A Faster Single-Atom Detector

A new single-atom detection system that uses two polarizations of light simultaneously through cavity mirrors is more than 99.7 percent accurate and can discern the arrival of a neutral atom in less than one-millionth of a second, about 20 times faster than previous methods.

The system, devised by researchers at the Joint Quantum Institute (JQI) in College Park (a research partnership between the University of Maryland and the National Institute of Standards and Technology) and the Universidad de Concepción in Chile, employs a novel means of altering the polarization of laser light trapped between two highly-reflective mirrors, in effect letting the scientists "see" atoms passing through by the individual photons that they scatter.

Fig. 1: In the first step of the single-atom detection system, a small population of atoms is trapped and cooled in a vacuum enclosure in such a way that they drop slowly, one at a time, through a hole 1.5 mm wide at the bottom of the trap. (Images: Joint Quantum Institute)

The ability to detect single atoms and molecules is essential to progress in many areas, including quantum information research, chemical detection and biochemical analysis.

"Existing protocols have been too slow to detect moving atoms, making it difficult to do something to them before they are gone. Our work relaxes that speed constraint," said coauthor David Norris of JQI. "Moreover, it is hard to distinguish between a genuine detection and a random 'false positive' without collecting data over a large period of time. Our system both filters the signal and reduces the detection time."

The scientists trap and cool a small population of atoms (rubidium is used in the current experiment) in a vacuum enclosure in such a way that they drop slowly, one at a time, through a hole 1.5 mm wide at the bottom of the trap. [See Fig. 1]

The atom then falls about 8 cm until it enters a tiny chamber, or cavity, that is fitted on opposite sides with highly reflective mirrors that face one another at a distance of about 2 mm. Passing through the center of both mirrors is a laser beam of wavelength 780 nm – just slightly longer than visible red light. The beam excites the atom as it falls between the mirrors, causing it to reradiate the light in all directions.

Fig 2: In step 2, the researchers use two polarizations of cavity light simultaneously: one (horizontal) which is pumped in to efficiently excite the atoms, and the other (vertical) which only appears when emitted by an atom inside the cavity.

That arrangement is a familiar one for labs studying the interaction of atoms and photons. The JQI system, however, has two distinctively unique features.

First, the researchers use two polarizations of cavity light simultaneously: one (horizontal) which is pumped in to efficiently excite the atoms, and the other (vertical) which only appears when emitted by an atom inside the cavity. [See Fig. 2] Although the descent of the atom through the chamber takes only 5 millionths of a second, that is 200 times longer than it takes for the atom to become excited and shed a photon, so this process can happen multiple times before the atom is gone.

Second, they create a magnetic field inside the cavity, which causes the laser light polarization to rotate slightly when an atom is present. Known as the Faraday effect, this phenomenon is typically very weak when observed with a single atom. However, since the light reflecting between the mirrors passes by the atom about 10,000 times, the result is a much larger rotation of a few degrees. This puts significantly more of the laser light into the vertical polarization, making the atoms easier to "see."

The light eventually escapes from the cavity and is fed through a polarizing beamsplitter which routes photons with horizontal polarization to one detector, and vertical polarization to another. Each arriving photon generates a unique time stamp whenever it triggers its detector. [See Fig. 3]

Fig 3: Each arriving photon generates a unique time stamp whenever it triggers its detector in the third step.

Although the detector for the vertically polarized light should only be sensitive to light coming from an atom in the cavity, it can be fooled occasionally by stray light in the room. But because there are multiple emissions from each atom, there will be a burst of photons whenever an atom passes between the mirrors. This is the signature that the researchers use to confirm an atom detection.

"The chief difficulty lies in verifying that our detector is really sensitive enough to see single atoms, and not just large groups of them," said team leader Luis A. Orozco of JQI. "Fortunately, the statistics of the light serve as a fingerprint for single-atom emission, and we were able to utilize that information in our system."

The system is described in the letter "Photon Burst Detection of Single Atoms in an Optical Cavity," appearing in an advance online publication on the Nature Physics Web site.

For more information, visit: www.nature.com/nphys/index.html