Jingfu Zhang

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Nuclear magnetic resonance implementation of universal quantum gate with constant Hamiltonian evolution

Applied Physics Letters, 2009

In this letter, we report an experimental demonstration of single-step realization of universal q... more In this letter, we report an experimental demonstration of single-step realization of universal quantum gate in a nuclear magnetic resonance quantum information processor. The scheme uses a constant Hamiltonian evolution and is robust against decoherence. Our work focuses on the implementation of the controlled-NOT gate. Several sets of parameters are obtained and they provide flexibility to suppress decoherence for quantum computation.

Optimal photon energies for initialization of hybrid spin quantum registers of nitrogen-vacancy centers in diamond

Physical Review A, 2020

Initializing quantum registers with high fidelity is a fundamental precondition for many applicat... more Initializing quantum registers with high fidelity is a fundamental precondition for many applications like quantum information processing and sensing. The electronic and nuclear spins of a Nitrogen-Vacancy (NV) center in diamond form an interesting hybrid quantum register that can be initialized by a combination of laser, microwave, and radio-frequency pulses. However, the laser illumination, which is necessary for achieving electron spin polarization, also has the unwanted sideeffect of depolarizing the nuclear spin. Here, we study how the depolarization dynamics of the 14 N nuclear spin depends on the laser wavelength. We show experimentally that excitation with an orange laser (594 nm) causes significantly less nuclear spin depolarization compared to the green laser (532 nm) typically used for excitation and hence leads to higher nuclear spin polarization. This could be because orange light excitation inhibits ionization of NV 0 into NV − and therefore suppresses one source of noise acting on the nuclear spin.

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Simulation of HeisenbergXYinteractions and realization of a perfect state transfer in spin chains using liquid nuclear magnetic resonance

Physical Review A, 2005

Experimental Protection of Two-Qubit Quantum Gates against Environmental Noise by Dynamical Decoupling

Physical review letters, Jan 11, 2015

Hybrid systems consisting of different types of qubits are promising for building quantum compute... more Hybrid systems consisting of different types of qubits are promising for building quantum computers if they combine useful properties of their constituent qubits. However, they also pose additional challenges if one type of qubits is more susceptible to environmental noise than the others. Dynamical decoupling can help to protect such systems by reducing the decoherence due to the environmental noise, but the protection must be designed such that it does not interfere with the control fields driving the logical operations. Here, we test such a protection scheme on a quantum register consisting of the electronic and nuclear spins of a nitrogen-vacancy center in diamond. The results show that processing is compatible with protection: The dephasing time was extended almost to the limit given by the longitudinal relaxation time of the electron spin.

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NMR implementation of a quantum scheduling algorithm

The improved quantum scheduling algorithm proposed by Grover has been generalized using the gener... more The improved quantum scheduling algorithm proposed by Grover has been generalized using the generalized quantum search algorithm, in which a unitary operator replaces the Walsh-Hadamard transform, and π/2 phase rotations replace the selective inversions, in order to make the quantum scheduling algorithm suitable for more cases. Our scheme is realized on a nuclear magnetic resonance (NMR) quantum computer. Experimental results show a good agreement between theory and experiment.

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Simulating decoherence behavior of a system in entangled state using nuclear magnetic resonance

By choosing H nucleus in Carbon-13 labelled trichloroethylene as one qubit environment, and two C... more By choosing H nucleus in Carbon-13 labelled trichloroethylene as one qubit environment, and two C nuclei as a two-qubit system, we have simulated quantum decoherence when the system lies in an entangled state using nuclear magnetic resonance (NMR). Decoupling technique is used to trace over the environment degrees of freedom. Experimental results show agreements with the theoretical predictions. Our experiment

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Experimental implementation of quantum gates through actuator qubits

Physical Review A, 2015

Hybrid magic state distillation for universal fault-tolerant quantum computation

Physical Review A, 2015

Experimental Quantum Simulation of Entanglement in Many-Body Systems

Physical Review Letters, 2011

We employ a nuclear magnetic resonance (NMR) quantum information processor to simulate the ground... more We employ a nuclear magnetic resonance (NMR) quantum information processor to simulate the ground state of an XXZ spin chain and measure its NMR analog of entanglement, or pseudoentanglement. The observed pseudo-entanglement for a small-size system already displays singularity, a signature which is qualitatively similar to that in the thermodynamical limit across quantum phase transitions, including an infinite-order critical point. The experimental results illustrate a successful approach to investigate quantum correlations in many-body systems using quantum simulators.

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Protected Quantum Computing: Interleaving Gate Operations with Dynamical Decoupling Sequences

Physical Review Letters, 2014

Implementing precise operations on quantum systems is one of the biggest challenges for building ... more Implementing precise operations on quantum systems is one of the biggest challenges for building quantum devices in a noisy environment. Dynamical decoupling (DD) attenuates the destructive effect of the environmental noise, but so far it has been used primarily in the context of quantum memories. Here, we present a general scheme for combining DD with quantum logical gate operations and demonstrate its performance on the example of an electron spin qubit of a single nitrogenvacancy center in diamond. We achieve process fidelities >98% for gate times that are 2 orders of magnitude longer than the unprotected dephasing time T2.

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Ground-state entanglement in a system with many-body interactions

Physical Review A, 2010

Direct observation of quantum criticality in Ising spin chains

Physical Review A, 2009

We use NMR quantum simulators to study antiferromagnetic Ising spin chains undergoing quantum pha... more We use NMR quantum simulators to study antiferromagnetic Ising spin chains undergoing quantum phase transitions. Taking advantage of the sensitivity of the systems near criticality, we detect the critical points of the transitions using a direct measurement of the Loschmidt echo. We test our simulators for spin chains of even and odd numbers of spins, and compare the experimental results to theoretical predictions.

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Iterative quantum-state transfer along a chain of nuclear spin qubits

Physical Review A, 2007

Transferring quantum information between two qubits is a basic requirement for many applications ... more Transferring quantum information between two qubits is a basic requirement for many applications in quantum communication and quantum-information processing. In the iterative quantum-state transfer proposed by Burgarth et al. ͓Phys. Rev. A 75, 062327 ͑2007͔͒, this is achieved by a static spin chain and a sequence of gate operations applied only to the receiving end of the chain. The only requirement on the spin chain is that it transfers a finite part of the input amplitude to the end of the chain, where the gate operations accumulate the information. For an appropriate sequence of evolutions and gate operations, the fidelity of the transfer can asymptotically approach unity. We demonstrate the principle of operation of this transfer scheme by implementing it in a nuclear magnetic resonance quantum-information processor.

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Experimental quantum error correction with high fidelity

Physical Review A, 2011

More than ten years ago a first step toward quantum error correction (QEC) was implemented [Phys.... more More than ten years ago a first step toward quantum error correction (QEC) was implemented [Phys. Rev. Lett. 81, 2152 (1998)]. The work showed there was sufficient control in nuclear magnetic resonance to implement QEC, and demonstrated that the error rate changed from to ∼ 2. In the current work we reproduce a similar experiment using control techniques that have been since developed, such as the pulses generated by gradient ascent pulse engineering algorithm. We show that the fidelity of the QEC gate sequence and the comparative advantage of QEC are appreciably improved. This advantage is maintained despite the errors introduced by the additional operations needed to protect the quantum states.

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Experimental implementation of a codeword-stabilized quantum code

Physical Review A, 2012

A five-qubit codeword stabilized quantum code is implemented in a seven-qubit system using nuclea... more A five-qubit codeword stabilized quantum code is implemented in a seven-qubit system using nuclear magnetic resonance (NMR). Our experiment implements a good nonadditive quantum code which encodes a larger Hilbert space than any stabilizer code with the same length and capable of correcting the same kind of errors. The experimentally measured quantum coherence is shown to be robust against artificially introduced errors, benchmarking the success in implementing the quantum error correction code. Given the typical decoherence time of the system, our experiment illustrates the ability of coherent control to implement complex quantum circuits for demonstrating interesting results in spin qubits for quantum computing.

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Speedup of quantum-state transfer by three-qubit interactions: Implementation by nuclear magnetic resonance

Physical Review A, 2006

Quantum data bus in dipolar coupled nuclear spin qubits

Physical Review A, 2009

We implement an iterative quantum state transfer exploiting the natural dipolar couplings in a sp... more We implement an iterative quantum state transfer exploiting the natural dipolar couplings in a spin chain of a liquid crystal NMR system. During each iteration a finite part of the amplitude of the state is transferred and by applying an external operation on only the last two spins the transferred state is made to accumulate on the spin at the end point. The transfer fidelity reaches one asymptotically through increasing the number of iterations. We also implement the inverted version of the scheme which can transfer an arbitrary state from the end point to any other position of the chain and entangle any pair of spins in the chain, acting as a full quantum data bus.

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Quantitative complementarity between local and nonlocal character of quantum states in a three-qubit system

Physical Review A, 2008

Broadband excitation by chirped pulses: application to single electron spins in diamond

New Journal of Physics, 2013

Digital quantum simulation of the statistical mechanics of a frustrated magnet

Nature Communications, 2012

many problems of interest in physics, chemistry and computer science are equivalent to problems d... more many problems of interest in physics, chemistry and computer science are equivalent to problems defined on systems of interacting spins. However, most such problems require computational resources that are out of reach with classical computers. A promising solution to overcome this challenge is quantum simulation. several 'analogue' quantum simulations of interacting spin systems have been realized experimentally, where ground states were prepared using adiabatic techniques. Here we report a 'digital' quantum simulation of thermal states; a three-spin frustrated magnet was simulated using a nuclear magnetic resonance quantum information processor, and we were able to explore the phase diagram of the system at any simulated temperature and external field. These results help to identify the challenges for performing quantum simulations of physical systems at finite temperatures, and suggest methods that may be useful in simulating thermal open quantum systems.

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