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

RESEARCHER INTERVIEW REPORTS

“The quantum computer” is attracting“The quantum computer” is attracting“The quantum computer” is attracting“The quantum computer” is attracting“The quantum computer” is attractinggrgrgrgrgreat expectations and attention for its in-eat expectations and attention for its in-eat expectations and attention for its in-eat expectations and attention for its in-eat expectations and attention for its in-novative operating principles and its capa-novative operating principles and its capa-novative operating principles and its capa-novative operating principles and its capa-novative operating principles and its capa-bility for solving prbility for solving prbility for solving prbility for solving prbility for solving problems that have beenoblems that have beenoblems that have beenoblems that have beenoblems that have beendifdifdifdifdifficult to solve for traditional computers.ficult to solve for traditional computers.ficult to solve for traditional computers.ficult to solve for traditional computers.ficult to solve for traditional computers.Although, until only a few years ago it wasAlthough, until only a few years ago it wasAlthough, until only a few years ago it wasAlthough, until only a few years ago it wasAlthough, until only a few years ago it wassaid that its implementation would take asaid that its implementation would take asaid that its implementation would take asaid that its implementation would take asaid that its implementation would take ahundrhundrhundrhundrhundred years or so, several pred years or so, several pred years or so, several pred years or so, several pred years or so, several promising romising romising romising romising re-e-e-e-e-searsearsearsearsearch rch rch rch rch results that seem to appresults that seem to appresults that seem to appresults that seem to appresults that seem to approach roach roach roach roach real-eal-eal-eal-eal-ization arization arization arization arization are now being announced fre now being announced fre now being announced fre now being announced fre now being announced from allom allom allom allom allover the world. Among these rover the world. Among these rover the world. Among these rover the world. Among these rover the world. Among these results, theesults, theesults, theesults, theesults, theachievements made by NEC’achievements made by NEC’achievements made by NEC’achievements made by NEC’achievements made by NEC’s Fundamen-s Fundamen-s Fundamen-s Fundamen-s Fundamen-tal and Envirtal and Envirtal and Envirtal and Envirtal and Environmental Researonmental Researonmental Researonmental Researonmental Research Laborato-ch Laborato-ch Laborato-ch Laborato-ch Laborato-ries arries arries arries arries are highly evaluated as forming the fore highly evaluated as forming the fore highly evaluated as forming the fore highly evaluated as forming the fore highly evaluated as forming the fore-e-e-e-e-frfrfrfrfront of the field. The details of this ront of the field. The details of this ront of the field. The details of this ront of the field. The details of this ront of the field. The details of this researesearesearesearesearchchchchchararararare dife dife dife dife difficult for the layman to understandficult for the layman to understandficult for the layman to understandficult for the layman to understandficult for the layman to understandbut in orbut in orbut in orbut in orbut in order to help explain it, we have in-der to help explain it, we have in-der to help explain it, we have in-der to help explain it, we have in-der to help explain it, we have in-terviewed Drterviewed Drterviewed Drterviewed Drterviewed Dr. Jawshen T. Jawshen T. Jawshen T. Jawshen T. Jawshen Tsai, a Researsai, a Researsai, a Researsai, a Researsai, a Research Fel-ch Fel-ch Fel-ch Fel-ch Fel-low at the laboratory about the significancelow at the laboratory about the significancelow at the laboratory about the significancelow at the laboratory about the significancelow at the laboratory about the significanceand the challenges of his rand the challenges of his rand the challenges of his rand the challenges of his rand the challenges of his researesearesearesearesearch.ch.ch.ch.ch.

* * * * * * * *

Road to theQuantum ComputerNow Found!Success in 2-bit logic computationsusing solid quantum bits

Dr. Jaw-shen TSAIResearch Fellow

Fundamental and Environmental ResearchLaboratories

What is “a quantum computer”?

Dr. Tsai works at NEC’s Fundamental and En-vironmental Research Laboratories in Tsukuba,Ibaraki Prefecture, Japan. When we pass the cor-ridor leading to his laboratory, we see severalNEC ad posters posted on the walls. One of themshows Chinese characters representing enormousvalues such as “兆(1012 =trillion), 京(1016), 垓(1020),...正(1040) and 載(1044)” in a cubically su-perposed design, and another shows an illustra-

tion with the motif of “Rubin’s Vase,” which is awell-known optical illusion.

The former poster seems to convey the messagethat the quantum computer is capable of instan-taneously factorizing a huge number by makinguse of the property of “superposition” of thequantum, and the latter poster seems to attemptto show an image of that “superposition.” In otherwords, a state may be expressed just like the pic-ture of the Rubin’s Vase, in which one can seetwo aspects, either the profiles of two people fac-ing each other or the silhouette of a vase, but can-not see one of these aspects if the other aspect is

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watched too much. This poster is not wrong as apictorial explanation of the logic, but it is not atall a representation of the “quantum computer”itself.

A large number of books and websites offerexplanations of the “quantum computer,” but thehurdle to be cleared is quite high in order for usto comprehend a real image of it. As outsiders,we may even be frustrated by the fact that thebasic concept of “superposition” (the state inwhich a single bit can be both 0 and 1) is hard tobe understood intuitively. This concept is quiteunlike those of classical physics. The “quantum,”which is a minimum mass of energy like a pho-ton or electron, can simultaneously feature both“particle-like properties” and “wave-like prop-erties.” So some people suggest that “superposi-tion” can be understood more easily in terms ofthe principle of “interference” that belongs to“wave-like properties.” This notion refers to thefact that, when interference is caused by wavespassing through two slits, one can know the slitthat the interference passed through only in termsof probability. Even when this figurative expla-nation makes us feel that we have understoodsomething, we then have to cross another hurdle,which is another basic concept of “entanglement”in the case of two quanta being present.

Quantum dynamics has a history of about a cen-tury, while computer science has a history of littlemore than a half century. Furthermore, the his-tory of the “quantum computer” is as short astwo decades and has been led exclusively byphysicists as a research subject. If a real thing ispresented to us and its function is described, webelieve that we should be able to grasp its imagein more detail......

Dr. Tsai et al. are reported be achieving steadyresults that can help realizing such aspirations.Our expectations in interviewing an expert of thestanding of Dr. Tsai were indeed great.

How NEC’s quantum computerresearch was begun

“In the beginning, our research was not consciousof the ‘quantum computer’ at all.” Dr. Tsai be-gan his talk with unexpected words.“I was originally a researcher of the Josephsondevice, a device that makes use of the tunnel ef-fect of electrons in the superconducting state. Myinvolvement was also influenced by a recently

undertaken national project on the subject. Thisproject can be regarded as having heralded“nanoelectronics”. By the way, I was the firstperson to use this term at NEC, back in 1986, butit was rather criticized as being ‘incomprehen-sible’ and unpopular (laughter). The actual situ-ation was so inadequate that we had difficulty inprocuring even the requisite electron beam ex-posure for producing fine devices, but we hadbeen aiming at a “single-electron transistor” withan ultra low current consumption. This transis-tor makes it possible to switch the output ON-OFF by moving only one electron, while currentlyavailable computers necessitate almost a millionelectrons to create an ON or OFF state (each bit).We started with fundamental research into thissubject and accumulated results little by little.”

A turning point came in the early 1990’s. It wasaround that time that Principal ResearcherYasunobu Nakamura who worked in the samelaboratory as Dr. Tsai joined NEC. When theychecked the electronic properties of a transistormade using aluminum electrodes, they found thatlike the electron, scattered energy levels werefeatured, in spite of it being an electric circuitoverall.“From around 1995, we noticed this characteris-tic and started an experiment by applying micro-waves to the circuit and checking the energy dif-ference between the two lowest energy levels. Itwas just like the spectrometric experiment ofapplying light on an atom and checking the spec-trum. Then, Dr. Nakamura contributed a paperon the results of this experiment to the journalPhysical Review Letters. This research was actu-ally the discovery of “quantum coherence,” thephenomenon leading to the verification of theprinciples of the “quantum computer.” However,we did not mention this because we were not yetat the stage of controlling the quantum state.”

Meanwhile, it seems that NEC documents usedfor the domestic invitation of public applicationprojects and in-house reports had started PR sug-gesting that the research would contribute to thequantum computer. This was a judgment of theresearch management team in consideration ofworld trends. Apart from this development, theresearch was gradually focused on the confirma-tion of the changes in time of the “superposition”states of quanta. The laboratory confirmed thebehavior of the qubit (quantum bit) in solid de-vices for the first time in the world in November’98, and succeeded in creating “superposition”

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arbitrarily in ’99 (Fig. 1). With regard to theabove, the limitation “in solid devices” is addedbecause major laboratories worldwide are cur-rently competing in the production of the qubit,using their own individually developed methods.These include the use of ions in a vacuum, mol-ecules in liquid and photons. The author hastherefore adopted the method “in solid devices”with the view that it is the most advantageousmethod.

There are several hurdles to be crossedbefore practical implementation

can be achieved

At this point, it may be necessary to offer a fur-ther explanation on the development of the“quantum computer.” Since the world of quan-tum dynamics is a place in which the conventionalwisdom of the macroscopic world does not workat all, understanding is difficult and we have toresort to multiple keywords. The bits (combina-tion of <0> and <1>) that are the basis of the “clas-sical computer,” are in the “superposition” states

with the qubits. When the “entangle-ment” state is created between thequbits to make them inseparable, a“quantum circuit” can be formed. Thisis the basic principle of quantum logicoperations and can function as a univer-sal gate playing the roles of AND, ORand NAND of traditional computers.The requirement at the device level isto enable these circuits to be controlledas desired.“It was an important hurdle to create the‘entanglement’ state between twoqubits, but we succeeded in achieving itin February 2003 (Fig. 2). Since then, wehave added improvements for the arbi-trary control of qubits in order to en-able logic operations and we have alsoPhoto 1 Dr. Tsai at his laboratory.

Fig. 1 The solid-state device consisted of aluminum thin film having a thickness ofabout 50 nanometers.

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identified the conditions of the applied voltagepulses,” says Dr. Tsai.

These achievements have led Dr. Tsai to winthe 2004 Nishina Memorial Award and theAgilent Europhysics Prize.

When we asked if the development of the“quantum computer” was at hand? The answerwas “We cannot yet say so”. “We cannot createin an instant, computers such as those that we areusing today. First of all, we must enable the re-tention of the ‘entanglement’ state, free from en-vironmental effects, as long as possible. We arestill experimenting under strictly controlled cryo-genic conditions. The next hurdle is to create alarge number of qubits. With the quantum com-puter, an increase in the number of qubits canincrease the ‘entanglement’ states to the orderof the power of 2N . It is possible to express 4 val-ues with 2 qubits, 8 values with 3 qubits and, with100 qubits, it is possible to express a number thatis a million times a trillion multiplied by a tril-lion.... This can be considered equal to the com-puting capability of traditional computers. Thereason that we selected the solid device was thatwe thought that the time would come in which aquantum computer containing many qubitswould be required (Fig. 3).”“We should also be advancing more importantresearch....,” continues Dr. Tsai.“In the first place, research into the kinds of prob-lems solved by the quantum computer and howthey are solved is still inadequate. This is research

into aspects of algorithms, programs and appli-cations. Basically, the quantum computer is suit-able for dealing with problems that are hard tobe dealt with by conventional computers becauseof the need for one-by-one solutions, so algo-rithms are proposed for solving the problems offactorization and database inquiries. However,when factorization becomes a very simple prob-lem for the quantum computer, the basis of thecurrent communication encryption system basedon the combination of prime numbers will nolonger be adequate. Based on this perspective,applications in the field of information commu-nication such as ‘quantum encryption’ are con-ceived, making use of the properties of quanta.NEC is also a leader in this field (see the articleon page 254). It will be only after the weak pointsof the quantum computer are identified and thedevelopment of hardware is coupled with thestudy of software such as algorithms, that a prac-tical quantum computer will be born.”

Does this mean that it might take a hundredyears as some people commonly assert?“No, we have made considerable advancements,at least in the last five years. New knowledge andsignificant breakthroughs are also expected in thenear future. So, I guess that a practical implemen-tation will come in ten or twenty years. However,both appearance and applications may be quitedifferent from the current computers. The resultmay not for example, take the form of a general-purpose computer.”

Fig. 2 Device structure used in the experiment, demonstrating the entangled state.

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The art of laboratory leadership:“Encourage originality and adopt an

open policy.”

It is said that some of the quantum computerresearchers of the past abandoned this subjectbecause of the inherent difficulties. It is no won-der that such pessimism was aroused because thisresearch is just like trekking in a forest of intelli-gence where intuition does not work. But thereare still many people in the world who are tack-ling this difficult subject with young, optimisticminds. The laboratory of Dr. Tsai is a typical ex-ample. Outsiders like us cannot understand ev-erything based on theory, but at least we can beencouraged by the brave endeavors of research-

ers such as Dr. Tsai.The laboratory led by Dr. Tsai employs many

talented researchers, who have achieved fame viathe societies and research journals in this field.These include Dr. Tsuyoshi Yamamoto (whodoubles as a RIKEN researcher), Dr. YuriPashkin (RIKEN researcher) and Dr. OlegAstafiev (similarly employed) as well as Dr.Nakamura mentioned above. Their brilliantachievements in the field of the quantum com-puter are also the results of the teamwork of thesevariously talented researchers. We asked Dr. Tsaiabout the knack of leading such a talented teamand achieving such fruitful results.“No special method” he replied. “I leave themalone so that they are free to exhibit their origi-nalities. To promote research effectively, it is

Fig. 3 Mechanism of a logical operation on the device.

(a) The case with the control bit in the 0 state.

(b) The case with the control bit in the 1 state.

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Photo 2 Quantum bit cooling device.

important to have an environment in whichpeople can stimulate each other by exchangingideas in great variety. To be more open in search-ing for human resources may be an essential in-put for encouraging Japanese science and tech-nology in general.”

Dr. Tsai is Chinese from Taiwan. As his fatherwas a diplomat, he spent the first half of his el-ementary school life in Japan. He came to knowhis wife who is Japanese when he was attendingthe State University of New York. His words,which are quoted above are backed by his ownexperiences in living in different cultural spheresin Taiwan, Japan and the United States, and alsoby the experience that he gained twenty years agoas the first non-Japanese researcher in NEC Cor-poration. He is also interested in the world of art,and his hobby is in the architectural designof houses. His plans for which are to earn areputation for being “open, roomy and personalbut at the same time, functional and easy to use.”This seems to be rather similar to his strategy asa laboratory head. We are confident that we mayexpect much from his research results in the com-ing years.

(Interviewed/compiled by Haruhito Tsuchiya)