Session K39: Characterizing and Controlling Superconducting Circuits I

Protecting quantum information from noise – a passive approach

The steady improvement in coherence times and gate fidelities over the past several years has largely been due to reductions in noise and energy loss mechanisms. Achieving highly integrated quantum hardware, however, may necessitate tolerance of noisier signals and dirtier materials. Over the past couple of years, we have been looking at practical ways to design noise-resilience into quantum devices. In this talk, I’ll present theoretical work on methods for performing gates that are robust to control noise and that reduce qubit overhead and coupling complexity, building off of Bacon and Flammia’s Adiabatic Gate Teleportation technique. I’ll also talk about more fully noise-protected qubits and gates using blocks of qubits coupled together in Bacon-Shor codes.   

Quantum Information Processing with Stroboscopic Qubit Interactions

Microwave dressing of superconducting qubits arranges energy level structures that allow bath-engineered cooling, single photon detection, and electromagnetically induced transparency. Application of an additional stroboscopic dispersive tone can induce interactions analogous both to back-action evading measurements in cavity optomechanics and to stimulated Raman transitions in trapped ions. With such stroboscopic interactions, we have recently demonstrated the ability to measure the state of a superconducting transmon qubit via synthetic longitudinal coupling, to adjust dynamically the measurement axes, and to improve such measurements using squeezed vacuum. We now apply the stroboscopic interaction to realize an entangling gate for superconducting qubits, similar to entangling gates routinely employed for trapped ion qubits. This gate generates simultaneous multi-qubit entanglement via a shared photonic mode. The introduction of such periodic interactions in multi-particle systems under constant drive may additionally permit studies of out-of-equilibrium many-body states.   

Ion-Trap Inspired Entangling Gate for Superconducting Qubits

High-fidelity gates entangling multiple qubits can be an invaluable resource for implementing efficient error correcting quantum codes. Trapped-ion qubits are routinely entangled with a high degree of connectivity with fidelity greater than 99% using the Mølmer-Sørensen gate. Multi-partite entanglement is mediated through the interaction of many qubits with a shared phonon mode combined with two-photon transitions induced by bi-chromatic fields. In this talk, we present experimental progress on developing an analogous protocol for entangling two superconducting qubits using a shared microwave photon mode. Such functionality can be extended to multi-qubit entanglement and harnessed for classical qubit stabilization feedback.
  

Correlated randomized benchmarking

As quantum circuits increase in size one of the main challenges is how to properly characterize gates in a way that is algorithmically useful. In particular, it is difficult to characterize crosstalk, i.e., unwanted interactions between qubits. These terms would show up in a full tomographic reconstruction, but such techniques are not scalable. The most successful scalable technique is randomized benchmarking (RB). In RB the ground state population is measured after applying a random identity operator constructed from n Clifford gates. The qubit population decays exponentially as the number of Clifford gates n which can be used to infer the average gate fidelity irrespective of preparation and measurement errors. This method has been widely adapted and extended to measure simultaneous gate error, leakage, purity and specific gates via interleaving. Here we will discuss how to measure crosstalk errors by analysing the decay of correlated observables in simultaneous RB data (correlation RB). We show that the decay of the correlated terms is a measure of correlated errors by applying this technique to a four qubit fixed-frequency transmon device.   

A qubit-based oscilloscope for characterizing the distortion of flux pulses for two-qubit gates in circuit QED

In circuit QED, conditional-phase gates between flux-tunable Transmon qubits can be performed in 40 ns by pulsing fast adiabatically in and out of an avoided crossing in the two-excitation manifold. However, the flux pulses required are subject to distortions due to electrical components both at room temperature and inside the fridge. The room-temperature distortions can be characterized using a conventional oscilloscope and corrected using linear filtering. Correcting the cryogenic distortions is more challenging. We demonstrate a novel protocol that uses the pulsed qubit to directly measure the flux distortions on-chip, creating a cryogenic oscilloscope. We use the cryo-scope and numerical optimization to tune conditional-phase gates to the limit set by decoherence.   

Optimal Quantum Acceleration of Frequency Estimation Using Adaptive Coherent Control

Precision measurements of frequency are critical to accurate timekeeping, and are fundamentally limited by quantum measurement uncertainties. While for time-independent quantum Hamiltonians, the uncertainty of any parameter scales at best as 1/T, where T is the duration of the experiment, recent theoretical works have predicted that explicitly time-dependent Hamiltonians can yield a 1/T² scaling of the uncertainty for an oscillation frequency. This quantum acceleration in precision requires coherent control, which is generally adaptive. We experimentally realize this quantum improvement in frequency sensitivity with superconducting circuits, using a single transmon qubit. With optimal control pulses, the theoretically ideal frequency precision scaling is reached for times shorter than the decoherence time. This result demonstrates a fundamental quantum advantage for frequency estimation.   

A Quantum Kernel and Configurable Electronic System for Scalable Quantum Control and Measurement

Quantum computing architectures rely on classical electronics for control and readout. As the number of qubits increase in a quantum processor, so does the number of electronic channels needed for qubit control (gating and flux tuning), and measurement. In this talk we present an electronic system for scalable quantum control and measurement with applications to quantum computing.

The platform is designed to be configurable by experimental groups to match their needs based on the design and architecture of their quantum device. The platform is also designed to have low latency with provisions to allow for feedback and feedforward loops.

We also present the accompanying software (Quantum Kernel^TM) that compiles a quantum algorithm into necessary microwave and DC pulses. The Quantum Kernel also operates the electronics and the quantum computer. The Quantum Kernel is designed to allow experimental groups to perform fundamental studies on a quantum device, as well as providing a user-friendly platform for quantum computation by others.   

Experimental Implementation of Randomized Compilation

Coherent errors in gate operations account for an increasing percentage of computational error budgets as the gate depths of quantum algorithms increase. Randomized compiling is an efficient method for converting these coherent errors into stochastic Pauli errors, whose aggregate imperfection scales more favorably with gate count. This conversion, known as Randomized Compiling, is achieved via substitution of portions of the algorithm of interest with easily pre-computed equivalent operations. The procedure allows for an increase in the total fidelity of an algorithm with minimal classical overhead. We demonstrate randomized compiling with an implementation of the HHL algorithm on 4 transmon qubits in the presence of artificial coherent noise, and compare performance to that of the non-randomized version of the algorithm.
  

Crosstalk and drift-detection on the IBM Quantum Experience

A wide variety of noise processes can corrupt the performance of a quantum information processor (QIP). Certain noise processes can be caused by changing external variables that should not (but do) affect the quantum computation; such noise makes the QIP’s behavior context dependent. Two examples of context-dependent noise are drift (behavior changing with time) and crosstalk (behavior changing with operations on, and states of, other qubits). We used a unified framework for context dependence detection to look for both drift and crosstalk on IBM’s publicly accessible Quantum Experience devices. Small but detectable levels of both drift and crosstalk for certain single-qubit operations were found, while two-qubit CNOT gates induced crosstalk with a much stronger signal. We conclude that our method of context dependence detection is suitable for use in currently extant quantum information processor systems.   

Evacuating entropy in full-stack quantum computers

As quantum computers advance toward real world applications, schemes to maintain accurate calibration become essential for trustworthy results. In this talk, we present scalable techniques to detect and quantify low frequency instability in full-stack quantum computers based on superconducting qubits. We find that drifts in the system can have a measurable contribution to overall operational fidelities. We discuss lightweight automated re-calibration routines to correct for these errors based on correlations between the sources of instability and on chip properties. This method of stabilization allows full-stack quantum computers to maintain high fidelity gates.   

Detecting Drift, Change, and Context Dependence in Qubit Experiments

Quantum information processors are sensitive and precisely controlled devices. As a result, their behavior — especially the operations implemented by “logic gate” pulses — is very easily influenced by external influences. This context dependence includes familiar failure modes such as drift (where the context is time) and crosstalk (where the context is the state or situation of a neighbor qubit). Detecting and characterizing context dependence is important for at least two reasons: (1) it can constitute a significant source of error on its own; and (2) it can corrupt protocols designed to probe other errors, like randomized benchmarking and tomography. We present two techniques that we've been using to detect and characterize context dependence in experimental systems. The first is simple, and generic for discrete contexts (e.g., whether a neighboring qubit is driven or not); the second is specifically adapted to tracking drift over time. Both protocols are specifically designed to analyze count data, as produced by (e.g.) randomized benchmarking or tomography, rather than requiring specialized experiments.