NEC RESEARCHER’S INTERVIEWNEC RESEARCHER’S INTERVIEW

254 NEC J. of Adv. Tech., Summer 2005

RESEARCHER INTERVIEW REPORTS

It has been widely considerIt has been widely considerIt has been widely considerIt has been widely considerIt has been widely considered that “Thered that “Thered that “Thered that “Thered that “Thereeeeeis no humanly pris no humanly pris no humanly pris no humanly pris no humanly produced encryption that isoduced encryption that isoduced encryption that isoduced encryption that isoduced encryption that isnot decipherable.” Even if a cipher that isnot decipherable.” Even if a cipher that isnot decipherable.” Even if a cipher that isnot decipherable.” Even if a cipher that isnot decipherable.” Even if a cipher that isbased on factorization theory such as thebased on factorization theory such as thebased on factorization theory such as thebased on factorization theory such as thebased on factorization theory such as thecurrcurrcurrcurrcurrent RSA encryption is not decipherable,ent RSA encryption is not decipherable,ent RSA encryption is not decipherable,ent RSA encryption is not decipherable,ent RSA encryption is not decipherable,it is because “It takes too much time usingit is because “It takes too much time usingit is because “It takes too much time usingit is because “It takes too much time usingit is because “It takes too much time usinga curra curra curra curra currently available computer” and not be-ently available computer” and not be-ently available computer” and not be-ently available computer” and not be-ently available computer” and not be-cause it is indecipherable in principle. How-cause it is indecipherable in principle. How-cause it is indecipherable in principle. How-cause it is indecipherable in principle. How-cause it is indecipherable in principle. How-evereverevereverever, when a code based on the “uncer, when a code based on the “uncer, when a code based on the “uncer, when a code based on the “uncer, when a code based on the “uncer-----tainty principle” of quantum mechanics istainty principle” of quantum mechanics istainty principle” of quantum mechanics istainty principle” of quantum mechanics istainty principle” of quantum mechanics isapplied, it is rapplied, it is rapplied, it is rapplied, it is rapplied, it is reported that an encryptedeported that an encryptedeported that an encryptedeported that an encryptedeported that an encryptedcommunication is rcommunication is rcommunication is rcommunication is rcommunication is realized that is absolutelyealized that is absolutelyealized that is absolutelyealized that is absolutelyealized that is absolutelyimpossible to be decrypted. This is the “ul-impossible to be decrypted. This is the “ul-impossible to be decrypted. This is the “ul-impossible to be decrypted. This is the “ul-impossible to be decrypted. This is the “ul-timate encryption,” which is a rtimate encryption,” which is a rtimate encryption,” which is a rtimate encryption,” which is a rtimate encryption,” which is a researesearesearesearesearch topicch topicch topicch topicch topicthat is prthat is prthat is prthat is prthat is presently moresently moresently moresently moresently more advanced than thee advanced than thee advanced than thee advanced than thee advanced than the“quantum computer” r“quantum computer” r“quantum computer” r“quantum computer” r“quantum computer” researesearesearesearesearch in the fieldch in the fieldch in the fieldch in the fieldch in the fieldof practical quantum-based rof practical quantum-based rof practical quantum-based rof practical quantum-based rof practical quantum-based researesearesearesearesearch. NECch. NECch. NECch. NECch. NECnow stands as a world leader in this rnow stands as a world leader in this rnow stands as a world leader in this rnow stands as a world leader in this rnow stands as a world leader in this re-e-e-e-e-searsearsearsearsearch, with successes at the highest levelch, with successes at the highest levelch, with successes at the highest levelch, with successes at the highest levelch, with successes at the highest levelin terms of length and speed of successivein terms of length and speed of successivein terms of length and speed of successivein terms of length and speed of successivein terms of length and speed of successivecryptographic key generation, under prac-cryptographic key generation, under prac-cryptographic key generation, under prac-cryptographic key generation, under prac-cryptographic key generation, under prac-tical conditions. This achievement is a verytical conditions. This achievement is a verytical conditions. This achievement is a verytical conditions. This achievement is a verytical conditions. This achievement is a veryinterinterinterinterinteresting theme. In this article, we interesting theme. In this article, we interesting theme. In this article, we interesting theme. In this article, we interesting theme. In this article, we inter-----viewed two rviewed two rviewed two rviewed two rviewed two researesearesearesearesearchers in this field who archers in this field who archers in this field who archers in this field who archers in this field who areeeeeworking in difworking in difworking in difworking in difworking in differferferferferent departments.ent departments.ent departments.ent departments.ent departments.

* * * * * * * *

Quantum Cryptography:A Reliable Security Guard!

Dr. Akihisa TOMITAPrincipal Researcher

Fundamental andEnvironmental Research

Laboratories

Mr. Akio TAJIMAAssistant ManagerSystem Platforms ResearchLaboratories

Quantum cryptography that“Cannot be broken unless the system

is physically destroyed”

Today, commercially based cryptography is ac-tively employed on the Internet and for othercommunications systems. But it is impossible forthe current technology to prevent “bugging” com-pletely. The “public key encryption methods”based on factorization theory (such as RSA) aresaid to be “undecipherable,” but this presupposesthe use of “principles that are inherent in the cur-

rently employed computers.” It is almost axi-omatic to say that, once the “quantum com-puter” becomes widely available (see thearticle on page 248 of this issue), it will be-come possible to factorize a 200-digit num-ber in a few minutes and even a 10,000-digitnumber could be factorized in a few hoursor days.

The breakthrough against such a threat ispossible only by exploring the lowermostconstraint of the information communicationlayer model, that is to say the physical layer.For example, in the case of optical fiber

“Ultimate encryption” quantum mechanicsguarantees security

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communication, this corresponds to the use of“photons.” Every photon can be transmitted anddetected individually. In the detection of bugging,there is the notion of monitoring whether or noteach photon is captured during its transmissionpath in terms of the classical understanding ofphoton behavior. Although this strategy is logi-cal, it is still inadequate. If the villain is equippedwith a perfect detector and generator and retrans-mits the signal, it is no longer possible to identifythe bugging. The conclusion is that no system iscertain except for the quantum cryptographymethod which, “cannot be broken unless the sys-tem is physically destroyed.” This is an idea pro-posed in 1984.“Quantum cryptography is also called the quan-tum key distribution system. This system consistsof the sharing of a random number sequence, ora secret key, by two remote parties connectedthrough an optical fiber. The “secret key” is alsoone of the keywords of the current symmetric-key encryption systems,” Dr. Tomita of the NECFundamental and Environmental ResearchLaboratories began the explanation.“The transmitter prepares a random bit sequencecomposed of <0> and <1> (random number se-quence) and transmits this by expressing it bymeans of a ‘polarization’ of photons. The sets ofthe photon polarization states used here includethe ‘horizontal polarization (0°)’ and ‘vertical

polarization (90°)’ of ‘+basis’ and ‘+45°’ and‘−45°’ states of the ‘x basis’ and the basis may beselected randomly. Similarly, the receiver decidesrandomly on the basis used for the polarizationstate detection of each photon, and determinesthe bit value. After the transmission, the senderand the receiver exchange the basis informationused for each photon detection. They keep onlythe bits where the receiver used the same basisas the sender, and forget about others. The aboveprocedure leaves identical random bit sequenceson the both parties. Bugging on the photons af-fects the polarization states, and results in errorsin the shared random bit sequence, that is, theobvious proof of bugging (Fig. 1).”

Photon reception systembased on scientific fundamentals

achieves major breakthrough

Dr. Tomita continues to expand this theme, butthe understanding of the interviewer reaches itslimit. We decided to return to the original aim ofthe interview and asked him, “After all, whatexactly is the contribution made by NEC throughthis research into quantum cryptography?”

Dr. Tomita then showed us Fig. 2.“First of all, we conceived a new operation prin-ciple of the ‘photon receiver,’ which features the

Fig. 1 Example of quantum key distribution.

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highest sensitivity yet achieved, and implementedit. Secondly, we invented the ‘alternative phaseshift modulation system’ that employs a fiber loopcircuit using a polarized beam splitter, a systemthat ensured independent polarization rotationfrom the changes in the ambient temperature. Wedemonstrated the superiority of our technology.Thirdly, we established the synchronization tech-nology that can absorb the effects of delay fluc-tuations in the transmission path. Fourthly, wedeveloped key generation software that can gen-erate the key concurrently with the photon trans-mission. Fifthly, we packaged all of the compo-nents, including the photon receiver, in a desk-top sized cabinet and operated it successfully.These are the factors that allowed us to achievesuccessive final key generation in a practical en-vironment.”

The explanation was continued by Mr. Tajimaof the NEC System Platforms Research Labora-tories.“The work assigned to my department was thedemonstration, development and packaging ofthe system. The system has the official name ofthe “quantum cryptosystem.” As is widely known,we have developed a “quantum cryptosystem”and succeeded in two-way quantum key commu-nication over 16.3km of commercial fiber opticcable that is actually installed outdoors. Morespecifically, we conducted a demonstrative ex-periment 24 hours a day for more than 14 days,and achieved the world’s highest final key gen-eration average speed of 13kbps, and also per-

Fig. 2 Contribution made by NEC Fundamental and Environmental Research Labs.and System Platforms Research Labs.

formed encrypted communication of various datausing the generated final key (Fig. 3). Prior tothis experiment, we also succeeded in an experi-ment for single photon transmission over 150kmand 100kbps raw key generation at 40km trans-mission. The reason that we selected outdooroperation in this experiment was because therehad been concern that climate changes, includ-ing the differences between the daytime andnighttime temperatures would vary the cablelength and thereby affect the photon bit synchro-nization. Naturally, we concede that there is thenecessity of additional considerations concern-ing the handling of the sync signal, so we appliedthe WDM (Wavelength Division Multiplexing)transmission method, which transmits the quan-tum signal and sync signal through a single fiber

Photo 1 External view of quantum cryptosystem.

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optic cable by multiplexing their wavelengths.”An experiment may always encounter unex-

pected events. One of the results obtained fromthis particular experiment was the fact that “evenvery low, previously ignored noise is capturedbecause the energy level is so low.” But each andevery problem was solved one by one, and a sig-nificant result was achieved. Through these re-sults we were awarded by the Ministry of Inter-nal Affairs and Communications.

What are the next issuesto be challenged?

We returned to question Mr. Tajima, who said,“Quantum cryptography research has now movedfrom the phase of proving the principles of pho-ton transmission to the phase of system packag-ing.”“We have already begun to challenge more ad-vanced topics. These can roughly be classifiedinto three topics. The first topic is to improve thestability, speed and long-distance capability fur-ther, and the key to this may lie in the applica-tion of the PLC (Planar Light-wave Circuit) tech-nology. The second is to package an applicationand perform a thorough verification of the en-cryption system from the viewpoint of system.And the third is to verify the unconditional secu-

rity of quantum cryptography, which has alreadybeen confirmed theoretically, also in the fieldtests.”

Listening to Mr. Tajima, Dr. Tomita explainedabout the topics from his own viewpoint.“Past experiments employed an optical fibercable connecting two points directly, but we alsoneed to perform the transmission in a complicatednetwork environment. This is the “quantum keydistribution network.” The design of repeaters isalso an important topic and in any case, we haveto study these systematically, by beginning fromtheoretical matters.”

For the allotment of work, Dr. Tomita is to es-tablish the fundamental principles and Mr.Tajima is to implement the system by consider-ing its practical use and commercialization in thefuture.

“Q-Gov. (Quantum Government)”could be the next topic to come after

“quantum cryptography”

We asked how it was that two laboratories withsuch different bases were able to develop suchexcellent cooperation.

According to Dr. Tomita, “Our lab began thestudy covering both ‘quantum’ and ‘communica-tion’ in around ’98, but it was in 2003 that we

Fig. 3 Successive final key generation.

NEC J. of Adv. Tech., Summer 2005258

FIg. 4 Network society based on quantum information technology.

brought this subject to the System Platforms Re-search Laboratories where Mr. Tajima is work-ing. There had been a gap of understanding be-tween the two laboratories in the beginning, butwe continued periodical exchanges of informa-tion by such means as the teleconference system.We sometimes spent very long hours talking onthe phone. Thus, it has gradually become clearthat the issue of ‘sync’ would be a significantbottleneck and that the ‘alternative phase shift’modulation might be the key to a breakthrough.”

On the other hand, Mr. Tajima says, “We havelong been researching into optical communica-tion, particularly the optical switch system. Wealso developed the optical Ethernet system sincethe lab was reorganized to emphasize practicalimplementation. When we had finished that workand felt as though we were in a vacuum, it wasproposed to us to do the ‘quantum cryptography.’But, when we considered the details, we felt that‘quantum’ would be fairly difficult. The subjectis hard to be understood, and no existing mea-suring instruments are usable (laughter). How-ever, fortunately, our lab had a very large varietyof staff members; some of them being strong invarious fabrication techniques, some were strongin the field of photonics and some were capableof making objective judgments. So we came to

think that we could all advance steadily by care-fully allotting the work. And I am the kind ofperson who rather loves to mix with people withsuch strong individualities.”

We also asked Dr. Tomita about his interests.“I originally majored in physics, but I tended tobe interested in many things, often rather exten-sively but sometimes superficially. I have alsobeen engaged in R&D of optical communicationdevices such as laser diodes and functional de-vices for photonics. But I felt like tackling some-thing ‘ultimate’ for the physicists. This idea natu-rally led to the ‘quantum,’ and I selected the topicof ‘quantum cryptography’ thinking that it in-volved ‘communication’ which was a currenttrend.”

We then asked this “trend-oriented” Dr. Tomitaabout the next topic to come after “quantum cryp-tography.”“We are today in the age of ‘electronic govern-ment,’ ‘electronic payment,’ ‘electronic comput-ers’ and ‘electronic memory.” And I firmly be-lieve that the time is near in which all of theseadjectives ‘electronic’ are replaced by ‘quantum.’Then, we will see the ‘Q-government (quantumgovernment),’ and everything beginning with‘Q-’ in place of ‘E-’ (laughter). When absolutesecurity, large-capacity communication and

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ultrahigh-speed computing backed by thetheory of quantum mechanics come to berealized, will it not be evident that lifestylesand industries could be changed com-pletely?” Saying this, Dr. Tomita showedus a panel of pictures attached to the doorof his lab (Fig. 4). Mr. Tajima from his sideseemed to be already aware of all this, say-ing, “private businesses are rather passivetoward investment into security, but pub-lic organizations need the ‘absolute secu-rity.’ We should absolutely appeal to thegovernment about the necessity of ‘quan-tum government.’ His words revealed to ushow it is that their cooperation is going sowell.

What kinds of effects will the synergy of thesetwo personalities bring about, and how will theybe developed commercially? The future leads usto expect much.

(Interviewed/compiled by Haruhito Tsuchiya)

Photo 2 Dr. Tomita and Mr. Tajima in the lobby of NEC Tsukuba.