Journal Club Presentation Quantum Information Science: Indistinguishable Photons from Separated Silicon-Vacancy Centers in Diamond [1]

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Diamond [1] Silvia Song Soorya Suresh Stella Sun University of Illinois

1 . Journal Club Presentation Quantum Information Science: Indistinguishable Photons from Separated Silicon-Vacancy Centers in Diamond [1] Silvia Song Soorya Suresh Stella Sun University of Illinois Urbana-Champaign 21 st November 2014 [1]A. Sipahigil et.al., Phys. Rev. Lett. 113, (2014).

2 Quantum computer and qubits Quantum computer is superior by using quantum bit (qubit) instead of classical bit Qubit: A memory element. It can hold not only the states '0' and '1' but a linear superposition of both states, a 1> + b 0>

3 Qubits carry more information Information stored in state coefficients of a qubit: For one qubit: >= a 1> +b 0> For two qubits: In order to present interference (i.e. entanglement) terms in these states, qubits must be indistinguishable, otherwise qubits become their classical kin

4 Power of Quantum Information N qubits can store bits of information when classical bits can store N bits only Qubits can be realized using many different methods: Electron spins with two eigenstates And etc.

5 Qubits using Nitrogen-Vacancies in diamond Vacancy centers in diamond is an example of using two electron spin states as the qubit states Further criteria [2] : they have efficient photoemission they can be made routinely on demand Therefore, the most useful ones [2] : Nitrogen- Vacancy (NV) Silicon-Vacancy (SiV) [2] A.D. Greentree, Physics(Viewpoint) 7,93 (2014)

6 Nitrogen-Vacancy Vs. Silicon-Vacancy NV [2,3, 4] SiV [1,2] manipulations of the spin states optical photon emission~4% manipulations of the spin states optical photon emission ~70% Indistinguishable photons ~ 74% (of optical photons) Spectral stability: over 4 hrs Table 1. Silicon vacancy can be a solution [1]! [3] A. Faraon, C. Santori, Z. Huang, V. M. Acosta, & R. G. Beausoleil,Phys. Rev. Lett. 109, (2012). [4] M. W. Doherty, N. B. Manson, P. Delaney, F. Jelezko, J. Wrachtrup, and L. C. L. Hollenberg, Phys. Rep. 528, 1 (2013).

7 Structure of SiV Center in the diamond Carbon atoms A split vacancy: Two empty lattice sites diamond bonds dangling bonds Figure 1. Negatively charged SiV center in diamond [1]

Energy Transitions of SiV Center in the diamond Two parity states (a qubit) of a SiV center Phonon Side Band (PSB): between the doublet in the excited states Zero Phonon Lines (ZPL): A, B, C, D

8 Energy Transitions of SiV Center in the diamond Two parity states (a qubit) of a SiV center Phonon Side Band (PSB): between the doublet in the excited states Zero Phonon Lines (ZPL): A, B, C, D transitions Figure 2. Energy transitions for a SiV [1]. Spin-orbit interaction accounts for the energy gap between the degenerate states. This is an optical transition producing the optical photons (~70%) Nitrogen has only 4% We want to find: Are optical photons indistinguishable (except position)?

Emission spectrum for ZPLs of a single SiV center [1] Use solid etalon to

9 ZPL spectrum for a single SiV center Off-resonant excitation: 532nm Temperature: 4.5K Figure 3. Emission spectrum for ZPLs of a single SiV center [1] Use solid etalon to filter out transitions A, B, D to have a single two-level transition Figure 4. Spectrum before(brown) and after(blue) using etalon filter [1]

10 Hong-Ou-Mandel Interference Experiment Goal: to test whether the two input photons are indistinguishable Photon 1 Output port 1 Photon 2 Beam splitter 50:50 Output port 2 Result 1: perfect photon bunching vanishing probability of detecting coincident photons Indistinguishable Result 2: otherwise, distinguishable

11 HOM Interference Experiment-method Optical construction of Mirrors Figure 5. HOM setup [1] Photoemission from SiV center I Photoemission from SiV center II During this process, the spectrum was found to be stable for over four hours of data acquisition

12 HOM Interference Experiment-results Evidence of indistinguishability Figure 6. Second order intensity correlation function for two different polarizations ~ ~ Pink data (blue fitted curve): photons with same polarizations Green data (brown fitted curve): photons with orthogonal polarizations

13 Authors conclusions Silicon vacancy centers in diamond are more efficient emitters [large fraction of ZPL emissions] SiV centers are emitters of indistinguishable photons that is, their emitted ZPL photons are mostly indistinguishable [a small value for ] SiV centers have stable spectrum, that is, they remain as coherent emitters for a long period of time [the emission spectrum was stable for at least 4 hrs]

14 Something else to consider There are 4 possible optical transitions, but the explicit explanation for using transition C was vague. Is the indistinguishability level high enough for quantum computing? How long do the photons stay indistinguishable after emission?

15 Citation History since Number of Citations per Month Nov. Sept. 3 Aug. 1 2 Oct. 1 Table 2. 7 direct citations: 3 published + 4 preprints. 3 Second generation citations.

16 How has the field evolved? SiV centers in diamond can efficiently generate indistinguishable photons from multiple distinct emitters, and this enables: Application of a diamond-based quantum computer [5] Coupling multiple spins via a common photonic mode [6] Photon flux consisting of completely identical photons [7] Photonic crystal nanocavities [8]

17 Citation lists [5] Rogers L J, Jahnke K D, Metsch M H, et al, arxiv preprint, arxiv: , (2014). [6] Pingault B, Becker J N, Schulte C H H, et al, arxiv preprint, arxiv: , (2014). [7] Aharonovich I, Nature Photonics,8(11): , (2014). [8] Bayn I, Mouradian S, Li L, et al, arxiv preprint, arxiv: , (2014).

18 Summary [paper] We agree SiV centers are more efficient coherent emitters of indistinguishable photons than NV centers [critique] The lifetime and quality of the indistinguishable photons need to be further explored [future] This exciting fundamental improvement on qubit construction brings us closer to the realisation of the quantum computer

19 Questions

Single Photon Generation & Application

Single Photon Generation & Application Photon Pair Generation: Parametric down conversion is a non-linear process, where a wave impinging on a nonlinear crystal creates two new light beams obeying energy