Subject B : Entanglement purification between distant nodes

158B-T02Quantum repeater based on optical-pulse-controlled quantum dot spins and single photons

Stanford University / National Information of Informatics
(Yoshihisa Yamamoto / Shoko Utsunomiya)

The two essential components of a quantum repeater system are (i) photons transmitting information between distant nodes and (ii) stationary quantum memories at the respective nodes. The latter requires both long coherence time and efficient interfacing to optical photons, for which single spins in optically bright quantum dots based on III-V semiconductors are promising candidates. Using ultrafast optical pulses and high magnetic fields, we are able to control, coherently and completely, a single quantum dot spin within a few tens of picoseconds. Further, the use of spin echo technique allows for hundreds of thousands of gate operations within the coherence time. With these abilities in hand, our project aims to (i) demonstrate entanglement between a single quantum dot spin and a single photon, and (ii) generate entanglement between a single spin and single photon in distant nodes, heralded by the coincidence detection of two indistinguishable single photons emitted from the respective nodes. Realization of the spin-photon entanglements will be major steps on the way toward a terrestrial and satellite-based quantum repeater.

Along with these experimental efforts, we carry out theoretical study on (i) methods for implementing quantum logic gates and projective measurements in a quantum dot system, (ii) protocols for high-speed and efficient entanglement generation between distant nodes using an intermediate EPR photon-pair source and (iii) designs for fault tolerant quantum repeater/computer architecture. In all cases we take realistic situations such as errors in the quantum memories and optical losses in the communication channel into account so that our theoretical analysis can be immediately reflected in actual implementation.

The following subthemes are pursued.

Subtheme 1: Coherent control of quantum dot spins

Benchmark the fidelities of initialization and single-qubit operation, develop techniques to reduce decoherence, and propose schemes for multiple-qubit operation

Subtheme 2: Wavelength-conversion technology

Develop technology for low-loss and high-efficiency single-photon wavelength-conversion from 0.9 um (wavelength of optical photons from quantum dots) to 1.5 um (compatible with low-loss optical fiber channel) using a periodically-poled lithium niobate (PPLN) waveguide device

Subtheme 3: Spin-photon entanglement

Demonstrate high-fidelity entanglement between a single quantum dot spin and a single optical photon

Subtheme 4: Projective measurement of quantum dot spins

Develop a technique for multi- to single-shot quantum non-demolition (QND) readout of a spin state, using an exciton-polariton resonance.

Subtheme 5: Entanglement between distant nodes

Demonstrate entanglement between a single spin and single photon in distant nodes, heralded by the coincidence detection of two indistinguishable single photons emitted from the distant nodes.

The relation among the subthemes are as follows; The results from subthemes 1 and 2 will be directly used to demonstrate the mid-term goal (subtheme 3). Then they will be combined with the result from subtheme 4 in order to achieve the project goal (subtheme 5).

Interim results for Team 158B-T02

Task title	Outcome	Date	Note
1: Coherent control of quantum dot spins	(i)Coherent control of quantum dot hole spins. (ii)Schemes for two-qubit logic gates for quantum dot spins are proposed. (iii)A design for layered quantum architecture is proposed.(iv)Coherent control of donor-bound spins.	Aug. 2011, Dec. 2011, June 2012, July 2012	(i)Nature Physics 7, 872(2011)(ii)Phys. Rev. B 84, 235307 (2011); Phys. Rev. B 85 241403 (2012);(iii)Phys. Rev. X 2, 031007 (2012);(iv)Nano Lett. 13, 116(2013)
2: Wavelength-conversion technology	Downconversion quantum interface for a quantum dot spin and low-loss optical channel is realized.	Dec. 2012, Feb. 2012	Opt. Exp. 20, 27510 (2012);Opt. Lett. 37, 476(2012)
3: Spin-photon entanglement	High-fidelity spin-photon entanglement is generated.	Nov. 2012, Jul. 2013, Sept. 2013	Nature 491, 421 (2012) Nat. Commun. 4, 2228 (2013) Rep. Prog. Phys. 76, 092501(2013)

Final results for Team 158B-T02

Task title	Outcome	Date	Note
4: Projective measurement of a single spin	Exciton-polariton meditated projective measurement scheme	June 2012, May 2013, Oct. 2014	Phys. Rev. B85, 241, 403(R)(2012) Nature 497,348(2013) Phys. Rev. B90, 155421(2014)
5: Entanglement between distant nodes	Develop a technique for entanglement swapping between spins and photons	July 2012 Nov. 2015	Nano Lett. 12, 4611(2012) Nature Commun. 6, 8955(2015)