Doppler cooling – build a quantum computer

Say you are going on a road trip and want to find the shortest route. Or you are punching holes in circuit boards and want to reduce the traveling time from hole to hole. Quantum computers are predicted to solve optimization problems like these much faster than conventional computers. But how do you build a quantum computer?

Use lasers to make quantum computers

A very promising way to make quantum computers is to use ultra-cold atoms as quantum bits (or “qubits”). Special lasers can freeze the atoms and hold them still mid-air.

Cold-atom systems are also useful for a long list of other applications, such as highly accurate atomic clocks for ultra-precise GPS systems or systems that can locate hidden objects based on their gravitational pull.

Cold-atom quantum gravity sensor from the UK National Quantum Technologies Programme


How do you make cold atoms?

When you want to cool and trap atoms with lasers, you need to know that – at the atomic scale – temperature makes atoms wiggle around. Reducing their movement is the same as reducing their temperature. You can cool atoms by carefully matching your atom with a laser that can emit light with the properties needed to cool that specific atom.

To cool an atom, you make it absorb energy only when it randomly moves towards the laser. After a short while, the atom begins to reemit the absorbed light but in random directions. On average, this makes the atom slow down in the direction towards the laser because it loses net kinetic energy in that direction.

Now you add a beam in all three dimensions, and the atom will be forced to slow down in all directions. This technique lets you cool the atom down to well below 1°K, depending on the atom used.

The goal is to make the atom absorb light only when it is moving in a specific direction. Atoms can only absorb light if the light is oscillating at one of the discrete frequencies allowed by the atom. For Rubidium, one of these frequencies corresponds to light with a wavelength of 780 nm. If the wavelength is longer, the Rubidium atom will not absorb it.

In a laser Doppler cooling system, the wavelength of the laser must be slightly longer than required for absorption. Now, when the atom moves towards the laser, the Doppler effect will cause the atom to experience the laser as emitting light at a shorter wavelength due to the Doppler effect. The atom will absorb a photon.

When moving away from the laser source, the atom experiences the wavelength as longer and nothing happens. You can use a magneto-optical trap (MOT) to shoot light at the atom from both directions and in all three dimensions. This cools Rubidium atoms down to a few µK. Now, they are ready to be put to work.

In some systems, the atoms are transferred to other types of laser cooling systems to further lower the temperature before they are used.

What kind of laser should you use?

Three parameters are important when choosing a laser for atom trapping and cooling:

  • A stable wavelength
  • A narrow linewidth
  • A high power

Stable wavelength

When picking a laser for atom cooling, it is important to ensure accurate wavelength control. To fine-tune the system, the absolute wavelength must be well-defined and adjustable – and it must not fluctuate. A laser’s linewidth describes how much the wavelength fluctuates over a short period.

Narrow linewidth

The laser’s linewidth must be significantly smaller than the natural linewidth of the atom. If not, the laser will be limiting the lowest temperature reachable rather than the atom itself.  Similarly, intensity fluctuations – typically expressed as relative intensity noise (RIN) – also heats the atom and limits its cooling rate.

High power

Finally, the laser’s power is very important. High power equals a high number of emitted photons. The number of atoms in a trap – as well as the probability of an atom absorbing a photon – depends on photon availability.

A system that checks all these boxes – and more – is the Koheras HARMONIK. We have developed it specifically for trapping and cooling of atoms for quantum optic applications.

Read more about atomic trapping and cooling.


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