# US 9,438,246 B1 · System and method for qubit readout > Agents: this data is available as JSON from the patentgrep API and MCP server — see https://patentgrep.com/llms.txt - Number: US 9,438,246 B1 - Document ID: US9438246B1 - Status: Granted - Kind: B1 - Application number: 14/845,981 - Filed: 2015-09-04 - Granted: 2016-09-06 - Earliest claimed: 2015-09-04 - Assignees: Northrop Grumman Systems Corporation - Applicants: Ofer Naaman - Inventors: Ofer Naaman - Web page: https://patentgrep.com/patent/US9438246B1 ## CPC - H03K 19/195: using superconductive devices - G06N 10/40: Physical realisations or architectures of quantum processors or components for manipulating qubits, e.g. qubit coupling or qubit control - G11C 11/44: using super-conductive elements, e.g. cryotron - H03K: PULSE TECHNIQUE (measuring pulse characteristics G01R; modulating sinusoidal oscillations with pulses H03C; transmission of digital information H04L; discriminator circuits detecting phase difference between two signals by counting or integrating cycles of oscillation H03D3/04; automatic control, starting, synchronisation or stabilisation of generators of electronic oscillations or pulses where the type of generator is irrelevant or unspecified H03L; coding, decoding or code conversion, in general H03M) - G06N: COMPUTING ARRANGEMENTS BASED ON SPECIFIC COMPUTATIONAL MODELS - G11C: STATIC STORES (semiconductor memory devices H10B) ## Abstract A system is provided for multiplexed readout of qubits. The system comprises a plurality of bandpass (BP) filter resonant sections that are each coupled to a different respective point on a read line, wherein each BP filter resonant section is coupleable to a respective qubit through a respective qubit readout resonator. The system further comprises a plurality of tunable couplers, wherein each tunable coupler is coupled between a respective BP filter resonant section and a qubit readout resonator, and a coupling controller that controls the coupling strength of each qubit to the read line by controlling the impedance of each tunable coupler of the plurality of tunable couplers. ## Claims ### Claim 1 A system for readout of qubits, the system comprising: a plurality of bandpass (BP) filter resonant sections that are each coupled to a different respective point on a read line, wherein each BP filter resonant section is coupleable to a respective qubit through a respective qubit readout resonator; a plurality of tunable couplers, wherein each tunable coupler is coupled between a respective BP filter resonant section and a qubit readout resonator; and a coupling controller that controls the coupling strength of each qubit to the read line by controlling the impedance of each tunable coupler of the plurality of tunable couplers. ### Claim 2 (depends on 1) The system of claim 1, wherein the impedance of each tunable coupler can be controlled by the coupling controller to provide one of strong coupling of its respective qubit to the read line, strong isolation of its respective qubit to the read line and intermediate coupling of its respective qubit to the read line. ### Claim 3 (depends on 2) The system of claim 2, wherein strong coupling of a respective qubit allows for a fast reset of that respective qubit removing any state information from the qubit, a strong isolation of a respective qubit allows for operations to be performed on the respective qubit without any coupling of the respective qubit to the read line, and the retention of state information for the respective qubit, and intermediate tunable coupling of the respective qubit allows for a high fidelity readout of the qubit state. ### Claim 4 (depends on 1) The system of claim 1, wherein each tunable coupler is galvanically coupled to its respective BP filter resonant section. ### Claim 5 (depends on 1) The system of claim 1, wherein each tunable coupler is a superconducting quantum interference device (SQUID) having at least one Josephson junction, such that the current through the at least one Josephson junction changes the inductance of the SQUID and the coupling strength of its respective qubit to the read line. ### Claim 6 (depends on 5) The system of claim 5, wherein each tunable coupler and its respective BP resonant section share a common inductance. ### Claim 7 (depends on 1) The system of claim 1, wherein each BP filter resonant section is formed from lumped elements comprising an inductor and a capacitor. ### Claim 8 (depends on 1) The system of claim 1, wherein each BP filter resonant section is formed from one or more transmission line stubs. ### Claim 9 (depends on 1) The system of claim 1, wherein the read line is formed from a plurality of series coupled coupling capacitors coupled to one another. ### Claim 10 (depends on 1) The system of claim 1, wherein one or more qubits to be read can be coupled to the read line at a time. ### Claim 11 (depends on 10) The system of claim 10, wherein the read line includes an input port and an output port, wherein an input signal provided at the input port is altered by a read of one or more qubits, such that the altered input signal can be provided as an output signal at an output port that can provide the state of the one or more qubits. ### Claim 12 A system for multiplexed readout of qubits, the system comprising: a plurality of bandpass (BP) filter resonant sections forming a BP filter, each BP filter resonant section being coupled between a different respective point on a read line and a respective qubit through its respective qubit readout resonator; a plurality of tunable couplers, each tunable coupler being galvanically coupled to a respective BP filter resonant section, wherein each tunable coupler is a superconducting quantum interference device (SQUID) having at least one Josephson junction, such that flux induced in the SQUID changes the inductance of the at least one Josephson junction and the coupling strength of its respective qubit to the read line; and a coupling controller that controls the coupling strength of each qubit to the read line by controlling an amount of flux induced in each SQUID. ### Claim 13 (depends on 12) The system of claim 12, wherein the coupling strength of each qubit to the read line can be one of strong coupling, strong isolation and intermediate coupling. ### Claim 14 (depends on 12) The system of claim 12, wherein each tunable coupler and its respective BP resonant section share a common inductance. ### Claim 15 (depends on 12) The system of claim 12, wherein the read line is formed from a plurality of series coupled coupling capacitors coupled to one another. ### Claim 16 (depends on 12) The system of claim 12, wherein the read line includes an input port and an output port, wherein an input signal provided at the input port is altered by a read of one or more qubits of the plurality of qubits, such that the altered input signal can be provided as an output signal at an output port that can provide the state of the one or more qubits. ### Claim 17 A method for multiplexed readout of qubits, the method comprising: providing a system comprising a plurality of bandpass (BP) filter resonant sections forming a BP filter, each BP filter resonant section being coupled between a different respective point on a read line and a respective qubit of a plurality of qubits through its respective qubit readout resonator of a plurality of qubit readout resonators; coupling one more qubits to the read line, while isolating each of the other of the plurality of qubits from the read line; providing an input signal to an input port of the read line; reading an output signal from an output port of the read line; and determining the state of the one or more qubits based on the change between the input signal and the output signal. ### Claim 18 (depends on 17) The method of claim 17, wherein the coupling of the one or more qubits to the read line comprises one of strong coupling of at least one qubit to the read line to reset the state of the at least one qubit, intermediate coupling of the at least one qubit to the read line to read the state of the at least one qubit, or strong isolation of the at least one qubit to the read line to allow for operations to be performed on the at least one qubit. ### Claim 19 (depends on 17) The method of claim 17, wherein the coupling of the one or more qubits to the read line comprises tunable intermediate coupling of the one or more qubits to the read line for high fidelity read out of the state of the one or more qubits. ### Claim 20 (depends on 17) The method of claim 17, wherein the providing a system comprises providing a plurality of superconducting quantum interference devices SQUIDs, each SQUID being galvanically coupled to a respective BP filter resonant section, and further comprising inducing an amount of flux in each SQUID that controls the coupling strength of its respective qubit to the read line.