The same pulling force that causes “tears” in a glass of wine also shapes embryos. It’s another example of how genes exploit mechanical forces for growth and development.
Classical clocks have to obey the second law of thermodynamics: the higher their precision, the more entropy they produce. For a while, it seemed like quantum clocks might beat this system, at least in theory. This is because although quantum fluctuations produce no entropy, if you can count those fluctuations as clock “ticks”, you can make a clock with nonzero precision. Now, however, a collaboration of researchers across Europe has pinned down where the entropy-precision trade-off balances out: it’s in the measurement process. As project leader Natalia Ares observes, “There’s no such thing as a free lunch.”
The clock the team used to demonstrate this principle consists of a pair of quantum dots coupled by a thin tunnelling barrier. In this double quantum dot system, a “tick” occurs whenever an electron tunnels from one side of the system to the other, through both dots. Applying a bias voltage gives ticks a preferred direction.
This might not seem like the most obvious kind of clock. Indeed, as an actual timekeeping device, collaboration member Florian Meier describes it as “quite bad”. However, Ares points out that although the tunnelling process is random (stochastic), the period between ticks does have a mean and a standard deviation. Hence, given enough ticks, the number of ticks recorded will tell you something about how much time has passed.
In any case, Meier adds, they were not setting out to build the most accurate clock. Instead, they wanted to build a playground to explore basic principles of energy dissipation and clock precision, and for that, it works really well. “The really cool thing I like about what they did was that with that particular setup, you can really pinpoint the entropy dissipation of the measurement somehow in this quantum dot,” says Meier, a PhD student at the Technical University in Vienna, Austria. “I think that’s really unique in the field.”
Calculating the entropy
To measure the entropy of each quantum tick, the researchers measured the voltage drop (and associated heat dissipation) for each electron tunnelling through the double quantum dot. Vivek Wadhia, a DPhil student in Ares’s lab at the University of Oxford, UK who performed many of the measurements, points out that this single unit of charge does not equate to very much entropy. However, measuring the entropy of the tunnelling electron was not the full story.
Timekeeping: Vivek Wadhia working on the clock used in the experiment. (Courtesy: Wadhia et al./APS 2025)
Because the ticks of the quantum clock were, in effect, continuously monitored by the environment, the coherence time for each quantum tunnelling event was very short. However, because the time on this clock could not be observed directly by humans – unlike, say, the hands of a mechanical clock – the researchers needed another way to measure and record each tick.
For this, they turned to the electronics they were using in the lab and compared the power in versus the power out on a macroscopic scale. “That’s the cost of our measurement, right?” says Wadhia, adding that this cost includes both the measuring and recording of each tick. He stresses that they were not trying to find the most thermodynamically efficient measurement technique: “The idea was to understand how the readout compares to the clockwork.”
This classical entropy associated with measuring and recording each tick turns out to be nine orders of magnitude larger than the quantum entropy of a tick – more than enough for the system to operate as a clock with some level of precision. “The interesting thing is that such simple systems sometimes reveal how you can maybe improve precision at a very low cost thermodynamically,” Meier says.
As a next step, Ares plans to explore different arrangements of quantum dots, using Meier’s previous theoretical work to improve the clock’s precision. “We know that, for example, clocks in nature are not that energy intensive,” Ares tells Physics World. “So clearly, for biology, it is possible to run a lot of processes with stochastic clocks.”
Researchers in the Netherlands have demonstrated the first chip-based device capable of splitting phonons, which are quanta of mechanical vibrations. Known as a single-phonon directional coupler, or more simply as a phonon splitter, the new device could make it easier for different types of quantum technologies to “talk” to each other. For example, it could be used to transfer quantum information from spins, which offer advantages for data storage, to superconducting circuits, which may be better for data processing.
“One of the main advantages of phonons over photons is they interact with a lot of different things,” explains team leader Simon Gröblacher of the Kavli Institute of Nanoscience at Delft University of Technology. “So it’s very easy to make them interface with systems.”
There are, however, a few elements still missing from the phononic circuitry developer’s toolkit. One such element is a reversible beam splitter that can either combine two phonon channels (which might be carrying quantum information transferred from different media) or split one channel into two, depending on its orientation.
While several research groups have already investigated designs for such phonon splitters, these works largely focused on surface acoustic waves. This approach has some advantages, as waves of this type have already been widely explored and exploited commercially. Mobile phones, for example, use surface acoustic waves as filters for microwave signals. The problem is that these unconfined mechanical excitations are prone to substantial losses as phonons leak into the rest of the chip.
Mimicking photonic beam splitters
Gröblacher and his collaborators chose instead to mimic the design of beam splitters used in photonic chips. They used a strip of thin silicon to fashion a waveguide for phonons that confined them in all dimensions but one, giving additional control and reducing loss. They then brought two waveguides into contact with each other so that one waveguide could “feel” the mechanical excitations in the other. This allowed phonon modes to be coupled between the waveguides – something the team demonstrated down to the single-phonon level. The researchers also showed they could tune the coupling between the two waveguides by altering the contact length.
Although this is the first demonstration of single-mode phonon coupling in this kind of waveguide, the finite element method simulations Gröblacher and his colleagues ran beforehand made him pretty confident it would work from the outset. “I’m not surprised that it worked. I’m always surprised how hard it is to get it to work,” he tells Physics World. “Making it to look and do exactly what you design it to do – that’s the really hard part.”
Prospects for integrated quantum phononics
According to A T Charlie Johnson, a physicist at the University of Pennsylvania, US whose research focuses on this area, that hard work paid off. “These very exciting new results further advance the prospects for phonon-based qubits in quantum technology,” says Johnson, who was not directly involved in the demonstration. “Integrated quantum phononics is one significant step closer.”
As well as switching between different quantum media, the new single-phonon coupler could also be useful for frequency shifting. For instance, microwave frequencies are close to the frequencies of ambient heat, which makes signals at these frequencies much more prone to thermal noise. Gröblacher already has a company working on transducers to transform quantum information from microwave to optical frequencies with this challenge in mind, and he says a single-phonon coupler could be handy.
One remaining challenge to overcome is dispersion, which occurs when phonon modes couple to other unwanted modes. This is usually due to imperfections in the nanofabricated device, which are hard to avoid. However, Gröblacher also has other aspirations. “I think the one component that’s missing for us to have the similar level of control over phonons as people have with photons is a phonon phase shifter,” he tells Physics World. This, he says, would allow on-chip interferometry to route phonons to different parts of a chip, and perform advanced quantum experiments with phonons.
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