Special Feature: Silicon Photonic Technologies Leading
the Way to a New Generation of Telecommunications

Entanglement Generation Using
Silicon Wire Waveguide
Hiroki Takesue†, Ken-ichi Harada, Yasuhiro Tokura,
Hiroshi Fukuda, Tai Tsuchizawa, Toshifumi Watanabe,
Koji Yamada, and Sei-ichi Itabashi
Abstract

Generating high-purity entangled photon pairs is an important step toward quantum information processing using photons. In this article, we describe the world’s first ever experiment on generating
entanglement using a spontaneous parametric process in a silicon wire waveguide.

1. IntroductionIn recent years, information processing using theprinciples of quantum mechanics, including quantum
cryptography and quantum computation, has beenstudied intensively. Quantum entanglement is an
essential resource for quantum information processing. In particular, an entangled photon pair [1], which
is an entangled state of photons, has been investigatedas a way of distributing entanglement over long distances. In this article, we describe the world’s firstever experiment on generating entangled photon pairs
using a silicon wire waveguide [2], [3].
2. Time-bin-entangled photon pair
An entangled photon pair is a state where two photons show a correlation that cannot be explained by
the classical theory. For example, a polarizationentangled photon pair is a state in which the polarization state of each photon is not fixed, but the relationship between the polarization states of the two photons is predetermined [1]. On the other hand, a timebin-entangled state is based on a time-bin qubit,
which is a superposition of two quantum states ofphotons in two distinguishable temporal positions. Ina similar manner to that for polarization-entangled† NTT Basic Research Laboratories
Atsugi-shi, 243-0198 Japanphotons, we can describe a time-bin-entangled photon pair as being a state in which the temporal position of each photon is not fixed, but the two photons
are always found in the same time slot. Here, we callthe photons of an entangled pair the signal photon andthe idler photon. The quantum state of a time-binentangled photon pair is given by|Φ = 1 |1 s|1 i + |2 s|2 i ,(1)√2 
where |x y denotes the quantum state where there is aphoton in the xth time slot and in mode y (=s: signal,i: idler).
In fact, we can get the situation in which the temporal position of each photon is not fixed, but the twophotons are always found in the same time slot using
two classically correlated photons. However, withclassically correlated photons, the correlation
between the measurement outcomes vanishes if wemeasure the photons using a basis that is not orthogonal to the original basis (for example, a polarization
measurement that discriminates ±45° linear polarization is non-orthogonal to a horizontal/vertical polarization measurement). An example of a measurementthat is non-orthogonal to the measurement that discriminates the states |1 y and |2 y is one that discriminates whether the photon is in a superposition of |1 yand |2 y with a relative phase of 0 or π. This corresponds to a measurement that determines whether thestate is one of the states |+ y or |– y defined below.

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AliceBob

Basis X

Entanglement source

SignalBasis Y

Basis X

Idler

Basis YClassical communication

Fig. 1. QKD using entangled photon pairs.

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> > > >|± y = 1 |1 y ± |2 y (2)√2 
Using the states defined in Eq. (2), we can rewrite Eq.
(1) as|Φ = 1 |+ s|+ i + |– s|– i .
(3)√2 
This equation clearly shows that the correlation of
the measurement outcomes for two photons is stillpreserved even when we use a non-orthogonal measurement basis. This is a very important characteristicof an entangled photon pair, which cannot be achieved
using two classically correlated photons.
3. Quantum key distribution using
entangled photon pairs
We can implement various quantum information
tasks using the peculiar characteristics of entangled
photons explained in the previous section. Here, as an
example of such a quantum information task, we
describe quantum key distribution (QKD), where asecret key for cryptography is distributed between
two distant parties. The configuration, in which a key
is established between Alice and Bob, is shown in
Fig. 1. We prepare many entangled pairs and send thesignal and idler photons to Alice and Bob, respectively, over conventional optical transmission lines
such as optical fibers. Both Alice and Bob are
equipped with two measurement setups that correspond to two non-orthogonal measurements. Here,
we call the measurement that discriminate states |1 yand |2 y the “X basis” and that for |+ y or |– y the “Ybasis”. Alice and Bob measure each incoming photonusing a basis randomly chosen from X and Y. They
record the measurement outcome and the basis usedfor each measurement. After measuring all the photons, they disclose which basis they used for eachmeasurement through a conventional communication>

Vol. 8 No. 2 Feb. 2010

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line. Note that they do not disclose the measurement
outcomes. Then, by extracting the events where theirmeasurement bases (accidentally) coincided, they
can share correlated measurement outcomes without
disclosing them. As a result, Alice and Bob can sharea random bit string by assigning, for example, theoutcomes (|1 y, |+ y ) as bit 0 and (|2 y , |– y ) as bit 1.
This bit string can be used as a key for one-time-padcryptography.
It is fundamentally impossible to eavesdrop on thekey created by QKD, so we can achieve unconditionally secure communication by using QKD. Intuitively, the security originates from the fact that any twoclassically correlated photons cannot fully mimic thecharacteristics of entangled photon pairs (of course,very careful treatment is required to prove the unconditional security of any QKD protocol). Currently,
secret communication based on QKD is expected toprovide ultimate network security for future communication networks.

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4. Entanglement generation using
silicon wire waveguideWe generated 1.5-μm-band entangled photon pairs
using a silicon wire waveguide [4]. A silicon wire
waveguide is a single-mode, single-crystal silicon
waveguide whose core diameter is typically 400 nm
wide and 200 nm thick. Thanks to this extremely
small core, the optical power density in the waveguide is very high, and as a result, we can observe ahuge nonlinear effect. The nonlinear coefficient of
the waveguide used in our experiment was five ordersof magnitude higher than that of a conventional silica
fiber. Since silicon is an inversion symmetric material, we can use it for a third-order nonlinearity experiment. In the present experiment, we generated timebin-entangled photon pairs by spontaneous four-waveSpecial Feature

SFWM

Pump pulse

ωp

ωi

ωp

ωsSilicon wire waveguide

Fiber Bragg grating
Arrayed waveguide grating

SignalSingle-photon detector

Idler

1-bit delayedinterferometer

Optical bandpass filters

Fig. 2. Setup for generating time-bin-entangled photon pairs.

mixing (SFWM), which is one of the third-order nonlinear effects.
When we input a 1.5-μm pump light whose angularfrequency is given by ωp into a silicon wire waveguide, we can generate a photon pair with angularfrequencies of ωs and ωi through SFWM. The following relationship exists between ωp, ωs, and ωi.
2ωp = ωs + ωi

(4)This equation implies that two pump photons areannihilated and a signal-idler photon pair is created.
Since the two photons are created simultaneously, wecan generate a temporally correlated photon pairstate, in which two photons are always observed in
the same time slot (for example, |1 s|1 i ).
Thus, we can obtain a temporally correlated state,but it is still not entangled, as shown by Eq. (1). To
generate an entangled state, we use a double pulse asa pump. Here, the phase difference between twopump pulses is fixed, so the first and second pulses
probabilistically create pair states |1 s|1 i and |2 s|2 i,
respectively. By using a relatively low pump power,
we can make the probability of simultaneous generation of both pair states very low. As a result, we canobtain a coherent superposition of the |1 s|1 i and> >> >> >> >> >|2 s|2 i states, which is a time-bin-entangled state

given by Eq. (1).
This technique enables us to make a simple, stableentanglement source in the 1.5-μm band. In addition,
the use of a silicon waveguide enables us to avoid
noise photons caused by spontaneous Raman scattering, which has been a serious problem for entanglement sources based on SFWM in an optical fiber[5].
5. Experiment
We generated time-bin-entangled photon pairs
using the setup shown in Fig. 2. We injected 1551.1nm double pulses into a silicon wire waveguide. Thewaveguide used in the experiment was 460 nm wide,
220 nm thick, and 1 cm long, and the waveguide loss
was approximately 1 dB. The SFWM process inside
the waveguide generated the quantum state shown byEq. (1). The photons generated from the waveguidewere first passed through a fiber Bragg grating to suppress the pump photons and input into an arrayedwaveguide grating to separate the signal and idler
photons. The wavelengths of the signal and idler photons were 1547.9 and 1554.3 nm, respectively. EachNTT Technical Review

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0.006

Coincidence rate

0.005

0.004

0.003

0.002

0.001

0π02π

3πIdler interferometer phase difference (rad.)Fig. 3. Results of coincidence measurements between the signal and idler photons. Squares and circles are thecoincidence fringes obtained for a signal interferometer phase differences of 0 and –π/2, respectively.

photon was passed through an optical bandpass filter
to further suppress the residual pump photons, andthen launched into a 1-bit delayed interferometer,
which is an unbalanced Mach-Zehnder interferometer whose path difference corresponds to the timeinterval between the two pump pulses. The photons
output from the interferometers were detected bysingle-photon detectors.
When a photon whose state spans two time slots islaunched into the 1-bit delayed interferometer, thesingle-photon detector connected to the interferometer output could detect the photon in any one of threetime slots. Photon detection in the second time slot
means that the input state is projected to a state givenby|Ψ y = 1 |1 y + e–iφy |2 y ,(5) √2 
where φy is the phase difference induced in the interferometer. Thus, we can implement measurement
bases that correspond to |+ y and |– y described above,by setting φy to 0 and –π, respectively.
To confirm the generation of time-bin entanglement, we set the phase difference of the signal interferometer to 0 (φs = 0) and observed the coincidence
counts of two photons as a function of idler interferometer phase difference φi. The results are shown bysquares in Fig. 3. We observed a clear sinusoidal
modulation of the coincidence counts, which indi->

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cates that the measurement results for the signal andidler photons were correlated. The waveform had
peak values at φi = 2 n π (n: integer). This means thatthe coincidence rate was maximum when the signal
and idler photons were both projected to |+ y, reflecting the characteristics suggested by Eq. (3).
We then set φs = –π/2, which let us implement a
non-orthogonal basis for measuring signal photons,
and measured the coincidence fringe. The results(circles in Fig. 3) clearly show that the measurement
results for two photons were again correlated evenwith the non-orthogonal basis. Thus, we successfully
confirmed the generation of an entangled state. The
visibilities of the coincidence fringes were 96.3% (φs= 0) and 95.2% (φs = –π/2), implying that we obtained
high-purity entanglement in the 1.5-μm band.

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6. Conclusion
NTT has performed the world’s first ever entanglement generation experiment using a silicon wirewaveguide. This technique is expected to provide apractical and high-purity entanglement source in the
1.5-μm telecommunication band, which will enable
us to achieve quantum communication over opticalfibers in the future.

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References
[1]

[2]
[3]

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H. Takesue, “Generation of polarization-entangled photon pairs in
1.5-μm telecommunication band,” NTT Technical Review Vol. 3, No.
12, pp. 52–60, 2005.
https://www.ntt-review.jp/archive/ntttechnical.php?contents=ntr2005
12052.pdf
H. Takesue, Y. Tokura, H. Fukuda, T. Tsuchizawa, T. Watanabe, K.
Yamada, and S. Itabashi, “Entanglement generation using silicon wirewaveguide,” Appl. Phys. Lett., Vol. 91, No. 20, 201108, 2007.
K. Harada, H. Takesue, H. Fukuda, T. Tsuchizawa, T. Watanabe, K.

[4]

[5]

Yamada, Y. Tokura, and S. Itabashi, “Generation of high-purity
entangled photon pairs using silicon wire waveguide,” Opt. Express,
Vol. 16, No. 25, pp. 20368–20373, 2008.
T. Tsuchizawa, K. Yamada, H. Fukuda, T. Watanabe, J. Takahashi, M.
Takahashi, T. Shoji, E. Tamechika, S. Itabashi, and H. Morita, “Microphotonics Devices Based on Silicon Microfabrication Technology,”IEEE J. of Select. Topics Quant. Electron., Vol. 11, No. 1, pp. 232–240, 2005.
H. Takesue and K. Inoue, “1.5-μm band quantum-correlated photon
pair generation in dispersion-shifted fiber: suppression of noise photons by cooling fiber,” Opt. Express, Vol. 13, No. 20, pp. 7832–7839,
2005.

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Hiroki Takesue

Tai Tsuchizawa

Senior Research Scientist, Quantum Optical
State Control Research Group, NTT Basic
Research Laboratories.
He received the B.E., M.E., and Ph.D. degrees
in engineering science from Osaka University in1994, 1996, and 2002, respectively. He joined
NTT Laboratories in 1996, where he engaged inresearch on lightwave frequency synthesis, optical access networks using wavelength divisionmultiplexing, and quantum communication.
From 2004 to 2005, he was a Visiting Scholar at
Stanford University, California, USA. He is amember of IEEE and the Japan Society ofApplied Physics (JSAP).

Senior Research Engineer, Nano Silicon Technology Research Group, NTT Microsystem
Integration Laboratories.
He received the Ph.D. degree from the Department of Pure and Applied Sciences, the University of Tokyo, in 1990. He joined NTT in 1990
and engaged in studies on ECR plasma and its
application to etching processes for microfabrication. He is currently engaged in studies onfabrication technology for silicon-based optoelectronic devices. He is a member of JSAP.

Ken-ichi Harada

Toshifumi Watanabe

Research Associate, Quantum Optical State
Control Research Group, NTT Basic ResearchLaboratories.
He received the B.S., M.S., and Ph.D. degrees
in physics from Kumamoto University in 2001,
2003, and 2006, respectively. He is currently
engaged in experimental studies on quantum
communication systems using silicon photonicwaveguides. He is a member of the PhysicalSociety of Japan (JPS) and JSAP.

Senior Research Engineer, Nano Silicon Technology Research Group, NTT Microsystem
Integration Laboratories.
He received the B.S. and M.S. degrees inapplied physics from Tokyo Institute of Technology in 1982 and 1984, respectively. He is currently working in NTT Laboratories, where he isengaged in layout processing studies for electron-beam pattern writers and for computeraided design data modulation for large-scale
integration and photonic circuits. His current
research interests include robust processing algorithms for very complex pattern layouts.

Yasuhiro Tokura

Koji Yamada

Executive Manager, Optical Science Laboratory, NTT Basic Research Laboratories.
He received the B.S., M.S., and Ph.D. degrees
in arts and science from the University of Tokyoin 1983, 1985, and 1998, respectively. Since
1985, he has been with NTT Laboratories, where
he has engaged in the research on condensedmatter physics and transport theory in lowdimensional systems. He is a member of JPS andJSAP.

Senior Research Engineer, DistinguishedTechnical Member, Nano Silicon Technology
Research Group, NTT Microsystem Integration
Laboratories.
He received the B.E., M.E., and Ph.D. degrees
in nuclear engineering from Kyushu University,Fukuoka, in 1986, 1988, and 2003, respectively.
He has been engaged in theoretical and experimental studies on beam dynamics in electron
synchrotrons. Recently, he has been engaged inresearch on silicon-based nanophotonic structures. He is a member of JSAP, the Atomic
Energy Society of Japan, the Particle Accelerator
Society of Japan, IEICE, and IEEE.

Hiroshi Fukuda

Sei-ichi Itabashi

Senior Research Engineer, Nano Silicon Technology Research Group, NTT Microsystem
Integration Laboratories.
He received the B.E. and M.E. degrees innuclear engineering from Tohoku University,Miyagi, in 1993 and 1995, respectively. He
joined NTT Laboratories in 1995 and has since
been engaged in research on microphotonicdevices. He is a member of the Institute of Electronics, Information and Communication Engineers (IEICE) of Japan.

Vol. 8 No. 2 Feb. 2010

Senior Research Engineer, Supervisor, Nano
Silicon Technology Research Group Leader,
NTT Microsystem Integration Laboratories.
He received the B.S. and M.S. degrees in physics from Tohoku University, Miyagi, in 1982 and1984, respectively, and the Ph.D. degree fromHokkaido University in 1998. He joined Nippon
Telegraph and Telephone Public Corporation(now NTT) in 1984. He has worked on the development of X-ray and extreme-ultraviolet sources
for X-ray lithography in NTT Laboratories. He
has also engaged in theoretical and experimental
studies on highly reflective X-ray mirrors andmultilayers. He is currently engaged in studies onsilicon photonics technology. He is a member ofJSAP.

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