Towards a quantum internet thanks to teleportation

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

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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