Researchers discover new fundamental quantum mechanical property

January 6, 2016
Researchers discover new fundamental quantum mechanical property
Schematic representation of the nonlocal electron interference experiment. A dc current is driven from the upper left to the lower left contact. A nonlocal, oscillating voltage is measured between the upper and lower right contacts due the magnetic-field induced single-electron interference in the 500 nanometer ring in the middle.

Nanotechnologists at the University of Twente research institute MESA+ have discovered a new fundamental property of electrical currents in very small metal circuits. They show how electrons can spread out over the circuit like waves and cause interference effects at places where no electrical current is driven. The geometry of the circuit plays a key role in this so called nonlocal effect. The interference is a direct consequence of the quantum mechanical wave character of electrons and the specific geometry of the circuit. For designers of quantum computers it is an effect to take account of. The results are published in the British journal Scientific Reports.

Interference is a common phenomenon in nature and occurs when one or more propagating waves interact coherently. Interference of sound, light or water waves is well known, but also the carriers of electrical current – electrons – can interfere. It shows that electrons need to be considered as waves as well, at least in nanoscale circuits at extremely low temperatures: a canonical example of the quantum mechanical wave-particle duality.

Gold ring

The researchers from the University of Twente have demonstrated electron in a gold ring with a diameter of only 500 nanometers (a nanometer is a million times smaller than a millimeter). One side of the ring was connected to a miniature wire through which an can be driven. On the other side, the ring was connected to a wire with a voltmeter attached to it. When a current was applied, and a varying magnetic field was sent through the ring, the researchers detected electron interference at the other side of the ring, even though no net current flowed through the ring.

This shows that the electron waves can "leak" into the ring, and change the electrical properties elsewhere in the circuit, even when classically one does not expect anything to happen. Although the gold ring is diffusive (meaning that the electron mean free path is much smaller than the ring), the effect was surprisingly pronounced.

Quantum information processing

The result is a direct consequence of the fact that the quantum equations of motion are nonlocal. That nature is nonlocal is also well-known from another kind of nonlocality: the counterintuitive ability of objects to instantaneously know about each other's state, even when separated by large distances. Einstein referred to it as: "spooky action at a distance". The Twente results help to further understand the first type of nonlocality, referred to as dynamical nonlocality, which plays a key role in all quantum interference experiments. It is very well known that quantum interference is affected by decoherence (where the physical environment causes loss of phase memory), and by performing a "which-path-measurement" (removing the dynamical nonlocality and hence destroying the interference pattern). Now the researchers from the University of Twente have discovered a new way to affect the dynamical noncality. Namely the geometry of the circuit. Understanding this fundamental effect is important for future . For example when creating a quantum computer.

Explore further: Quantum interference fine-tuned by Berry phase

More information: E. Strambini et al. Geometric reduction of dynamical nonlocality in nanoscale quantum circuits, Scientific Reports (2016). DOI: 10.1038/srep18827

Related Stories

Quantum interference fine-tuned by Berry phase

July 5, 2012

(Phys.org) -- A team from the University of Bristol’s Centre for Quantum Photonics (CQP) has experimentally demonstrated how to use Berry’s phase to accurately control quantum interference between different photons.

Electrons always find a (quantum) way

November 17, 2015

Scientists from the Swiss Nanoscience Institute and the Department of Physics at the University of Basel have demonstrated for the first time how electrons are transported from a superconductor through a quantum dot into ...

Recommended for you

Chemists explore outer regions of periodic table

August 25, 2016

A little known—and difficult to obtain—element on the fringes of the periodic table is broadening our fundamental understanding of chemistry. In the latest edition of the journal Science, Florida State University Professor ...

More to rainbows than meets the eye

August 25, 2016

In-depth review charts the scientific understanding of rainbows and highlights the many practical applications of this fascinating interaction between light, liquid and gas.

Measuring tiny forces with light

August 25, 2016

Photons are bizarre: They have no mass, but they do have momentum. And that allows researchers to do counterintuitive things with photons, such as using light to push matter around.

DNA chip offers big possibilities in cell studies

August 25, 2016

A UT Dallas physicist has developed a novel technology that not only sheds light on basic cell biology, but also could aid in the development of more effective cancer treatments or early diagnosis of disease.

Understanding nature's patterns with plasmas

August 23, 2016

Patterns abound in nature, from zebra stripes and leopard spots to honeycombs and bands of clouds. Somehow, these patterns form and organize all by themselves. To better understand how, researchers have now created a new ...

18 comments

Adjust slider to filter visible comments by rank

Display comments: newest first

Protoplasmix
4 / 5 (4) Jan 06, 2016
A dc current is driven from the upper left to the lower left contact. A nonlocal, oscillating voltage is measured between the upper and lower right contacts due the magnetic-field induced single-electron interference in the 500 nanometer ring in the middle.
Wow, an oscillating voltage from a dc current? What was the frequency?

Double wow, the oscillations are, "periodic in the magnetic flux quantum, h/e (h is Planck's constant and e is the electronic charge)." - quote from the Nature article

Fascinating insight into the fundamentals - which path does the particle take? Looks like all of them (within the geometrical constraints).
Hyperfuzzy
2.3 / 5 (3) Jan 06, 2016
Why is this new? Looks like multiple paths; hence, a reflection phenomenon if not exactly equal paths. Using a pulse, watch it decay.
Frosted Flake
3 / 5 (2) Jan 07, 2016
Well, I clearly understand that I do not clearly understand. I'm not sure if that means I'm smart or not. So I am going to mark this page with this comment, so I may watch the discussion unfold.

I have great expectations. But for now I am troubled by the resemblance of the device to an electrical generator. Moving magnetic field, Conductor. Current. We learn of this in grade school. And learn it at the grade school level. So it's in the way. Rather like a Camel in a restaurant.
Frosted Flake
5 / 5 (3) Jan 07, 2016
Postscript : It is a great pleasure to have access to the actual paper. Thanks very much.
Multivac jr_
5 / 5 (3) Jan 07, 2016
Well, I clearly understand that I do not clearly understand. I'm not sure if that means I'm smart or not.


That's okay, you're in good company...

I think I can safely say that nobody understands quantum mechanics. (Richard Feynman)
antialias_physorg
5 / 5 (4) Jan 07, 2016
Wow, an oscillating voltage from a dc current? What was the frequency?

If I read the paper correctly: Since they use a lock-in amplifier at 17.77Hz then that would likely be the frequency at which they measure a signal (see section "Experimental setup")
Protoplasmix
5 / 5 (3) Jan 07, 2016
If I read the paper correctly: Since they use a lock-in amplifier at 17.77Hz then that would likely be the frequency at which they measure a signal (see section "Experimental setup")
That's a reference signal to "reduce 1/f noise and interference" in the measurement. Lock-in amplifiers are used to detect and measure really small AC signals, down to just a few nanovolts, "even when the small signal is obscured by noise sources many thousands of times larger." They use a method called "phase-sensitive detection" which requires a reference frequency. (Refer to Section 3 of the "Model SR830 DSP Lock-in Amplifier" manual from Stanford Research Systems.)

From the introduction (Nature article), "The corresponding Fourier spectrum (Fig. 1d) clearly reveals the AB period h/e, as well as higher harmonics up to h/4e."
Protoplasmix
5 / 5 (3) Jan 07, 2016
Why is this new?
In previous experiments, like the well known double-slit, the choice of paths is external to the experimental apparatus. In this experiment, the choice of "conduction channel" occurs inside the apparatus.

From the Nature article:
We have thus shown a new aspect of the dynamical nonlocality of electrons in a quantum nanoscale circuit that is solely governed by geometric aspects and not by external measurement.
swordsman
5 / 5 (2) Jan 07, 2016
"Beautiful"! This is evidence to support Planck's assertion of the "action of the electron upon itself". It is one of the basic tenets of physics that has not been investigated over the past century, until recently. I have proven that the electron begins to move in a circular path as it approaches the speed of light. This is the mechanism that allows a proton and an electron to form a hydrogen atom. (ref: "The Birth of an Atom - How Matter is Formed in the Universe", 2010, Chapter VII, "Electron Capture and the Birth of an Atom").
axemaster
4.4 / 5 (7) Jan 07, 2016
You're making lock-in amps sound much more mysterious than they are.

Imagine you're taking a measurement of a signal Vs that has background noise Vbn.
Vsignal = Vs+Vbn
So you disconnect the signal for a moment and measure just the background Vbn by itself. Then you do:
Vs = Vsignal - Vbn

A lock-in amp just does this in analog. Though to be fair they are surprisingly tricky to build, at least if you want a decent frequency response.

You can also build them in digital, provided you use a sufficiently fast and precise (>=24 bit) ADC.

The advantage, obviously, is that they remove the background very effectively (>60dB improvement), including background from the electronics themselves.

Another way to view a lock-in amp is that you modulate the signal with a very very narrow frequency, then have a receiver with an incredibly narrow bandpass filter at the same frequency.
Hyperfuzzy
1 / 5 (2) Jan 07, 2016
Consider the multiple paths and the sum at the junction and the time of arrival of "what signal" based on the geometry! Basic EE!
Protoplasmix
5 / 5 (2) Jan 08, 2016
You're making lock-in amps sound much more mysterious than they are.
You're right, sorry about that. After reading more of the manual, the reference frequency has to be locked to the signal, so AA must be right about it "likely being 17.77Hz." I don't understand it quite as well as I thought I did...
vlaaing peerd
5 / 5 (1) Jan 08, 2016
Well, I clearly understand that I do not clearly understand. I'm not sure if that means I'm smart or not.

Probably the first. In my language there is a saying: "self-knowledge graces humanity".
Hyperfuzzy
1 / 5 (3) Jan 08, 2016
come on, oscillating current require an oscillating moving charge, electrons move all the time. and they follow well known rules. There is no reason to invent s#it you can't explain. You are excited about stupidity. QM effects, really, vs actual effects defined! Try looking at the effect of the 3rd order of a non-linear resistor, temperature controlled and relaxed automatically or magnetic field response, oscillating only creates a non-linear resistor. Nothing new, here. You have a time varying resistor and probably reflections from using multiple paths.
Hyperfuzzy
1 / 5 (2) Jan 08, 2016
come on, oscillating current require an oscillating moving charge, electrons move all the time. and they follow well known rules. There is no reason to invent s#it you can't explain. You are excited about stupidity. QM effects, really, vs actual effects defined! Try looking at the effect of the 3rd order of a non-linear resistor, temperature controlled and relaxed automatically or magnetic field response, oscillating only creates a non-linear resistor. Nothing new, here. You have a time varying resistor and probably reflections from using multiple paths.

Resistor, constant at the quantum level? This doesn't even make sense. Nothing new here, just not knowing why is silly. How many ways does resistance alter and from what effects? Jeez.
Hyperfuzzy
not rated yet Jan 08, 2016
Even a crack has a 3rd harmonic response, relax, separate, don't relax, in how much time. New?
Protoplasmix
5 / 5 (1) Jan 09, 2016
come on, oscillating current require an oscillating moving charge,... You are excited about stupidity. QM effects, really, vs actual effects defined... Nothing new, here...
No, the rules for quantum mechanics are quite different from those of the classical mechanics that we experience macroscopically. And the transition from micro to macro and connections between those different sets of rules has not been easy to discern and interpret, especially when the measurement itself is enough to change the outcome. So this work showing the actual connections between dynamical nonlocality and local physical geometry, independent of measurement, is pretty exciting.

And this "dynamical nonlocality" looks like wave mechanics. With waves traveling faster than energy itself can propagate. Faster than c, Hyperfuzzy. Put that in your nothing new circuit and flip the switch. Maybe information is composed of particles -- FTL "infotons" ?
artichoke33
not rated yet Jan 12, 2016
(post removed)

Please sign in to add a comment. Registration is free, and takes less than a minute. Read more

Click here to reset your password.
Sign in to get notified via email when new comments are made.