Scientists develop methods to speed up quantum measurements using space-time trade-offs

To speed up quantum measurement, the study of a new physics review letter proposes a space-time trade-off scheme that is very useful for quantum computing applications.

Quantum computing has several challenges, including error rates, qubit stability, and scalability beyond a few qubits. However, one lesser known challenge is the face of quantum computing, fidelity and speed of quantum measurement.

The researchers in this study address this challenge by using additional or auxiliary kibits to significantly reduce measurement times while maintaining or improving measurement quality.

The work, led by Christopher Collett, Professor Noah Linden and Dr. Paul Skulziptzik of the University of Bristol, was a joint effort that included the University of Oxford, Strathclyde and Sorbonne.

Phys.org spoke to Corlett, Professor Linden and Dr. Skrzypczyk about their work.

“The measurement process of quantum mechanics is one of its most important and attractive features. It is also important for future quantum technology,” explained Corlett.

“Accurate and fast quantum measurements are important for the development of emerging quantum technologies. The recent important results of quantum error correction demonstrate the need for fast and accurate measurements to facilitate error decoding.

Measurement Challenge

There are endless measurements that can be performed with qubit. Of particular importance is to determine whether it is one of two natural states. 0 or 1. Accurately performing this measurement usually involves investigating Qubit for a long time.

These longer measurements usually provide greater accuracy, but they also provide significant overhead and delay. In particular, there is a problem with the central circuit measurements required for quantum error correction. Furthermore, long measurements introduce noise and decoherence that can accumulate during this period.

Researchers explain this by analogy.

“Imagine seeing two glasses of water, one glass with 100 mL of 1 glass, the other with 90 mL of photos.

“If you’re only showing a photo for a second, you might have a hard time telling which glass is full. But if you’re showing the photo for two seconds, you might feel more confident about which glass is full,” Corlett explained.

The researchers used auxiliary qubit to amplify the amount of information the measurement could collect at a fixed time.

It’s like double the volume of each glass. A 20 mL difference would be easier to observe than a 10 mL difference. It gives you more confidence in the answer. As this process continues and the amount of information continues, the time required for responses decreases.

“If we continue to use the analogy, adding a second auxiliary qubit triples the volume to 300 ml and 270 ml, and we can confidently distinguish it in 0.66 seconds. In this way, we can get a linear increase in read speed,” explained Professor Collett.

Space trading hours

The researcher’s scheme is based on previous protocols that use repeated codes for error correction. This method involves the target qubit (to perform the measurement) using the auxiliary key bits.

More specifically, the target qubit is entangled with the N-1 auxiliary kibit. Information from the target qubit is copied into all auxiliary keybits using so-called CNOT gates.

This is where innovation is. Instead of measuring the target qubit at time t, all n-chrysanthemum (target and auxiliary) are measured simultaneously for T/N time. Next, all measurements are added for the composite results, providing the same statistical reliability as a longer single measurement.

Space (the number of Qubits used) is being traded for time. A single kit measurement for 5 seconds is the same as measuring five qubits simultaneously for one second.

“Amazingly, this allows us to maintain or enhance the quality of our measurements. The scheme is widely applied to a wide range of major quantum hardware platforms, such as cold atoms, confined ions, and superconducting Qubits,” Corlett said.

Robust to noise

The researchers first investigated the scheme in ideal conditions with no noise, then realistic noise models. They found that the ideal case showed a total linear speedup with the number of Qubits.

Additionally, the noise model showed significant speedups and could be better than linear improvements. Researchers have shown that their approach can achieve higher maximum measurement quality than before.

“To ensure that the scheme is robust to this noise is extremely important as it ensures that noise is useful for inevitable real-world implementations,” Professor Linden said.

Researchers want to see experimental implementations of their schemes and are working to develop them in greater detail for specific systems such as superconducting Qubits.

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