Research Roundup for May 2022


from dr Chris Mansell


Title (1): Ytterbium nuclear spin qubits in an optical tweezer array
Organizations (1): JILA; University of Colorado; National Institute for Standards and Technology
Link (1):
Title (2): Universal Gate Operations on Nuclear Spin Qubits in an Optical Tweezer Array of 171-Yb Atoms
Organization (2): Princeton University
Link (2):

Most cold atom quantum information experiments have focused on alkali atoms because they have a relatively simple energy level structure. Now that the quantum states of such atoms can be manipulated with great precision, it is time to explore the potential benefits of atoms with richer, more complex energy level structures. Against this background, two different research groups have reported experiments with ytterbium-171. The first publication describes the record-breaking efficiency of loading atoms into optical traps. The second publication details a new “magic” trapping wavelength that kept atoms coherent two to three orders of magnitude longer than alkali metals. Taken together, the results show that reconfigurable arrays of ytterbium-171 atoms are a very promising qubit system to which fast, high-precision, single-qubit logic gates can be applied, although more work needs to be done to realize the two-qubit improve goals.

Title: Construction and coherent control of a registry of nuclear spin qubits
Organization: Atom Computing, Inc.
Other results on cold-atom processors include recent work on the arrangement and coherent control of Sr-87 atoms. This isotope of strontium has been used in atomic clocks and could make an effective quantum processor due to its two valence electrons and non-zero nuclear spin. The researchers performed 10^5 operations within the coherence time of their individually addressable system called Phoenix. This ratio of coherence time to gate time is not as high as in ion trap quantum computers, but better than in solid state platforms. They achieved this without the use of magnetic shielding, dynamic decoupling, composite pulses, or even pulse shape optimization. They estimate that adding these techniques could allow 10 8 gates before decoherence occurs.

Title: High-Fidelity Three-Qubit iToffoli Gate for Fixed-Frequency Superconducting Qubits
Organizations: Lawrence Berkeley National Laboratory; University of California, Berkeley
The Toffoli gate implements a controlled-controlled-NOT operation which, in combination with the Hadamard gate, is universal for quantum computers. One of the main advantages of multiqubit gates like the Toffoli is that they can reduce the total number of gates in quantum circuits. In general, gates that rely on the simplest physical mechanisms for their implementation have the highest fidelity and are the most useful. In this work on a superconducting processor, cycle benchmarking was used to determine that a single-level “iToffoli” gate scheme has higher process fidelity—just over 98%—than previous three-qubit superconducting gates. This accuracy could be further increased by studying different techniques to suppress the ZZ interactions identified as the main cause of decoherence. Importantly, the gate could be easily applied to commercial quantum chips accessible via the cloud.

Title (1): Intracavity Rydberg Superatom for Optical Quantum Engineering: Coherent Control, Single-Shot Detection, and Optical π Phase Shift
Organizations (1): PSL University; Sorbonne University
Link (1):
Title (2): Quantum logic gate between two optical photons with an average efficiency of over 40%
Organization (2): Max Planck Institute for Quantum Optics
Link (2):

The efficiency of a two-photon quantum logic gate is the probability that neither the control photon nor the target photon will be lost during the process. The record for this efficiency since 2003 is 11%. Attempts to improve it have involved either optical cavities or collective excitations of cold atoms. Even combinations of these approaches failed to break the record. That is, until this month, when two groups achieved efficiencies in excess of 40%. They could also manipulate the phase of the target photon by up to 180 degrees and determine the state of the system non-destructively with 95% single-shot accuracy. Deciding what the next steps are will be extremely interesting because there are so many possibilities: optimizing its function as a two-qubit or multi-qubit logic gate; Development of a quantum repeater for quantum communication networks; or adapt it to coherently convert photons between the microwave and optical regimes so that superconductors and cold atoms can be bonded together.

Title: Qubit teleportation between non-neighboring nodes in a quantum network
Organizations: QuTech and Kavli Institute of Nanoscience, Delft University of Technology
The paper describes how the researchers were able to realize quantum teleportation between distant, non-neighboring nodes in a quantum network. The network uses three optically connected nodes based on solid-state spin qubits. The teleporter is prepared by establishing remote entanglement on the two links, followed by an entanglement exchange on the middle node and storage in a storage qubit. They demonstrate that once the successful preparation of the teleporter is announced, arbitrary qubit states can be teleported beyond the classical limit with accuracy, even with unit efficiency. These results are made possible by key innovations in the qubit readout process, active storage qubit protection during entanglement generation, and tailored announcements that reduce remote entanglement infidelity.
Link: Qubit teleportation between non-neighboring nodes in a quantum network | Nature


Title: Beyond Barren Plateaus: Quantum Variation Algorithms Are Awash With Traps
Organization: Massachusetts Institute of Technology
Classical neural networks are fairly easy to train, also because their loss functions have local minima that are very similar to the global minimum. Unfortunately, parameterized quantum algorithms appear to be more difficult to train in many circumstances. For example, deep quantum circuits can have regions in their optimization landscapes with extremely shallow gradients, making it difficult to figure out where the minima might be. While intense research into these “barren plateaus” is underway, this month’s novel research has found classes of quantum circuits where local minima cluster far away from the global minimum. The researchers showed that this can happen even on flat stretches that don’t have barren plateaus. Nevertheless, there is hope if the optimization starts at a cleverly chosen point in the damage landscape or if the problem is highly symmetrical.

Title: Fault Tolerant Quantum Computing with Low Overhead and Long Distance Connectivity
Organization: University of Sydney
If error correction protocols cannot be improved, fault-tolerant quantum computers may require millions of physical qubits to solve problems of practical interest. The authors of this paper show how quantum low-density parity check (LDPC) codes combined with long-range interactions could significantly reduce the resource requirements for fault tolerance. Of course, implementing high-fidelity entanglement operations between distant qubits is a considerable challenge, but many quantum architectures have made strides on this front. Rather than analyzing how their scheme behaves when approaching a theoretical limit, the authors estimate the significant resource savings it would allow for today’s typical device sizes. Importantly, the work on LDPC codes seems to offer room for further development and improvement.

Title: Deep Learning of Quantum Many-Body Dynamics by Random Driving
Organisations: Max Planck Institute for the Physics of Light; University of Erlangen-Nuremberg; Shanghai Jiao Tong University; Shanghai Quantum Science Research Center
Predicting quantum dynamics with a classical setup is extremely computationally intensive. However, when a quantum system has only local interactions, most of the quantum states cannot be reached on a practical time scale in the exponentially large Hilbert space of the system. Tracking this truncated set of states makes classical simulation a little more workable. This approach can be taken to extremes by completely ignoring the quantum states and only using the past evolution of expectation values ​​(ie measurement results) to predict their future evolution. A classical algorithm that follows this approach would have to find an implicit and compressed representation of the information in the quantum state. Fortunately, neural networks have had great success in autonomously discovering compressed forms of other data streams. In this paper, the authors successfully trained a deep recurrent network to predict the behavior of different spin systems. They write that their work could be used to predict the response of qubits to optimized impulses and feedback-based control schemes.

Title: Industrial Applications of Quantum Computing with Neutral Atoms to Solve Independent Set Problems
Organization: QuEra Computing Inc.
If you’ve heard that many areas of a business could improve their operations by solving problems of their own, but never found a helpful introduction to the topic, then this is the paper for you. The distinction z. B. between using a variational algorithm to find the maximum independent set to optimize the placement of radio antennas and using a sampling algorithm to find the maximum An independent set to optimize the construction of retail stores may seem quite subtle. However, everything is made very clear. Individual sample applications are detailed and linked to the latest quantum protocols, allowing the potential benefits of a quantum approach to be assessed.

Title: Error Resistant Quantum Amplitude Estimation from Parallel Quantum Phase Estimation
Organization: JoS QUANTUM
This paper shows how phase and amplitude estimation algorithms can be parallelized. This can reduce the gate depth of the quantum circuits to that of a single Grover operator with little overhead. Furthermore, we show that for quantum amplitude estimation, parallelization can lead to significant improvements in quantum error resilience. The resiliency is not caused by the smaller gate depth, but by the structure of the algorithm. Even in cases with errors that make it impossible to extract the exact or approximate solutions from traditional amplitude estimation, our parallel approach provided the correct solution with a high probability. The error resilience results apply to the standard and shallow versions of the quantum amplitude estimation.

May 28, 2022


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