By Dr. Chris Mansell

Hardware

Title (1): Ytterbium Nuclear-Spin Qubits in an Optical Tweezer Array
Organizations (1): JILA; University of Colorado; National Institute of Standards and Technology
Link (1): https://journals.aps.org/prx/abstract/10.1103/PhysRevX.12.021027
Title (2): Universal Gate Operations on Nuclear Spin Qubits in an Optical Tweezer Array of 171-Yb Atoms
Organization (2): Princeton University
Link (2): https://journals.aps.org/prx/abstract/10.1103/PhysRevX.12.021028

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 high fidelity, the time is right to explore the possible benefits of atoms with richer, more complex energy level structures. Given this, two different research groups have reported on experiments with Ytterbium-171. The first paper describes record-breaking efficiency for the loading of atoms into optical traps. The second paper gives the details of a new “magic” trapping wavelength that kept the atoms coherent for two to three orders of magnitude longer than alkali metals. Taken together, the results indicate that reconfigurable arrays of Ytterbium-171 atoms are a very promising qubit system, on which fast, high-fidelity single-qubit logic gates can be applied, although more work needs to be done to improve the two-qubit gates.

Title: Assembly and coherent control of a register of nuclear spin qubits
Organization: Atom Computing, Inc.
Further results on cold atom processors include recent work on the assembly and coherent control of Sr-87 atoms. This isotope of strontium has been used in atomic clocks and could constitute 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, named Phoenix. This coherence time to gate time ratio is not as high as in ion trap quantum computers but is better than those found in solid-state platforms. They achieved this without the use of magnetic shielding, dynamic decoupling, composite pulses or even optimising the pulse shape. They estimate that adding in these techniques could enable 10^8 gates before decoherence occurs.
Link: https://www.nature.com/articles/s41467-022-29977-z

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 that, in combination with the Hadamard gate, is universal for quantum computation. One of the key benefits of multiqubit gates like the Toffoli is that they can reduce the total gate count of quantum circuits. In general, gates that rely on the simplest physical mechanisms for their implementation have the highest fidelities and are the most useful. In this work on a superconducting processor, cycle benchmarking was used to find that a single-step “iToffoli” gate scheme had a higher process fidelity – a little over 98% – than previous three-qubit superconducting gates. This fidelity could be further increased by investigating different techniques to suppress the ZZ interactions that were identified as the main cause of decoherence. Importantly, the gate could readily be applied to commercial quantum chips that are accessible via the cloud. 
Link: https://www.nature.com/articles/s41567-022-01590-3

Title (1): Intracavity Rydberg Superatom for Optical Quantum Engineering: Coherent Control, Single-Shot Detection, and Optical π Phase Shift
Organizations (1): PSL University; Sorbonne Université
Link (1): https://journals.aps.org/prx/abstract/10.1103/PhysRevX.12.021034
Title (2): Quantum-Logic Gate between Two Optical Photons with an Average Efficiency above 40%
Organization (2): Max-Planck-Institut für Quantenoptik
Link (2): https://journals.aps.org/prx/abstract/10.1103/PhysRevX.12.021035

The efficiency of a two-photon quantum logic gate is the probability that neither the control photon nor the target photon are lost during the procedure. The record for this efficiency has stood at 11% since 2003. Attempts to improve upon it have involved either optical cavities or collective excitations of cold atoms. Even combinations of these approaches have failed to break the record. That is, until this month when two groups achieved efficiencies of over 40%. They could also manipulate the phase of the target photon by up to 180 degrees and nondestructively determine the state of the system with a single-shot fidelity of 95%. Deciding what the next steps are will be extremely interesting because there are so many possibilities: optimising its function as a two-qubit or multiqubit logic gate; developing it into a quantum repeater for quantum communication networks; or adapting it to coherently convert photons between the microwave and optical regimes so that superconductors and cold atoms can interface with each other.

Title: Qubit teleportation between non-neighbouring 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 remote, non-neighbouring 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 entanglement swapping on the middle node and storage in a memory qubit. They demonstrate that, once successful preparation of the teleporter is heralded, arbitrary qubit states can be teleported with fidelity above the classical bound, even with unit efficiency. These results are enabled by key innovations in the qubit readout procedure, active memory qubit protection during entanglement generation and tailored heralding that reduces remote entanglement infidelities.
Link: Qubit teleportation between non-neighbouring nodes in a quantum network | Nature

Software

Title: Beyond Barren Plateaus: Quantum Variational Algorithms Are Swamped With Traps
Organization: Massachusetts Institute of Technology
Classical neural networks are quite easy to train, partly because their loss functions have local minima that are very similar to the global minimum. Unfortunately, in many circumstances, parameterised quantum algorithms seem to be harder to train. For example, deep quantum circuits can have regions in their optimisation landscapes with extremely shallow gradients, making it challenging to find where the minima might be. While intensive research into these “barren plateaus” is ongoing, this month’s novel research has found classes of quantum circuits where the local minima cluster far from the global minimum. The researchers showed this can even happen for shallow circuits that have no barren plateaus. Despite this, hope can still be found if the optimisation starts from a cleverly chosen place in the loss landscape or if the problem is highly symmetric.
Link: https://arxiv.org/abs/2205.05786

Title: Low-overhead fault-tolerant quantum computing using long-range connectivity
Organization: University of Sydney
Unless error correction protocols can be improved, fault-tolerant quantum computers may need millions of physical qubits to solve problems of practical interest. The authors of this article show how quantum low-density parity-check (LDPC) codes combined with long-range interactions could substantially reduce the resource requirements for fault tolerance. Of course, implementing high fidelity entangling operations between distant qubits is a considerable challenge but many quantum architectures have been making progress on this front. Instead of analysing how their scheme performs as a theoretical limit is approached, the authors estimate the considerable resource savings that it would enable for today’s typical device sizes. Importantly, work on LDPC codes appears to have room for further development and improvement.
Link: https://www.science.org/doi/10.1126/sciadv.abn1717

Title: Deep Learning of Quantum Many-Body Dynamics via Random Driving
Organizations: Max-Planck-Institut für die Physik des Licht; University of Erlangen-Nuremberg; Shanghai Jiao Tong University; Shanghai Research Center for Quantum Sciences
Predicting quantum dynamics using a classical set-up is extremely compute-intensive. However, when a quantum system only has local interactions, most of the quantum states in the system’s exponentially large Hilbert space cannot be reached on a practical timescale. Keeping track of this truncated set of states makes the classical simulation a bit more feasible. This approach can be taken to the extreme by ignoring the quantum states entirely and just using the past evolution of expectation values (i.e. measurement results) to predict their future evolution. A classical algorithm taking 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 article, the authors successfully trained a deep recurrent network to predict the behaviour of different spin systems. They write that their work could be used to predict the response of qubits to optimised pulses and feedback-based control schemes.
Link: https://quantum-journal.org/papers/q-2022-05-17-714/

Title: Industry applications of neutral-atom quantum computing solving independent set problems
Organization: QuEra Computing Inc.
If you have ever heard that many business sectors could improve their operations by solving independent set problems but have never found a helpful introduction to the topic, then this is the right paper for you. The distinction between, say, using a variational algorithm to find the maximum independent set for optimising the placement of radio antennae and using a sampling algorithm to find the maximal independent set for optimising where retail stores are built may seem quite subtle. However, it is all made extremely clear. Individual example applications are thoroughly described and connected to the latest quantum protocols so that the potential advantages of a quantum approach can be assessed.
Link: https://arxiv.org/abs/2205.08500

Title: Error Resilient 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 a small overhead. Further, we show that for quantum amplitude estimation, the parallelization can lead to vast improvements in resilience against quantum errors. The resilience is not caused by the lower gate depth, but by the structure of the algorithm. Even in cases with errors that make it impossible to read out the exact or approximate solutions from conventional amplitude estimation, our parallel approach provided the correct solution with high probability. The results on error resilience hold for the standard version and for low depth versions of quantum amplitude estimation.
Link: https://arxiv.org/abs/2204.01337

May 28, 2022