Our aim was to develop the concepts and perform the experiments to establish a clear path towards useful quantum coherent computation using superconducting flux-based qubits. We have concentrated upon using superconducting single flux quantum (SFQ) circuitry integrated on the qubit chip to act as the interface between a quantum circuit and laboratory electronics. We have developed SFQ circuits for initialization and for read-out of the quantum state of a qubit; for precise timing of duration and exact biasing all with picosecond-scale precision. We developed a set of techniques to design such qubit control circuitry while preserving proper isolation (not significantly contributing to decoherence). We also designed an SFQ experiment to investigate quantum behavior using foundry-fabricated chips at higher temperatures. We acquired and custom set-up a He3 refrigerator, and demonstrated SFQ circuit functionality at 300 mK for the first time. We have investigated a new method for quantum control both analytically and numerically, using isolated SFQ pulses instead of resonant excitation, and shown that the minimum operation time can be dramatically decreased ii some cases. However, the complete lack of custom fabrication has prevented us from making progress towards our experimental goals.
Our aim was to develop the concepts and perform the experiments to establish a clear path towards useful quantum coherent computation using superconducting flux-based qubits. We have concentrated upon using superconducting single flux quantum (SFQ) circuitry integrated on the qubit chip to act as the interface between a quantum circuit and laboratory electronics. We have developed SFQ circuits for initialization and for read-out of the quantum state of a qubit; for precise timing of duration and exact biasing all with picosecond-scale precision. We developed a set of techniques to design such qubit control circuitry while preserving proper isolation (not significantly contributing to decoherence). We also designed an SFQ experiment to investigate quantum behavior using foundry-fabricated chips at higher temperatures. We acquired and custom set-up a He3 refrigerator, and demonstrated SFQ circuit functionality at 300 mK for the first time. We have investigated a new method for quantum control both analytically and numerically, using isolated SFQ pulses instead of resonant excitation, and shown that the minimum operation time can be dramatically decreased ii some cases. However, the complete lack of custom fabrication has prevented us from making progress towards our experimental goals.
We report on the development and test of an integrated system composed of a flux qubit and a rapid single-flux quantum (RSFQ) circuit that allows qubit manipulation. The goal is to demonstrate the feasibility of control electronics integrated on the same chip as the qubit, in view of the application in quantum computation with superconducting devices. RSFQ logic relies on the storage and transmission of magnetic flux quanta and can be profitably used with superconducting qubits because of the speed, scalability, compatibility with the qubit fabrication process and low temperature environment. While standard RSFQ circuitry is well assessed, the application to quantum computing requires a complete rescaling of parameter values, in order to preserve the qubit coherence and reduce the power dissipation. In the system presented in this paper, the qubit role is played by a superconducting loop interrupted by a small dc SQUID, usually called a double SQUID, which behaves as a tunable rf-SQUID. Its energy potential has the shape of a double well, with the barrier between the wells controlled by magnetic flux applied to the inner dc SQUID. Here for the first time we report measurements at a base temperature of 370 mK in which flux control pulses with desired characteristics were supplied by a RSFQ circuit fabricated using non-standard parameters in the same chip as the qubit.
Fast on-chip control of superconducting qubits has engaged complex and power consuming RSFQ circuits that currently pose more of an experimental burden than an asset. Measurements of quantum coherent oscillations of qubits require dilution refrigerator temperatures. The motivation of this design is to minimize the necessary bias leads and power dissipation for an SFQ based control circuit. Elimination of redundant circuit elements by innovative use of fundamental elements allows small-scale control circuitry.
Superconducting single flux quantum (SFQ) circuit can operate at very low temperature. This is suitable for controlling a quantum computing system with Josephson junctions. However, it is difficult to integrate both SFQ circuits and qubits into a single-chip, because of the dissipative characteristics of SFQ circuits. Therefore, we have developed a multi-chip packaging technology for a qubit control module. The module consists of SFQ circuit chips, qubit chips, and a substrate all of which are fabricated with Nb and Al technology. The chips are flip-chip bonded with superconducting solder bumps. We also investigated SFQ control circuits for superconducting qubits and circuit parameter optimization for sub-Kelvin temperature operation. Using both multi-chip packaging and optimized SFQ control circuit makes the design of qubit control module more flexible.
An approach for scalable quantum computing infrastructure based on the use of low-power digital superconducting single flux quantum (SFQ) circuits is described. Rather than replicating the room-temperature microwave control and measurement infrastructure solutions dominating the current systems, we use the inherent to superconducting technology methods - the use of SFQ pulses directly at the base temperature. For qubit control, we irradiate qubits with the coherent SFQ pulse sequences computed using optical control theory. For qubit measurement, Josephson photon counter performs projective quantum measurement, the result of which is converted to digital SFQ output. These operations are aided by a high-speed digital SFQ coprocessor located at higher temperature stage (e.g., 3 K) to process the measurement results and load new control sequences to 20 mK SFQ quantum-classical interface circuits.
The convergence of quantum computing and high-performance computing (HPC) necessitates innovative approaches to qubit control and coherence for scalable quantum systems. In this work, we explore the integration of superconducting Single-Flux Quantum (SFQ) digital circuits to enhance quantum state manipulation, leveraging their potential for high-speed, energy-efficient classical control. Our approach maps SFQ pulse trains and configurable SFQ-based switches to discrete phase shifts, enabling efficient qubit state transformations with minimal decoherence overhead. We introduce a classically driven SFQ-based quantum gate implementation, where SFQ pulses orchestrate the state evolution of qubits. To support dynamic and flexible qubit control within hybrid quantum-HPC architectures, we design and implement a multi-frequency SFQ pulse generator, capable of generating tailored pulse sequences that seamlessly interface with SQUID-based resonator-qubit cavities. This work demonstrates a promising path toward integrating quantum accelerators with classical HPC infrastructure, opti-mizing quantum-classical workflows, and enabling scalable, high-performance quantum computation.
In the quest to build a fault-tolerant quantum computer, superconducting circuits based on Josephson junctions have emerged as a leading architecture. Coherence times have increased significantly over the last two decades, and processors with ∼ 50 qubits have been experimentally demonstrated. These systems traditionally utilize microwave frequency control signals, and heterodyne based detection schemes for measurement. Both of these techniques rely heavily on room temperature microwave generators, high-bandwidth lines from room temperature to millikelvin temperatures, and bulky non-reciprocal elements such as cryogenic microwave isolators. Reliance on these elements makes it impractical to scale existing devices up a single order of magnitude, let alone the 5-6 orders of magnitude needed for performing fault-tolerant quantum algorithms. Here, I present results that suggesting superconducting digital logic, namely Single Flux Quantum (SFQ) logic, can replace analog control and measurement techniques, avoiding the significant overhead involved. I describe a scheme for measuring qubits with a device known as a Josephson Photomultiplier (JPM), which crucially stores the result of a qubit measurement in a classical circulating supercurrent within the device and allows for integration with SFQ detection circuitry. This technique is experimentally demonstrated, with single-shot measurement fidelity of 92%. Two methods for accessing this measurement result are presented, one utilizing ballistic fluxons, and another utilizing flux comparison. Initial experimental results of the latter are presented. In addition, I describe a scheme for controlling qubits with sequences of digital SFQ pulses. This method is then used to control a qubit without a microwave signal generator, with results of an average single-qubit gate fidelity of around 95%. When combined, these techniques form a nearly fully digital interface to superconducting qubits, which could allow these systems to scale much more easily.