An international research team led by Skoltech and IBM has created an
extremely energy-efficient optical switch that could replace electronic
transistors in a new generation of computers manipulating photons rather
than electrons. In addition to direct power saving, the switch requires no
cooling and is really fast: At 1 trillion operations per second, it is
between 100 and 1,000 times faster than today's top-notch commercial
transistors. The study comes out Wednesday in Nature.
"What makes the new device so energy-efficient is that it only takes a few
photons to switch," the first author of the study, Dr. Anton Zasedatelev
commented. "In fact, in our Skoltech labs we achieved switching with just
one photon at room temperature. That said, there is a long way to go before
such proof-of-principle demonstration is utilized in an all-optical
co-processor," added Professor Pavlos Lagoudakis, who heads the Hybrid
Photonics Labs at Skoltech.
Since a photon is the smallest particle of light that exists in nature,
there is really not much room for improvement beyond that as far as power
consumption goes. Most modern electrical transistors take tens of times more
energy to switch, and the ones that use single electrons to achieve
comparable efficiencies are way slower.
Besides performance issues the competing power-saving electronic transistors
also tend to require bulky cooling equipment, which in turn consumes power
and factors into the operating costs. The new switch conveniently works at
room temperature and therefore circumvents all these problems.
In addition to its primary transistor-like function, the switch could act as
a component that links devices by shuttling data between them in the form of
optical signals. It can also serve as an amplifier, boosting the intensity
of an incoming laser beam by a factor of up to 23,000.
How it works
The device relies on two lasers to set its state to "0" or "1" and to switch
between them. A very weak control laser beam is used to turn another,
brighter laser beam on or off. It only takes a few photons in the control
beam, hence the device's high efficiency.
The switching occurs inside a microcavity—a 35-nanometer thin organic
semiconducting polymer sandwiched between highly reflective inorganic
structures. The microcavity is built in such a way as to keep incoming light
trapped inside for as long as possible to favor its coupling with the
cavity's material.
This light-matter coupling forms the basis of the new device. When photons
couple strongly to bound electron-hole pairs—aka excitons—in the cavity's
material, this gives rise to short-lived entities called exciton-polaritons,
which are a kind of quasiparticles at the heart of the switch's operation.
When the pump laser—the brighter one of the two—shines on the switch, this
creates thousands of identical quasiparticles in the same location, forming
so-called Bose-Einstein condensate, which encodes the "0" and "1" logic
states of the device.
To switch between the two levels of the device, the team used a control
laser pulse seeding the condensate shortly before the arrival of the pump
laser pulse. As a result, it stimulates energy conversion from the pump
laser, boosting the amount of quasiparticles at the condensate. The high
amount of particles in there corresponds to the "1" state of the device.
The researchers used several tweaks to ensure low power consumption: First,
efficient switching was aided by the vibrations of the semiconducting
polymer's molecules. The trick was to match the energy gap between the
pumped states and the condensate state to the energy of one particular
molecular vibration in the polymer. Second, the team managed to find the
optimal wavelength to tune their laser to and implemented a new measurement
scheme enabling single-shot condensate detection. Third, the control laser
seeding the condensate and its detection scheme were matched in a way that
suppressed the noise from the device's "background" emission. These measures
maximized the signal-to-noise level of the device and prevented an excess of
energy from being absorbed by the microcavity, which would only serve to
heat it up through molecular vibrations.
"There's still some work ahead of us to lower the overall power consumption
of our device, which is currently dominated by the pump laser that keeps the
switch on. A route toward that goal could be perovskite supercrystal
materials like those we're exploring with collaborators. They have proven
excellent candidates given their strong light-matter coupling which in turn
leads to a powerful collective quantum response in the form of
superfluorescence," the team comments.
In the larger scheme of things, the researchers see their new switch as but
one in the growing toolkit of all-optical components they have been
assembling over the past few years. Among other things, it includes a
low-loss silicon waveguide for shuttling the optical signals back and forth
between transistors. The development of these components takes us ever
closer to optical computers that would manipulate photons instead of
electrons, resulting in vastly superior performance and lower power
consumption.
Reference:
Zasedatelev, A.V., Baranikov, A.V., Sannikov, D. et al. Single-photon
nonlinearity at room temperature. Nature 597, 493–497 (2021).
DOI: 10.1038/s41586-021-03866-9
Tags:
Physics