Monday dawns. Headlines report that âStar Wars: Episode VIIâ has earned more money, during its opening weekend, than I hope to earn in my lifetime. Trading the newspaper for my laptop, I learn that a friend has discovered ThinkGeekâs BB-8 plushie.Â âI want one!â she exclaims in a Facebook post. âBecause BB-8 definitely needs to be hugged.â
BB-8 plays sidekick to Star Wars hero Poe Dameron. The droid has a spherical body covered with metallic panels and lights.Mr. Gadget and Frosty the Snowman could have spawned such offspring. BB-8 has captured viewersâ hearts, and its chirps have captured cell-phone ringtones.
Still, I scratch my head over my friendâs Facebook post. Hugged? Why would she hugâ¦
Iâve mentally verbalized âBB-8â as âBB84.â BB84 denotes an application of quantum theory to cryptography. Cryptographers safeguard information from eavesdroppers and tampering. Iâve been thinking that my friend wants to hug a safety protocol.
Charles Bennett and Gilles Brassard invented BB84Â in 1984.Â Imagine wanting to tell someone a secret. Suppose I wish to coordinate, with a classmate, the purchase of a BB-8 plushie for our friend the droid-hugger. Suppose that the classmate and I can communicate only via a public channel on which the droid-hugger eavesdrops.
Cryptographers advise me to send my classmate a key. A keyÂ is a random string of letters, such as CCCAAACCABACA. Iâll encode my message with the string, with which my classmate will decode the message.
I have to transmit the key via the public channel. But the droid-hugger eavesdrops on the public channel. Havenât we taken one step forward and one step back? Why would the key secure our information?
Because quantum-information science enables me to to transmit the key without the droid-huggerâs obtaining it. I wonât transmit random letters; Iâll transmit quantum states. That is, Iâll transmit physical systems, such as photons (particles of light), whose properties encode quantum information.
A nonquantum letter has a value, such as A or B or C.Â Each letter has one and only one value, regardless of whether anyone knows what value the letter has. You can learn the value by measuring (looking at) the letter. We canât necessarily associate such a value with a quantum state. Imagine my classmate measuring a state I send. Which value the measurement device outputs depends on chance and on how my classmate looks at the state.
If the droid-hugger intercepts and measures the state, sheâll change it. My classmate and I will notice such changes. Weâll scrap our key and repeat the BB84 protocol until the droid-huggerÂ quits eavesdropping.
BB84 launched quantum cryptography, the safeguarding of information with quantum physics. Todayâs quantum cryptographers rely on BB84 as you rely, when planning a holiday feast, on a carrot-cake recipe that passed your brother’s taste test on his birthday. Quantum cryptographers construct protocols dependent onÂ lines like âThe message sender and receiver are assumed to share a key distributed, e.g., via the BB84 protocol.â
BB84 has become a primitive task, a solved problem whose results we invoke in more-complicatedÂ problems. Other quantum-information primitives include (warning: jargon ahead) entanglement distillation, entanglement dilution,Â quantum data compression,Â and quantum-state merging.Â Quantum-information scientists solved many primitive problems during the 1990s and early 2000s. You can apply those primitives, even if youâve forgotten how to prove them.
Those primitives appear to darken quantum informationâs horizons. The spring before I started my PhD, an older physicist asked me why I was specializing in quantum information theory. Havenât all the problems been solved? he asked. Isnât quantum information theory âdeadâ?
Imagine discovering how to power plasma blades with kyber crystals. Would you declare, âProblem solvedâ and relegate your blades to the attic? Or would you apply your tool to defending freedom?
Primitive quantum-information tools are unknotting problems throughout physicsâin computer science; chemistry; optics (the study of light); thermodynamics (the study of work, heat, and efficiency); and string theory. My advisor has tracked how uses of âentanglement,â a quantum-information term, have swelled in high-energy-physics papers.
A colleague of that older physicist views quantum information theory as a toolkit, a perspective, a lens through which to view science. During the 1700s, the calculus invented by Isaac Newton and Gottfried Leibniz revolutionized physics. Emmy Noether (1882â1935) recast physics in terms of symmetries and conservation laws. (If the forces acting on a system donât change in time, for example, the system doesnât gain or lose energy. A constant force is invariant under, or symmetric with respect to, the progression of time. This symmetry implies that the systemâs energy is conserved.) We can cast physics instead (jargon ahead) in terms of the minimization of a free energy or an action.
Quantum information theory, this physicist predicted, will revolutionize physics as calculus, symmetries, conservation, and free energy have. Quantum-information tools such as entropies, entanglement, and qubits will bleed into subfields of physics as Lucasfilm has bled into the fanfiction, LEGO, and Halloween-costume markets.
BB84, and the solution of other primitives, have not killed quantum information. Theyâve empowered it to spreadâthankfully, to this early-career quantum information scientist. Never mind BB-8; Iâd rather hug BB84. Perhaps I shall. Engineers have realized technologies that debuted on Star Trek;Â quantum mechanics has secured keyÂ sharing; bakers have crafted cakes shaped like the Internet; and a droidâs popularity rivals R2D2âs. Maybe next Monday will bring a BB84 plushie.