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Can we make solid state quantum networks?


The secure exchanging of cryptographic keys over fibre [1] or free space [2] is now approaching a commercial reality through advanced quantum cryptography systems. One key limitation to all present quantum key distribution systems is the finite range of a single quantum link and the inability to amplify the fading signal by classical means. The quantum repeater [3] has been suggested as a possible solution to this exponential decrease of bit rate with distance. Ideal repeater schemes extend the distance using “entanglement swapping” and “teleportation” and by concatenating short entanglement swapping sub-sections it is in principle possible to generate entangled (correlated) bits over very long distances with bit rate only limited by the losses in one short section. If realised this would extend quantum key distribution out to distances of thousands of kilometres.  Each sub-section is linked to the next by an optical circuit which performs a ‘Bell’ measurement between photons arriving from each direction. Proof of principle experiments [4] carried out to date have been limited to using quantum interference effects at a beamsplitter to perform a limited Bell measurement with 25% success rate when photons arrive simultaneously at the beamsplitter. These quantum ‘relays’ are extremely inefficient and cannot extend the range in practical system.

In this talk we will introduce the concept of a spin photon interface [5] where an incoming photon leaves its information in a spin memory until the next photon arrives whereupon the Bell measurement can be carried out with in principle 100% success probability thus making a near ideal quantum repeater [6]. This leads us to propose an all solid state quantum network where the sources of single photons and entangled photon pairs are realised by quantum dot emission into a microcavity while the repeater itself is a quantum dot strongly coupled to a cavity system.

I will go on to describe early experiments using high-Q micro-pillar cavitites containing single quantum dots where we are beginning to prove the feasibility of this concept [7].

[1] N. Gisin, et al, Rev. Mod. Phys. 74 , 145 (2002).

[2] R. Ursin, et al Nature Physics 3 , 481 - 486, (2007).

[3] H.-J. Briegel et al, Phys. Rev. Lett. 81 , 5932  (1998).

[4] H. de Riedmatten et al, Phys. Rev. Lett. 92 , 047904 (2004).

[5] C.Y. Hu, et al ., Phys. Rev. B 78 , 085307  (2008).

[6] C.Y. Hu and J. G. Rarity, Phys. Rev. B 83 , 115303 (2011).

[7] A. Young et al Phys. Rev. A 78 , 125318 (2011).