Graphene-based Josephson junction microwave bolometer The MIT Faculty has made this article openly available. Please share how this access benefits you. Your story matters. Citation Lee, Gil-Ho et al. “Graphene-based Josephson junction microwave bolometer.” Nature, 586, 7828 (September 2020): 42–46 © 2020 The Author(s) As Published 10.1038/s41586-020-2752-4 Publisher Springer Science and Business Media LLC Version Author's final manuscript Citable link https://hdl.handle.net/1721.1/129674 Terms of Use Article is made available in accordance with the publisher's policy and may be subject to US copyright law. Please refer to the publisher's site for terms of use. Graphene-based Josephson junction microwave bolometer Gil-Ho Lee,1, 2 Dmitri K. Efetov,3 Woochan Jung,2 Leonardo Ranzani,4 Evan D. Walsh,5, 6 Thomas A. Ohki,4 Takashi Taniguchi,7 Kenji Watanabe,7 Philip Kim,1 Dirk Englund,5 and Kin Chung Fong4, ∗ 1Department of Physics, Harvard University, Cambridge, MA 02138 2Department of Physics, Pohang University of Science and Technology, Pohang 790-784, Republic of Korea 3ICFO-Institut de Ci`enciesFot`oniques, The Barcelona Institute of Science and Technology, 08860 Castelldefels (Barcelona), Spain 4Raytheon BBN Technologies, Quantum Information Processing Group, Cambridge, Massachusetts 02138, USA 5Department of Electrical Engineering and Computer Science, Massachusetts Institute of Technology, Cambridge, MA 02139 6School of Engineering and Applied Sciences, Harvard University, Cambridge, MA 02138 7National Institute for Materials Science, Namiki 1-1, Tsukuba, Ibaraki 305-0044, Japan (Dated: November 6, 2020) Sensitive microwave detectors are criti- internal energy from absorbed photons to evade leak- cal instruments in radioastronomy [1], dark age through optical phonon emission [24]; its weak matter axion searches [2], and supercon- electron-phonon coupling can keep the electrons ducting quantum information science [3, 4]. thermally isolated from the lattice [10, 15, 16, 25{ The conventional strategy towards higher- 29]; most importantly, at the charge neutrality point sensitivity bolometry is to nanofabricate an (CNP), graphene has a vanishing density of states. ever-smaller device to augment the thermal This results in a small heat capacity and electron- response [5{7]. However, this direction is in- to-phonon thermal conductance which are highly creasingly more difficult to obtain efficient desirable material properties for bolometers and photon coupling and maintain the material calorimeters, while maintaining a short thermal re- properties in a device with a large surface- sponse time [19]. Although the bolometric response to-volume ratio. Here we advance this con- of graphene has been tested in devices based on cept to an ultimately thin bolometric sensor noise thermometry [16, 18, 19], their performance based on monolayer graphene. To utilize its is severely hampered by the degrading thermome- minute electronic specific heat and thermal ter sensitivity when the electron temperature rises conductivity, we develop a superconductor- upon photon absorption [18]. Here, we overcome graphene-superconductor (SGS) Josephson this challenge by adopting a fundamentally differ- junction [8{13] bolometer embedded in a mi- ent measurement technique: we integrate monolayer crowave resonator of resonant frequency 7.9 graphene simultaneously into a microwave resonator GHz with over 99% coupling efficiency. From and a Josephson junction, and upon absorbing mi- the dependence of the Josephson switch- crowave radiation into the resonator, the rise of ing current on the operating temperature, the electron temperature in graphene suppresses the charge density, input power, and frequency, switching current of the SGS Josephson junction. we demonstrate a noise equivalent power This mechanism can function as the bolometer read- (NEP) of 7 10−19 W/Hz1=2, corresponding out and provide us a way to study the thermal re- to an energy× resolution of one single photon sponse of this bolometer. at 32 GHz [14] and reaching the fundamental Inspired by the demonstration of using heating limit imposed by intrinsic thermal fluctuation or quasiparticle injection to control the supercur- at 0.19 K. rent in superconductor-normal-superconductor junc- Many attractive electrical and thermal properties tions in the DC regime [30, 31], we design our in graphene make it a promising material for bolom- microwave bolometer with a orthogonal-terminal arXiv:1909.05413v3 [cond-mat.mes-hall] 5 Nov 2020 etry and calorimetry [15{22]. It can absorb pho- graphene-based Josephson junction (GJJ) as shown tons from a wide frequency bandwidth efficiently by in Fig. 1a and b. The monolayer graphene is encap- impedance matching [23]; the electron-electron scat- sulated on the top and bottom by hexagonal boron- tering time is short and can quickly equilibrate the nitride (hBN). The proximitized Josephson junction (green color) is formed by edge-contacting NbN su- perconductors to the graphene such that dissipation- ∗ [email protected] less Josephson current can flow along the JJ direc- (a) (b) (d) Jose 1 mm phson resonator junction hBN encapsulated 1 µm graphene resonator (c) dc quarter-wave directional resonator coupler circulator LNA quarter-wave graphene resonator Connections to Connection to dc Josephson 20 dB local gate attenuator junction FIG. 1. (a) Device concept of the superconducting-graphene-superconducting (SGS) Josephson junction (JJ) mi- crowave bolometer. The hBN-encapsulated SGS JJ (1 µm wide and with a gap of '0.3 µm) is embedded simulta- neously in a half-wave resonator to allow microwave coupling (blue) and DC readout (green) of the JJ. For clarity, the local gate is not shown. (b) Scanning electron microscope image of the orthogonal-terminal JJ. (c) Schematics of the detector setup. The graphene flake is located at the current antinode of the half-wave microwave resonator. Test microwave power is coupled to the detector through the 20 dB directional coupler and highly attenuated coaxial cables from room temperature. Two stages of inductors and capacitors form a low-pass filter network for the DC measurement. (d) False-colored optical image of the actual device. tion [11]. A dissipative microwave current can flow current is swept from 1.5 to -1.5 µA at device tem- along the direction perpendicular to that of the junc- peratures between 0.19 and 0.9 K. Our GJJ shows tion, with the graphene extended out by 0.8 µm from hysteretic switching behavior: the switching current each side of the GJJ before connecting to quarter- Is, at which the junction switches from the dissipa- wave resonators (blue color) to form a half-wave res- tionless state to the normal state, is different from onator using a NbN microstrip with a characteristic the retrapping current, Ir. Such hysteresis is pre- impedance of 86 Ω (Fig. 1c and d). This extension sumably due to self-Joule heating when the junction is narrow and long to prevent Josephson coupling turns normal [10]. The averaged switching currents to the microstrips and positions the graphene at the Is are plotted at various gate voltages Vgate and current antinode of the resonator. temperaturesh i in Figs. 2b and c. The drop of I as h si Microwave power is applied to the resonator temperature rises is an important feature that can through a 200 fF coupling capacitor. We can charac- determine the sensitivity of the GJJ as a bolome- terize our GJJ-embedded resonator by reflectometry ter as well as the quantum efficiency and dark count using a directional coupler. All test power is deliv- of the future microwave single photon detector [23]. ered via the heavily-attenuated microwave coaxial Fig. 2d plots the normal-state junction resistance cables to filter the thermal noise from room temper- Rn as a function of gate voltage, indicating that the ature. To decouple the GJJ DC measurement from CNP is at -0.9 V. We note that the unusual rise of the microwave resonator, two stages of LC low-pass Rn at around 2 to 3 V of Vgate may be due to the filters are implemented to form a high-impedance formation of a Moir´esuperlattice with the hBN sub- line at high frequency. The 1 nH inductors are made strate (see Method). The Is Rn product is on the h i of narrow meandered wires and are shunted by 530 order of 0.16 mV, which is comparable to other GJJs fF capacitor plates. of similar size in the long diffusive limit [23]. We study the GJJ switching as a function of tem- The coupling efficiency can be characterized using perature and gate voltage. Fig. 2a shows the typical reflectometry (see Fig. 3a). We design the resonator voltage drop across the junction VJJ as the DC bias to be critically coupled at about 7.9 GHz. The dis- 2 (a) (b) 1.2 0.15 1.0 (a) 0 Ir T (K) 0.1 1 0.5 0.05 Is A) -10 µ ( 0.1 0 0.8 (mV) i s 1.0 (dB) I h JJ -0.05 V -20 0.6 11 -0.1 T (K) 0.5 S 0.1 V -0.15 1.3 V 0.1 0.4 -30 -1.5 -1 -0.5 0 0.5 1 1.5 0 1 2 3 0 Ibias (µA) Vgate (V) (b) (c) 1.2 1.3 V (d) 0.1 V -0.5 V 1 0.2 1 -4 I s (nA) A) R Ω) µ k i n ( 0.8 ( s n i (mV) -8 I s 0.5 0.1 R I h h 0.6 ∆ -12 0.4 0 0 0 0.2 0.4 0.6 0.8 -2 0 2 4 6 8 10 12 T (K) Vgate (V) Frequency (GHz) FIG. 2. Characterizing the graphene-based Josephson junction (GJJ) switching current. (a) GJJ voltages with FIG. 3. Demonstration of the device's operation as a sweeping of bias current and (b) the averaged switching bolometer and measuring the detector efficiency.