Fidelity of 99-! Superconducting Quantum Bit Control Precision Breaks Through Limits

Single flux quantum (SFQ) digital logic has been proposed for scalable control of next-generation superconducting quantum bit arrays; scalable systems for controlling quantum bits have extremely low error rates, which is critical for fabricating practical devices.

A major obstacle to the development of practical quantum computers has been the difficulty of scaling up to make devices with a large number of quantum bits and providing accurate results even in the presence of ambient noise.

Now, a joint team of researchers from the University of Wisconsin-Madison, Syracuse University, the National Institute of Standards and Technology (NIST), the University of Colorado, and others report a significant improvement in the accuracy of a technique known to scale up much more easily than conventional techniques: this alternative technique uses a magnetic flux quanta called the " a unit of magnetic flux to control conventional superconducting quantum bits; the physical separation of the control circuit from the quantum bits reduces the error rate.

With further improvements, magnetic flux quanta technology could provide a superior pathway to practical quantum computing.

On July 24, the experimental results were published as "Single Flux Quantum-Based Digital Control of Superconducting Qubits in a Multichip Module" in PRX QUANTUM.

Quantum Classical Multichip Module (MCM). (a) Micrograph of a quantum bit chip. (b) Micrograph of SFQ driver chip. (c) Photograph of the assembled MCM stack; the quantum bit chip is outlined in red and the SFQ chip in blue. (d) Circuit diagram of a pair of quantum bit-SFQs.

Much of the current work performing quantum logic operations (the basic unit of computation) uses short microwave pulses to control quantum bits; however, it is currently difficult to scale this technique beyond 1000 quantum bits. According to some estimates, the presence of ambient noise requires that error correction methods rely on a large number of quantum bits: perhaps a million or more, in order to obtain an effective error correction system that can perform useful computations.

In an alternative approach to building quantum systems, a single flux quanta-the smallest unit of magnetic flux produced in a superconducting device-is used to control the quantum bits. The researchers believe that this quantum bit control technique is easier to scale up than microwave control because the hardware consumes much less power, which reduces the amount of cryogenic cooling power required - a major problem for large quantum computing systems.

Any quantum computation involves a sequence of elementary logic operations, each of which changes the state of a quantum bit in a specific way. A key challenge in developing single-flux quantum technology is demonstrating the ability to perform these operations accurately.

In previous research, Robert McDermott and colleagues at the University of Wisconsin-Madison demonstrated 91 percent accuracy, a result that others have since improved to nearly 98 percent. Now, McDermott, graduate student Chuan-Hong Liu, and their colleagues have taken the technique a step further: achieving more than 99 percent fidelity by placing the flux-generating quantum device on a chip that is physically different from the one that supports the quantum bits that perform the operations.

The research team says, "This physical separation reduces the interference between the flux pulse generator and the quantum bits."

To demonstrate this improvement, the researchers fabricated two flat chips placed in parallel to form a "sandwich"-like structure. On the upper chip, they created two transmon quantum bits, each capable of storing a quantum piece of information using the magnetic flux in a superconducting circuit. The lower chip has two similar superconducting circuits, each of which forms the basis of a single magnetic flux quantum generator that can change the state of the quantum bits on the upper chip by sending pulses of magnetic flux. The researchers connected the two wafers through a series of narrow indium (indium) bridges - a superconducting connection designed to prevent undesirable physical disturbances from the generator, in particular electron-vacancy excitations and vibrational quanta known as phonons, from affecting the quantum bits.

Micrographs show the two chips, the top one containing two quantum bits (left) and the bottom one containing two devices capable of emitting individual magnetic flux quanta (right). These magnetic flux quanta can cause controlled changes in the quantum bits to perform logical operations. Placing the generators and quantum bits on separate chips reduces the likelihood of errors by minimizing unwanted interference with the quantum bits.

In a series of tests, the researchers measured the ability of the flux pulse generator to trigger precise logic operations on the quantum bits when various operating parameters were changed. These parameters included the drive current of the pulse generator and its operating frequency. After finding the optimal parameter settings, they tested the accuracy of the pulse generator in driving the desired logic operations and averaged the results of trials with a range of initial quantum bit states.

Experimental line diagram

Complete quantum bit control In their initial work to optimize the flux-quantum driver, the researchers tested their ability to control quantum bits using two interleaved sequences of flux-quantum pulses that perform complementary operations. In the circular image, the radial distance from the center outward reflects the time interval between the pulses, while the angle around the circle shows the relative phase or time offset between the two different sequences. The colors correspond to changes in the state of the quantum bits: blue indicates no change at all; red indicates a complete flip to an orthogonal state. The results show that there are eight equivalent pulse timing and phase choices that can cause a complete flip, meaning that the researchers have complete control over the state of the quantum bit.

Characterization of incoherent errors.

Overall, the team found that the flux generator produced erroneous results in 1.2% of all cases-an order of magnitude less than the gate error when controlling quantum bits based on SFQ; and twice as much as the data reported last year by another research group (2.1% ).

In what is undoubtedly a great achievement, the experimental team has advanced the development of SFQ-based digital control of superconducting quantum bits. By separating quantum bits and classical control elements on different chips in the MCM architecture, the team achieved a Clifford gate error of 1.2(1)%.

In future work, the team intends to improve the setup to further minimize interference between the chips. "With these optimizations, we should be able to achieve 99.9 percent gate fidelity, or even 99.99 percent gate fidelity by optimizing the pulse sequence."

Reference link:

[1]https://physics.aps.org/articles/v16/128

[2]https://journals.aps.org/prxquantum/abstract/10.1103/PRXQuantum.4.030310

[3]https://journals.aps.org/prxquantum/abstract/10.1103/PRXQuantum.3.010350

[4]https://research.ibm.com/publications/superconducting-qubit-control-with-single-flux-quantum-pulses-in-a-multi-chip-module

[5]https://arxiv.org/abs/2301.05696