of + or -. The electron can be in a state that’s a superposition of + and -, but if you measure the spin of the electron along that axis, you will always get a result of either + or -, never anything in between: Even if it was in a superposition before, it isn’t afterwards. Now suppose that after making the measurement, you turn your measurement device 90 degrees, so you’re measuring along a different axis: Now, along that new axis, the electron is again in a superposition of + and -. Measure it again, and it falls into a definite state along that axis, but now it’s a superposition relative to the original axis.
Thank you for your feedback. Are electrons also visible? In “The Age of Wonder” by Richard Holmes, it states that they cannot be seen. Is this incorrect?
Yep. And my physics textbook shows they’re purple!
A more serious answer: for something to be seen *directly *with your eyes using an optical microscope, it must have a physical size (length, width, height, whatever) that is greater or equal to the wavelength of visible light. The wavelength of visible light spans from 400 nm to 700 nm, and particles such as protons, neutrons, and electrons have diameters a lot smaller than 400 nm. And I believe the electron doesn’t even have a diameter; it is essentially 0 nm.
http://newton.dep.anl.gov/newton/askasci/1993/physics/PHY118.HTM
“The reason for No is that it is actually impossible for anybody to “see” an individual atom, since all atoms are thousands of times smaller than the smallest light waves we can see using our eyes. The reason for Yes is that, even though they cannot be seen directly with our eyes there is so much evidence for atoms, and we know so much about them, that it is impossible to say they do not exist.”
Is this information now dated?
davidmich
It’s a slight misnomer to say an atom is in more than one state, because at any given time the atom is in only one quantum state. However what quantum mechanics does state is that any linear superposition of quantum states is also a state, so if state A corresponds to the atom definitely being in one position and state B corresponds to it definitely being in another position, then state A+B is an allowable state and many would say it corresponds to the atom being in both positions A and B. The laws of quantum measurement mean that when we make a measurement though we will only observe the atom to be in position A or B and never A and B though.
The theory behind quantum computing is quite complicated, but if an individual atom is a qubit, then you must be able to directly measure some or other property of the atom.
Just to clarify the terminology…
An atom (or any quantum mechanical system) is always in one state. That state can be a superposition of other states, but that isn’t to say that the atom is “in multiple states”. It is in one state. A superposition is a valid state.
The reason superpositions are talked about is that certain states have special properties(), and if the atom’s state doesn’t have a desired special property, it might be convenient to view it as a superposition of states that do.
() For instance, some states have well-defined energies, and some don’t. Some have well-defined spin, and some don’t.
ETA: Ninja’d by Asymptotically fat
I have difficulty understanding the word ‘state’ as it is used in quantum mechanics. Does it imply movement (‘spin’) or polarity (+/-) ? Please give me an easy-to-follow explanation. Can someone come up with a better word than ‘spin’ if that is not exactly what we normally understand as ‘spin’? Thank you.
davidmich
The quantum mechanical state of an atom (or more broadly a quantum mechanical system) completely specifies all information that it is theoretically possible to know about that atom - this is one of the basic ideas (or postulates) of quantum mechanics. As stated above, some states correspond to a certain measurement having an outcome that can be definitely predicted in advance (assuming you know the quantum state), whereas other states can be superpositions of these states in which case we will only be able to assign a probability to each possible outcome of the measurement.
Spin is another name for ‘intrinsic angular momentum’ and perhaps it’s best seen as just a fundamental property. Spin is merely used as an example as measuring the spin along a certain axis will always result in a discrete set of possible outcomes which is very useful for illustrating certain properties of QM.
Well, sort of. Even in e ground state, an atomic nucleus may have very slightly different energy states (the hyperfine structure of the nucleus).
With regard to the questions of the o.p., a thorough explanation would be of large article length. Rather than digress endlessly I’m going to recomend that the o.p. check out Jim al-Kalili’s Quantum: A Guide for the Perplexed as a good starting point for a layman’s guide to quantum mechanics.
Stranger
This site has a pretty good explanation for starters.
Researchers suggest one can affect an atom's spin by adjusting the way it is measured.
I’ll get Jim al-Kalili’s Quantum: A Guide for the Perplexed. Looks like a very useful guide. Thanks.
Thanks everyone
davidmich
I would describe that as a single ground state, with a separate excited state available at just slightly higher energy. Though it is also possible to have a truly degenerate ground state, of course (for instance, for an atom with nonzero total angular momentum, in the absence of an external magnetic field).
Well the article is right - we can’t see atoms, but then again we can’t see pressure waves either. What we can do is map differences in pressure to different colours. The same thing is done with scanning/tunnelling electron microscope. Basically the tunneling current values are mapped to a colour and voila! you can "see"an atom.