New work asserts that a key technique used to probe quantum systems may not be so quantum after all, according to Perimeter postdoctoral researcher Joshua Combes and his colleague Christopher Ferrie.
Over the past 20 years, a strange idea called a “weak value” has taken root in quantum information science.
How the result of a coin toss can be 100 heads: First, you preselect on heads. Then, a friend performs a weak measurement and occasionally flips the coin. When the coin comes back tails, then you calculate (using the mathematical steps in Ferrie and Combes’ paper) what your friend measured. The calculation says they measured not just heads, but 100 heads!
Many of the things you can do with quantum technologies entail being able to gain information from quantum systems. But there is a quantum conundrum: we can’t say what a particle is doing when we’re not looking at it, but when we do look at it, we change its behaviour.
But what if we could look “a little”? Well, that’s a weak measurement, a concept which is central to the notion of a weak value. The basic idea of weak measurement is to gain a little bit of information about a quantum system by only disturbing it a little bit; by doing this many times, one can ultimately gain quite a bit of information about the system. Weak measurements have applications in quantum information technologies such as quantum feedback control and quantum communications.
Obtaining a weak value involves taking a weak measurement of a particle. It also – counterintuitively – depends on throwing out the majority of the results, carefully selecting only a few to keep in an effort to screen out particles which were knocked off-course by the act of measurement.
In this way, researchers believe they can gradually build up a picture of the typical behaviour of particles even between measurements. When these carefully gathered and screened measurements produce something unexpected and (apparently) quantum, that’s called a weak value. Weak values are a whole new window into the quantum world.
Unless, of course, they’re not. What if weak values aren’t quantum at all?
“We’re skeptical of the whole field,” says Joshua Combes. Combes is a postdoctoral fellow at Perimeter and the University of Waterloo’s Institute for Quantum Computing (IQC), and he has just published a Physical Review Letters paper critical of weak measurement.
“On the one hand, the quantum world can be weird, of course,” he says. “But on the other hand, we need to work carefully to distinguish between genuinely quantum effects and effects that can be replicated classically.”
In the new paper, Combes worked with Christopher Ferrie of the University of New Mexico on just such a problem: finding a classical analogue of a weak value presented in the field’s seminal paper.
In the original paper, Yakir Aharonov (now a Distinguished Visiting Research Chair at Perimeter), David Albert, and Lev Vaidman laid down the principles of weak measurement, arguing for the power of extracting only a “little bit” of information from each measurement, and for throwing most of that away. Their procedure went something like this.
Say you want to measure the spin of some particles. You would prepare particles in some particular state, say “spin up,” throwing away the data from particles that are “spin down.” This is called “pre-selection.” Later, you would detect the particles in a final state, again throwing away those that aren’t in a desired state. This is called “post-selection.”
You also make a measurement in between, but in the quantum world, any measurement has the potential to disrupt the system. Aharonov et al. argued that you should measure the spin as gently – as weakly – as possible. This measurement is by nature imprecise, so you must then average over a large number of trials.
By cleverly combining pre-selection, post-selection, and weak measurement, Aharonov and colleagues invented a new and apparently fundamentally quantum way of measuring quantum properties. Their landmark 1988 paper is called, “How the measurement of a component of the spin of a spin-½ particle can turn out to be 100.” The weak value is the spin quantity that is equal to 100.
A particle whose spin should be either +½ or -½ having a spin of 100? Combes and Ferrie wouldn’t put money on that.
Facing the field’s giants head on, they outline a parallel process – the same pre-selection, post-selection, and weak measurement – to show that you can get the same odd result out of the world’s simplest random system: a coin flip. As a poke in the eye, they call their paper: “How the result of a single coin toss can turn out to be 100 heads.”
Combes demonstrates. He has you flip coins, then hand him only the coins that come up heads, without telling him what you got. (That’s pre-selection.) He glances at each coin too quickly to be entirely sure what it says. (That’s weak measurement.) Some percentage of the time, he gives each coin a nudge, which might occasionally flip it. (Even a weak measurement can sometimes disturb a quantum system, and the nudge imitates that.) Finally, he hands it back to you. If it’s heads, the trial is discarded (that’s the post-selection step); if it comes back tails, he asks you to predict what he measured.
It seems straightforward when the coin is in front of you: if it’s tails, you would intuitively predict he probably saw heads (since you only handed him coins that came up heads). In the paper, Ferrie and Combes outline the mathematics of your prediction step-by-step using the same sequence of operations that resulted in Aharonov et al.’s weak value. The bizarre result? If the coin comes back to you showing tails, after performing the same math, you too would predict that he measured it as reading 100 heads.
The quantum calculations performed by Aharonov et al. to get that strange 100 result are highly technical; what’s important, say Combes and Ferrie, is that when the same calculations are done classically, they give the same bizarre result.
“If you don’t find that convincing, then why would you find the quantum equivalent of that convincing?” asks Combes.
Quantum effects can sometimes be strange. But, as the authors write, “Where a classical explanation exists, no quantum explanation is required. This is the guiding principle of quantum foundations research.”
The authors believe that anomalous values found via weak measurement are not a truly quantum effect, but an artifact of classical statistics and classical disturbances. As any fan of mathematical puzzles could tell you, problems based on who knows what and when can produce surprising results. The Monty Hall problem is the most famous example, and it tricks game show contestants and mathematics professors alike.
“Statistics can fool you,” says Combes. “We think this particular weak value puzzle is a statistical question, not a fundamentally quantum question. There might be something genuinely quantum about weak values, but to my eye that’s not clear yet.”
Reflecting on the central paradox of weak values – that by measuring things somewhat precisely, we can build up a better picture of the quantum world – Combes gets philosophical.
“Don’t get me wrong: mystery is great,” he says. “I want there to be mystery – that’s why I’m in this field. But shouldn’t we be trying to get to the bottom of things, rather than making them more mysterious than they really are? I think we need to carefully question what weak values really tell us. Chris and I are hoping this paper will spark some of that questioning.”
That, at least, seems like a very good bet. – Erin Bow
Eamon O'Flynn | Eurek Alert!
Hope to discover sure signs of life on Mars? New research says look for the element vanadium
22.09.2017 | University of Kansas
22.09.2017 | Forschungszentrum MATHEON ECMath
Plants and algae use the enzyme Rubisco to fix carbon dioxide, removing it from the atmosphere and converting it into biomass. Algae have figured out a way to increase the efficiency of carbon fixation. They gather most of their Rubisco into a ball-shaped microcompartment called the pyrenoid, which they flood with a high local concentration of carbon dioxide. A team of scientists at Princeton University, the Carnegie Institution for Science, Stanford University and the Max Plank Institute of Biochemistry have unravelled the mysteries of how the pyrenoid is assembled. These insights can help to engineer crops that remove more carbon dioxide from the atmosphere while producing more food.
A warming planet
Our brains house extremely complex neuronal circuits, whose detailed structures are still largely unknown. This is especially true for the so-called cerebral cortex of mammals, where among other things vision, thoughts or spatial orientation are being computed. Here the rules by which nerve cells are connected to each other are only partly understood. A team of scientists around Moritz Helmstaedter at the Frankfiurt Max Planck Institute for Brain Research and Helene Schmidt (Humboldt University in Berlin) have now discovered a surprisingly precise nerve cell connectivity pattern in the part of the cerebral cortex that is responsible for orienting the individual animal or human in space.
The researchers report online in Nature (Schmidt et al., 2017. Axonal synapse sorting in medial entorhinal cortex, DOI: 10.1038/nature24005) that synapses in...
Whispering gallery mode (WGM) resonators are used to make tiny micro-lasers, sensors, switches, routers and other devices. These tiny structures rely on a...
Using ultrafast flashes of laser and x-ray radiation, scientists at the Max Planck Institute of Quantum Optics (Garching, Germany) took snapshots of the briefest electron motion inside a solid material to date. The electron motion lasted only 750 billionths of the billionth of a second before it fainted, setting a new record of human capability to capture ultrafast processes inside solids!
When x-rays shine onto solid materials or large molecules, an electron is pushed away from its original place near the nucleus of the atom, leaving a hole...
For the first time, physicists have successfully imaged spiral magnetic ordering in a multiferroic material. These materials are considered highly promising candidates for future data storage media. The researchers were able to prove their findings using unique quantum sensors that were developed at Basel University and that can analyze electromagnetic fields on the nanometer scale. The results – obtained by scientists from the University of Basel’s Department of Physics, the Swiss Nanoscience Institute, the University of Montpellier and several laboratories from University Paris-Saclay – were recently published in the journal Nature.
Multiferroics are materials that simultaneously react to electric and magnetic fields. These two properties are rarely found together, and their combined...
19.09.2017 | Event News
12.09.2017 | Event News
06.09.2017 | Event News
22.09.2017 | Life Sciences
22.09.2017 | Medical Engineering
22.09.2017 | Physics and Astronomy