A tunable resonator-resonator coupling circuit (400, 500, 600) comprising: - a first resonator (402, 502, 602), the first resonator (402, 502, 602) being linear or nonlinear and having a first frequency; - a second resonator (404, 504, 604), the second resonator (404, 504, 604) being linear or nonlinear and having a second frequency, the first resonator (402, 502, 602) and the second resonator (404, 504, 604) having a direct coupling therebetween; - a tunable coupling element (406, 506, 606) arranged to provide an indirect coupling between the first resonator (402, 502, 602) and the second resonator (404, 504, 604), the tunable coupling element (406, 506, 606) having an idling frequency that is below each of the first frequency and the second frequency; - a first readout resonator configured to readout the first resonator (402, 502, 602), the first readout resonator having a first operating frequency; and - a second readout resonator configured to readout the second resonator (404, 504, 604), the second readout resonator having a second operating frequency; wherein the tunable coupling element (406, 506, 606) comprises a first superconducting island (408, 508, 608) and a second superconducting island (410, 510, 610) which are both ungrounded; wherein the indirect coupling comprises: (i) a first coupling between the first resonator (402, 502, 602) and the first superconducting island (408, 508, 608), (ii) a Josephson coupling between the first superconducting island (408, 508, 608) and the second superconducting island (410, 510, 610), and (iii) a second coupling between the second resonator (404, 504, 604) and the second superconducting island (410, 510, 610); and wherein each of the first operating frequency and the second operating frequency is above each of the idling frequency of the tunable coupling element (406, 506, 606), the first frequency of the first resonator (402, 502, 602) and the second frequency of the second resonator (404, 504, 604). - a first resonator (402, 502, 602), the first resonator (402, 502, 602) being linear or nonlinear and having a first frequency; - a second resonator (404, 504, 604), the second resonator (404, 504, 604) being linear or nonlinear and having a second frequency, the first resonator (402, 502, 602) and the second resonator (404, 504, 604) having a direct coupling therebetween; - a tunable coupling element (406, 506, 606) arranged to provide an indirect coupling between the first resonator (402, 502, 602) and the second resonator (404, 504, 604), the tunable coupling element (406, 506, 606) having an idling frequency that is below each of the first frequency and the second frequency; - a first readout resonator configured to readout the first resonator (402, 502, 602), the first readout resonator having a first operating frequency; and - a second readout resonator configured to readout the second resonator (404, 504, 604), the second readout resonator having a second operating frequency; wherein the tunable coupling element (406, 506, 606) comprises a first superconducting island (408, 508, 608) and a second superconducting island (410, 510, 610) which are both ungrounded; wherein the indirect coupling comprises: (i) a first coupling between the first resonator (402, 502, 602) and the first superconducting island (408, 508, 608), (ii) a Josephson coupling between the first superconducting island (408, 508, 608) and the second superconducting island (410, 510, 610), and (iii) a second coupling between the second resonator (404, 504, 604) and the second superconducting island (410, 510, 610); and wherein each of the first operating frequency and the second operating frequency is above each of the idling frequency of the tunable coupling element (406, 506, 606), the first frequency of the first resonator (402, 502, 602) and the second frequency of the second resonator (404, 504, 604).