PDF (double-sided) - Physics Department, UCSB - University of ...
PDF (double-sided) - Physics Department, UCSB - University of ... PDF (double-sided) - Physics Department, UCSB - University of ...
6.3 Quantum Measurements at 25 mK 6.3.1 Dilution Refrigerator Once a sample has been deemed likely to perform, it can be prepared for the real cool-down in the “Dilution Refrigerator” (DR). This refrigerator reaches 25 mK in four stages. It consists of a vacuum that houses one tank filled with liquid nitrogen (LN 2 ) and one with liquid helium (LHe). Just like water at standard pressure boils at 100 ◦ C no matter how much energy is put into it, LN 2 boils at 77 K and LHe boils at 4 K. This keeps the two reservoirs at these temperatures as long as they are kept full of liquid. (This need for keeping the reservoirs filled is one of the reasons why the DR is much more costly to operate than the ADR.) Attached to the LHe reservoir is a thin tube that slowly feeds LHe into a small volume called the “Pot”. This pot is pumped on by a vacuum pump to lower the boiling point of the LHe further to 1.5 K. The remaining cooling to 25 mK is achieved with a closed system that cycles a mixture of He-3 and He-4. Its cooling power results from the entropy increase when He-3 mixes with He-4. A continuous mixing is achieved with a two-chamber design consisting of a destillation chamber (“Still”) that selectively removes He-3 by evaporation and a mixing chamber where the He-3 is allowed to mix back in with the He-4. The entropy increase in the mixing chamber cools the chamber down to around 25 mK. Since this process 112
happens continuously, the DR can theoretically stay cold forever. The rather large cooling power of the mixing process (∼ 20-50 µW) allows for a large sample stage (∼ 150 in 2 ) with several hundred electrical connections. The DR we use in our lab is a custom design, but it is also possible to buy pre-built DRs from companies like Oxford and Janis. 6.3.2 Sample Mount Even though it might seem trivial at first thought, the exact design of the sample mount used for connecting the sample inside the DR is actually quite crucial. This is due to the fact that the states of the qubit correspond to electromagnetic oscillations inside the circuit at GHz frequencies. Therefore, any box-modes that the sample holder might have that resonate in this frequency range will couple to the qubit and degrade its performance. Also, several microwave drive lines converge in a 0.25” square die that need to be electrically isolated in a way that each qubit can be addressed individually with minimal electrical crosstalk from its neighbors. Another concern is the reaction of the box to internal and external magnetic fields. A printed-circuit-board (PCB) design with a centered hole in which the qubit chip is placed, for example, is not a useful design since the current loop formed by the PCB interacts with the flux biases applied to the qubit and leads to flux settling times in the many 10’s of microseconds. An optimal design 113
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6.3 Quantum Measurements at 25 mK<br />
6.3.1 Dilution Refrigerator<br />
Once a sample has been deemed likely to perform, it can be prepared for<br />
the real cool-down in the “Dilution Refrigerator” (DR). This refrigerator reaches<br />
25 mK in four stages. It consists <strong>of</strong> a vacuum that houses one tank filled with liquid<br />
nitrogen (LN 2 ) and one with liquid helium (LHe). Just like water at standard<br />
pressure boils at 100 ◦ C no matter how much energy is put into it, LN 2 boils at<br />
77 K and LHe boils at 4 K. This keeps the two reservoirs at these temperatures<br />
as long as they are kept full <strong>of</strong> liquid. (This need for keeping the reservoirs filled<br />
is one <strong>of</strong> the reasons why the DR is much more costly to operate than the ADR.)<br />
Attached to the LHe reservoir is a thin tube that slowly feeds LHe into a small<br />
volume called the “Pot”. This pot is pumped on by a vacuum pump to lower<br />
the boiling point <strong>of</strong> the LHe further to 1.5 K. The remaining cooling to 25 mK is<br />
achieved with a closed system that cycles a mixture <strong>of</strong> He-3 and He-4. Its cooling<br />
power results from the entropy increase when He-3 mixes with He-4. A continuous<br />
mixing is achieved with a two-chamber design consisting <strong>of</strong> a destillation chamber<br />
(“Still”) that selectively removes He-3 by evaporation and a mixing chamber where<br />
the He-3 is allowed to mix back in with the He-4. The entropy increase in the<br />
mixing chamber cools the chamber down to around 25 mK. Since this process<br />
112
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