Research

Developing reliable quantum hardware capable of executing quantum algorithms is paramount. One crucial metric assessing the efficacy of a quantum information storage unit is the relaxation time constant, denoted as T1, which essentially signifies the duration a unit can maintain information before complete loss. Conventional two-dimensional (2D) superconducting qubits, notably transmons, exhibit T1 values below 1 millisecond. In contrast, 3D superconducting cavities can achieve T1 values surpassing 1 second, presenting a significant three-order-of-magnitude improvement in lifetimes. Leveraging this substantial enhancement, we are in the process of constructing a quantum processor poised to deliver superior performance.

Quantum error correction (QEC) would be indispensable when scaling up qubits for building a full-fledged quantum computer. Autonomous quantum error correction (AQEC) is a hardware-efficient way to realize QEC without fast digital feedback control. Our scheme, called the star code, requires only two-photon processes to protect quantum information from single-photon loss error. The star code can be implemented using any linear coupler that can parametrically drive qutrit-qutrit red and blue sidebands. We demonstrate an improvement in the lifetime of the logical qubit when the error correction is turned on compared to the free decay case in a parametrically coupled two-transmon system.

References: Theory

Experiment

Grover's search algorithm provides quadratic speedup for an unstructured database search over classical algorithms. However, the original protocol is non-deterministic - there exists a finite chance of getting an incorrect answer. We have developed a modified version of Grover's search algorithm to obtain the correct answer deterministically without having user control over the quantum oracle. Our deterministic 2-parameter or "D2p" protocol utilizes only two phase parameters for the amplification step while using standard phase-flip for the oracle implementation. We achieve determinism with no more than one extra iteration compared to the original non-deterministic Grover's search providing identical quadratic speedup. This method works for an N-qudit system as well.

Reference: Phys. Rev. Research 4, L022013 (2022)

The strong ZZ coupling provided by the "Trimon" enables high-fidelity implementation of three-qubit gates like controlled-controlled-NOT (CCNOT). Moreover, the generalized controlled-controlled-phase (CCZ) gates can be realized with near perfection due to its virtual nature. Armed with these features, we demonstrate several three-qubit algorithms, including the famous Grover's search which provide quadratic speed-up. As examples, Deutsch-Jozsa, Bernstein-Vazirani and Grover’s search algorithms show success probabilities of 92%, 63% and 49% respectively, significantly exceeding the corresponding classical bounds of 50%, 25% and 25%.

Reference: Phys. Rev. Applied 14, 014072 (2020)

Achieving high-fidelity multi-qubit gates demand good connectivity and strong coupling between the qubits in a quantum processor. We introduced the concept of using multiple longitudinally coupled qubits as a building block for larger systems. These blocks, nicknamed "multimon", are comprised of multi-mode superconducting circuits providing strong all-to-all coupling. The multimons allow high-fidelity implementation of generalized controlled-NOT and error-free controlled-phase gates and measurement is done using joint dispersive readout schemes.

Reference: Phys. Rev. A 98, 052318 (2018)

We devised a new superconducting circuit, nicknamed "trimon", comprising of three oscillator modes which act as three transmon like qubits with pairwise ZZ coupling. This property leads to transition frequency of one qubit dependent on the state of the other qubits, making it very easy to realize controlled rotations. In the first part of this project, we demonstrated realization two-qubit controlled-NOT, swap and transfer gates along with preparation of entangled states with high fidelity. We also showed the variable coupling of the three qubits with a neighboring transmon qubit.

Reference: Phys. Rev. Applied 7, 054025 (2017)

We have developed a simple technique to enhance the bandwidth of a single SQUID-based JPA based on impedance engineering. The idea is to modify the input admittance as seen by the JPA in such a way that it cancels the inherent frequency dependence of gain and provides much larger bandwidth beyond what is allowed by natural gain-bandwidth product. We achieved nearly flat 640 MHz of bandwidth, which is about an order of magnitude larger than typical devices, at 20 dB gain. An image of our transformer along with the device performance are shown below. 

Reference: Appl. Phys. Lett. 107, 262601 (2015)