Towards a quantum internet thanks to teleportation

This article is part of our spe­cial issue « Quan­tum : the second revo­lu­tion unfolds ». Read it here

Resear­chers at QuTe­ch’s Ronald Han­son Lab (a col­la­bo­ra­tion bet­ween Delft Uni­ver­si­ty of Tech­no­lo­gy in the Nether­lands and TNO) are wor­king on the trans­mis­sion of quan­tum infor­ma­tion using quan­tum bits (qubits) in dia­mond. They recent­ly demons­tra­ted that they could trans­fer this infor­ma­tion bet­ween two non-direct­ly connec­ted nodes by quan­tum tele­por­ta­tion – a first. Ulti­ma­te­ly, this type of tele­por­ta­tion could be used to create a quan­tum inter­net, as it is robust and “unbrea­kable”.

Quan­tum tele­por­ta­tion often makes us think of Star Trek. While tele­por­ta­tion is not pos­sible for objects such as human beings, it is pos­sible for quan­tum states enco­ded on par­ticles that behave accor­ding to quan­tum mecha­nics. The pro­cess does not involve any phy­si­cal trans­fer of mat­ter, but the ins­tan­ta­neous trans­fer of a quan­tum state bet­ween par­ticles sepa­ra­ted by an immense dis­tance. It is era­sed on the sen­der’s site and appears imme­dia­te­ly on the reci­pient’s site.

Spoo­ky action at a distance

The basic idea behind tele­por­ta­tion is that two net­work nodes, tra­di­tio­nal­ly cal­led Alice and Bob, share a pair of entan­gled par­ticles (in quan­tum cryp­to­gra­phy, Alice is the sen­der of a mes­sage and Bob the recei­ver). Entan­gled par­ticles are those that remain lin­ked in a way that is impos­sible in clas­si­cal phy­sics, no mat­ter how far apart they are. Albert Ein­stein cal­led this effect “spoo­ky action at a dis­tance”. Alice then inter­acts with a third par­ticle – in an unk­nown state – with her half of the entan­gled pair, mea­sures the out­come of the inter­ac­tion and informs Bob of the result via a clas­si­cal chan­nel. Armed with this infor­ma­tion and a mea­su­re­ment of his half of the entan­gled pair, Bob can recons­truct the ori­gi­nal unk­nown state, which is the one that was teleported.

Tele­por­ta­tion was first pro­po­sed theo­re­ti­cal­ly in 1993 and demons­tra­ted expe­ri­men­tal­ly for the first time in 1997 with the tele­por­ta­tion of the pola­ri­sa­tion of a pho­ton. Since then, seve­ral teams of resear­chers have tele­por­ted the states of ato­mic spins, nuclear spins and trap­ped ions, to cite but three examples. Resear­chers have also suc­cee­ded in tele­por­ting “two degrees of free­dom” – spin and orbi­tal angu­lar momen­tum – bet­ween indi­vi­dual photons.

Ronald Han­son and his col­leagues recent­ly pro­du­ced the first ever three-node quan­tum net­work using “nitro­gen vacan­cy centres” (NVs) in dia­mond as qubits. Nitro­gen vacan­cy centres are defects in the lat­tice of car­bon atoms in the mate­rial in which a nitro­gen atom has sub­sti­tu­ted for a car­bon atom. Each node contains a com­mu­ni­ca­tion qubit and one node also contains a memo­ry qubit that can store the quan­tum infor­ma­tion in the node.

To tele­port quan­tum infor­ma­tion from a sen­der to a recei­ver, their res­pec­tive qubits need to be entan­gled. When a “Bell state mea­su­re­ment” is per­for­med on the sen­der’s qubit, its quan­tum state is tele­por­ted, that is, it disap­pears from the sen­der’s node and appears at the recei­ver’s node. This quan­tum state, which arrives in encryp­ted form, can then be decryp­ted using the result of the Bell state mea­su­re­ment, that is, by sen­ding it to the recei­ver via a conven­tio­nal chan­nel, such as an opti­cal fibre.

Until now, this pro­cess had only been demons­tra­ted for two adja­cent net­work points, Alice and Bob. Adding a third point (cal­led Char­lie) is not easy, as the entan­gle­ment bet­ween Alice and Char­lie must be crea­ted via Bob. The entan­gle­ment must also be of a high fide­li­ty for tele­por­ta­tion to succeed.

Ronald Han­son and his col­leagues achie­ved this by ins­tal­ling addi­tio­nal detec­tors that bet­ter iden­ti­fy “false” signals from unwan­ted pho­tons emit­ted in their sys­tem. They have also impro­ved the memo­ry used to store infor­ma­tion by pro­tec­ting the memo­ry qubit from inter­ac­tions with the com­mu­ni­ca­tion qubit and the crys­tal­line envi­ron­ment. These inter­ac­tions cause a phe­no­me­non known as deco­he­rence that makes the qubit lose the quan­tum infor­ma­tion it contains. Final­ly, they impro­ved qubit memo­ry-rea­dout by fil­te­ring out ‘bad’ rea­douts in real time, which ulti­ma­te­ly increases fidelity.

All these mea­sures allow them to tele­port quan­tum infor­ma­tion bet­ween the non-adja­cent Alice and Char­lie nodes. To do this, they first entangle Ali­ce’s and Char­lie’s qubits via Bob’s qubit. Char­lie then stores part of the entan­gled states at his memo­ry qubit and pre­pares the quan­tum state to be tele­por­ted on his com­mu­ni­ca­tion qubit. Applying the Bell state mea­su­re­ment at Char­lie tele­ports the state to Alice.

The resear­chers are cur­rent­ly wor­king on increa­sing the num­ber of memo­ry qubits, which will allow for more com­plex pro­to­cols to be exe­cu­ted. They are also consi­de­ring inte­gra­ting conven­tio­nal opti­cal fibres into their expe­riment. This would help take the tech­no­lo­gy out of the labo­ra­to­ry and into net­works alrea­dy in use in the real world. Final­ly, the deve­lop­ment of a quan­tum net­work “control stack”, simi­lar to that used in today’s inter­net, will also be neces­sa­ry for a future func­tio­nal quan­tum internet.

Interview by Isabelle Dumé

Refe­rences :

The Ronald Han­son Lab

TNO

Publi­ca­tion dans Nature