## SORCE – What it’s all about

The development of optical atomic clocks and optical frequency standards is foundational to the advancement of space-based missions in geodesy, deep-space satellite navigation, and fundamental physics. Optical clocks fluctuate very rapidly ($\sim 10^{14}$ times per second), so the interval between “ticks” of an optical atomic clock is a few femtoseconds. Since the scale of time is divided into such small intervals, optical atomic clocks provide exceptional timing resolution and stability. However, most of these existing clocks are not compact enough to be suitable for space-based applications.

The Stratospheric Optical Rubidium Clock Experiment (SORCE) aims to demonstrate a portable optical atomic clock on a stratospheric balloon platform, as a first step towards a functioning space-based system. This will be the first demonstration of a remotely operated, portable optical atomic clock.

SORCE will utilize the $^5S_{1/2}$ to $^5D_{5/2}$ two-photon transition in rubidium to reach a fractional stability of $5\times10^{-13}/\sqrt{\tau(s)}$ in a 35 x 35 x 25 cm package. A rubidium vapour cell is interrogated with counterpropagating 778 nm laser beams, and a fluorescence signal at 420 nm is detected using a photomultiplier tube.  This signal is used to lock the laser to the atomic transition.

SORCE will fly on a high-altitude balloon as part of the 2019 STRATOS campaign (CSA/CNES) in late August from Timmins, Ontario, Canada.

## What’s in the box? Part 1 – lasers

There’s less than one month until we launch our optical clock into the stratosphere and next week we’ll be assembling all the components of our experiment into the compact box that will strapped to the balloon gondola. Want to know what’s in the box? We’ll go through it, part by part!

The basis for our clock, the very thing that ticks, is a laser. What exactly is a laser and what’s so special about it? The word laser actually started off as an acronym, standing for “Light amplification by stimulated emission of radiation”. Let’s break that down.

Stimulated emission is the process in which a photon, a particle of light, bumps into an atom that happens to be in an excited state. (Remember excited atoms? We talked about them here if you need a reminder!) When this happens, the photon knocks the atom back to the ground state and in the process, the photon stimulates the atom to emit a photon that is identical to the one that collided with it! When I say identical, I mean that it has the same color, the same polarization, and it even travels in the same direction. Now there are two photons. These may very well bump into two new atoms, stimulating the emission of two more photons. Now there are four photons! This process can occur many many times leading to the amplification of the light that was there originally.

A laser consists of a “gain medium”, which just means a tube or chamber filled with excited atoms, that is usually placed between two mirrors. The mirrors allow the photons to bounce back and forth many times through the gain medium so that a very large number of photons can be produced. A small hole in one of the mirrors allows a stream of these photons to exit in a narrow beam, what we see as the laser beam.

Laser light is special because it differs from “regular” light, such as sunlight or light from an incandescent light bulb, in two ways:

1. Laser light is monochromatic. All of the photons in a laser beam have the same colour which is related to the wavelength of the light waves. Sunlight consists of photons of all the colours of the rainbow, which our eye interprets as white.
2. Laser light is all “in phase”. This means that the crests of the waves all line up, and the troughs of the waves all line up. This is the property that gives laser beams their “punch”, enabling technology like laser cutters and laser eye surgery.

Here is a picture of one of our lasers. The laser itself is the little gold block in the centre. The light exits from the small round glass window in the side of the laser package. Surrounding it are a lot of electronics that enable us to control the frequency of this laser so precisely and use it as a clock!

## Tabletop physics – small experiments with big results

### An old-is-new-again approach to fundamental physics

Pretend you know absolutely nothing about how a car works. (Maybe you don’t have to pretend – I know I don’t.) How might you go about figuring it out? Here’s an idea: smash two cars together and take a look at the carnage. This might give you some clue about the interior workings. Maybe you’ll find the engine, axles, brake pads, etc. If you smash the cars even harder, maybe the engine itself will fall apart and you’ll find pistons and a crankshaft.

Sounds ridiculous? Well, this is more-or-less how modern high energy experimental physics works. Particles are accelerated to tremendous speeds and smashed together in colliders in order to analyze the particles that are created as byproducts. This actually works really well. A lot of important and fascinating new physics has been discovered this way. The problem is that in order to learn more and more using this method, you need to collide particles at higher and higher energies, and this is really hard (and expensive!) to do.

But there’s another way to figure out how a car works: you could pop the hood and examine it with a magnifying glass. Sure, you could only look at one small part at a time, but you could get a really good look.

This is one of the great motivations for doing precision measurements. By performing very careful measurements you can get a close up look at the inner workings of particles. Searching for subtle deviations from the results that physics as-we-know-it would predict is a great way to discover exotic new physics.

These experiments don’t require kilometeres-long tunnels or massive apparatus. In fact, they usually fit on a single table in a lab, reminiscent of the early days of experimental physics, and they’re becoming more and more popular as scientists turn to precision measurements to probe more deeply into fundamental physics. These tabletop experiments might be small in scale but they’re big in impact!

## Are fundamental constants actually constant?

Physical constants are quantities which we believe to be universal and unchanging. But what if this isn’t true? The constancy of certain fundamental constants is a cornerstone of the Standard Model of physics, the best description of the universe we have today. Evidence that these quantities are not actually constant would be indicative of exotic and exciting new physics that we’ve never seen before. One of the scientific goals of SORCE is search for variation of fundamental physical constants.

Fundamental constants come in two types:

1. Measured quantities – These are quantities that have units, such as the speed of light or the charge of an electron. The numerical value of these quantities depends on the system of units you use. For example, the speed of light is 300 million metres per second, but 670 million miles per hour.
2. Dimensionless quantities – These quantities are unitless numbers, usually describing ratios of measured quantities, such as the ratio of the proton mass to the electron mass. These quantities are the same no matter what system of units you use.

How do we know if physical constants change with time, or are different in different locations? It’s very difficult to know if measured quantities are actually constant because we define our system of units based on them!

Here’s an illustration of what I mean: Have you ever heard of the smoot? In 1958 a fraternity at MIT took freshman pledge Oliver R. Smoot, Jr. and rolled him head over heels the entire length of the Harvard Bridge in order to measure the length of the bridge in terms of his height, defining the unit of length the “smoot”. They measured the bridge to be be exactly 364.4 smoots (plus or minus an ear).  (As a fabulous endnote to this story, Oliver Smoot later became chairman of the American National Standards Institute and then president of the International Organization for Standardization.)

It’s 61 years later now. Let’s pretend you go out and remeasure the length of the bridge in smoots, carefully laying down the 80 year old Oliver Smoot on the bridge over and over. Let’s pretend you found the bridge to be 370 smoots. Here’s the question – did the bridge expand or did Mr. Smoot shrink? Maybe a little of both? This is the problem with measured quantities – it’s really hard to know whether the quantity you’re measuring is changing or your ruler is changing. So we try to make our rulers based on quantities that we think really shouldn’t change, like the speed of light, and hope for the best.

Dimensionless quantities don’t suffer from this problem. We can unambiguously determine if dimensionless physical constants actually change through experiments. SORCE is one such experiment that could detect the changing of fundamental constants. How? More on this in an upcoming post… stay tuned!

## A beginner’s guide to atomic physics (and why we like rubidium)

Our optical clock is based on rubidium atoms. Rubidium is an atom with the symbol Rb and atomic number 37. Rubidium is a very soft, silvery-white metal. It’s called “rubidium” from the Latin word rubidus, or ruby-red, because when put into fire it causes the flame to burn red.

Rubidium is an alkali metal – this means that of its 37 electrons, 36 are paired up in closed atomic shells leaving a single electron to have all the fun. Rubidium behaves a lot like a very simple atom with a big nucleus and a single electron, much like hydrogen which is the simplest of all atoms.

Like all atoms and molecules, rubidium has a number of energy levels. Figuring out exactly what all those energy levels are is the job of quantum mechanics. But that’s why we like rubidium – because it basically looks like an atom with just one electron, the math and physics is significantly simpler.

The energy level with the smallest amount of energy is called the “ground state”, while all of the other energy levels are called “excited states”. An individual atom can “jump” from one energy level to another if it absorbs some photons of just the right amount of energy (see our post here for more information). An atom in an excited state will eventually fall back down to the ground state, emitting photons in the process to get rid of the excess energy. In our clock, the rubidium atoms absorb two infrared photons (778 nm) to jump to an excited state, and when the atoms fall back down to the ground state they emit a blue photon (at 420 nm). We know that our infrared lasers are tuned to just the right energy when the rubidium atoms start glowing blue. As long as the atoms keep glowing blue, we know exactly what frequency that infrared laser is and we can use it to keep time!

## What is a stratospheric balloon?

The stratospheric balloon that SORCE will fly on is launched by the Canadian Space Agency’s STRATOS campaign from their balloon base in Timmins, ON. The following excellent description is excerpted from the CSA’s website:

Stratospheric balloons are high-altitude balloons that are released into the stratosphere. They are the only type of balloons that can be operated in this region of the atmosphere (15 to 45 km in altitude), which is too low for satellites, too high for aircraft and cleared too quickly by rockets. The Canadian Space Agency uses stratospheric balloons to test and validate new technologies developed for long-duration space missions and to perform scientific experiments in a near-space environment.

Stratospheric balloons are typically made out of ultra-thin plastic filled with helium and can stretch into a gigantic upside-down “teardrop” shape more than half as tall as the CN Tower, or about the height of the Eiffel Tower. They are equipped with several gondolas suspended on the flight chain. The gondolas can carry science, astronomy, atmospheric chemistry, weather forecasting and technological demonstration payloads weighing up to 1.1 tons altogether.

These balloons require no engine and no fuel and are fully recovered after each flight. They can reach altitudes of up to 42 km, holding their instrument packages aloft for several hours. Some balloons can even conduct long-duration flights, lasting days, weeks and even months.

Stratospheric balloons are a platform of choice for scientists and engineers, as they can be used to test and advance space science for far less than the cost of a satellite (up to 40 times less) and provide an opportunity to carry out concrete scientific experiments in a short period of time and obtain results quickly.

## So what’s an optical clock anyway?

A clock is something that ticks. A good clock ticks at a very stable rate. To get a clock to tick at a stable rate, it is slaved to a natural frequency. For example, sundial clocks rely on the steady rotation of the earth; grandfather clocks rely on the regular oscillations of a pendulum; quartz clocks use the natural vibrations of quartz crystals.

The stability of an optical atomic clock relies on the quantum oscillations within atoms. The thing that “ticks” is a laser, which is an oscillating electromagnetic field – this is the “optical” part of optical atomic clocks. The atoms absorb photons from the laser beam in order to jump from one energy level to another. Only when the frequency of the laser is tuned just right will the atoms be able to absorb the photons. This is how the laser frequency is slaved to the natural atomic frequency.

What’s so special about optical atomic clocks? These natural oscillations within atoms occur at frequencies in the 100s of terahertz, meaning that these clocks tick about a quadrillion (one million billion) times per second! This is what makes these the most precise clocks in existence today. These clocks only gain or lose a second once in 10 billions years!

What does anyone need all this precision for? Precise and stable atomic clocks are interesting because they can be used to search for evidence of dark matter and dark energy. Such clocks can also be used to build a gravitational wave telescope. They can also be used to perform precise surveys of the earth’s gravitational field, yielding new insight into what lies below the surface of the earth and providing input into models used to study and forecast the effects of climate change.

The most precise optical clocks that exist today are not ideal for field applications, due to their size and complexity. We’re working to develop an optical atomic clock that is both precise enough for these exciting applications, and compact and portable enough that it could be put on a satellite or transported from location to location.

Our clock design makes use of a “two-photon transition”: in the energy jump that the atoms make, they simultaneously absorb two photons. This is important because usually when an atom absorbs a photon it gets physically kicked because the photon has momentum. This kick can mess up the clock, but if the atom absorbs two photons from two opposite directions, the kicks cancel out! Clocks that don’t use a special two-photon transition have to use complicated (and bulky) methods to correct for the kick. Since our clock doesn’t have a kick problem, we can build it in a smaller, simpler way – small enough to take into the field, put aboard a stratospheric balloon, and even put on a satellite!