In an important step towards more practical quantum
information processing, researchers from NIST; the University
of California, San
Diego; and the Politecnico di Milano in Milan, Italy,
have demonstrated the first heralded single photon source made from silicon.
This source complements two other recently developed silicon-based
technologies—interferometers for manipulating the entanglement of photons and
single photon detectors—needed to build a quantum optical circuit or a secure
quantum communication system.
The line between interesting and practical in advanced
electronics and optics often comes down to making the new device compatible
with existing technology. According to NIST scientist Kartik Srinivasan, the
new 0.5 mm by 0.05 mm-sized heralded photon generator meshes with existing
technology in three important ways: it operates at room temperature; it
produces photons compatible with existing telecommunications systems (wavelengths
of about 1,550 nm); and it’s in silicon, and so can be built using standard,
scalable fabrication techniques.
A “heralded” photon is one of a pair whose existence is
announced by the detection of its partner—the “herald” photon. To get heralded
single photons, the group built upon a technique previously demonstrated in
silicon called photon pair generation.
In photon pair generation, a laser pumps photons into a
material whose properties cause two incoming pump photons to spontaneously
generate a new pair of frequency-shifted photons. However, while these new
photons emerge at precisely the same time, it is impossible to know when that
“Detecting one of these photons, therefore, lets us know to
look for its partner,” says Srinivasan. “While there are a number of
applications for photon pairs, heralded pairs will sometimes be needed, for
example, to trigger the storage of information in future quantum-based computer
According to Srinivasan, the group’s silicon-based device
efficiently produced pairs of single photons, and their experiment clearly demonstrated
they could herald the presence of one photon by the detection of the other.
While the new device is a step forward, it is not yet
practical, according to co-author Professor Shayan Mookherjea at UC San Diego,
because a single source is not bright enough and a number of other required
functions need to be integrated onto the chip. However, putting multiple
sources along with their complementary components onto a single chip—something
made possible by using silicon-based technology—could supply the performance
needed for practical applications.