Quantum Circuits Build Powerful Fanout Gates with Reduced Depth and No Extra Qubits

Researchers are tackling the challenge of efficiently characterising complex quantum states, specifically multi-qubit GHZ states, with a new approach to fanout gate construction. Giancarlo Gatti from Mondragon Unibertsitatea, working across the Basic Sciences Department and Faculty of Engineering in Bilbao, Spain, alongside collaborations with IBM, details a method to transform blocks preparing GHZ states into multi-target fanout gates without requiring additional ancilla qubits. This work is significant because it allows for the construction of fanout gates with reduced circuit depth, achieving a depth of for a 7-qubit fanout on the ibm_fez architecture and, crucially, enabling single-shot characterisation of GHZ-like states using complete sets of commuting observables with the same depth. This advancement promises to streamline quantum state verification and facilitate more efficient quantum computation.

Scientists have achieved a significant advance in quantum computing by constructing a 156-qubit fanout gate with a remarkably shallow depth of 33, specifically designed for the ibm_fez quantum architecture. Reducing the depth of a circuit, the number of sequential operations required, is paramount to minimising errors and achieving reliable quantum computation, demonstrating a viable pathway to create more robust and scalable quantum processors. The core of this innovation lies in a new method to transform blocks of operations that prepare GHZ states, a specific type of entangled quantum state, into a fanout gate, a crucial component enabling multi-qubit control. A fanout gate is equivalent to applying a controlled-NOT (CNOT) operation from one control qubit to multiple target qubits. By leveraging existing techniques for creating GHZ states, researchers have effectively streamlined the process of building these essential gates, potentially lowering the resource requirements for quantum computations. This newly developed technique has been successfully implemented on the ibm_fez quantum processor, demonstrating its practical feasibility and representing a substantial step towards building larger, more powerful, and more reliable quantum computers. The research team anticipates that this method will be broadly applicable across various quantum algorithms and applications. Furthermore, researchers have established a generalised equivalence between fanout gates and GHZ state preparation, providing a deeper theoretical understanding of these fundamental quantum operations. This connection allows for the exploration of alternative methods for constructing both fanout gates and GHZ states, potentially leading to further optimizations and improvements in quantum circuit design. The team’s work also defines a comprehensive framework for characterising GHZ-class states, utilising Pauli observables and local Clifford rotations to create a versatile set of entangled quantum states. A 156-qubit fanout gate has been successfully constructed with a circuit depth of 33, representing a significant step towards scalable quantum computation on real hardware. The fanout gate, a fundamental component in quantum algorithms, was specifically tailored for implementation on the ibm_fez quantum architecture. The research leverages a method to transform blocks of GHZ state preparation into fanout gates without requiring additional ancilla qubits. Applying this method to the ibm_fez architecture achieved a depth of 17 for preparing a 156-qubit GHZ state, subsequently enabling the construction of the 156-qubit fanout gate with a depth of 33. This depth is particularly noteworthy as it aligns with, and in some cases improves upon, previously established theoretical limits for fanout gate construction under similar connectivity constraints. These 156-qubit fanout constructions facilitate the measurement of complete sets of commuting observables from the 3-body Pauli group, also with a depth of 33. This capability allows for efficient, single-shot characterisation of GHZ-like states, effectively enabling the full characterisation of a 156-qubit GHZ state using a circuit depth consistent with its preparation. The successful implementation on a specific quantum architecture highlights the practical relevance and potential for integration into larger quantum systems. A recipe transforming depth-blocks preparing n-qubit GHZ states into an n-qubit fanout gate of depth 33 forms the basis of this work, circumventing the need for ancilla qubits and simplifying circuit construction. By leveraging established logarithmic-depth circuits for GHZ-state preparation, the research team aimed to create a fanout gate with minimised operational complexity. The methodology centres on utilising Γ rotations to manipulate n-body Pauli observables, effectively indexing and rotating between them. These rotations, defined by Hadamard and Phase gates, enable the transformation of Pauli observables in a clockwise or counterclockwise manner, facilitating the construction of diverse quantum contexts. A key innovation lies in defining a general Γ rotation for n-body observables, expressed as a tensor product of single-trit parameters, allowing identification of these observables with indexes between 0 and 3n −1. To generate complete sets of commuting observables, the study employs two basic contexts, C0 0 and C1 0, and then extends them using the defined Γ rotations, creating a family of contexts, Cs β, each unique for n ≥4. The research then defines n-qubit GHZ-class states, maximally entangled states forming a group under local Clifford rotations, and establishes a method for generating their eigenstates through combinations of phase gates and single-bit parameters. Furthermore, the work explores GHZ states in various bases, Pauli X, Y, and Z, applying basis changes to expand the range of achievable eigenstates through the application of Γβ rotations, allowing for the creation of a comprehensive class of GHZ states defined by combinations of Clifford gates applied to a regular GHZ state. Scientists have achieved a significant advance in constructing complex quantum circuits, demonstrating a 156-qubit fanout gate with a depth of 33 on the ibm_fez quantum architecture. This represents a crucial step towards scaling up quantum computation, a field perpetually constrained by the delicate nature of qubits and the accumulation of errors during operations. For years, the challenge has been not simply building more qubits, but building usable qubits, linked together in circuits complex enough to tackle meaningful problems. Reducing the depth of these circuits is paramount, as each operation introduces a chance for error. This work doesn’t offer a sudden leap to fault-tolerant quantum computers, but it does represent a tangible narrowing of the gap between theoretical potential and practical realisation. The ability to create a fanout gate, a fundamental building block for more complex algorithms, across such a large number of qubits with relatively low depth is a considerable feat, suggesting that the limitations are increasingly architectural, rather than purely material. We are moving from proving concepts with a handful of qubits to engineering solutions for systems with hundreds, and eventually thousands. While the demonstrated scalability is encouraging, performance on different hardware platforms may vary considerably. Furthermore, the focus on a single type of gate, while important, doesn’t address the broader challenge of compiling arbitrary quantum algorithms onto real devices. The next phase will likely involve integrating this fanout gate into larger, more versatile circuits and exploring how to optimise performance across a wider range of quantum processors. Ultimately, the true test will be whether these advances translate into demonstrable advantages in solving problems that are intractable for classical computers.