Quantum Repeaters and Memories

Transcription

1 Quantum Repeaters and Memories Nicolas Gisin and Mikael Afzelius Group of Applied Physics Geneva University, Switzerland Quantum Repeaters Quantum memories 1 click Quantum Entanglement 1

2 QKD over 307 km with real time secret key distillation and finite InGaAs key APDs analysis Integration into ATCA blades Standard telecom format B. Korzh, C. W. Lim et al., Nature Photonics 9, (2015)

3 QKD over 307 km with real time secret key distillation and finite InGaAs key APDs analysis Integration into ATCA blades A vision of a QKD engine producing 1 Gb/s of provably secret bits is on the horizon. Standard telecom format B. Korzh, C. W. Lim et al., Nature Photonics 9, (2015)

4 How far can one send a photon? Secret Key Rate Rate 10MHz P2P + WDM +4 Years 1MHz 100kHz Today Lab Today Commercial 10kHz 1kHz 1Hz Distance [km] There is a hard wall around 400 km! With the best optical fibers, perfect noise-free detectors and ideal 10 GHz single-photon sources, it would take centuries to send 1 qubit over 1000 km! 4

5 Quantum teleportation Entanglement = quantum teleportation channel 5

6 Beating the hard wall: Teleportation of entanglement Q teleportation Entanglement Entanglement Entanglement over twice the distance Entanglement between photons that never interacted Q Teleportation over long distances: Nature 421, 509 (2003) Teleportation of Entanglement at telecom wavelength: PRA 71, (2005) 6

7 Teleportation of Entanglement This sounds like magic. I don t want to base my security on magic. The fact is that it is physics and you can use it without understanding entanglement and teleporation. 7

8 Quantum Repeater - Requirements A Z Distribution of entanglement over long distances Quantum memories Teleportation of telecom 8

9 Distribution of entanglement over long distances N W E S Satigny 18.0 km Jussy δ Geneva Nature 454, 861,

10 N-photon quantum communication: quantum networks, quantum internet Q repeaters We need the capability to store entanglement: Developing quantum memories is a grand challenge! 10

11 Time to distribute an entangled pair (s) How far can one send a photon? Q repeaters with atomic ensembles and linear optics Direct transmission DLCZ, Nature (2001) Too slow Sangouard et al, PRA (2008) 11

12 Time to distribute an entangled pair (s) How far can one send a photon? Storing N modes in ONE memory using time, spatial or frequency multiplexing will reduce this time with a factor N Q repeaters with atomic ensembles and linear optics ms / Direct transmission DLCZ, Nature (2001) Sangouard et al, PRA (2008) 12

13 Quantum Repeater - Requirements A Z Distribution of entanglement over long distances Multi-mode quantum memories Teleportation of telecom C. Simon, H. de Riedmatten, M. Afzelius, N. Sangouard, H. Zbinden and N. Gisin, Phys. Rev. Lett. 98, (2007) 13

14 Quantum memory Goal: controlled and reversible mapping of a photonic quantum state onto a long lived atomic ensemble. photon in crystal doped with billions of ions photon out at desired time in same Q state The quantum state of the photon is now coded Today s in a huge efficiencies entangled states of billions of «atoms» 20 % Nature 456, 773,

15 Absorption Absorption N j 1 e ikr i t 1... e j... Δ continuous Dephasing Controlling the Dephasing! j e j g Im( P j ) g N P j e i t j Re( P j ) Frequency Δ periodic Rephasing Im( P j ) Re( P j ) Frequency m k, 0, 1, 2,... k m k Rephasing after T 1 15

16 Entanglement-preserving quantum memory H HH H VV V V (a) 100 Normalized intensity (arb. units) input Optical depth transmitted d d Optical detuning [MHz] echo 20% Bandwidth: 680 MHz Time ( s) 0

17 Teleportation from a telecom photon to a memory over 25 km 12.5 km 883 nm Analysis of teleported qubit D km Qubit to teleport D 2 D 1 D 2 C. Clausen, F. Bussières, A. Tiranov, M. Afzelius et. al., Nature Photonics 8, 775 (2014)

18 normalized counts GAP Optique Geneva University normalized counts Multi-mode storage in Nd 3+ :Y 2 SiO 5 n < 1 per mode Input modes Mapping 64 input modes onto one crystal Normalized counts1.0output modes x time modes can be used to code 32 time-bin qubits! Input mode Time ( s) Output mode x time [ s] Input mode Output mode x time [ s] I. Usmani et al., Nature Communications 1,12 (2010) 18

19 Quantum memory - dream and reality Property Desired performance Efficiency State-of-the-art (quantum & classical state storage) Fidelity* Multi-mode storage capacity high Pulse duration ns ~100 ps Storage time > sec >40 sec Universal entanglementpreserving Complexity simple 64 (1000) modes atomic vapor & RE-crystal Different storage media and protocols (for qubits) * post selected Hedges et al, Nature (2010); Hosseini et al, Nature Phys. (2011), Usmani et al, Nature Comm (2010), Saglamyurek, WT et al, Nature (2011), Heinze et al, Phys Rev Lett. (2013), Jin et al, quant-ph (2010), Clausen et al, Nature (2011), Rempe et al., Nature (2011), Lvovsky, Sanders, WT, Nature Phot. (2010) 19

20 Conclusions Direct Quantum Key Distribution over telecom optical fibers is ultimately limited to about km. Quantum Repeaters require - long distance distribution of entanglement, - multi-mode quantum memories - Entanglement swapping at telecom. Towards a multimode quantum memory Ensembles of atoms or ions provide a promising light-matter quantum interface at the single photon level with excellent (conditional) fidelity and multimode capability. Second-long quantum memories are on the horizon (a few years)! Then it will only be a matter of (non-trivial) engineering. 20

21 REPEATER RELAY Longer distances Q relays, repeater & memory * Bell measurement.. * entanglement entanglement * Bell measurement entanglement * *?? memory Bell measurement. QND measurement + Q memory J. D. Franson et al, PRA 66,052307(2002); D. Collins et al., J.Mod.Opt. 52,735,2005 H. Briegel, W. Dür, J. I. Cirac and P. Zoller, Phys. Rev. Lett. 81, 5932 (1998) Teleportation QND measurement 21

22 Bit rate of the 1 st transatlantic telegram How long did it take to transmit this congratulation in 1858? "The Queen desires to congratulate the President upon the successful completion of this great international work, in which the Queen has taken the deepest interest. The Queen is convinced that the President will join with her in fervently hoping that the electric cable, which now connects Great Britain with the United States, will prove an additional link between the two places whose friendship is founded upon their common interests and reciprocal esteem. The Queen has much pleasure in thus directly communicating with the President, and in renewing to him her best wishes for the prosperity of the United States." 17 hours! (1 letter took 2 minutes)

23 Entanglement-preserving quantum memory CHSH = 2.52±0.13 nonlocal quantum correlations

24 Coherent spin control for long-lived quantum memories Counts/min 50 Input / +T S // x5 Output 5% Time ( s) Memory efficiency (%) 10 Mean photon number in input Optical-to-spin conversion : 50% 4 2 Mean spin excitation 1 stored among Eu ions Spin-wave storage time T S (ms) High SNR=10 2 in output!

25 Cavity-enhanced AFC quantum memories Experiment by the group of Stefan Kröll (Lund, Sweden) Mirror coatings on crystal surfaces Theory: M. Afzelius and C. Simon, PRA 82, (2010) 56% efficiency (<1% efficiency without cavity) M. Sabooni et al., PRL 110, (2013)