Research interest: Quantum error correction Quantum devices

# Quantum memory

Quantum data must be stored within a physical system. One readily accessible physical system is the Heisenberg ferromagnet, which has as its ground space the set of all symmetric states. By storing quantum data within a symmetric code, one can benefit from (1) the fact that such codes are left unchanged by the underlying dynamics of the Heisenberg ferromagnet, and (2) the quantum error correction properties of these symmetric codes.

I calculate bounds on the error of such quantum storage under physically realistic conditions. This calculation does not rely on any specific properties of symmetric codes apart from the number of errors they can correct. Hence, all symmetric codes with QEC properties can be used. Also I use the fact that the spectral gap of the Heisenberg ferromagnet can grow with the system size.

**Related papers:**

2019 *Quantum storage in quantum ferromagnets *

Y. Ouyang http://arxiv.org/abs/1904.01458

2019 *Computing spectral bounds of the Heisenberg ferromagnet from geometric considerations *

# Quantum Metrology

In the quantum metrology problem, one wishes to use a quantum resource to measure a signal with known structure but unknown strength. Quantum metrology does not require the full power of quantum computation, and might serve as an early use-case for quantum technologies.

The quantum resource in quantum metrology are probe states. Probe states are unfortunately fragile, and need to be designed to be robust against errors. Surprisingly, entanglement in a probe state, while necessary for quantum metrology, is not sufficient to allow robust metrology. The search for robust probe states is non-trivial, and symmetric probe states are good candidates.

Eventually quantum metrology will integrate with quantum error correction, to allow measurements to attain their maximum precision, namely via fault tolerant quantum metrology.

**Related papers:**

2020 * Weight distribution of classical codes influences quantum metrology*

Yingkai Ouyang, N. Rengaswamy http://arxiv.org/abs/2007.02859

2019 *Tight bounds on the simultaneous estimation of incompatible parameters *

J. S. Sidhu, Yingkai Ouyang, E. T. Campbell, P. Kok http://arxiv.org/abs/1912.09218

2019 *Robust quantum metrology with explicit symmetric states*

Yingkai Ouyang, N. Shettell, D. Markham http://arxiv.org/abs/1908.02378

# Quantum computer

With respect to quantum computations, I am interested in (1) permutational quantum computation, (2) quantum simulation and (3) the resource theory of quantum computation.

**Related Papers:**

2020 *Quantifying quantum speedups: improved classical simulation from tighter magic monotones*

J.R. Seddon, B. Regula, H. Pashayan, Y. Ouyang, E.T. Campbell https://arxiv.org/abs/2002.06181

2020 *Compilation by Hamiltonian Sparsification*

Yingkai Ouyang, D.R. White, E.T. Campbell Quantum ,4 235

2019 *Faster quantum computation with permutations and resonant couplings *

Yingkai Ouyang, Y. Shen, L. Chen Linear Algebra and its Applications 592, 270-286

2018 *Classical verification of quantum circuits containing few basis changes*

T.F. Demarie, Yingkai Ouyang, J.F. Fitzsimons Physical Review A 97, 042319

# Quantum security

I have also worked in quantum cryptography, using my expertise of quantum information and quantum coding theory.

Related papers:

2020 Homomorphic encryption of linear optics quantum computation on almost arbitrary states of light with asymptotically perfect security

Yingkai Ouyang, S.H. Tan, J.F.Fitzsimons, P.P. Rohde Physical Review Research 2, 013332

2018 Quantum homomorphic encryption from quantum codes

Yingkai Ouyang, S.H. Tan, J. Fitzsimons Physical Review A 98, 042334

2018 Practical quantum somewhat-homomorphic encryption with coherent states

S.H. Tan, Yingkai Ouyang, P. Rohde Physical Review A 97, 042308

2017 Computing on quantum shared secrets

Yingkai Ouyang, S.H. Tan, L. Zhao, J. Fitzsimons Physical Review A 96, 052333

2016 A quantum approach to homomorphic encryption

S.H. Tan, J. Kettlewell, Yingkai Ouyang, L. Chen, J. Fitzsimons Scientific Reports 6, 042340