A wireless technique enables a super-cold quantum computer to send and receive data
without generating too much error-causing heat.
Heat causes errors in the qubits that are the building blocks of a quantum computer,
so quantum systems are typically kept inside refrigerators that keep the temperature just
above absolute zero (-459 degrees Fahrenheit).
But quantum computers need to communicate with electronics outside the refrigerator,
in a room-temperature environment.
The metal cables that connect these electronics bring heat into the refrigerator, which has
to work even harder and draw extra power to keep the system cold.
Plus, more qubits require more cables, so the size of a quantum system is limited by
how much heat the fridge can remove.
To overcome this challenge, an interdisciplinary team of MIT researchers has developed a wireless
communication system that enables a quantum computer to send and receive data to and from
electronics outside the refrigerator using high-speed terahertz waves.
A transceiver chip placed inside the fridge can receive and transmit data.
Terahertz waves generated outside the refrigerator are beamed in through a glass window.
Data encoded onto these waves can be received by the chip.
That chip also acts as a mirror, delivering data from the qubits on the terahertz waves
it reflects to their source.
This reflection process also bounces back much of the power sent into the fridge, so
the process generates only a minimal amount of heat.
The contactless communication system consumes up to 10 times less power than systems with
metal cables.
The researchers’ square transceiver chip, measuring about 2 millimeters on each side,
is placed on a quantum computer inside the refrigerator, which is called a cryostat because
it maintains cryogenic temperatures.
These super-cold temperatures don’t damage the chip; in fact, they enable it to run more
efficiently than it would at room temperature.
The chip sends and receives data from a terahertz wave source outside the cryostat using a passive
communication process known as backscatter, which involves reflections.
An array of antennas on top of the chip, each of which is only about 200 micrometers in
size, act as tiny mirrors.
These mirrors can be “turned on” to reflect waves or “turned off.”
The terahertz wave generation source encodes data onto the waves it sends into the cryostat,
and the antennas in their “off” state can receive those waves and the data they
carry.
When the tiny mirrors are turned on, they can be set so they either reflect a wave in
its current form or invert its phase before bouncing it back.
If the reflected wave has the same phase, that represents a 0, but if the phase is inverted,
that represents a 1.
Electronics outside the cryostat can interpret those binary signals to decode the data.
This backscatter technology is not new.
For instance, RFIDs are based on backscatter communication.
The researcher says that they borrow that idea and bring it into this very unique scenario.
The data are transmitted using high-speed terahertz waves, which are located on the
electromagnetic spectrum between radio waves and infrared light.
Because terahertz waves are much smaller than radio waves, the chip and its antennas can
be smaller, too, which would make the device easier to manufacture at scale.
Terahertz waves also have higher frequencies than radio waves, so they can transmit data
much faster and move larger amounts of information.
But because terahertz waves have lower frequencies than the light waves used in photonic systems,
the terahertz waves carry less quantum noise, which leads to less interference with quantum
processors.
Importantly, the transceiver chip and terahertz link can be fully constructed with standard
fabrication processes on a CMOS chip, so they can be integrated into many current systems
and techniques.
The researchers were able to transmit data at 4 gigabits per second with their prototype,
but they say the sky is nearly the limit when it comes to boosting that speed.
The downlink of the contactless system posed about 10 times less heat load than a system
with metallic cables, and the temperature of the cryostat fluctuated up to a few millidegrees
during experiments.
Now that the researchers have demonstrated this wireless technology, they want to improve
the system’s speed and efficiency using special terahertz fibers, which are only a
few hundred micrometers wide.
The research group has shown that these plastic wires can transmit data at a rate of 100 gigabits
per second and have much better thermal insulation than fatter, metal cables.
The researchers also want to refine the design of their transceiver to improve scalability
and continue boosting its energy efficiency.
Generating terahertz waves requires a lot of power, but the research group is studying
more efficient methods that utilize low-cost chips.
Incorporating this technology into the system could make the device more cost-effective.
The transceiver chip was fabricated through the Intel University Shuttle Program.