We have studied the impact of low-frequency magnetic flux noise upon superconducting transmon qubits with various levels of tunability. We find that qubits with weaker tunability exhibitdephasing that is less sensitive to flux noise. This insight was used to fabricate qubits where dephasing due to flux noise was suppressed below other dephasing sources, leading to flux-independent dephasing times T2* ~ 15 us over a tunable range of ~340 MHz. Such tunable qubits have the potential to create high-fidelity, fault-tolerant qubit gates and fundamentally improve scalability for a quantum processor.
Nonreciprocal microwave devices such as circulators are useful in routing quantum signals in quantum networks and protecting quantum systems against noise coming from the detectionchain. However, commercial, cryogenic circulators, used nowadays, are unsuitable for scalable superconducting quantum architectures due to their appreciable size, loss, and inherent magnetic field. In this work, we report on the measurement of a key nonreciprocal element, i.e., the gyrator, which can be used to realize a circulator. Unlike state-of-the-art gyrators, which use a magneto-optic effect to induce a phase shift of π between transmitted signals in opposite directions, our device uses the phase nonreciprocity of a Josephson-based three-wave mixing device. By coupling two of these mixers and operating them in noiseless frequency conversion mode, we show that the device acts as a nonreciprocal phase shifter, whose phase shift is controlled by the phase difference of the microwave tones driving the mixers. Such a device could be used to realize a lossless, on-chip, superconducting circulator suitable for quantum information processing applications.
For superconducting qubits, microwave pulses drive rotations around the Bloch sphere. Here we show that the phase of these drives can be used to generate zero-duration arbitrary Z-gateswhich, combined with two Xπ/2 gates, can generate any SU(2) gate. We perform randomized benchmarking using a Clifford set of Hadamard and Z-gates and show that the error per Clifford is reduced versus a set consisting of standard finite-duration X and Y gates. Z-gates can also correct unitary rotation errors for weakly anharmonic qubits as an alternative to pulse shaping techniques such as DRAG. We investigate leakage and show that a combination of DRAG pulse shaping to minimize leakage and Z-gates to correct rotation errors (DRAGZ) realizes a 13.3ns Xπ/2 gate characterized by low error (1.95[3]×10−4) and low leakage (3.1[6]×10−6). Ultimately leakage is limited by the finite temperature of the qubit, but this limit is two orders-of-magnitude smaller than pulse errors due to decoherence.
Josephson parametric converters (JPCs) are superconducting devices capable of performing nondegenerate, three-wave mixing in the microwave domain without losses. One drawback limitingtheir use in scalable quantum architectures is the large footprint of the auxiliary circuit needed for their operation, in particular, the use of off-chip, bulky, broadband hybrids and magnetic coils. Here, we realize a JPC which eliminates the need for these bulky components. The pump drive and flux bias are applied in the new device through an on-chip, lossless, three-port power divider and on-chip flux line, respectively. We show that the new design considerably simplifies the circuit and reduces the footprint of the device while maintaining a comparable performance to state-of-the-art JPCs. Furthermore, we exploit the tunable bandwidth property of the JPC and the added capability of applying alternating currents to the flux line in order to switch the resonance frequencies of the device, hence demonstrating time-multiplexed amplification of microwave tones that are separated by more than the dynamical bandwidth of the amplifier. Such a measurement technique can potentially serve to perform time-multiplexed, high-fidelity readout of superconducting qubits.
The resonator-induced phase (RIP) gate is a multi-qubit entangling gate that allows a high degree of flexibility in qubit frequencies, making it attractive for quantum operations inlarge-scale architectures. We experimentally realize the RIP gate with four superconducting qubits in a three-dimensional (3D) circuit-quantum electrodynamics architecture, demonstrating high-fidelity controlled-Z (CZ) gates between all possible pairs of qubits from two different 4-qubit devices in pair subspaces. These qubits are arranged within a wide range of frequency detunings, up to as large as 1.8 GHz. We further show a dynamical multi-qubit refocusing scheme in order to isolate out 2-qubit interactions, and combine them to generate a four-qubit Greenberger-Horne-Zeilinger state.
A challenge for constructing large circuits of superconducting qubits is to balance addressability, coherence and coupling strength. High coherence can be attained by building circuitsfrom fixed-frequency qubits, however, leading techniques cannot couple qubits that are far detuned. Here we introduce a method based on a tunable bus which allows for the coupling of two fixed-frequency qubits even at large detunings. By parametrically oscillating the bus at the qubit-qubit detuning we enable a resonant exchange (XX+YY) interaction. We use this interaction to implement a 183ns two-qubit iSWAP gate between qubits separated in frequency by 854MHz with a measured average fidelity of 0.9823(4) from interleaved randomized benchmarking. This gate may be an enabling technology for surface code circuits and for analog quantum simulation.
We present improvements in both theoretical understanding and experimental implementation of the cross resonance (CR) gate that have led to shorter two-qubit gate times and interleavedrandomized benchmarking fidelities exceeding 99%. The CR gate is an all-microwave two-qubit gate offers that does not require tunability and is therefore well suited to quantum computing architectures based on 2D superconducting qubits. The performance of the gate has previously been hindered by long gate times and fidelities averaging 94-96%. We have developed a calibration procedure that accurately measures the full CR Hamiltonian. The resulting measurements agree with theoretical analysis of the gate and also elucidate the error terms that have previously limited the gate fidelity. The increase in fidelity that we have achieved was accomplished by introducing a second microwave drive tone on the target qubit to cancel unwanted components of the CR Hamiltonian.
The main promise of quantum computing is to efficiently solve certain problems that are prohibitively expensive for a classical computer. Most problems with a proven quantum advantageinvolve the repeated use of a black box, or oracle, whose structure encodes the solution. One measure of the algorithmic performance is the query complexity, i.e., the scaling of the number of oracle calls needed to find the solution with a given probability. Few-qubit demonstrations of quantum algorithms, such as Deutsch-Jozsa and Grover, have been implemented across diverse physical systems such as nuclear magnetic resonance, trapped ions, optical systems, and superconducting circuits. However, at the small scale, these problems can already be solved classically with a few oracle queries, and the attainable quantum advantage is modest. Here we solve an oracle-based problem, known as learning parity with noise, using a five-qubit superconducting processor. Running classical and quantum algorithms on the same oracle, we observe a large gap in query count in favor of quantum processing. We find that this gap grows by orders of magnitude as a function of the error rates and the problem size. This result demonstrates that, while complex fault-tolerant architectures will be required for universal quantum computing, a quantum advantage already emerges in existing noisy systems
The technological world is in the midst of a quantum computing and quantum information revolution. Since Richard Feynman’s famous „plenty of room at the bottom“ lecture,hinting at the notion of novel devices employing quantum mechanics, the quantum information community has taken gigantic strides in understanding the potential applications of a quantum computer and laid the foundational requirements for building one. We believe that the next significant step will be to demonstrate a quantum memory, in which a system of interacting qubits stores an encoded logical qubit state longer than the incorporated parts. Here, we describe the important route towards a logical memory with superconducting qubits, employing a rotated version of the surface code. The current status of technology with regards to interconnected superconducting-qubit networks will be described and near-term areas of focus to improve devices will be identified. Overall, the progress in this exciting field has been astounding, but we are at an important turning point where it will be critical to incorporate engineering solutions with quantum architectural considerations, laying the foundation towards scalable fault-tolerant quantum computers in the near future.
Decoherence of superconducting transmon qubits is purported to be consistent with surface loss from two-level systems on the substrate surface. Here, we present a study of surface lossin transmon devices, explicitly designed to have varying sensitivities to different surface loss contributors. Our experiments also encompass two particular different sapphire substrates, which reveal the onset of a yet unknown additional loss mechanism outside of surface loss for one of the substrates. Tests across different wafers and devices demonstrate substantial variation, and we emphasize the importance of testing large numbers of devices for disentangling di?erent sources of decoherence.