Enhancing the Yield of Bucket Brigade Quantum Random Access Memory using Redundancy Repair

TL;DR

This paper proposes a novel quantum memory architecture that compensates for defective qubits by introducing redundant qubits, and demonstrates that for the qRAM comprising 1024 logical qubits, eight redundant logical qubits improved the yield by 95.92% from that of qRAM not employing the redundant repair scheme.

Abstract

Quantum Random Access Memory (qRAM) is an essential computing element for running oracle-based quantum algorithms. qRAM exploits quantum superposition to access all data stored in the memory cells simultaneously and guarantees the superior performance of quantum algorithms. A qRAM memory cell comprises logical qubits encoded through quantum error correction technology for successful operation against various quantum noises. In addition to quantum noise, the low-technology nodes based on silicon technology can increase the qubit density and may introduce defective qubits. As qRAM comprises many qubits, its yield will be reduced by defective qubits; these qubits must be handled using QEC scheme. However, the QEC scheme requires numerous physical qubits, which burdens resource overhead. In this paper, to resolve this overhead problem, we propose a novel quantum memory architecture that compensates for defective qubits by introducing redundant qubits. We also analyze the yield improvement offered by our proposed quantum memory architecture by varying the ideal fabrication error rate from 0.5% to 1% for different numbers of logical qubits in the qRAM. We demonstrate that for the qRAM comprising 1,024 logical qubits, eight redundant logical qubits improved the yield by 95.92% from that of qRAM not employing the redundant repair scheme.

Figures (7)

• Figure 1: Bucket Brigade qRAM architecture example (left) and occurrence of fabrication defects on surface code (right)
• Figure 2: Fabrication of qRAM with wafers (left) and the relationship between the number of logical qubits qRAM yield (right)
• Figure 3: Overall architecture of our proposed qRAM with redundant repair scheme and an external device ATE. All the data and states transferred to/from different parts are quantum. The ATE sends information of faulty addresses and spare addresses from the FAT to the quantum oracle of the redundant repair circuit. Based on two different types of address information, quantum oracle implements address comparison and address replacement parts, respectively. When the superposition of input addresses is given as input to qRAM, address comparison compares faulty addresses from FAT with each of the input addresses and sets the value of the repair flag. Checking the repair flag, the address replacement decides whether to replace or not replace the input address with the spare address. After replacing all faulty addresses, the redundant repair circuit passes the superposition of memory addresses to the address routing circuit for routing. Memory locations are then given to the memory cells, which communicate with read/write circuit data to read the memory cell data (Quantum) and write the input data (Quantum) to the memory cell.
• Figure 4: Example of our proposed redundant repair algorithm. When the superposition of addresses including faulty addresses is given as input the quantum oracle does address comparison and address replacement based on FAT. The output of the quantum oracle is the superposition of addresses as well. For faulty addresses, the repair flag qubit is activated to route spare memory qubits. original memory qubits will be routed.
• Figure 5: Quantum circuit example of the proposed qRAM consisting of redundancy recovery circuit, address routing circuit, read/write circuit, and memory cell. This circuit is an example of an input address represented by two logical qubits. Parts highlighted with a yellow background are essential to support redundant repair schemes.