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Turns out, two atomic clocks are indeed better than one. And what role does sleep play in memory suppression?

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Michael: For record-breaking timekeeping, the combination of two atomic clocks can be better than one. At least, according to research announced this week from the National Institute of Standards and Technology, or NIST, in the U.S.

Atomic clocks are named for the atoms inside that oscillate rapidly and reliably between two energy states. These atomic oscillations are so consistent that, in 1967, one second was officially defined in terms of cesium oscillations. Cesium atoms oscillate at microwave frequencies, and are usually paired with a vibrating quartz crystal to keep time. But the newest generation of timepieces are optical atomic clocks.

They use ytterbium or strontium – atoms that oscillate at visible light frequencies – and sync up lasers to the atomic oscillations for timekeeping. The extreme accuracy of atomic clocks might be a little excessive for your wristwatch. But we need this kind of timekeeping for GPS navigation, and research into the fundamental nature of the universe, like the search for dark matter.

So, atomic clocks have two major engineering challenges: precision and stability. A few years ago, NIST built a strontium clock that broke the world record for precision – that’s how accurately a clock tracks atomic oscillations. This time, NIST focused on stability, or how identical the ticks are.

Specifically, the researchers worked on a problem known as dead-time noise, which limits how fast the clock reaches peak stability. This happens because atoms aren’t checked by the laser 100% of the time. There are dead-time periods, like when the atoms are being prepared in a quantum state.

During this “dead” time, the laser keeps going, but it can lose track and not oscillate exactly in time. To counteract this, physicists engineered a double-clock system, with two sets of ytterbium atoms being sampled by the same laser. It was built so there’s always some laser light sampling the atoms, and the results are combined into one smooth, stable clock, with no dead-time noise.

The researchers call it a ZDT clock, for Zero Dead Time, and it can reach a higher stability ten times faster than the previous record-holder. So it’s the most stable optical atomic clock ever built, with pretty high precision too. Sure, this double-clock model is a bit bulky, but the researchers think they can improve on this design to develop even smaller, high-stability atomic clocks in the future.

Now, when it’s time for sleep, your brain is ready to start consolidating some memories. Normally, this is super useful, but what if there are things you’d rather forget? Conditions like depression or PTSD, for example, can lead to unwanted memories intruding into your everyday life. Memories can sometimes be forgotten on purpose, but whether sleep plays a part isn’t well understood.

This week, though, Chinese and American researchers published an article in Nature Communications about how sleep might reduce our ability to suppress bad memories. They trained student participants to remember 26 neutral faces paired with aversive images – things like human suffering, or rotting animal carcasses. After 24 hours and a self-reported good night’s sleep, the students learned another set of images.

Half an hour later, they had their brains scanned in an fMRI machine, while they tried to suppress some of these new memories in something called the Think-NoThink procedure. Students were shown the neutral faces they learned that day or the day before, along with a red or green rectangle. Green meant “Think”.

Think hard about the aversive image that accompanied that face. Red meant “NoThink” – or, try to keep the aversive image from entering your mind, to hopefully suppress the memory. Another 30 minutes later, the students were tested on their memory of the pairings, by looking at faces and describing the paired aversive image.

For recent memories, the results showed that they had some control over what they did and didn’t remember. In other words, for recent memories, there was a significant difference in the images people could describe from Think pairs and the suppressed-memory NoThink pairs. But for the pairings they learned over 24 hours prior, that level of control went down.

The fMRI scans also showed changes in their brain activity while processing recent and day-old memories. When retrieving a new memory, there was more brain activity in the hippocampus. Voluntary suppression of these image pairings, like in the NoThink part of the procedure, activated areas in the prefrontal cortex, which is a part of the brain involved in things like planning.

This inhibited activity in the hippocampus, and the amygdala, a brain region involved in emotions. But, when retrieving memories from the day before, brain activity was more spread out in the neocortex. Plus, the prefrontal cortex seemed to work extra hard during the NoThink parts, but didn’t inhibit activity in the hippocampus and amygdala as much.

Basically, the overnight memories of aversive images seemed to stay stronger than the newer memories. There’s still more work to do, to figure out whether it really was a night’s sleep that made a difference, or just time passing. With more experiments, the researchers hope their work will lead to clinical applications, like helping people with PTSD keep good memories without the torment of the bad ones.

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