Ultralong Coherence Times Research Articles

Interference of single photons with ultralong coherence time

Interference of single photons with ultralong coherence time

Hyperfine states of erbium doped yttrium orthosilicate for long-coherence-time quantum memories

Due to their unique optical transition at 1.5 μm, erbium-doped crystals are one of the best candidates for the realization of quantum memories at telecommunication wavelengths. While the hyperfine states of erbium doped yttrium orthosilicate hold the possibility of achieving ultra-long coherence times for a quantum memory, the complexity of their hyperfine structures leads to inaccurate theoretical prediction, thus hindering the extending of the coherence times. Here, we report a set of spin-Hamiltonian parameters with sufficient predictive power on the coherence times of the erbium hyperfine states. Our calculation simultaneously takes into account both the data measured in Electron-Paramagnetic-Resonance and Raman-heterodyne experiments. Based on the new parameters, the coherence properties of the hyperfine ground states are discussed. At zero applied magnetic field, all hyperfine transitions between the ground states are transitions with zero-first-order-Zeeman (ZEFOZ) effect, manifesting both long coherence times and strong transition strengths. By turning up the magnetic field, it is possible to further extend the coherence times to the scale of hours. The long coherence times at ZEFOZ transitions can be an important step forward in creating long-lived telecommunication quantum memories.

Electron spin coherence on a solid neon surface

A single electron floating on the surface of a condensed noble-gas liquid or solid can act as a spin qubit with ultralong coherence time, thanks to the extraordinary purity of such systems. Previous studies suggest that the electron spin coherence time on a superfluid helium (He) surface can exceed 100 s. In this paper, we present theoretical studies of the electron spin coherence on a solid neon (Ne) surface, motivated by our recent experimental realization of single-electron charge qubit on solid Ne. The major spin decoherence mechanisms investigated include the fluctuating Ne diamagnetic susceptibility due to thermal phonons, the fluctuating thermal current in normal metal electrodes, and the quasi-statically fluctuating nuclear spins of the 21Ne ensemble. We find that at a typical experimental temperature about 10 mK in a fully superconducting device, the electron spin decoherence is dominated by the third mechanism via electron–nuclear spin–spin interaction. For natural Ne with 2700 ppm abundance of 21Ne, the estimated inhomogeneous dephasing time is around 0.16 ms, already better than most semiconductor quantum-dot spin qubits. For commercially available, isotopically purified Ne with 1 ppm of 21Ne, can be 0.43 s. Under the application of Hahn echoes, the coherence time T 2 can be improved to 30 ms for natural Ne and 81 s for purified Ne. Therefore, the single-electron spin qubits on solid Ne can serve as promising new spin qubits.

Environment spectrum and coherence behaviours in a rare-earth doped crystal for quantum memory

We theoretically investigate the dynamics of environment and coherence behaviours of the central ion in a quantum memory based on a rare-earth doped crystal. The interactions between the central ion and the bath spins suppress the flip-flop rate of the neighbour bath spins and yield a specific environment spectral density S(ω). Under dynamical decoupling pulses, this spectrum provides a general scaling for the coherence envelope and coherence time, which significantly extend over a range on an hour-long time scale. The characterized environment spectrum with ultra-long coherence time can be used to implement various quantum communication and information processing protocols.

Quantum metrology with single spins in diamond under ambient conditions

AbstractThe detection of single quantum systems can reveal information that would be averaged out in traditional techniques based on ensemble measurements. The nitrogen-vacancy (NV) centers in diamond have shown brilliant prospects of performance as quantum bits and atomic sensors under ambient conditions, such as ultra-long coherence time, high fidelity control and readout of the spin state. In particular, the sensitivity of the NV center spin levels to external environmental changes makes it a versatile detector capable of measuring various physical quantities, such as temperature, strain, electric fields and magnetic fields. In this paper, we review recent progress in NV-based quantum metrology, and speculate on its future.

A long-lived Zeeman trapped-ion qubit

We demonstrate a coherence time of 2.1(1)~s for electron spin superposition states of a single trapped $^{40}$Ca$^+$ ion. The coherence time, measured with a spin-echo experiment, corresponds to residual rms magnetic field fluctuations $\leq$~2.7$\times$10$^{-12}$~T. The suppression of decoherence induced by fluctuating magnetic fields is achieved by combining a two-layer $\mu$-metal shield, which reduces external magnetic noise by 20 to 30~dB for frequencies of 50~Hz to 100~kHz, with Sm$_2$Co$_{17}$ permanent magnets for generating a quantizing magnetic field of 0.37~mT. Our results extend the coherence time of the simple-to-operate spin qubit to ultralong coherence times which so far have been observed only for magnetic insensitive transitions in atomic qubits with hyperfine structure.

Coherence time limit of the biphotons generated in a dense cold atom cloud.

Biphotons with narrow bandwidth and long coherence time can enhance light-atom interaction, which leads to strong coupling between photonic and atomic qubits. Such strong coupling is desirable in quantum information processing, quantum storage and communication. In particular, paired photons with a long coherence time over submicroseconds facilitate the direct manipulation of biphoton wavefunction. In this paper, we report the narrow-band biphotons with a coherence time of 2.34 μs generated from spontaneous four-wave mixing (SFWM) in a dense cold atom cloud, in which the anti-Stokes photons go through a narrow electromagnetically-induced transparency (EIT) window. In our knowledge, this is the best record of coherence time for paired photons achieved so far. A number of factors limiting the coherence time are analyzed in detail. We find the EIT coherence plays an essential role in determining the coherence time for paired photons. The EIT dephasing rate is the ultimate limit to the coherence time, and an ultra-long coherence time above ten microseconds is possible by further improvement of the dephasing rate below 100 kHz.

基于室温单自旋磁共振技术的量子精密测量

The ability of penetrating deeply into single quantum system can help reveal the unique information of the quantum individuals used to be averaged by the ensemble statistics in traditional macroscopic measurement. The nitrogen-vacancy (NV) center in diamond, which has an ultralong coherence time and can be readout and manipulated through ODMR (optically detected magnetic resonance) techniques under room temperature, has been an important platform achieving quantum precision measurement and quantum information processing. Particularly in magnetometry, the researches on single spin imaging are in full swing. In this review we retrospect the work about quantum precision metrology based on the NV center, and introduce the progress in sensing magnetic field, electric field, mechanical system and temperature. We also discuss the possible developing directions in this area.

Ultrafast Nonlinear Spectroscopy of Strain-compensated Quantum Dots

We investigate the dephasing mechanism of optically-induced exciton coherence in an ensemble of InAs self-assembled quantum dots (QDs) using a two-pulse photon-echo (PE) technique. The sample was fabricated on an InP (311)B substrate using strain compensation to control the emission wavelength. This technique also enabled us to fabricate a 150-layer stacked QD structure for enhancing the PE signal intensity. We show the measured decay time of PE signal intensity for resonant excitation of ground-state transition is extremely long, which corresponds to ultralong coherence times of excitons in our QDs. We confirmed that non-radiative population relaxation and pure dephasing are considerably smaller than the radiative relaxation at low temperature. Furthermore, we demonstrate the ultrafast coherent control of QD exciton polarization by changing the pulse areas (Rabi oscillations) in the PE experiment. We also propose a new scheme for implementing QD quantum memory by using the PE technique.

Decoherence in Josephson vortex quantum bits: Long-Josephson-junction approach to a two-state system

We investigated decoherence of a Josephson vortex quantum bit (qubit) in dissipative and noisy environment. As the Josephson vortex qubit (JVQ) is fabricated by using a long Josephson junction (LJJ), we use the perturbed sine-Gordon equation to describe the phase dynamics representing a two-state system and estimate the effects of quasiparticle dissipation and weakly fluctuating critical and bias currents on the relaxation time ${T}_{1}$ and on the dephasing time ${T}_{\ensuremath{\phi}}$. We show that the critical current fluctuation does not contribute to dephasing of the qubit in the lowest order approximation. Modeling the weak current variation from magnetic field fluctuations in the LJJ by using the Gaussian colored noise with long correlation time, we show that the coherence time ${T}_{2}$ is limited by the low frequency current noise at very low temperatures. Also, we show that a ultra-long coherence time may be obtained from the JVQ by using experimentally accessible value of physical parameters.

Ultralong coherence times in the purely electronic zero-phonon line emission of single molecules

We report the observation of ultralong coherence times in the purely electronic zero-phonon line emission of single terrylenediimide molecules at 1.4K. Vibronic excitation and spectrally resolved detection with a scanning Fabry–Perot spectrum analyzer were used to measure a linewidth of 65MHz. This is within a factor of 1.6 of the transform limit. It therefore indicates that single molecule emission may be suited for applications in linear optics quantum computation. Additionally, it is shown that high-resolution spectra taken with the spectrum analyzer allow for the investigation of fast spectral dynamics in the emission of a single molecule.