Telescope watching the sky and falling star.3d rendering

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I have been searching for examples of a quantum memory and how these might be used to do useful computational and other tasks. I recently found an article from the American Physical Society that provided an interesting example of a quantum memory to improve astronomical observations.

First, what is a quantum memory? A quantum memory is a device that stores the quantum state of a photon or elementary particle, allowing it to be retrieved later without losing information. Because quantum states can be disrupted easily and once read, the quantum state disappears, quantum memories are by nature, a kind of volatile memory.

The more starlight a telescope collects, the more resolution in the image it captures. Larger telescopes provide better images, but there are physical limits on how big a telescope can be. Astronomers get around the limit on the physical size of telescopes by using a technique called interferometry, where images are combined from multiple telescopes to create an interference pattern. A single image of much higher resolution can be obtained using this interference pattern and so these multiple telescopes can act together like they are part of a much larger telescope gathering more starlight.

This method was used by the radio astronomers of the Event Horizon Telescope, EHT, to capture the first image of a black hole in 2017. The receivers at multiple observatories act like a virtual radio receiver with a diameter of the entire Earth. However, using this interferometer method with shorter wavelength radiation, such as visible light, is much more difficult. The largest optical interferometer, CHARA, consists of six telescopes at the Mount Wilson Observatory in California with an effective aggregate diameter of 330 m.

At the recent Global Physics Summit in Denver, researchers from Harvard University talked about how quantum memories can be used to increase the baseline, or effective diameter, of optical interferometry. Rather than spatially combining photos to make them interfere, like for CHARA, the researchers record a telescope’s detected photon’s quantum information in a quantum memory and then use quantum entanglement with another quantum memory that has information from another photon from a different telescope to create an interference pattern.

The two telescopes used for this experiment were located in two laboratories 6 m apart in the same building but they are connected by 1.5 km of spooled fiber. Each telescope’s quantum memory is a chip with a nanometer scale cavity made of diamond. These diamonds contain a defect known as a silicon vacancy where two carbons in the diamond crystal lattice are replaced by a silicon atom and a hole. An electron spin and a nuclear spin in the defect each serve as a qubit that stores and manipulates quantum information. Prior to each observation run, the researchers entangle the nuclear spins of the two crystals using light signals.

A weak laser beam was used to emulate starlight in the two telescopes. An incoming photon interacts with an electron in one quantum memory at one telescope. That electron spin then interacts with the silicon’s nuclear spin, passing the incoming photon’s quantum information on the nuclear spin. Because this nuclear spin is entangled with the nuclear spin of the second quantum memory at the other telescope, the researchers reproduced the photon’s state at the second quantum memory by making measurements of the electron and nuclear spins.

This teleportation allows them to combine the photon signals from the two telescopes and produce an interference pattern even though the two telescopes are effectively 1.5 km away, over 4X further than the CHARA. This method improves the effective diameter of the combined two telescopes by avoiding the photon loss from the conventional approach of sending light signals from the two telescopes to a central interferometer, like in CHARA. This photon loss is what limits the maximum baseline of conventional interferometry.

Quantum memories can be used to create low loss and high effective diameter arrays of optical telescopes to increase the resolution of the effective telescope and let us see deeper into our universe.