Press Release
February 3, 2010
Developing of Hybrid Entanglement Photon Pair Source
－ Key Device for Space Fiber Hybrid Communication Link －
Quantum ICT group and Space Communications Group of National Institute of Information and Communications Technology (NICT, President: Hideo Miyahara) and NEC Corporation (NEC, President: Kaoru Yano) have succeeded in developing of hybrid entanglement photon pair source for free space-fiber hybrid Quantum Key Distribution (QKD) system. The entanglement is hybrid in the sense that a time-bin qubit at 1550 nm is entangled with a polarization qubit at 810 nm. We obtained coincidence between two qubits with visibilities over 88%. This result means that it is possible to distill a secure key in QKD.
The result is published in Applied Physics Letters.
URL: http://apl-beta.aip.org/applab/v95/i26
The result is published in Applied Physics Letters.
URL: http://apl-beta.aip.org/applab/v95/i26
- [Background]
- The Quantum Key Distribution (QKD) allows two users, to communicate in absolute security. Unconditional security is guaranteed by the fundamental law of physics. An entanglement photon pair source will be used in a next generation QKD system because of the advantage in security proof. A quantum key distribution length in an optical fiber is limited within 300 km due to the losses in the fiber and a noise of a single photon detector. On the other hand, a free space QKD can encompass the entire global by using an earth orbiter. Moreover, free space QKD system enables us to build a flexible link. However, there is no inter operability between a free space and fiber based QKD systems, because the optimal entangled state is different due to the difference of characteristics in propagation channels.
- [Results]
- A newly developed hybrid entanglement photon pair source consists of a format transformer devised by NICT and a non-degenerated photon pair source developed by NEC. The non-degenerate down conversion photon pairs (1550 and 810 nm) are input to asymmetric Mach-Zehnder Interferometer with a 2.5ns time delay (Figs. 1, 2 (a) and (b)), and time-bin entanglement was formed. An 810 nm photon is input to a transformer, and superposed polarization entanglement information. We obtained coincidence between two qubits with visibilities over 88% (in Fig.3 ). These values are well above the threshold of 70.7% needed to violate a Bell inequality and allow distilling a secure key in the QKD.
- [Prospects]
- Our scheme simplifies the free space receiver structure to be robust to the temperature changes and mechanical disturbance. Such a free space receiver is suitable for a mobile terminal of a QKD system. Our hybrid entanglement photon pair source can be used in integrated free space-fiber QKD network.
< Technical Inquiries >
Mikio FUJIWARA
Quantum ICT Group
New Generation Network Research Center
Advanced ICT Research Center
Tel ：+81-42-327-7552
Mikio FUJIWARA
Quantum ICT Group
New Generation Network Research Center
Advanced ICT Research Center
Tel ：+81-42-327-7552
< Public Relations >
Sachiko HIROTA
Public Relations Office
Strategic Planning Department
Tel ：+81-42-327-6923
Sachiko HIROTA
Public Relations Office
Strategic Planning Department
Tel ：+81-42-327-6923
< Terminology and Interpretations >
- Quantum Key Distribution (QKD)
- Quantum Key Distribution (QKD) allows two users, Alice and Bob, to communicate in absolute security. Unconditional security is guaranteed by the fundamental law of physics. BB 84 is the most famous protocol of QKD, and in such system, signals originated from single photon pulses emitted by Alice are guaranteed to be secure.
- Quantum Entanglement
- Quantum entanglement is a property of a quantum state of a system of two or more objects in which the quantum states of the constituting objects are linked together so that one object can no longer be adequately described without full mention of its counterpart—even if the individual objects are spatially separated.
- Applied Physics Letters
- URL: http://apl-beta.aip.org/applab/v95/i26
- Performance of hybrid entanglement photon pair source for quantum key distribution
M. Fujiwara, M. Toyoshima, M. Sasaki, K. Yoshino, Y. Namb, and A. Tomita
Vol.95, Issue 26, 261103, 2009. Dec. 29 online published.
- Mach-Zehnder Interferometer
- The Mach-Zehnder interferometer is a device used to determine the relative phase shift between two collimated beams from a coherent light source.
- Time-bin Entanglement
- Time-bin-entangled photons are photon pairs created in a coherent superposition of two discrete emission times. As time-bin entanglement can easily be protected from decoherence as encountered in optical fibers.
- Periodically Poled Lithium Niobate (PPLN)
- PPLN is a domain-engineered lithium niobate crystal, used mainly for achieving quasi-phase-matching in nonlinear optics.
< Appendix >
Fig.1：
Conceptual View of the QKD System Setup with the Hybrid Entanglement Photon Pair Source
Fig.2：
(a) Conceptual View and (b) Equivalent Optical Circuit of the Format Transformer
The first (second) time-bin which passes through the short (long) arms of a format transformer is superimposed the polarization information as |H〉 (|V〉). Instead of a normal PBS, the Glan laser prism is selected to improve an extinction ratio
Fig.3：
Coincidence Counts of (a) Polar (z axis) State as a Function of Delay Time, and (b) Equatorial (x axis)
State as a Function of the PLC Operation Temperature
Coincidence of Z0(Alice)-Z0(Bob) and Z1(Alice)-Z1(Bob) are detected in center peaks of (a) . And the visibilities are 95.8±0.2% are obtained. The visibilities of (b) are 88±1%, calculated from fringes.
The hybrid entanglement source is constructed of a non-degenerate photon pair source and a format transformer from time encoding to polarization encoding. The non-degenerate down-conversion entangled photon pair source is shown in Fig. 1. A 532 nm contiguous wave laser adjusted at 160 µW is incident on a 30 mm long Periodically Poled Lithium Niobate (PPLN) crystal that is quasi-phase-matched to create co-polarized entangled photons at 810 and 1550 nm.
The 810 nm photons and the 1550 nm photons are separated using a Dichroic Mirror (DM). We use long-wavelength pass filters to reduce stray photons. After down conversion, a 1550 nm photon is incident a decoder set at Bob side. The decoder is a two-input and four-output silica-based PLC on a silicon substrate (15), featured by an Asymmetric Mach-Zehnder interferometer (AMZI) with a 2.5 ns time delay. 50 cm long spiral pattern was fabricated to make the 2.5 ns time delay.