[This is a “literature impression“.]
Masahiro Hotta has a series of paper about what he calls “quantum energy teleportation (QET)”, modeled after the well-known notion of quantum teleportation (of information). Although it sounds like crazy crack pot stuff, and they contain the red-flag term “zero-point energy”, the basic physics of Hotta’s work are sound. But they don’t appear to have important consequences for energy transmission.
The idea is to exploit the fact that the ground state of the vacuum in QFT is, in principle, entangled over arbitrary distances. In a toy Alice and Bob model with respective systems and , you assume a Hamiltonian for which the ground state is unique and entangled. Then, Alice makes a local measurement on her system . Neither of the two conditional global states for the joint system — conditional on the outcome of the measurement — are eigenstates of the Hamiltonian, and so therefore the average energy must increase for the joint system. The source of this energy is the device Alice used to make the measurement. Now, if Bob were to independently make a measurement of his system, he would find that energy would also necessarily flow from his device into the joint system; this follows from the symmetry of the problem. But if he waits for Alice to transmit to him the outcome of her result, it turns out that he can apply a local unitary to his system and a subsequent local measurement that leads to a net average energy flow to his equipment. The fact that he must wait for the outcome of Alice’s measurement, which travels no faster than the speed of light, is what gives this the flavor of teleportation.… [continue reading]
I have often been frustrated by the inefficiency of reading through the physics literature. One problem is that physicists are sometimes bad teachers and are usually bad writers, and so it can take a long time of reading a paper before you even figure out what the author is trying to say. This gets worse when you look at papers that aren’t in your immediate physics niche, because then the author will probably use assumptions, mathematical techniques, and terminology you aren’t familiar with. If you had infinite time, you could spend days reading every paper that looks reasonably interesting, but you don’t. A preferred technique is to ask your colleagues to explain it to you, because they are more likely to speak your language and (unlike a paper) can answer your questions when you come up against a confusion. But generally your colleagues haven’t read it; they want you to read it so you can explain it to them. I spend a lot of time reading papers that end up being uninteresting, but it’s worth it for the occasional gems. And it seems clear that there is a lot of duplicated work being done sorting through the chaff.
So on the one hand we have a lengthy, fixed document from a single, often unfamiliar perspective (i.e. the actual paper in a different field) and on the other hand we have a breathing human being in your own field who will patiently explain things to you. An intermediate solution would be to have a few people in different fields read the paper and then translate the key parts into their field’s language, which could then be passed around.… [continue reading]
[This was originally posted at the Quantum Pontiff.]
People sometimes ask me what how my research will help society. This question is familiar to physicists, especially those of us whose research is connected to every-day life only… shall we say…tenuously. And of course, this is a fair question from the layman; tax dollars support most of our work.
I generally take the attitude of former Fermilab director Robert R. Wilson. During his testimony before the Joint Committee on Atomic Energy in the US Congress, he was asked how discoveries from the proposed accelerator would contribute to national security during a time of intense Cold War competition with the USSR. He famously replied “this new knowledge has all to do with honor and country but it has nothing to do directly with defending our country except to help make it worth defending.”
Still, it turns out there are philosophers of practical ethics who think a few of the academic questions physicists study could have tremendous moral implications, and in fact might drive key decisions we all make each day. Oxford philosopher Nick Bostrom has in particular written about the idea of “astronomical waste“. As is well known to physicists, the universe has a finite, ever-dwindling supply of negentropy, i.e. the difference between our current low-entropy state and the bleak maximal entropy state that lies in our far future. And just about everything we might value is ultimately powered by it. As we speak (or blog), the stupendously vast majority of negentropy usage is directed toward rather uninspiring ends, like illuminating distant planets no one will ever see.
These resources can probably be put to better use. … [continue reading]
I’ve submitted my papers (long and short arXiv versions) on detecting classically undetectable new particles through decoherence. The short version introduces the basic idea and states the main implications for dark matter and gravitons. The long version covers the dark matter case in depth. Abstract for the short version:
Detecting Classically Undetectable Particles through Quantum Decoherence
Some hypothetical particles are considered essentially undetectable because they are far too light and slow-moving to transfer appreciable energy or momentum to the normal matter that composes a detector. I propose instead directly detecting such feeble particles, like sub-MeV dark matter or even gravitons, through their uniquely distinguishable decoherent effects on quantum devices like matter interferometers. More generally, decoherence can reveal phenomena that have arbitrarily little classical influence on normal matter, giving new motivation for the pursuit of macroscopic superpositions.
This is figure 1:
Decoherence detection with a Mach-Zehnder interferometer.
is placed in a coherent superposition of spatially displaced wavepackets
that each travel a separate path and then are recombined. In the absence of system
, the interferometer is tuned so that
will be detected at the bright port with near unit probability, and at the dim port with near vanishing probability. However, if system
, these two paths can decohere and
will be detected at the dim port 50% of the time.
Below are some FAQs I have received.
Won’t there always be momentum transfer in any nontrivial scattering?
For any nontrivial scattering of two particles, there must be some momentum transfer. But the momentum transfer can be arbitrarily small by simply making the mass of the dark particle as tiny as desired (while keeping its velocity fixed). … [continue reading]
Physics StackExchange user QuestionAnswers asked the question “Is the preferred basis problem solved?“, and I reproduced my “answer” (read: discussion) in a post last week. He had some thoughtful follow-up questions, and (with his permission) I am going to answer them here. His questions are in bold, with minor punctuation changes.
How serious would you consider what you call the “Kent set-selection” problem?
If a set of CHs could be shown to be impossible to find, then this would break QM without necessarily telling us how to correct it. (Similar problems exist with the breakdown of gravity at the Planck scale.) Although I worry about this, I think it’s unlikely and most people think it’s very unlikely. If a set can be found, but no principle can be found to prefer it, I would consider QM to be correct but incomplete. It would kinda be like if big bang neucleosynthesis had not been discovered to explain the primordial frequency of elements.
And what did Zurek think of it, did he agree that it’s a substantial problem?
I think Wojciech believes a set of consistent histories (CHs) corresponding to the branch structure could be found, but that no one will find a satisfying beautiful principle within the CH framework which singles out the preferred set from the many, many other sets. He believes the concept of redundant records (see “quantum Darwinism”) is key, and that a set of CHs could be found after the fact, but that this is probably not important. I am actually leaving for NM on Friday to work with him on a joint paper exploring the connection between redundancy and histories.… [continue reading]
Now I would like to apply the reasoning of the last post to the case of verifying macroscopic superpositions of the metric. It’s been 4 years since I’ve touched GR, so I’m going to rely heavily on E&M concepts and pray I don’t miss any key changes in the translation to gravity.
In the two-slit experiment with light, we don’t take the visibility of interference fringes as evidence of quantum mechanics when there are many photons. This is because the observations are compatible with a classical field description. We could interfere gravitational waves in a two-slit set up, and this would also have a purely classical explanation.
But in this post I’m not concentrating on evidence for pure quantum mechanics (i.e. a Bell-like argument grounded in locality), or evidence of the discrete nature of gravitons. Rather, I am interested in superpositions of two macroscopically distinct states of the metric as might be produced by a superposition of a large mass in two widely-separated positions. Now, we can only call a quantum state a (proper) superposition by first identifying a preferred basis that it can be a superposition with respect to. For now, I will wave my hands and say that the preferred states of the metric are just those metric states produced by the preferred states of matter, where the preferred states of matter are wavepackets of macroscopic amounts of mass localized in phase space (e.g. L/R). Likewise, the conjugate basis states (e.g. L+R/L-R) are proper superpositions in the preferred basis, and these two bases do not commute.
There are two very distinct ways to produce a superposition with different states of the metric: (1) a coherent superposition of just gravitational radiation Note that we expect to produce this superposition by moving a macroscopic amount of matter into a superposition of two distinct position or momentum states. … [continue reading]
Suppose we are given an ensemble of systems which are believed to contain coherent superposition of the metric. How would we confirm this?
Well, in order to verify that an arbitrary system is in a coherent superposition, which is always relative to a preferred basis, it’s well known that we need to make measurements with respect to (at least?) two non-commuting bases. If we can make measurement M we expect it to be possible to make measurement M` = RM for some symmetry R.
I consider essentially two types of Hilbert spaces: the infinite-dimensional space associated with position, and the finite-dimensional space associated with spin. They have a very different relationship with the fundamental symmetries of spacetime.
For spin, an arbitrary rotation in space is represented by a unitary which can produce proper superpositions. Rotating 90 degrees about the y axis takes a z-up eigenstate to an equal superposition of z-up and z-down. The rotation takes one basis to another with which it does not commute.
In contrast, for position, the unitary representing spatial translation is essentially just a permutation on the space of position eigenstates. It does not produce superpositions from non-superpositions with respect to this basis.
You might think things are different when you consider more realistic measurements with respect to the over-complete basis of wavepackets. (Not surprisingly, the issue is one of preferred basis!) If you imagine the wavepackets as discretely tiling space, it’s tempting to think that translating a single wavepacket a half-integer number of tile spacing will yield an approximate superposition of two wavepackets. But the wavepackets are of course not discrete, and a POVM measurement of “fuzzy” position (for any degree of fuzziness σ) is invariant under spatial translations.… [continue reading]
Unfortunately, physicists and philosophers disagree on what exactly the preferred basis problem is, what would constitute a solution, and how this relates (or subsumes) “the measurement problem” more generally. In my opinion, the most general version of the preferred basis problem was best articulated by Adrian Kent and Fey Dowker near the end their 1996 article “On the Consistent Histories Approach to Quantum Mechanics” in the Journal of Statistical Physics. Unfortunately, this article is long so I will try to quickly summarize the idea.
Kent and Dowker analyzed the question of whether the consistent histories formalism provided a satisfactory and complete account of quantum mechanics (QM). Contrary to what is often said, consistent histories and many-worlds need not be opposing interpretations of quantum mechanics Of course, some consistent historians make ontological claims about how the histories are “real”, where as the many-world’ers might say that the wavefunction is more “real”. In this sense they are contradictory. Personally, I think this is purely a matter of taste.a . Instead, consistent histories is a good mathematical framework for rigorously identifying the branch structure of the wavefunction of the universe Note that although many-worlders may not consider the consistent histories formalism the only way possible to mathematically identify branch structure, I believe most would agree that if, in the future, some branch structure was identified using a completely different formalism, it could be described at least approximately by the consistent histories formalism. Consistent histories may not be perfect, but it’s unlikely that the ideas are totally wrong.b . Most many-world’ers would agree that unambiguously describing this branch structure would be very nice (although they might disagree on whether this is “necessary” for QM to be a complete theory).… [continue reading]