Cryptography: a computer scientist and a quantum physicist win the Turing Award

Charles H. Ben­nett and Gilles Brass­ard have been awar­ded the A.M. Tur­ing Award “for their essen­tial role in estab­lish­ing the found­a­tions of quantum inform­a­tion sci­ence and trans­form­ing secure com­mu­nic­a­tions and com­put­ing.” Gilles Brass­ard is a com­puter sci­ent­ist at the Uni­ver­sity of Montreal in Canada, and Charles Ben­nett a phys­i­cist at IBM Research in York­town Heights, New York, in the United States.

The A.M. Tur­ing Award, often referred to as the “Nobel Prize of com­put­ing,” is awar­ded by the Asso­ci­ation for Com­put­ing Machinery¹ (ACM) and is worth one mil­lion dol­lars. It is named after Alan Tur­ing, the Brit­ish math­em­atician who laid the math­em­at­ic­al found­a­tions of com­puter sci­ence, and is sponsored by Google.

Many of us use clas­sic­al cryp­to­graphy in our daily lives — for example, to trans­fer sens­it­ive inform­a­tion such as bank details — and it has become cru­cial for mod­ern com­puter and com­mu­nic­a­tion net­works. The inform­a­tion being sent is kept secret thanks to an encryp­tion algorithm com­bined with a “key” that the sender uses to scramble the mes­sage into a form that is incom­pre­hens­ible to an eaves­drop­per. The receiv­er of the mes­sage then uses the same key with a decryp­tion algorithm to read it. How­ever, this stand­ard encryp­tion has one major draw­back: both parties must know the key, and it must be estab­lished securely.

This is where quantum cryp­to­graphy, or quantum key estab­lish­ment, comes in. This Quantum Key Dis­tri­bu­tion, or QKD as it is known, is a mis­nomer accord­ing to the invent­or Gilles Brass­ard him­self, who believes that the term should be Quantum Key Estab­lish­ment. This pro­cess exploits the quantum prop­er­ties of particles and enables secret keys to be estab­lished using stand­ard com­mu­nic­a­tion fibres (or even satel­lites). These keys are then used in con­ven­tion­al encryp­tion pro­cesses. QKD is inher­ently secure and allows keys to be changed fre­quently, mak­ing crypt­ana­lys­is much more dif­fi­cult, or even math­em­at­ic­ally impossible if the keys obtained via QKD are as long as the mes­sages to be transmitted.

It was in 1984 that Charles Ben­nett and Gilles Brass­ard pub­lished the first meth­od for estab­lish­ing secret keys encoded in such quantum states. In their now fam­ous “BB84²” pro­tocol, they rep­res­en­ted a bit of inform­a­tion by the polar­isa­tion state of a single photon — “0” when the photon was polar­ised hori­zont­ally or at 45°, for example, and “1” when it was polar­ised ver­tic­ally or at −45°. In this pro­cess, the sender trans­mits a string of polar­ised single photons to the receiv­er. By then car­ry­ing out a series of meas­ure­ments and, fol­low­ing a pub­lic dis­cus­sion, they are able to estab­lish a shared key and veri­fy wheth­er any inform­a­tion has been inter­cep­ted by a mali­cious third party along the way.

Charles Ben­nett and Gilles Brass­ard drew on the work of the late phys­i­cist Steph­en Wies­ner car­ried out in the 1960s³. Wies­ner had real­ised that the quantum nature of particles such as photons — which had pre­vi­ously been regarded as a poten­tial obstacle to applic­a­tions — could in fact be put to good use.

Their next break­through came in 1993, when they intro­duced the concept of quantum tele­port­a­tion⁴ in col­lab­or­a­tion with oth­er col­leagues, includ­ing anoth­er Que­beck­er, Claude Crépeau. This is a way of trans­fer­ring a quantum state from one entity to anoth­er without actu­ally send­ing a particle in that state.

Quantum tele­port­a­tion is a fun­da­ment­al build­ing block of quantum com­put­ing and quantum com­mu­nic­a­tion and is based on quantum entan­gle­ment. This appar­ent (but not real) “spooky action at a dis­tance”⁵, to quote Albert Ein­stein him­self, allows two particles that have inter­ac­ted — one owned by a sender and the oth­er by a receiv­er — to share a quantum state and thus remain linked in a way that is impossible in clas­sic­al phys­ics. It holds no mat­ter how far the particles are from each oth­er. Entan­gle­ment can there­fore be used as a way to trans­port quantum inform­a­tion from the sender to the receiver.

This pro­cess bears the catchy name of tele­port­a­tion because the state of the particle at the sender is des­troyed by the meas­ure­ment pro­cess before it can appear intact at the receiver.

In more detail, quantum tele­port­a­tion begins when the sender and the receiv­er share a pair of entangled particles (for example, photons). The sender then inter­acts their half of the entangled pair with a third particle whose state is unknown. They then meas­ure the out­come of this inter­ac­tion and com­mu­nic­ate it to the receiv­er via a clas­sic­al chan­nel. Armed with this inform­a­tion, the receiv­er can then recon­struct the state of the particle with the unknown state that has been tele­por­ted. This pro­cess bears the catchy name of tele­port­a­tion because the state of the particle at the sender is des­troyed by the meas­ure­ment pro­cess before it can appear intact at the receiver.

Quantum tele­port­a­tion was demon­strated exper­i­ment­ally for the first time in 1997 when research­ers suc­ceeded in tele­port­ing the polar­isa­tion of a photon. Since then, they have man­aged to tele­port the spin states of atoms, nuc­lei and trapped ions, to name just three examples. In 2022, a phys­i­cist was awar­ded the Nobel Prize in Phys­ics, among oth­er things, for his exper­i­ment­al demon­stra­tion of this process.

Quantum tele­port­a­tion has allowed for the devel­op­ment of secure com­mu­nic­a­tion sys­tems and in the future will be used to trans­fer inform­a­tion with­in a quantum com­puter as well as to trans­mit data from one device to anoth­er. The ulti­mate goal is to build on the work of Charles Ben­nett and Gilles Brass­ard to con­struct quantum net­works and a quantum inter­net cap­able of trans­mit­ting quantum inform­a­tion between cit­ies and bey­ond, much like today’s inter­net, which is used to trans­mit clas­sic­al information.

As well as quantum cryp­to­graphy and com­mu­nic­a­tion, their work has also had an impact on algorithm design, the devel­op­ment of fault-tol­er­ant quantum com­puters and math­em­at­ic­al phys­ics. “Ben­nett and Brass­ard fun­da­ment­ally changed our under­stand­ing of inform­a­tion itself,” said ACM Pres­id­ent Yan­nis Ioan­nid­is in an ACM press release⁶. “Their insights expan­ded the bound­ar­ies of com­put­ing and set in motion dec­ades of dis­cov­ery across dis­cip­lines. The glob­al momentum behind quantum tech­no­lo­gies today under­scores the endur­ing import­ance of their con­tri­bu­tions.”

“Charles Ben­nett and Gilles Brassard’s vis­ion­ary insights laid the ground­work for one of the most excit­ing fron­ti­ers in sci­ence and tech­no­logy,” adds Jeff Dean, Chief Sci­ent­ist at Google Deep­Mind and Google Research, in the same press release. “Their work con­tin­ues to influ­ence both fun­da­ment­al research and real-world innov­a­tion. Google is proud to sup­port the ACM A.M. Tur­ing Award and hon­our the pion­eers shap­ing the future of com­put­ing.”

Isabelle Dumé