The quantum state of a single photon stands amongst the most fundamental and intriguing manifestations of quantum physics. At the same time single photons and pairs of single photons are important building blocks in the fields of linear optical based quantum computation and quantum repeater infrastructure. These fields possess enormous potential and much scientific and technological progress has been made in developing individual components, like quantum memories and photon sources using various physical implementations. However, further progress suffers from the lack of compatibility between these different components. Ultimately, one aims for a versatile source of single photons and photon pairs in order to overcome this hurdle of incompatibility. Such a photon source should allow for tuning of the spectral properties (wide wavelength range and narrow bandwidth) to address different implementations while retaining high efficiency. In addition, it should be able to bridge different wavelength regimes to make implementations compatible. Here we introduce and experimentally demonstrate such a versatile single photon and photon pair source based on the physics of whispering gallery resonators. A disk-shaped, monolithic and intrinsically stable resonator is made of lithium niobate and supports a cavity-assisted triply-resonant spontaneous parametric down-conversion process. Measurements show that photon pairs are efficiently generated in two highly tunable resonator modes. We verify wavelength tuning over 100 nm between both modes with a controllable bandwidth between 7.2 and 13 MHz. This compact source provides unprecedented possibilities to couple to different physical quantum systems and renders it ideal for the implementation of quantum repeaters and optical quantum information processing.
Technology Review - A photon gun capable of reliably producing single photons of different colours could become an important building block of a quantum internet
One of the enabling technologies for a quantum internet is a reliable photon gun that can fire single photons on demand. That's not easy.
One of the significant weaknesses of current quantum cryptographic systems is the finite possibility that today's lasers emit photons in bunches rather than one at a time. When this happens, an eavesdropper can use these extra photons to extract information about the data being transmitted.
So there's no shortage of interest in developing photon guns that emit single photons and indeed various groups have made significant progress towards this.
The researchers have a photon emitter with a range of properties that make it far more flexible, efficient and useful than any before--a kind of photon supergun.
The gun is a disc-shaped crystal of lithium niobate zapped with 582nm light from a neodymium-doped yttrium aluminium garnet (Nd:YAG) laser. Lithium niobate is a nonlinear material that causes single photons to spontaneously convert into photon pairs.
So the 582nm photons ricochet around inside the disc and eventually emerge either as unchanged 582nm photons or as a pair of entangled photons with about twice the wavelength (about 1060nm). This entangled pair don't have quite the same wavelength and so all three types of photon can be easily separated.
The 582 nm photons are ignored. Of the other pair, one is used to transmit information and the other is picked up by a detector to confirm that the other photon is ready form transmission.
So what's so special about this photon gun? First and most important is that the gun emits photons in pairs. That's significant because the detection of one photon is an unambiguous sign that another has also been emitted. It's like a time stamp that says a photon is on its way.
This so-called photon herald means that there can be no confusion over whether the gun is secretly leaking information to a potential eavesdropper.
This gun is also fast, emitting some 10 million pairs of photons per second per mW and also two orders of magnitude more efficient than other photon guns.
These guys can also change the wavelength of the photons the gun emits by heating or cooling the crystal and thereby changing its size. This rainbow of colours stretches over 100nm (OK, not quite a rainbow but you get the picture).
That's important because it means the gun can be tuned to various different atomic transitions allowing physicists and engineers to play with a variety of different atoms for quantum information storage.
All in all, an impressive feat and clearly an enabling step along the way to more powerful quantum information processing tools.
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