Frequency multiplexing for qubit readout

1. A system, comprising:

a quantum processor comprising a plurality of qubits;

for each of at least two of the plurality of qubits:

a circulator operative to receive a control signal and an output signal from the qubit;

an isolator coupled to an output of the circulator; and

a quantum-limited amplifier coupled to an output of the isolator and configured to provide an output of the qubit; and

a multiplexor (MUX) configured to frequency multiplex the outputs of the at least two of the plurality of qubits as a single output of the quantum processor, wherein the MUX comprises one or more 90-degree hybrid couplers.

2. The system of claim 1 , wherein the control signal is at a different frequency for each qubit.

3. The system of claim 1 , further comprising:

a filter coupled to the qubit; and

a readout resonator coupled between the qubit and the filter.

4. The system of claim 1 , wherein the quantum-limited amplifier is a Josephson parametric converter (JPC).

5. The system of claim 1 , wherein the quantum-limited amplifier is a traveling wave parametric amplifier (TWPA).

6. The system of claim 1 , wherein the quantum processor, the MUX, a filter, the circulator and the quantum-limited amplifier of each qubit, are on a single printed circuit board.

7. The system of claim 1 , wherein the quantum processor, the MUX, a filter, the circulator and the quantum-limited amplifier of each qubit, are configured in a dilution refrigerator.

8. The system of claim 1 , wherein

each 90-degree hybrid coupler is terminated with a resistor to ground.

9. The system of claim 8 , wherein the MUX has multiple tapered stages configured to collectively provide a single output for all outputs of the plurality of qubits.

10. The system of claim 8 , wherein each 90-degree hybrid coupler comprises transmission lines embedded in traces of a printed circuit board.

11. A system for combining the outputs of a plurality of qubits, comprising:

a coupler having two inputs and two outputs;

a first circulator coupled to a first qubit and configured to route a first signal to a first qubit and route an output of the first qubit in response to the first signal, to a first input of the coupler; and

a second circulator coupled to a second qubit and configured to route a second signal to a second qubit and route an output of the second qubit in response to the second signal, to a second input of the coupler, wherein a first output of the coupler is operative to provide an output that combines the output of the first qubit and the output of the second qubit by way of frequency multiplexing.

12. The system of claim 11 , wherein the coupler is a 90-degree hybrid.

13. The system of claim 11 , wherein a second output of the coupler is terminated to ground via a resistor.

14. The system of claim 11 , wherein the system is in a refrigerated environment of a dilution refrigerator.

15. The system of claim 11 , further comprising:

a third circulator coupled between the first circulator and the first input of the coupler;

a fourth circulator coupled between the second circulator and the second input of the coupler;

a first amplifier coupled to the third circulator and configured to amplify the output of the first qubit before it is provided to the first input of the coupler;

a second amplifier coupled to the fourth circulator and configured to amplify the output of the second qubit before it is provided to the second input of the coupler; and

a third amplifier at the first output of the coupler.

16. The system of claim 11 , wherein:

the first and second amplifiers are Josephson parametric converters (JPCs); and

the third amplifier is a wave parametric amplifier (TWPA).