Maintaining Quantum Coherence for Quantum Information Applications

Project Information

MQC

Grant agreement ID: 785902

DOI

10.3030/785902

Project closed

EC signature date 7 March 2018

Start date 1 June 2018

End date 31 May 2020

Funded under

• EXCELLENT SCIENCE - Marie Skłodowska-Curie Actions

Total cost

€ 212 194,80

EU contribution

€ 212 194,80

Coordinated by

AARHUS UNIVERSITET
Denmark

Periodic Reporting for period 1 - MQC (Maintaining Quantum Coherence for Quantum Information Applications)

Reporting period: 2018-06-01 to 2020-05-31

Summary of the context and overall objectives of the project

Quantum information technologies have attracted much attention in recent years. Advanced fabrication technologies have
made it possible to develop quantum architectures, such as trapped ions, color-defects in crystals (nitrogen-vacancy in
diamond), and Rydberg atoms, where quantum information applications can be implemented. At the heart of this growing
field stands quantum coherence.

In physics, coherence is maintained as long as waves preserve their relative phase, thus enabling the interference phenomenon. The same applies to quantum coherence, which is maintained when the quantum superposition (phase and amplitude) is kept stable. Quantum coherence is at the heart of quantum information technology. The latter can only be realised as long as quantum oherence is preserved. In fact, as quantum applications increase in complexity, coherence time needs to be extended. In a similar manner, longer coherence times reveal higher performance and higher quantum operation fidelity, which is important for ealizations of interesting quantum applications, varying from quantum gates for quantum computation, through quantum simulation of classical intractable systems, to quantum sensing schemes for medical applications.

Noise, leakage and decay channels constitute the main sources for decoherence, which limit the fidelity of the desired quantum operations. In this project my main goal is to theoretically investigate refocusing schemes to maintain coherence in the quantum systems mentioned above, while realizing different quantum applications:

Work performed from the beginning of the project to the end of the period covered by the report and main results achieved so far

We theoretically proposed a scheme for entanglement distribution in quantum networks. By sending a single photon between quantum nodes – each consisting of an atomic qubit embedded in a cavity – we manage to generate a multi control phase gate between the atoms. This is an important universal gate, needed for quantum search protocols. One of the experimental obstacles lies in maintaining the optical phase of the photon due to optical length fluctuations. We overcame that problem by sending multiple photons to the quantum network: We used a pulsed version of the well-known refocusing secheme called dynamical decoupling to refocus the random optical phase. (Published in Phys. Rev. A)

In a theory-experiment collaboration with the Weizmann Institute group led by Ofer Firstenberg, we demonstrated that continuous dynamical decoupling can be applied to protect a collective excitation in warm atoms against Doppler broadening. When a photon (or a weak coherent pulse) is absorbed by atomic vapour, this global atomic excitation performs as a spin wave. Each atom possesses a phase according to its position, and the momentum of the absorbed photon. Due to random atomic velocities, the global excitation is subjected to Doppler decoherence that destroys the desired atomic phase. To tackle this, we introduce an additional ancillary sensor state, having an opposite sensitivity to the same Doppler mechanism. By smartly driving the transition between the excited and senor states, we couple them and obtain a protected dressed state that is insensitive to Doppler noise. The coherence time is prolonged. (Sent to publication in Phys. Rev. X)

The EU funding is acknowledged in all the articles published and will be acknowledged in future articles with the results of this research.

Progress beyond the state of the art and expected potential impact (including the socio-economic impact and the wider societal implications of the project so far)

More and more quantum computer companies begin to appear lately. Our scheme of deterministically distributing entanglement is extremely important for the growth of quantum computation with trapped ions, and superconducting qubits. Both are the leading architectures for quantum computers now. Particularly, our scheme or a deviation of it can be useful for trapped ion computation, where entangling separated ion chains remains a major challenge. Our theoretical proposal can be verified in several experimental groups having a quantum node made of an atom embedded in a cavity. For example, the Max Plank group led by Pempe, and the Weizmann group led by Dayan, the Sussex group led by Keller. Importantly, our scheme can be extremely beneficial for quantum computation platforms of superconducting qubits. As we have introduced an efficient and robust way to generate a universal multi-control gate – a building block needed for quantum search.


Our manuscript about the compensation for Doppler dephasing has been sent to publication in Phys. Rev. X, and has not been accepted yet. In addition to quantum memory application shown in our manuscript, our scheme can be used to increasing the entanglement gate fidelity of Rydberg atom architecture, which is currently, a less mature candidate for quantum computer in comparison with trapped ions and superconducting qubits. Moreover, our scheme is a new continuous dynamical decoupling scheme, which can be used for many other platforms, and against many other noise sources.

quantum network design, where a single photon scatters through the quantum nodes