Single photons are ideal carriers of quantum information, especially because they are fast, they barely interact with the environment or each other, and large-scale infrastructure for their distribution is already widely available.
Now, before networks can really turn quantum, suitable quantum memories for single photons are needed. It is required for everything from buffering communication to synchronizing processor operations. Ideally, such memories should be fast, efficient, and simple, operating near room temperature without the need for complex technology such as cryogenics or ultra-high vacuum.
In a recent study, researchers at the University of Basel in the group of Prof. Philipp Treutlein have now developed a quantum memory that is based on an atomic gas inside a glass cell.
The atoms do not have to be specially cooled, which makes the memory easy to produce and versatile, even for satellite applications. Moreover, the researchers have realized a single photon source which allowed them to test the quality and storage time of the quantum memory. Their results were recently published in the scientific journal PRX Quantum.
“In our work we demonstrate storage and retrieval of single photons at high bandwidth in a room-temperature platform, consisting of a single-photon source based on spontaneous parametric downconversion (SPDC) and a matched quantum memory in a hot atomic vapor.” Study mentions.
Warm atoms in vapor cells
“The suitability of warm atoms in vapor cells for quantum memories has been investigated for the past twenty years,” says Gianni Buser, who worked on the experiment as a Ph.D. student. “Usually, however, attenuated laser beams – and hence classical light – were used.” In classical light, the number of photons hitting the vapor cell in a certain period follows a statistical distribution; on average, it is one photon, but sometimes it can be two, three, or none.
Activating quantum memory at the right moment
To test the quantum memory with “quantum light” – that is, always precisely one photon – Treutlein and his co-workers developed a dedicated single photon source that emits exactly one photon at a time. The instant when that happens is heralded by a second photon, which is always sent out simultaneously with the first one. This allows the quantum memory to be activated at the right moment.
The single photon is then directed into the quantum memory, where, with the help of a control laser beam, the photon causes more than a billion rubidium atoms to take on a so-called superposition state of two possible energy levels of the atoms.
The photon itself vanishes in the process, but the information contained in it is transformed into the superposition state of the atoms. A brief pulse of the control laser can then read out that information after a certain storage time and transform it back into a photon.
Reducing read-out noise
“Up to now, a critical point has been noise – additional light that is produced during the read-out and that can compromise the quality of the photon,” explains Roberto Mottola, another Ph.D. student in Treutlein’s lab. Using a few tricks, the physicists were able to reduce that noise sufficiently so that after storage times of several hundred nanoseconds, the single-photon quality was still high.
“Those storage times are not very long, and we didn’t actually optimize them for this study,” Treutlein says, “but already now they are more than a hundred times longer than the duration of the stored single-photon pulse.” This means that the quantum memory developed by Basel researchers can already be employed for interesting applications. For instance, it can synchronize randomly produced single photons, which can then be used in various quantum information applications.
Features of the study
Researchers have reported the storage and retrieval of single photons in a ground-state atomic vapor cell quantum memory. Their memory scheme suppresses read-out noise by exploiting polarization selection rules in the atomic hyperfine structure and by operating at a bandwidth much higher than the excited state’s radiative decay rate.
They interface the atomic memory with a single-photon source based on cavity-enhanced spontaneous parametric down-conversion (SPDC), which they built for this purpose with improved operation and performance characteristics compared to their earlier work.
Single photons from this source are stored in the atomic memory and retrieved with decidedly nonclassical photon-number statistics, opening up many further possibilities for quantum networking experiments at high bandwidth in a room-temperature system.
Through simulations in the study, they have laid a road map for future improvements that will simultaneously realize state-of-the-art efficiency.
- Single-Photon Storage in a Ground-State Vapor Cell Quantum Memory; Gianni Buser, Roberto Mottola, Björn Cotting, Janik Wolters, and Philipp Treutlein. PRX Quantum 3, 020349 DOI: 10.1103/PRXQuantum.3.020349