Single Photon Frequently Asked Questions

Is this the first time an on-chip single-photon source has been made?
To our knowledge, it is the first such source for microwave photons, and the first source to generate and guide photons entirely within an electrical circuit. There have been sources demonstrated before for single photons in the optical or infrared, which can also be used for quantum communication, and may be better for long haul runs (i.e. guiding on fibers). Some of these earlier works were solid-state devices using semiconductors, see for example work from Yamamoto's group at Stanford, Nature 419, p. 594, 2002. What is new in our work is that the photons generated are much lower energy and longer wavelength (about 5 GHz, or a few centimeters wavelength, as opposed to infrared photons with 900 nanometer wavelength). Also, because the photons in our work are guided by wires, they are more readily useable for communicating quantum information between different parts of a quantum computer.
What is the difficulty with making single-photon sources?
While it is not very difficult to generate signals with one photon on average, it is very difficult to generate exactly one photon all the time. A weak conventional source will sometimes emit two or more photons, and sometimes none. Unfortunately, to encode quantum information on photons, you often want there to be exactly one. It is the certainty in photon number which makes this problem difficult.
Of course, there are other challenges, especially dealing with such low-energy photons. A single photon generated in our apparatus is an incredibly weak signal: a typical cell phone, in comparison, emits 1023 (100,000,000,000,000,000,000,000) photons every second. With such weak signals, it’s quite difficult to isolate your single photon from the environment, and to even detect your photon at all! Efficiency is also a challenge. It is difficult to collect your photon and guide it to where you want it. We detect a single photon 40% of the time we attempt to generate one, which is a very high efficiency for this type of source.
Why are microwave photons important?
Microwaves are used for classical communication all the time. Cell phones use microwaves to carry information over large distances, and microwave signals on wires connect different parts inside a computer. Just as with classical computers, microwaves could also be used to connect different parts of a quantum computer. However, to use microwaves for quantum communication, you need to be able to generate single photons, and to impart quantum information to single microwave photons. These are the steps accomplished in this work.
Could you describe, in simple terms, how you created the single microwave photons and why this is different than previous devices?
It has been known for a long time that energetic atoms can radiate energy by emitting photons. If you could isolate a single atom and give that atom only enough energy to emit one photon, then this could be a single photon source. Because there’s only one atom with a single photon’s energy, this can produce at most one photon. Of course, it’s possible that the energy is lost in some other way, and no photons are emitted. Therefore, you must also make sure that the atom emits its photon faster than any source of leakage in the system.
This is the basic principle of our single photon source. Rather than using a real isolated atom, we fabricate an artificial atom, or qubit, using superconducting tunnel junctions. Because we have to make each qubit individually, we can ensure that there is exactly one. The qubit is coupled to a superconducting microwave cavity that enhances the rate at which the qubit emits photons and efficiently collects the light. Whenever we want a photon, we simply excite our qubit and wait for it to emit light, which typically occurs within one ten millionth of a second after we excite it. Previous single photon sources have used either single atoms, or single quantum dots. We believe this is the first single photon source based on a superconducting artificial atom.
What will be the difference between your microwave single-photon emitter and one that is eventually used in a quantum information device? In other words, what still needs to be done?
We have demonstrated only the first half of quantum communication on a chip: quantum information is efficiently transferred from a stationary quantum bit (qubit) to a photon (sometimes called a flying qubit). However, for on-chip quantum communication to become a reality, we need to be able to transfer information from the photon back to a qubit. Such efficient detection remains an open challenge, though one could envision a detector that operates very much like our source, using a second superconducting qubit as a detector.