The belief is this could one day significantly improve the accuracy and performance of navigation on smartphones and other consumer devices.
Today’s most accurate time-keepers are atomic clocks which rely on the steady resonance of atoms, when exposed to a specific frequency, to measure exactly one second.
Several such clocks are installed in all GPS satellites. By “trilaterating” time signals broadcast from these satellites — a technique like triangulation, that uses 3D dimensional data for positioning — your smartphone and other ground receivers can pinpoint their own location.
But, atomic clocks are large and expensive and therefore, smartphones contain a much less accurate internal clock that relies on three satellite signals to navigate and can still calculate wrong locations. Errors can be reduced with corrections from additional satellite signals, if available, but this degrades the performance and speed of your navigation. When signals drop or weaken — such as in areas surrounded by signal-reflecting buildings or in tunnels — a phone primarily relies on its clock and an accelerometer to estimate the user’s location and where they’re going.
According to the researchers from MIT’s Department of Electrical Engineering and Computer Science (EECS) and Terahertz Integrated Electronics Group, they have built an on-chip clock that exposes specific molecules — not atoms — to an exact, ultrahigh frequency that causes them to spin. When the molecular rotations cause maximum energy absorption, a periodic output is clocked — in this case, a second. As with the resonance of atoms, this spin is reliably constant enough that it can serve as a precise timing reference.
In experiments, the molecular clock averaged an error under 1 microsecond per hour, comparable to miniature atomic clocks and 10,000 times more stable than the crystal-oscillator clocks in smartphones. Because the clock is fully electronic and doesn’t require bulky, power-hungry components used to insulate and excite the atoms, it is manufactured with the low-cost, complementary metal-oxide-semiconductor (CMOS) integrated circuit technology used to make all smartphone chips.
The chip relies on measuring the rotation of the molecule carbonyl sulfide (OCS), when exposed to certain frequencies. Attached to the chip is a gas cell filled with OCS. A circuit continuously sweeps frequencies of electromagnetic waves along the cell, causing the molecules to start rotating. A receiver measures the energy of these rotations and adjusts the clock output frequency accordingly. At a frequency very close to 231.060983GHz, the molecules reach peak rotation and form a sharp signal response. The researchers divided down that frequency to exactly one second, matching it with the official time from atomic clocks.
“The output of the system is linked to that known number — about 231 gigahertz,” Associate Professor Ruonan Han of EECS says. “You want to correlate a quantity that is useful to you with a quantity that is physical constant, that doesn’t change. Then your quantity becomes very stable.”
The researchers developed custom metal structures and other components that increase the efficacy of transistors, in order to shape a low-frequency input signal into a higher-frequency electromagnetic wave, while using as little power as possible. The chip consumes only 66milliwatts of power. For comparison, common smartphone features — such as GPS, Wi-Fi, and LED lighting —can consume hundreds of milliwatts during use.
The chip-scale molecular clock can also be used for more efficient time-keeping in operations that require location precision but involve little to no GPS signal, such as underwater sensing or battlefield applications, the team adds.