Bit Of Quantum Information Research Articles

Exploring noiseless subsystems via nuclear magnetic resonance

Noiseless subsystems offer a general and efficient method for protecting quantum information in the presence of noise that has symmetry properties. A paradigmatic class of error models displaying non-trivial symmetries emerges under collective noise behavior, which implies a permutationally-invariant interaction between the system and the environment. We describe experiments demonstrating the preservation of a bit of quantum information encoded in a three qubit noiseless subsystem for general collective noise. A complete set of input states is used to determine the super-operator for the implemented one-qubit process and to confirm that the fidelity of entanglement is improved for a large, non-commutative set of engineered errors. To date, this is the largest set of error operators that has been successfully corrected for by any quantum code.

Experimental realization of noiseless subsystems for quantum information processing.

We demonstrate the protection of one bit of quantum information against all collective noise in three nuclear spins. Because no subspace of states offers this protection, the quantum bit was encoded in a proper noiseless subsystem. We therefore realize a general and efficient method for protecting quantum information. Robustness was verified for a full set of noise operators that do not distinguish the spins. Verification relied on the most complete exploration of engineered decoherence to date. The achieved fidelities show improved information storage for a large, noncommutative set of errors.

Power of One Bit of Quantum Information

In standard quantum computation, the initial state is pure and the answer is determined by making a measurement of some of the bits in the computational basis. What can be accomplished if the initial state is a highly mixed state and the answer is determined by measuring the expectation of $\sigma_z$ on the first bit with bounded sensitivity? This is the situation in high temperature ensemble quantum computation. We show that in this model it is possible to perform interesting physics simulations which have no known efficient classical algorithms, even though the model is less powerful then standard quantum computing in the presence of oracles.

Multiple-particle interference and quantum error correction

The concept of multiple particle interference is discussed, using insights provided by the classical theory of error correcting codes. This leads to a discussion of error correction in a quantum communication channel or a quantum computer. Methods of error correction in the quantum regime are presented, and their limitations assessed. A quantum channel can recover from arbitrary decoherence of x qubits if K bits of quantum information are encoded using n quantum bits, where K/n can be greater than 1-2 H(2x/n), but must be less than 1 - 2 H(x/n). This implies exponential reduction of decoherence with only a polynomial increase in the computing resources required. Therefore quantum computation can be made free of errors in the presence of physically realistic levels of decoherence. The methods also allow isolation of quantum communication from noise and evesdropping (quantum privacy amplification).

Single Qubit Quantum Gates Implemented with Silicon Micro Ring Resonators

Quantum computers offer a new way of doing information processing by harnessing the unique properties of quantum mechanics, opening new possibilities for solving computationally difficult but useful problems more efficiently than a traditional classical computer (such as simulating molecular interactions). There are several ways of physically implementing a quantum computer, each with its own advantages and disadvantages. An approach which uses photons (i.e., particles of light), known as Linear Optical Quantum Computing (LOQC), has gained traction in the last decade. This approach uses integrated photonic technologies to design chips that can manipulate bits of quantum information – known as qubits – which are encoded in light. My undergraduate thesis research has focused on the investigation of new implementations of single qubit quantum gates – the physical structures which manipulate single qubits to do computation. Using a nano-scale silicon photonic device known as a micro-ring resonator, I have developed a novel configuration which in theory, should be able to implement any single qubit operation. Realizing single qubit gates using micro ring resonators could prove to provide a large improvement in the scalability of an integrated photonic quantum computer. My research has shown an almost 200 times increase in the on-chip density of single qubit gates over the current state of the art in the literature can be achieved by using a ring resonator architecture. This research may lay the foundation for future work on a new scalable implementation of quantum computer that uses light to solve the world’s most difficult problems.