Quantum State Transfer Protocols: A Giant Leap Forward?

Jeff Phillips, Code Siren, LLC

13 August 2023 - Quantum Computing

The latest big development in quantum computing is the ability to directly transfer quantum bits (qubits) between quantum computer microchip modules. This was demonstrated by researchers from the University of Sussex and Universal Quantum in February 2023. The breakthrough resolves a major challenge in building large and powerful quantum computers.

Previously, qubits could only be transferred between quantum computers using quantum teleportation. This process is efficient but requires a shared entanglement between the two quantum computers. Entanglement is a quantum phenomenon that allows two particles to be linked to share the same fate, even if a large distance separates them.

The new method of transferring qubits does not require entanglement. Instead, it uses a process called quantum state transfer (QST). QST is a more complex process but more robust and can transfer qubits over longer distances.

One way to scale up quantum computers is to connect multiple chips. However, this is not a trivial task, as it requires a way to transfer qubits between chips without losing their quantum properties.

In the subject research paper, ‘A high-fidelity quantum matter-link between ion-trap microchip modules,’ the authors proved a new technique for transferring qubits between chips. This technique, called UQ Connect, uses electric field links to enable qubits to move from one chip to another with unprecedented speed and precision[2].

The development of QST is a breakthrough in quantum computing. It opens up the possibility of building quantum computers that are much larger and more powerful than anything currently possible. QST is a process of transferring the quantum state of a qubit from one location to another. This is a fundamental operation in quantum computing, as it allows for the distribution of quantum information between different parts of a quantum network[3].

Probabilistic QST protocols typically create an entangled state between the two qubits. An entangled state is a state in which the quantum states of the two qubits are correlated so that they cannot be described independently. This correlation allows for transferring the quantum state of one qubit to the other with a certain probability.

  • Figure: Illustration of the process for connecting two modules (Module A and Module B) for the purposes of a modular ion-trap QC.

  • The UQ Connect technique creates a potential barrier between the two chips. When a voltage is applied to the barrier, the electric field creates a tunnel through which the qubit can pass. The qubit is then transferred to the other chip, which can continue to be used for quantum computing operations.

    The researchers demonstrated the UQ Connect technique with a pair of quantum computer chips containing 10 qubits. They could transport qubits between the chips with a 99.999993% success rate and a connection rate of 2424/s.

    The UQ Connect technique is a major step forward in developing quantum computing. It could help to make quantum computers a reality, and it could profoundly impact many different industries.

    In addition to the UQ Connect technique, several other promising approaches to scaling up quantum computers exist. These include using optical interconnects, microwave interconnects, and superconducting interconnects.

    Developing new techniques for scaling up quantum computers is an active area of research. As these techniques mature, quantum computers will become more powerful and accessible. This will open up new possibilities for quantum computing applications in cryptography, mathematics, and materials science.

    The UQ Connect technique is based on quantum teleportation, which allows the state of a qubit to be transferred to another qubit without physically moving it. The UQ Connect technique is still in its early stages of development, but it has the potential to be used to build practical quantum computers shortly.

    The development of the UQ Connect technique is a breakthrough in quantum computing and could significantly impact the future of quantum computing research and quantum charge-coupled device (QCCD) technology.

  • Figure: Illustration of a small section of a modular version of the QCCD quantum computer architecture.

  • There are two main types of QST: deterministic and probabilistic. Deterministic QST protocols guarantee that the quantum state will be transferred perfectly, while probabilistic protocols only have a certain probability of success.

    One of the most common deterministic QST protocols is the SWAP gate. The SWAP gate is a two-qubit gate that swaps the quantum states of the two qubits. This means that if the first qubit is in the state |0⟩ and the second qubit is in the state |1⟩, then after the SWAP gate, the first qubit will be in the state |1⟩ and the second qubit will be in the state |0⟩[4].

    Another common deterministic QST protocol is the Adiabatic quantum state transfer (AQT) protocol. The AQT protocol is more complex than the SWAP gate but can sometimes be more efficient. The AQT protocol has been demonstrated in various physical systems, including trapped ions, superconducting qubits, and semiconductor quantum dots. It is a promising technique for quantum state transfer, and it is being investigated for use in various quantum communication and computing applications.

    This is because the AQT protocol relies on the adiabatic theorem, which states that a quantum system will remain in its ground state if the Hamiltonian of the system is slowly changed. The Hamiltonian contains the operations associated with the kinetic and potential energies, and a particle in one dimension can be written. The Hamiltonian of the system in the AQT protocol is a time-dependent Hamiltonian that is given by:

    H (t) = H0 + H1 (t)


    H0    is the static Hamiltonian of the system, which is the Hamiltonian of the system in the absence of any time-dependent terms, meaning the system does change with time; it can still be possible to use the AQT protocol to transfer quantum states.

    H1 (t)    is the time-dependent Hamiltonian, which is the Hamiltonian responsible for the system's adiabatic change. In thermodynamics, an adiabatic process is without heat transfer between the system and its surroundings. This means that the only way energy can be transferred into or out of the system is through work.

    This means that the AQT protocol can be used to transfer a quantum state over long distances without the need for any intermediate operations.

    Here are some other notable developments in quantum computing in 2023 that Code Siren, LLC will be writing about:

    - IBM announced the development of a 1,121-qubit quantum computer called Condor.

    - Google announced the development of a new quantum computing language called Cirq.

    - The European Union launched the Quantum Flagship, a €1 billion project to develop quantum technologies.

    These are just a few of the many developments that are happening in the field of quantum computing. As technology continues to mature, we can expect to see even more exciting advances in the future.


  • [1] Akhtar, M., Bonus, F., Lebrun-Gallagher, F.R. et al. A high-fidelity quantum matter-link between ion-trap microchip modules. Nat Commun 14, 531 (2023).
  • [2] https://hackaday.com/2023/02/17/quantum-interconnects-get-faster/
  • [3] "Recent Innovations in Solid-State and Molecular Qubits for Quantum Information Applications" Robert H. Lavroff, Doran L. Pennington, Ash Sueh Hua, Barry Yangtao Li, Jillian A. Williams, and Anastassia N. Alexandrova. The Journal of Physical Chemistry Letters 2021 12 (44), 10742-10745 DOI: 10.1021/acs.jpclett.1c03269
  • [4] https://quantumcomputing.stackexchange.com/questions/22228/what-quantum-gates-admit-a-basis-independent-interpretation-of-their-action