INSIGHTS | PERSPECTIVES
1532 23 DECEMBER 2016 • VOL 354 ISSUE 6319 sciencemag.org SCIENCE
By William J. Munro1,2 and Kae Nemoto2
Optical circuits for information pro- cessing offer higher data rates (chan- nel capacities) and lower power consumption as compared with those of electronic circuits. Many optical devices (such
as switches and isolators) have
analogous electronic components
(transistors and diodes), but some
have special capabilities. For example, optical circulators form
part of an unusual class of components known as nonreciprocal devices (1)—their optical properties
depend on the direction of light
passing through them. A three- or
four-port optical circulator routes
light entering from any port to
exit only from the next. Bulk optical implementations rely typically
on nonreciprocal polarization
rotation via the Faraday effect,
in which a magnetic field breaks
symmetry (2). However, the drive
to miniaturization that has led to
nanophotonic integrated circuits
and waveguides has not included
optical circulators until now. On
page 1577 of this issue, Scheucher
et al. (3) demonstrate a fiber-integrated photonic circulator that
can work even at the single-photon
The fiber-integrated circula-
tor used in this demonstration
(see the figure) is formed from a
single-spin–polarized atom (whose
quantum state controls the opera-
tional direction of the circulator)
coupled to the evanescent field of
a whispering-gallery-mode microresonator
(4), with two coupling fibers interfacing it. In
order to understand how this device works,
we can begin with the whispering-gallery-
mode microresonator. The evanescent field
that surrounds the resonator (a field that di-
minishes with distance from the resonator)
exhibits a substantial polarization compo-
nent along the light’s propagation direction,
the clockwise sense, the electric field vector
rotates counterclockwise—it is s– polarized.
If the field propagates in the reverse sense,
the field vector rotates clockwise and is s+
The spin-polarized atom acts as a polar-ization-dependent scatterer that has dramatically different interaction cross sections
for the s+- and s–-polarized light fields. By
introducing such a spin-polarized atom into
the resonator, the resonator light field can
be suppressed for one sense of circulation.
At a given frequency, light can only circulate
4) are routed to port i + 1 (2, 3, 4, 1)
in the clockwise or counterclockwise sense,
depending on the spin state of the atom. In
order to exploit this effect, Scheucher et al.
interface the resonator with two tapered fi-
ber couplers (see the figure). Depending on
its propagation direction in the coupling fi-
ber, the light then either couples into the res-
onator and exits the latter via the
other fiber, or it cannot enter the
resonator and continues on its way.
This process selects for the desired
functionality of the optical circula-
tor: Photons from port i (i = 1, 2, 3,
but not port i − 1 (or vice versa for
the opposite spin state).
fiber-integrated circulator is controlled by the quantum state of a
single atom and can operate at the
single-photon level, unlike many
potential alternatives. Given state-of-the-art technology, this device
can in principle be extremely low
loss (0.3 dB) and have near-unity
efficiency. These features make
this router ideally suited for quantum information processing tasks
including quantum communication (5), computation (6), and
simulation and could be integrated
into quantum photonic chips. Further, the principles underlying this
nonreciprocal device could be used
to construct optical diodes (
isolators) (7) and fiber switches (8).
The most likely and intriguing application would be fully functional
quantum-entangling gates (9),
both photon-photon and photon-atom if the atom is in a spin superposition state. This latter case is
particularly interesting because it
gives excellent ways to bring nonlinearities
to single-photon quantum technologies in an
efficient and compact manner. j
1. C. Hogan, Proc.IRE 56, 1345 (1956).
2. D. Meschede, Optics, Light and Lasers (Wiley-VCH, 2007).
3. M.Scheucher,A.Hilico,E. Will,J.Volz,A.Rauschenbeutel,
Science354, 1577 (2016).
4. A.Matsko, V.Ilchenko, IEEE J. Sel. Top. Quantum Electron.
12, 3 (2006).
5. J. L. O’Brien, Science 318, 1567 (2007).
6. H. J. Kimble,Nature453, 1023 (2008).
7. C.Sayrin etal., Phys.Rev.X 5,041036(2015).
8. D. O’Shea et al ., Phys. Rev. Lett. 111, 193601 (2013).
9. K. Nemoto et al ., Phys. Rev. X 4, 031022 (2014).
1N TT Basic Research Laboratories, NTT Corporation, 3-1
Morinosato-Wakamiya, Atsugi, Kanagawa, 243-0198, Japan.
2National Institute of Informatics, 2-1-2 Hitotsubashi,
Chiyoda-ku, Tokyo 101-8430, Japan. Email: william.munro@
Optical circulators reach the quantum level
The spin state of a single atom can route light signals in different directions
The spin state of the atom determines the paths of pulses through
the circulator. The paths shown are for spin up; all paths reverse
direction for spin down.
Photonic optical circulator
Downsizing optical circulators
A large conventional optical ciruclator (top) is shown in contrast to a
photonic four-port circulator (bottom), in which a rubidium atom is
coupled to the whispering-gallery mode of a bottle microresonator with
two tapered optical fibers.