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