Quantum key distribution experiment through a PLC matrix switch

Quantum key distribution experiment through a PLC matrix switch

Optics Communications 263 (2006) 120–123 www.elsevier.com/locate/optcom Quantum key distribution experiment through a PLC matrix switch T. Honjo a a...

267KB Sizes 2 Downloads 77 Views

Optics Communications 263 (2006) 120–123 www.elsevier.com/locate/optcom

Quantum key distribution experiment through a PLC matrix switch T. Honjo a

a,*

, K. Inoue

a,b

, A. Sahara c, E. Yamazaki c, H. Takahashi

d

NTT Basic Research Laboratories, NTT Corporation, 3-1, Morinosato-Wakamiya, Atsugi, Kanagawa 243-0198, Japan b Division of Electrical, Electronic and Information Engineering, Osaka University, Suita, Osaka 565-0871, Japan c NTT Network Innovation Laboratories, NTT Corporation, Musashino, Tokyo 180-8585, Japan d NTT Photonics Laboratories, NTT Corporation, Atsugi, Kanagawa 243-0198, Japan Received 5 October 2005; received in revised form 28 December 2005; accepted 12 January 2006

Abstract Quantum key distribution (QKD) is being studied as a way to provide unconditionally secure communications. Several experiments have shown its feasibility. However, most experiments have used a point-to-point occupied optical link. In order to use QKD for secure communications on a real network, it is preferable to be able to change parties on demand and to have quantum transmission and ordinary optical transmission share the optical network. In this work, QKD through a silica-based planar lightwave circuit (PLC) 8 · 8 nonblocking matrix switch was investigated. We found that an interferometer type switch can work even for a single-photon-level light and that a multi-user QKD network can be constructed using a silica-based PLC 8 · 8 non-blocking matrix switch. In addition, single-photon-level transmission and ordinary optical transmission can share the same 8 · 8 non-blocking matrix switch, so which shows the possibility of sending quantum signals through current optical networks.  2006 Elsevier B.V. All rights reserved.

1. Introduction Quantum key distribution (QKD) promises to provide communications with unconditional security on the basis of the physical principles of quantum mechanics, and this technology is in the spotlight now [1]. Several experiments have shown its feasibility [2]. However, most experiments have used a point-to-point occupied optical link. In order to use QKD for secure communications on a real network, it would be better if many parties could communicate with each other and to have quantum communications and conventional optical communications share the optical network. To the best of our knowledge, Townsend et al. performed the first QKD experiment for multi-user optical fiber networks [3]. In their experiment, they used a pas-

*

Corresponding author. Tel.: +81 46 240 3416; fax: +81 46 240 4726. E-mail address: [email protected] (T. Honjo).

0030-4018/$ - see front matter  2006 Elsevier B.V. All rights reserved. doi:10.1016/j.optcom.2006.01.018

sive optical network (PON). Though the sender Alice can establish a secret and unique individual key with each receiver Bob, Alice cannot switch from one communication party to another actively. When Alice wants to create a secret key with a certain Bob, they cannot necessarily avoid consuming photons, which are sent to another receiver. Toliver et al. performed a QKD experiment through transparent optical switch elements [4]. In their experiment, they used several optical switch elements, such as lithium niobate (LiNbO3) switches, microelectromechanical systems (MEMS) switches, and optomechanical. However, the optical switch scales were not so large. In this paper, we report a successful QKD experiment through a silica-based planar lightwave circuit (PLC) 8 · 8 non-blocking matrix switch, which has high reliability and controllability and is used in optical crossconnect experiments. We also show that quantum level transmission and conventional data transmission can share the same matrix switch.

T. Honjo et al. / Optics Communications 263 (2006) 120–123

121

an optical photonic MPLS router [9]. In this experiment, we used an silica-based PLC 8 · 8 non-blocking matrix switch, which includes 128 PLC Mach–Zehnder switches.

2. Differential-phase-shift QKD 2.1. Differential-phase-shift QKD Differential-phase-shift QKD (DPS-QKD) is a new quantum key distribution scheme, which was proposed by one of the authors [5]. The DPS-QKD scheme uses weak coherent pulses, e.g., 0.1 photon/pulse on average, which are randomly phase-modulated by {0, p}. That is, the DPS-QKD scheme utilizes two non-orthogonal coherent states. In this sense the DPS-QKD scheme is similar to the original proposal of B92 [6]. This scheme has several advantages, including suitability for fiber transmission and highly efficient key generation. We have already achieved a stable operation using a PLC Mach–Zehnder interferometer [7]. Fig. 1 shows the DPS-QKD scheme. Alice randomly phase-modulates a pulse train of weak coherent states by {0, p} for each pulse and sends it to Bob with an average photon number of less than one per pulse. Bob measures the phase difference between two sequential pulses using a Mach–Zehnder interferometer and photon detectors and records the arrival time of photon and which detector clicked. After raw transmission, Bob tells Alice the time instances at which a photon has been counted. From this time information and her modulation data, Alice knows which detector clicked at Bob’s site. Under the agreement that a click by detector 1 denotes ‘‘0’’ and a click by detector 2 denotes ‘‘1’’, for example, Alice and Bob obtain an identical bit string. 2.2. A silica-based PLC matrix switch A silica-based PLC matrix switch is one kind of space division optical switch [8]. Though a silica-based PLC is essentially a passive device, it can be used to as an optical switch by combining the thermooptic effect and an interferometer configuration. PLC switches have low insertion loss and polarization dependence so that they are easy to attach to fiber and suitable for realistic production. They are also stable and highly reliable. Recently, a silica-based PLC matrix switch has been used as a main component of

3. Experiments 3.1. Multi-user QKD experiment First, we performed a multi-user QKD experiment. Fig. 2 shows the experimental setup. In this setup, Alice is connected to both Bob1 and Bob2 by way of the 8 · 8 non-blocking matrix switch. In the first transmission experiment, a matrix switch is set to connect Alice and Bob1. Alice randomly phase-modulated a pulse train of coherent light by 0, p for each pulse. The pulse train was created by intensity-modulating continuous wave (CW) light from an external-cavity laser diode (wavelength: 1551 nm). The pulse width was 125 ps and the repetition rate was 1 GHz. The phase modulation was imposed with a LiNbO3 modulator driven by a pulse pattern generator. Then, the light power was attenuated to be 0.1 photon per pulse on average and injected into the 8 · 8 non-blocking matrix switch through a 10-km fiber. One of eight output ports was connected to Bob1 through a 5-km fiber. The propagation loss of the switch was about 5.7 dB. Bob1 measured the phase difference between two sequential pulses using a PLC Mach–Zehnder interferometer. The path length difference was 20 cm, which corresponds to 1-bit delay of 1 ns. Photon detectors were placed at the two outputs of the interferometer. With an appropriate phase in the interferometer, detector 1 clicked for 0 phase difference between two consecutive pulses and detector 2 clicked for p phase difference. Avalanche photodiodes (APDs) were used as the photon detectors, which were gated at 5 MHz. The gate pulse was synchronized with the light pulse. The quantum efficiency was about 11.8% and the dark count probability was about 2.0 · 10 5 per gate. In detecting photons, Bob recorded the photon arrival time and which detector clicked. Using the above setup, Alice and Bob1 created raw secret keys at their respective sites, following the protocol described in the previous section. The quantum bit

Alice

Bob Mach-Zehnder Interferomter Fiber

LD

IM

PM

ATT

DET1

DET2

Fig. 1. The DPS-QKD scheme: IM, intensity modulator; PM, phase modulator; ATT, attenuator; and DET, photon detector.

122

T. Honjo et al. / Optics Communications 263 (2006) 120–123

Fig. 2. Experimental setup for multi-user QKD: IM, intensity modulator; PM, phase modulator; ATT, attenuator; and DET, photon detector.

error rate (QBER) was estimated from the difference between the created keys. We obtained a key generation rate of 2.2 kbit/s and a QBER of 6.4%. A sufficient QBER was obtained to create a secret key after error correction and privacy amplification. In the second transmission experiment, the 8 · 8 nonblocking matrix switch was set to connect Alice and Bob2. Bob2’s setup was almost same as Bob1’s. A key creation rate of 2.3 k bit/s with a QBER of 6.2% was obtained. According to the above experiment, we can conclude that a multi-user QKD network by using a silica-based PLC 8 · 8 non-blocking matrix switch is realizable. In addition, we can also say that this kind of a large-scale interferometric switch can work even for a single-photonlevel light. 3.2. Quantum key distribution along with a conventional data transmission Second, we performed an experiment where single-photon-level signals and conventional data transmission signals passed through the switch simultaneously.

Before performing the QKD transmission experiment, we measured the cross talk of our matrix switch. The average of the cross talk was 52.2 dB. In the following experiment, we used the input and output port pair that had the worst cross talk value, 42.4 dB. Fig. 3 shows the experimental setup. Alice’s setup was almost the same as in the previous experiment, except the wavelength was 1555 nm. Bob’s setup was also almost the same, except that Bob was directly connected to the matrix switch and a fiber grating filter, which suppressed the 1551 nm light inserted in front of his Mach–Zehnder interferometer. The suppression performance of this filter was about 52.1 dB. For conventional data transmission signals, CW light from an external cavity laser diode (wavelength: 1551 nm) was pseudo-randomly intensity-modulated at a rate of 10 Gbit/s. The intensity-modulated light was directly put into the matrix switch. The light power was 11.1 dBm. While these conventional data signals were transmitted, the QKD transmission experiment was performed. We obtained a key creation rate of 2.0 kbit/s with a QBER of 6.0%. Though quantum level signals and conventional data transmission signals crossed in the 8 · 8

Fig. 3. Experimental setup for quantum key distribution along with a large-bandwidth data transmission: IM, intensity modulator; PM, phase modulator; ATT, attenuator; and DET, photon detector.

T. Honjo et al. / Optics Communications 263 (2006) 120–123

(i) (ii) (iii)

QBER (%)

40

20

0

–40

–20 0 Input Power (dbm)

20

Fig. 4. QBER as a function of input power of conventional data transmission: (i) No filters applied. (ii) The 1551 nm band suppression filter applied only for the quantum channel. (iii) The 1551 nm band suppression filters and the 1551 nm band pass filter applied for quantum channels and for the conventional data transmission channel, respectively.

non-blocking matrix switch, the conventional data transmission signals did not disturb the single photon level signals. We can therefore say that quantum level signals and conventional transmission signals can share the same 8 · 8 non-blocking matrix switch. Note that conventional transmission signals were not sent over the long optical fiber in this experiment. If we transmit them over the long optical fiber, the spontaneous Raman scattering noise is generated by conventional transmission signals, which may disturb the quantum channel [10]. We also evaluated the dependence of the QBER on the input power of conventional data transmission signals. Though the center wavelength of the conventional signal light was 1551 nm, it also had residual spectral components other than 1551 nm, which degrade the QKD signal. To suppress these residual components, we used a band pass filter for 1551 nm at the output of the conventional signal light source, which consisted of a fiber grating filter and a circulator. This filter suppressed 1555 nm light by 40 dB. We also used a fiber grating filter to suppress 1551 nm light at the input of the QKD receiver as described above. The following three cases were investigated: (i) No filters; (ii) the 1551 nm band suppression filter applied only for the quantum channel; (iii) the 1551 nm band suppression filters and 1551 nm band pass filter applied for quantum channels and for the conventional data transmission channel, respectively. In these experiments, the conventional data transmission signal was not a pseudorandom bit stream but a CW light. Fig. 4 shows the experimental results.

123

The leaked light from the conventional data transmission channel increased the QBER. When no filters were applied, it was hard to send single-photon-level signals and conventional data transmission signals simultaneously. Case (ii) corresponds to the multi-user QKD experiment. Considering the QBER of our system fluctuates less than a few percent, almost the highest power light was used for the conventional data transmission signal in the previous multi-user experiment. If we had sent higher power light for conventional data transmission, it would have been hard to obtain sufficiently low QBER. We will be able to increase the light power for a conventional data transmission if we use a band suppression filter and a band pass filter for both channels. In case (iii), we obtained relatively low QBER compared with cases (i) and (ii). In our QKD system, we have to severely adjust the interferometer, so that in the first two experiments, the adjustment of the interferometer must not be good enough. However, these differences are not the main issues in these experiments. 4. Summary Quantum key distribution through a silica-based PLC 8 · 8 non-blocking matrix switch was reported. We showed that it is possible to construct a multi-user QKD network using a Mach–Zehnder interferometer-based optical matrix switch, which can work even for single-photon-level light. We also showed that a quantum level transmission and conventional data transmissions can share the same matrix switch. References [1] N. Gisin, G. Ribordy, W. Tittel, H. Zbinden, Rev. Mod. Phys. 74 (2002) 145. [2] A. Muller, T. Herzog, B. Huttner, W. Tittel, H. Zbinden, N. Gisin, Appl. Phys. Lett. 70 (1997) 793. [3] P.D. Townsend, Nature 385 (1997) 47. [4] P. Toliver, R.J. Runser, T.E. Chapuran, J.L. Jackel, T.C. Banwell, M.S. Goodman, R.J. Hughes, C.G. Peterson, D. Derkacs, J.E. Nordholt, L. Mercer, S. McNown, A. Goldman, J. Blake, IEEE Photon. Technol. Lett. 15 (2003) 1669. [5] K. Inoue, E. Waks, Y. Yamamoto, Phys. Rev. A 68 (2003) 022317. [6] C.H. Bennet, Phys. Rev. Lett. 68 (1992) 3121. [7] T. Honjo, K. Inoue, H. Takahashi, Opt. Lett. 29 (2004) 2797. [8] A. Himeno, K. Kato, T. Miya, IEEE J. Sel. Top. Quantum Electron. 4 (1998) 913. [9] S. Aisawa, A. Watanabe, T. Goh, Y. Takigawa, M. Koga, H. Takahashi, IEEE Commun. Mag. 41 (9) (2003) 54. [10] P. Toliver, R.J. Runser, T.E. Chapuran, S. McNown, M.S. Goodman, J. Jackel, R.J. Hughes, C.G. Peterson, K. McCabe, J.E. Nordholt, K. Tyagi, P. Hiskett, N. Dallman, in: Proceedings of the IEEE LEOS Annual Meeting 2004, WE1, Rio Grande, Puerto Rico, 2004.