EP-4433956-B1 - QUANTUM PROCESSING UNIT, QUANTUM COMPUTING SYSTEM, AND METHOD FOR READING OUT STATES OF QUBITS

Inventors

• Ockeloen-Korppi, Caspar
• HEINSOO, Johannes
• SARSBY, Matthew
• Lähteenmäki, Pasi

Dates

Publication Date

20260513

Application Date

20211115

Claims (11)

1. A quantum processing unit, comprising a first qubit, a plurality of other qubits and a readout resonator, wherein: - each of said first and plurality of other qubits is configured to acquire, as a result of a quantum computing operation, a quantum state specific to that qubit, - said first qubit is located closer to said readout resonator than any of said plurality of other qubits, - the quantum processing unit comprises a plurality of couplers, said couplers being configured to, in response to respective swap gate control signals, selectively perform swap gates between respective pairs among said first and plurality of other qubits to make the qubits of said respective pairs swap states, - said couplers are configured to make, by repeatedly performing said swap gates, the acquired quantum state of each of said plurality of other qubits appear in turn in said first qubit, - said first and plurality of other qubits constitute a first subset of all qubits in said quantum pro-cessing unit, - the quantum processing unit comprises one or more other subsets of qubits, - the quantum processing unit comprises a plurality of readout resonators, of which at least a respective one is available for being dedicated for each said subset of qubits, - the quantum processing unit comprises a two-dimensional area on a substrate, and - said subsets of qubits span said two-dimensional area, characterized in that - said first subset comprises tuned qubits and either parked qubits or fixed frequency qubits in an alternating pattern, parked qubits being qubits kept on a respective resonance frequency, - resonance frequencies of the parked qubits or fixed frequency qubits in said alternating pattern follow a rotating pattern of three different frequencies, - a second subset comprises tuned and either parked qubits or fixed frequency qubits in an alternating pattern, with resonance frequencies of the parked qubits or fixed frequency qubits in said alternating pattern following the same rotating pattern of three different frequencies as in the first subset, - the tuned qubits and parked or fixed frequency qubits of said first and second subsets are located at vertices of a hexagonal grid so that the three nearest neighbours of each parked or fixed frequency qubit in said grid are tuned qubits and the three nearest neighbours of each tuned qubit in said grid are parked or fixed frequency qubits of which each has a unique one of said three different frequencies.
2. A quantum processing unit according to claim 1, wherein there are more than 10, preferably more than 100, and most preferably more than 1000 qubits in said first and plurality of other qubits.
3. A quantum processing unit according to any of claims 1 or 2, wherein said couplers are configured to controllably change an order in which they make the acquired quantum state of each of said plurality of other qubits appear in said first qubit.
4. A quantum processing unit according to claim 1, wherein: - said couplers are configured to controllably change the dedication of readout resonators of said plurality of readout resonators to subsets of qubits.
5. A quantum processing unit according to any of claims 1 or 4, wherein said plurality of readout resonators are located at one or more edges of said two-dimensional area.
6. A quantum processing unit according to any of claims 1 or 4, wherein at least some of said plurality of readout resonators are distributed across said two-dimensional area.
7. A quantum computing system, comprising: - a quantum processing unit according to any of claims 1 to 6, - a control arrangement, and - a plurality of signal paths between said quantum processing unit and said control arrangement; wherein said control arrangement is configured to provide said swap gate control signals to the couplers in said quantum processing unit.
8. A quantum computing system according to claim 7, wherein said control arrangement is configured to: - dynamically determine a readout order of at least some of the plurality of other qubits in the quantum processing unit, and - provide said swap control signals to the couplers in conformity with the dynamically determined readout order, to make the acquired quantum state of each of said at least some of the plurality of other qubits appear in said first qubit in the determined readout order.
9. A quantum computing system according to any of claims 7 or 8, wherein: - the qubits in the quantum processing unit comprise computing qubits and ancilla qubits, and - the control arrangement is configured to read out the states of at least some of said ancilla qubits multiple times before reading out the states acquired by the computing qubits.
10. A quantum computing system according to claim 9, wherein said computing qubits constitute a first matrix pattern on a surface of the quantum pro-cessing unit and said ancilla qubits constitute a second matrix pattern, intertwined with said first matrix pattern, on said surface of the quantum processing unit.
11. A method for reading out states of qubits in a quantum computing system, the method comprising: - repeatedly performing swap gates among pairs of qubits in a set including a first qubit and a plurality of other qubits to make the state acquired, as a result of a quantum computing operation, by each of said plurality of other qubits appear in turn in said first qubit, - repeatedly performing readout operations on said first qubit using a readout resonator to which said first qubit is closer than any of said plurality of qubits, thus sequentially reading out the acquired state of each of said plurality of other qubits that was made to appear in turn in the first qubit, - performing said readout operations in a first subset of all qubits in said quantum processing unit and one or more other subsets of qubits, using at least a respective one of a plurality of readout resonators dedicated for each said subset of qubits, - in said first subset, using tuned qubits and either parked qubits or fixed frequency qubits in an alternating pattern, wherein parked qubits are qubits kept on a respective resonance frequency and wherein resonance frequencies of the parked qubits or fixed frequency qubits in said alternating pattern follow a rotating pattern of three different frequencies, and - in a second subset, using tuned and either parked qubits or fixed frequency qubits in an alternating pattern, wherein resonance frequencies of the parked qubits or fixed frequency qubits in said alternating pattern follow the same rotating pattern of three different frequencies as in the first subset; wherein the tuned qubits and parked or fixed frequency qubits of said first and second subsets are located at vertices of a hexagonal grid so that the three nearest neighbours of each parked or fixed frequency qubit in said grid are tuned qubits and the three nearest neighbours of each tuned qubit in said grid are parked or fixed frequency qubits of which each has a unique one of said three different frequencies.

Description

FIELD OF THE INVENTION The invention relates to the technical field of quantum computing. In particular, the invention relates to reading out the states of a plurality of qubits in a quantum processing unit. BACKGROUND OF THE INVENTION A basic functional unit of quantum computing is the qubit, of which there may be a large number on a quantum processing unit. Throughout this description, the term quantum processing unit and its acronym QPU refer to a piece of hardware in which a plurality of circuit elements, at least some of which are suitable and designed for quantum computing, exist in a physical form suitable for being operated in the cryogenically cooled environment that is required for quantum computing. The term quantum circuit refers to a configurable abstraction of quantum gates performed during quantum computation. The term quantum computing system refers to a larger entity that comprises one or more QPUs, the control arrangement located outside the cryogenically cooled environment, and the signal paths between the two. Each qubit used for a quantum computation assumes a superposition of two basis states. For concise reference, the superposition is often referred to as the quantum state, or simply just state, of the qubit. In general, a multi-qubit system is in a superposition of multi-qubit eigenstates. In order to obtain a useful result of a quantum computation, a readout operation must be performed. The readout operation causes the quantum state of a single qubit to collapse into one of the possible basis states, resulting in a classical state that can be represented as a digital one or a digital zero. A representative characteristic of any quantum circuit is the coherence time, during which the readout operation must be performed to avoid losing the information represented by the quantum state. A known way of performing a readout on a qubit involves using a readout resonator. The qubit is weakly coupled to an adjacent readout resonator, and the energy within the qubit causes, by means of qubit nonlinearity, a small shift in the scattering parameters of the combined system consisting of the qubit and the readout resonator. This shift can be detected by transmission of a so-called readout signal, which is a microwave pulse on resonance with the readout resonator. Assuming that the qubit is a transmon, the interaction between the state of the qubit and the readout signal injected into the readout resonator causes an observable effect in the amplitude and phase of the transmitted signal. This effect is indicative of the classical state observed in the readout operation. From the readout resonator there are further signal paths that eventually transfer the obtained classical state out of the cryogenically cooled environment where the quantum processing unit resides. The readout resonator must be located close to the qubit, the state of which is to be read. Building the signal paths between the readout resonator in the cryogenically cooled environment and the pro-cessing electronics in the surrounding room temperature environment is non-trivial, as it requires transmission lines operable at gigahertz frequencies with proper filtering and thermal anchoring to cold bodies in the cryostat. Frequency multiplexing may be utilised to share a common transmission line among about ten readout resonators in practice, the limit being related to gate speed and available bandwidth. Slower gates would allow more channels to be frequency-multiplexed if the hardware and software support it, but slower gates are against the overall goal of taking the best advantage of the limited coherence times of the qubits. In a system where the quantum processing unit comprises only a small number of qubits, these are not big problems. However, with the number of qubits in the quantum processing unit increasing it has been found that the physical space requirements of known readout systems as well as the conducted heat and cost related to a large quantity of wiring may become prohibitively large. Prior art documents that handle known technology on this technical field include PECHAL M ET AL: "Characterization and tomography of a hidden qubit", ARXIV.ORG, CORNELL UNIVERSITY LIBRARY, 201 OLIN LIBRARY CORNELL UNIVERSITY ITHACA, NY 14853, 17 November 2020, as well as A. J. SIGILLITO ET AL: "Coherent transfer of quantum information in a silicon double quantum dot using resonant SWAP gates", NPJ QUANTUM INFORMATION, vol. 5, no. 1, 29 November 2019, DOI: 10.1038/s41534-019-0225-0. SUMMARY It is an objective to present a quantum pro-cessing unit, a quantum computing system, and a method that enable reading out the states of a plurality of qubits without requiring extensive space for the readout-related circuitry. Another objective is to present a quantum processing unit, a quantum computing system, and a method that allow tailoring the readout operations for specific needs in systems that have a large number of qubits. A yet further obj