[3분논문] 제26회 - 플라즈몬 회로에서 양자적 간섭

[3분논문] 제26회 - 플라즈몬 회로에서 양자적 간섭

황용섭의 3분논문 제26회입니다.

총시간은 5분 4초입니다.

재미있게 들어보세요.

# 방송 바로 듣기

1. 26_3분논문_제26회.mp3

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# 논문표지

# 간단한 정보

글쓴이: Reinier W. Heeres, Leo P. Kouwenhoven and Valery Zwiller

제목: Quantum interference in plasmonic circuits

학술지: Nature Nanotechnology

발행년월: 2013년 8월 

이어가기: http://dx.doi.org/10.1038/nnano.2013.150

# 용어

- time-resolved correlation measurement: 두 입자에 시간 차이를 두어 얼만큼의 correlation을 보이는지 측정하는 방법

# 발췌

1. 

Our work shows that it is feasible to shrink quantum optical experiments to the nanoscale and offers a promising route towards subwavelength quantum optical networks.

2.

We first characterized the indistinguishability of the produced photon pairs in a HOM experiment using a fibre beamsplitter by performing a time-resolved correlation measurement.

# 그림

Figure 2: Plasmon interference device and experimental set-up.

a, SEM image of a gold plasmonic directional coupler device. Integrated SSPDs (coloured yellow) are located ~20 nm below the output waveguides. Top-right: zoom-in of the plasmon detection region. The width and spacing of the superconducting meander are both 100 nm and the thickness is ~5 nm. The 150-nm-thick gold waveguide is 600 nm wide and aligned for optimal absorption of the plasmonic mode in the SSPD (Supplementary Fig. S11). Bottom-right: zoom-in of the coupling region, with a waveguide width of 250 nm and gap between waveguides of 100 nm. The cross-coupling ratio can be controlled by varying the interaction length L. b, Photon-pair source based on degenerate type-II collinear spontaneous parametric downconversion. An ~400 mW pump laser (532 nm) is focused to a ~70-μm-diameter spot in a 2-cm-long potassium titanyl phosphate (KTP) crystal producing photon pairs. The polarizing beamsplitter (PBS) separates the H- and V-polarized photons from a pair to be collected in different optical fibres. A motorized delay line in one of the arms allows the time delay between the two photons to be controlled with ~16 fs resolution. c, Experimental set-up. Two beams from optical fibres are focused to separate spots (spot size diameter, ~1 µm) on the sample by a microscope objective. Their polarization can be rotated using half-wave plates to optimize plasmon excitation (Supplementary Fig. S2). Objective and sample are at 4 K in a dipstick filled with helium exchange gas and with a window for free-space optical access.

Figure 4: Quantum interference measurements.

a, Time-resolved correlation measurements to characterize the photon-pair source using the scheme in Fig. 1a (fibre beamsplitter-based). The x-axis spans the range from −4 to 4 ns, much larger than the interphoton delay dt (indicated for each curve). For an interphoton delay of >3 ps, the central peak around t = 0, well above the uncorrelated flat background, corresponds to a second-order correlation function g⁽²⁾(0) > 1 due to the correlated source. The peak width is limited by a jitter of ~600 ps in the APDs. Quantum interference causes these correlations to disappear almost completely when the photons overlap in time. Integration time per curve is 8 s. b, HOM interference dip of the photon pair source. The coincidences on the vertical axis are equal to the integrated number of counts at particular delay times dt between the two photons from a single pair (that is, shaded areas in a). The full-width at half-maximum of the Gaussian fit to the HOM dip gives a coherence time τc = (2.51 ± 0.03) ps and visibility V = 0.92 ± 0.01. c, Time-resolved correlation measurements of plasmon interference using the scheme in Fig. 1b. The x-axis now only spans from −1 to 1 ns because the SSPDs have significantly improved timing resolution (jitter, ~120 ps). This results in a lower background and a correspondingly larger g⁽²⁾(0). (The time resolution is still larger than the indicated interplasmon delay.) Integration time of 15 min per curve. d, HOM interference dip showing quantum interference of surface plasmons in two devices (as shown in Fig. 2a) with a 50/50 directional coupling strength (gap 150 nm, L= 0). Coherence time τc = (2.60 ± 0.11) ps and visibility V = 0.43 ± 0.02 for device 1 (integration time 30 min per point, pairs of traces from c combined);τc = (2.49 ± 0.08) ps and V = 0.39 ± 0.01 for device 2 (integration time 20 min per point). Compared to the photon-pair source properties in b, the measured coherence time is identical but the visibility is reduced.

# 결론

플라즈몬 회로에서 발생하는 양자적 간섭을 실제로 관측해서, 양자통신을 표면 플라즈몬을 이용하여 할 수 있음을 구체적으로 보였다는 점에서 매우 의미가 크다.

끝.

2013년 8월 30일