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IEEE Transactions on Applied Superconductivity

Picosecond On-Chip Qubit Control Circuitry

Thomas Ohki, M. Wulf, Mark F. Bocko

Published Jun 1, 2005• 17 citations
Publisher

Abstract

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.

DOI
10.1109/tasc.2005.850076

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IEEE Transactions on Applied Superconductivity•Jun 1, 2007•6 citations
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.
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.
Abstract—We believe that the best chance to observe macroscopic quantum coherence (MQC) in an rf-SQUID qubit is to use on-chip RSFQ digital circuits for preparing, evolving and reading out the qubit’s quantum state. This approach allows experiments to be conducted on a very short time scale (sub-nanosecond) without the use of large bandwidth control lines that would couple environmental degrees of freedom to the qubit, thus contributing to its decoherence. In this paper we present our design of an RSFQ digital control circuit for demonstrating MQC in an rf-SQUID. We assess some of the key practical issues in the circuit design including the achievement of the necessary flux bias stability. We present an "active" isolation structure to be used to increase coherence times. The structure decouples the SQUID from external degrees of freedom, and then couples it to the output measurement circuitry when required, all under the active control of RSFQ circuits. Index Terms—superconducting qubit, macroscopic quantum coherence, RSFQ control.
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.
We believe that the best chance to observe macroscopic quantum coherence (MQC) in an rf-SQUID qubit is to use on-chip RSFQ digital circuits for preparing, evolving and reading out the qubit's quantum state. This approach allows experiments to be conducted on a very short time scale (sub-nanosecond) without the use of large bandwidth control lines that would couple environmental degrees of freedom to the qubit, thus contributing to its decoherence. In this paper we present our design of an RSFQ digital control circuit for demonstrating MQC in an rf-SQUID. We assess some of the key practical issues in the circuit design including the achievement of the necessary flux bias stability. We present an "active" isolation structure to be used to increase coherence times. The structure decouples the SQUID from external degrees of freedom, and then couples it to the output measurement circuitry when required, all under the active control of RSFQ circuits.
Superconducting single flux quantum (SFQ) technology is well known for digital logic operations below 4.2 K, which are compatible with the Josephson junction qubit. Operation speed and signal sensitivity of circuits in SFQ are also sufficient to control qubits. Therefore, the SFQ circuit is a candidate technology for multiple qubit control. However, the self-heating effect of the SFQ circuit is a potential barrier to development of an SFQ-supported multiple qubit system because the qubit operating temperature is lower than the conventional 4.2 K. The multi-chip approach is a possible solution because the qubit chip can be thermally isolated from the SFQ circuitry. We report on recent progress in SFQ circuit and packaging technology for operation at sub-Kelvin temperatures. As for multi-chip module technology, a test sample of the multi-chip module shows good reliability of electrical connections between chips, and each solder bump has a bandwidth wide enough for signal transmission. We have designed lower power SFQ circuits for sub-Kelvin temperatures. The critical currents of Josephson junctions were scaled down, inductances were scaled up, and bias voltage was reduced to make power consumption lower. Based on our fabrication technology, critical current densities were optimized by considering trade-off relations between fabrication availability and operation speed. We successfully confirmed its correct operations at around 0.35 K. The combination of the multi-chip module and optimized SFQ circuits is very effective way of controlling qubit.