By Artur Ekert

ACCORDING to a popular story,when the Mongols ruled China inthe 14th century, the success of anuprising against them hinged on se-cret messages hidden in moon-cakes.

The successful revolution de-pended on keeping the method ofcommunication secret.

Not such a good idea. No mat-ter how creative we are in hidingmessages, our tricks will be foundout sooner or later.

Can we reveal our methods ofsecret communication withoutcompromising secrecy? Yes. Mod-ern methods of secret communica-tion can be described as sendingmessages in locked boxes.

The sender locks his message ina box and sends it to his ally, whounlocks the box and reads it.

Everybody knows how the box-es are constructed but only thesender and recipient hold thekeys. If an eavesdropper got holdof the key, he could open the box-es in transit and read the messagesundetected. The legitimate usersknow about this risk, and try tochange their keys frequently be-cause the security relies entirelyon keeping them secret.

Here, boxes are mathematicalmethods of disguising messages.Locking a box is called encryp-tion, and real cryptographic keysare secret sequences of numbersused by senders and recipients to

encrypt and decrypt their messag-es. Ultimate security is possible,but only if each message is en-crypted with its own unique keythat is truly random and never re-used. This method is known as theone-time pad and requires asmany keys as messages.

The snag is, users may be milesapart, so how do they exchangekeys? The problem of getting thekey from sender to recipient with-out an eavesdropper interceptingit – the “key distribution prob-lem” – has become an expensivelogistical issue for banks, govern-ments and the military.

If the same key is used for toolong, encryption can be broken inmany ingenious ways.

Physicists relate the key distri-bution problem to eavesdroppingabilities. Suppose an eavesdrop-per is tapping a telephone lineused for key distribution. Anymeasurement on the signal in theline may disturb it and leave trac-es. However, any eavesdropperwith superior technology can es-cape detection.

The way around this probleminvolves quantum physics – phys-ics that describes the world of ele-mentary particles such as photonsand atoms.

If, for example, some quantumproperties of photons are used tocarry the information, then, ac-cording to quantum principles,one cannot pick up all the encod-ed information and go undetec-ted. No matter how complex and

subtle the advanced technologyand computing power available tothe eavesdropper, the “quantumnoise” caused will expose each at-tempt to gain even partial informa-tion about the key. This is the ba-sic idea behind quantum key distri-bution.

One approach, which I pro-posed in 1991 during my studentdays in Oxford, involves entangle-ment, a quantum property whichimplies that two separated objectsbehave as if they were two partsof the same entity.

That this behaviour has not

been “pre-programmed” is one ofthe most surprising and profoundthings we have learnt about thephysical reality in the past few dec-ades.

This is also what protects theentanglement-based key distribu-tion. With no pre-determined out-come of a measurement, no eaves-dropper may know the outcome.

Convincing the world ofacademia was easy, but it took a

bit longer for quantum entangle-ment to enter the banking sector.In 2004, Bank Austria Creditan-stalt finally put some trust intoquantum theory by allowing Aus-trian researchers to carry out thefirst demonstration of a banktransfer protected by entangledphotons.

Singapore is not lagging be-hind; on the contrary, many of themost advanced quantum experi-ments have been performed here,on our small “Quantum Island”.

Research in quantum technolo-gy here goes back to 1998, when

local researchers Lai Choy Heng,Oh Choo Hiap, Kwek LeongChuan, Kuldip Singh and, subse-quently, Dagomir Kaszlikowski,decided to meet after hours to dis-cuss weird quantum phenomenaand their possible applications incomputing.

They formed a vibrant group,which I remember fondly frommy first visits to Singapore in2000.

A few years later, support fromA*Star’s consolidated research ef-forts in the field and the NationalUniversity of Singapore faculty ap-pointments of professors Berge En-glert, Christian Kurtsiefer, and An-tia Lamas Linares strengthenedthe group. The last two deserve aspecial mention – over the pastfour years they managed to putSingapore at the frontiers of exper-imental research in quantum tech-nologies.

It was their team of local youngresearchers that developed excel-lent sources of entangled photonsand demonstrated very stablefree-space quantum key distribu-tion between buildings on theNUS campus. These and many oth-er exciting experiments are nowcarried out at the Centre for Quan-tum Technologies (CQT), which isthe first Research Centre of Excel-lence here, funded jointly by theNational Research Foundationand Ministry of Education, andhosted by NUS.

One truly remarkable featureof the entanglement-based key dis-tribution is that there is no needfor sender or recipient to controlthe source of entangled photons.

It does not have to be protect-ed at all.

One can imagine sources of en-tangled photons in public placessuch as rooftops, to which anytwo parties willing to establishcryptographic keys can just tunein.

Indeed, a team of European re-searchers, with the European

Space Agency, is working on thepossibility of distributing quan-tum keys worldwide by satellite.

An even more puzzling featureof entanglement-based quantumcryptography was recently demon-strated here. CQT’s Valerio Scara-ni, together with European col-leagues, showed that the entangle-ment-based key distributionscheme is far more general andmuch more powerful than I origi-nally anticipated; it works evenwith devices of dubious prove-nance.

This means that you and yourfriend may purchase cryptograph-ic equipment from any company,even from your competitors or en-emies, and still be able to establisha secret key.

This amazing feat has just beendemonstrated by CQT’s quantumoptics group. The experimentopens a new chapter of moderncryptography.

Quantum information technolo-gy is a fundamentally new way ofharnessing nature. It is too earlyto say how important this willeventually be, but we can reasona-bly speculate about its impact ondata security – the future of se-cure military and commercial com-munication will probably go quan-tum.The writer is the director of the Centrefor Quantum Technologies at theNational University of Singapore, thefirst Research Centre of Excellencehere. He is one of the inventors ofquantum cryptography and has made anumber of contributions to quantuminformation science.

PHOTO: ISTOCKPHOTO

ULTIMATE SECURITY?: Entangled photons may be harnessed to keep information secure – no matter how complex and subtle the advanced technology and computing power available to the eavesdropper.

FOILING EAVESDROPPERS

Any eavesdropper with superior technology canescape detection. The way around this probleminvolves quantum physics – physics thatdescribes the world of elementary particlessuch as photons and atoms.

Use of quantum properties ensures unauthorised attemptsto access encoded information do not go undetected

The unbreakable code: Is this the lock?

S11 SCIENCE THE STRAITS TIMES SATURDAY, JULY 26 2008