Yuichiro Matsuzaki

Towards realizing a quantum memory for a superconducting qubit: storage and retrieval of quantum states.

We have built a hybrid system composed of a superconducting flux qubit (the processor) and an ensemble of nitrogen-vacancy centers in diamond (the memory) that can be directly coupled to one another, and demonstrated how information can be transferred from the flux qubit to the memory, stored, and subsequently retrieved. We have established the coherence properties of the memory and succeeded in creating an entangled state between the processor and memory, demonstrating how the entangled states coherence is preserved. Our results are a significant step towards using an electron spin ensemble as a quantum memory for superconducting qubits.

Observation of dark states in a superconductor diamond quantum hybrid system.

The hybridization of distinct quantum systems has opened new avenues to exploit the best properties of these individual systems. Superconducting circuits and electron spin ensembles are one such example. Strong coupling and the coherent transfer and storage of quantum information has been achieved with nitrogen vacancy centres in diamond. Recently, we have observed a remarkably sharp resonance (~1 MHz) at 2.878 GHz in the spectrum of flux qubit negatively charged nitrogen vacancy diamond hybrid quantum system under zero external magnetic field. This width is much narrower than that of both the flux qubit and spin ensemble. Here we show that this resonance is evidence of a collective dark state in the ensemble, which is coherently driven by the superposition of clockwise and counter-clockwise macroscopic persistent supercurrents flowing in the flux qubit. The collective dark state is a unique physical system and could provide a long-lived quantum memory.

Proposed Robust Entanglement-Based Magnetic Field Sensor Beyond the Standard Quantum Limit.

Recently, there have been significant developments in entanglement-based quantum metrology. However, entanglement is fragile against experimental imperfections, and quantum sensing to beat the standard quantum limit in scaling has not yet been achieved in realistic systems. Here, we show that it is possible to overcome such restrictions so that one can sense a magnetic field with an accuracy beyond the standard quantum limit even under the effect of decoherence, by using a realistic entangled state that can be easily created even with current technology. Our scheme could pave the way for the realizations of practical entanglement-based magnetic field sensors.

Quantum Zeno effect with a superconducting qubit

Detailed schemes are investigated for experimental verification of Quantum Zeno effect with a superconducting qubit. A superconducting qubit is affected by a dephasing noise whose spectrum is

Using the quantum Zeno effect for suppression of decoherence

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Observation of quantum Zeno effect in a superconducting flux qubit

and so the decay process of a superconducting qubit shows a naturally nonexponential behavior due to an infinite correlation time of

Improving the lifetime of the NV center ensemble coupled with a superconducting flux qubit by applying magnetic fields

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Enhanced energy relaxation process of a quantum memory coupled to a superconducting qubit

noise. Since projective measurements can easily influence the decay dynamics having such nonexponential feature, a superconducting qubit is a promising system to observe quantum Zeno effect. We have studied how a sequence of projective measurements can change the dephasing process and also we have suggested experimental ways to observe quantum Zeno effect with a superconducting qubit. It would be possible to demonstrate our prediction in the current technology.

Magnetic-field sensing with quantum error detection under the effect of energy relaxation

Projective measurements are an essential element of quantum mechanics. In most cases, they cause an irreversible change of the quantum system on which they act. However, measurements can also be used to stabilize quantum states from decay processes, which is known as the quantum Zeno effect (QZE). Here, we demonstrate this effect for the case of a superposition state of a nuclear spin qubit, using an ancilla to perform the measurement. As a result, the quantum state of the qubit is protected against dephasing without relying on an ensemble nature of NMR experiments. We also propose a scheme to protect an arbitrary state by using QZE.

A hybrid-systems approach to spin squeezing using a highly dissipative ancillary system

When a quantum state is subjected to frequent measurements, the time evolution of the quantum state is frozen. This is called the quantum Zeno effect. Here, we observe such an effect by performing frequent discrete measurements in a macroscopic quantum system, a superconducting quantum bit. The quantum Zeno effect induced by discrete measurements is similar to the original idea of the quantum Zeno effect. By using a Josephson bifurcation amplifier pulse readout, we have experimentally suppressed the time evolution of Rabi oscillation using projective measurements, and also observed the enhancement of the quantum state holding time by shortening the measurement period time. This is a crucial step to realize quantum information processing using the quantum Zeno effect.