Modular PCB architecture incorporating a high density of interconnects needed for multi-qubit readout and control. (a) The cryostat-PCB is fixed to the cold finger at the mixing chamber of a dilution refrigerator. (b) The high-frequency custom cables and feed-throughs entering the cold finger. (c) Top view and (d) bottom view cartoon of the coupled PCB set (rendered using Solidworks CAD software package). (e) The device-PCB, which houses the chip, connects to the cryostat-PCB with the use of MSMP connectors and bullets. The device-PCB has 31 filtered dc lines, matching circuits, and two microwave frequency connections together with 14 rf lines that are passed through from the cryostat-PCB. The ground-PCB pushes on to the device-PCB from the other side, allowing for “make-before-break” connections. (f) Terraced bond pads and recess for housing the qubit chip.
(a) Layout of the 5 individual device-PCB layers showing ground-planes that are used to electrically partition high-frequency connections. (b) Cross-section of the device-PCB in a region close to the chip. The square recess in the device-PCB is created by layers of laminate shaped to frame the chip and allow for short bond wires for high-frequency interconnects. The low dielectric constant of Rogers 3003 laminate suppresses crosstalk between adjacent signal tracks (see text for details).
(a) Crosstalk between the two ESR signal tracks on the high-frequency layer of the device-PCB (image of the PCB layer shown as an inset). The crosstalk between the signal lines (shown in red) remains below −40 dB for frequencies below 10 GHz. Transmission data for the ESR microwave lines (shown in blue) is obtained by linking the lines together using aluminium bond wires. These are required for the transmission measurement but introduce small parasitic resonances. The data shown is the total loss, from ESR 1 to ESR 2, divided by 2. (b) Crosstalk between the nearest neighbour high-frequency lines (with ports indicated on the image shown as an inset). Transmission is again measured by linking the end of the tracks with bond wires. (c) Crosstalk between two tank-circuit transmission lines (ports again shown in the inset). Crosstalk measured without inductors soldered to the pads is shown in blue. Measurements taken with inductors in place are shown in red. We note corresponding peaks in the crosstalk spectrum at the resonant frequencies of the individual tank circuits. (d) Measured crosstalk between various different transmission lines on the device-PCB [see Fig. 2(a) for port labels]. Maximum crosstalk occurs between vertically adjacent tank-circuit lines and high frequency tracks (shown in red). The majority of crosstalk is produced in the region close to the chip where the ground-planes have been removed.
The effect of the fencing-via technique on the EM coupling and crosstalk between signal lines. (a) Layout of the high-frequency layer of the device-PCB showing zoom and electric field strength obtained using EM simulation software36 [see scale bar in (c)]. An input voltage amplitude of 1 V is applied to the port SMP 1 at a frequency of 40 GHz. (b) Crosstalk between high-frequency ports SMP 1 and SMP 6 in a bandwidth 0–6 GHz. Simulation software is used to evaluate crosstalk when fencing-vias are used (red dashed line) and not used (blue dashed line). (c) Numerical simulation of the field strength in a cross-section of the high-frequency layer of the device-PCB, again at 40 GHz. The magnitude of the electric field is shown with (top) and without (bottom) fencing-vias. (d) Shows magnetic field strength comparing the coupling between ports, with and without fencing-vias for conditions as in (c). A reduction in the coupling strength of electric and magnetic field is seen when fencing-vias are implemented. A via diameter of 0.38 mm is used with a via centre-to-centre spacing of ∼0.6 mm.
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