PDF (double-sided) - Physics Department, UCSB - University of ...
PDF (double-sided) - Physics Department, UCSB - University of ... PDF (double-sided) - Physics Department, UCSB - University of ...
seems to be a box machined out of solid aluminum with coaxial feeds for the microwave lines. This allows the entire box to become superconducting and shield the sample from external magnetic fields. The box needs to support the chip with minimal contact above a cavity to not form a ground-plane underneath the chip that capacitively disperses microwaves across the chip. Nevertheless, the chip needs to be very well grounded to the box, which is achieved by hundreds of closely spaced wire bonds (see Section 6.3.3) that reduce the overall inductance in the grounding. The microwave lines should maintain their 50 Ω impedance throughout the box to minimize pulse distortion caused by reflections. This makes the sample mount one of the few parts that we have not been able to just buy off the shelf. But after the box was designed in a CAD program, it was very straightforward to have it machined to sufficient accuracy by the university’s machine shop. 6.3.3 Wire Bonding To be able to wire the qubit into the DR, the sub-millimeter traces of the qubit chip need to be connected to macroscopic coaxial cables that can be managed by hand. This can be done with a device called a “Wire Bonder”. Wire bonding works by feeding a thin (∼ 1 mil) aluminum wire through a tip such that the tip can push the wire down onto the surface that it is meant to adhere to. While applying pressure to the wire, the tip is vibrated rapidly with ultrasonic waves. 114
This melts the aluminum wire and bonds it to the surface. This allows the system to run wires between pads without the use of solder. This technique makes it possible to make contact to pads that are only a few hundred µm wide. 6.3.4 Dilution Refrigerator Wiring The other non-off-the-shelf component needed is the wiring inside the DR (see Figure 6.2). It consists of several different parts that have to be chosen/built to manage heat loads, noise, and cryogenic properties. Starting at the outside of the qubit box, the squid is connected immediately to a ∼ 30 Ω shunting resistor. This resistor is needed to limit the voltage generated in the squid when it switches to the voltage state as explained in Chapter 4.1.3. If this resistor is omitted, the squid switching generates a large amount of quasiparticle excitations in the qubit circuit which reduce qubit performance. The next component along the squid line is a copper powder filter (Cu). This device consists of a wire wound into a spiral that is sitting in a cavity filled with copper powder and epoxy for thermal contact. Electrically, it functions as a very quiet low-pass filter that absorbs most noise coming down the squid line. The ∼ 30 Ω resistor and the copper powder filter are both mounted at the 25 mK stage of the DR to prevent them from creating noise due to their temperature. Along the squid line, at the 4 K stage of the DR is a resistor network that splits the line into a bias and 115
- Page 92 and 93: Decoherence consists of two parts:
- Page 94 and 95: Note the difference in signs of the
- Page 97 and 98: Chapter 4 Designing the Phase Qubit
- Page 99 and 100: mutual inductance between the qubit
- Page 101 and 102: During the measurement, the | 1 〉
- Page 103 and 104: the right impedance transformation
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- Page 107 and 108: Figure 4.3: Squid I/V Traces - a) L
- Page 109 and 110: Figure 4.5: Qubit Integrated Circui
- Page 111 and 112: The geometry of the qubit junction
- Page 113 and 114: squid loop. Thus, this tool can be
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- Page 118 and 119: Figure 5.1: L-Edit Mask Layout Tool
- Page 120 and 121: Figure 5.2: Fabrication Building Bl
- Page 122 and 123: Figure 5.3: Photolithography and Et
- Page 124 and 125: times the removal can be a bit tric
- Page 126 and 127: Figure 5.4: Clearing Vias from Nati
- Page 128 and 129: 5.6 Junction Layers 5.6.1 Oxidation
- Page 130 and 131: top wiring layer to protect all low
- Page 132 and 133: 104
- Page 134 and 135: 6.1 Physical Quality Control during
- Page 136 and 137: 6.1.3 Atomic Force Microscopy To re
- Page 138 and 139: Figure 6.1: 4-Wire Measurement - a)
- Page 140 and 141: 6.3 Quantum Measurements at 25 mK 6
- Page 144 and 145: Figure 6.2: Dilution Refrigerator W
- Page 146 and 147: cessing data. This protects the vol
- Page 148 and 149: 6.3.9 Anritsu Microwave Source The
- Page 150 and 151: 122
- Page 152 and 153: ment, the scalability requirements,
- Page 154 and 155: people without any formal training
- Page 156 and 157: 7.2.4 Performance Last, but certain
- Page 158 and 159: or a Client Module. Client Modules
- Page 160 and 161: second Module talks to all these an
- Page 162 and 163: puters to talk to each other. Usual
- Page 164 and 165: 7.3.4 Performance Addressing the Pe
- Page 166 and 167: is designed such that the LabRAD Ma
- Page 168 and 169: Table 7.3: LabRAD Type Annotations
- Page 170 and 171: listed in Table 7.3. For transmissi
- Page 172 and 173: Architecture to manage network conn
- Page 174 and 175: Manager. In fact, in our lab, the o
- Page 176 and 177: waiting for their completion. The C
- Page 178 and 179: mentation of pipelining and certain
- Page 180 and 181: Since the API guarantees that all R
- Page 182 and 183: one microwave line for X/Y-rotation
- Page 184 and 185: 7.5.3 DC Rack Server The DC Rack Se
- Page 186 and 187: data taking on the lab servers and
- Page 188 and 189: keys and the ability to set Context
- Page 190 and 191: a certain time. 7.5.9 Optimizer Cli
seems to be a box machined out <strong>of</strong> solid aluminum with coaxial feeds for the microwave<br />
lines. This allows the entire box to become superconducting and shield<br />
the sample from external magnetic fields.<br />
The box needs to support the chip<br />
with minimal contact above a cavity to not form a ground-plane underneath the<br />
chip that capacitively disperses microwaves across the chip. Nevertheless, the chip<br />
needs to be very well grounded to the box, which is achieved by hundreds <strong>of</strong> closely<br />
spaced wire bonds (see Section 6.3.3) that reduce the overall inductance in the<br />
grounding. The microwave lines should maintain their 50 Ω impedance throughout<br />
the box to minimize pulse distortion caused by reflections. This makes the sample<br />
mount one <strong>of</strong> the few parts that we have not been able to just buy <strong>of</strong>f the shelf.<br />
But after the box was designed in a CAD program, it was very straightforward to<br />
have it machined to sufficient accuracy by the university’s machine shop.<br />
6.3.3 Wire Bonding<br />
To be able to wire the qubit into the DR, the sub-millimeter traces <strong>of</strong> the qubit<br />
chip need to be connected to macroscopic coaxial cables that can be managed by<br />
hand. This can be done with a device called a “Wire Bonder”. Wire bonding<br />
works by feeding a thin (∼ 1 mil) aluminum wire through a tip such that the tip<br />
can push the wire down onto the surface that it is meant to adhere to. While<br />
applying pressure to the wire, the tip is vibrated rapidly with ultrasonic waves.<br />
114
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