Shane Dooley

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.

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

Squeezed states of spin systems are an important entangled resource for quantum technologies, particularly quantum metrology and sensing. Here we consider the generation of spin squeezed states by interacting the spins with a dissipative ancillary system. We show that spin squeezing can be generated in this model by two different mechanisms: one-axis twisting and driven collective relaxation. We can interpolate between the two mechanisms by simply adjusting the detuning between the dissipative ancillary system and the spin system. Interestingly, we find that for both mechanisms, ancillary system dissipation need not be considered an imperfection in our model, but plays a positive role in spin squeezing. To assess the feasibility of spin squeezing we consider two different implementations with superconducting circuits. We conclude that it is experimentally feasible to generate a squeezed state of hundreds of spins either by one-axis twisting or by driven collective relaxation.

Quantum computation mediated by ancillary qudits and spin coherent states.

Models of universal quantum computation in which the required interactions between register (computational) qubits are mediated by some ancillary system are highly relevant to experimental realizations of a quantum computer. We introduce such a universal model that employs a d -dimensional ancillary qudit. The ancilla-register interactions take the form of controlled displacements operators, with a displacement operator defined on the periodic and discrete lattice phase space of a qudit. We show that these interactions can implement controlled phase gates on the register by utilizing geometric phases that are created when closed loops are traversed in this phase space. The extra degrees of freedom of the ancilla can be harnessed to reduce the number of operations required for certain gate sequences. In particular, we see that the computational advantages of the quantum bus (qubus) architecture, which employs a field-mode ancilla, are also applicable to this model. We then explore an alternative ancilla-mediated model which employs a spin ensemble as the ancillary system and again the interactions with the register qubits are via controlled displacement operators, with a displacement operator defined on the Bloch sphere phase space of the spin coherent states of the ensemble. We discuss the computational advantages of this model and its relationship with the qubus architecture.

Robust quantum sensing with strongly interacting probe systems

In the field of quantum metrology and sensing, a collection of quantum systems (e.g., spins) are used as a probe to estimate some physical parameter (e.g., magnetic field). It is usually assumed that there are no interactions between the probe systems. We show that strong interactions between them can increase robustness against thermal noise, leading to enhanced sensitivity. In principle, the sensitivity can scale exponentially in the number of probes—even at non-zero temperatures—if there are long-range interactions. This scheme can also be combined with other techniques, such as dynamical decoupling, to give enhanced sensitivity in realistic experiments.Quantum metrology: better sensing through interactionsUsing collections of quantum probes that continuously interact with each other could enable more precise frequency measurements. Without considering losses, the performance of quantum measurement devices can be improved by measuring for longer. In practice, this is not possible because coupling to the thermal environment introduces decoherence that limits the measurement time. Shane Dooley and colleagues from the National Institute of Informatics, Japan, have investigated theoretically how introducing strong mutual interactions to an ensemble of quantum systems can improve their collective sensitivity. The strong coupling changes how the environment affects the quantum coherence, allowing the sensing time to be extended by a factor that increases exponentially with the interaction strength and number of sensors. A simple, experimentally-realistic device with only two superconducting qubits could gain a 50% sensitivity increase using their new approach.

Hybridization: A route to state engineering for quantum-enhanced metrology

The hybridization of distinct quantum systems has now reached the stage when we can actually engineer the properties of the composite system to be better than the individual parts. One natural application of hybridization is the generation of non-classical states, which are extremely important in emerging quantum technologies such as quantum metrology and sensing. In this presentation we consider the generation of spin squeezed states in a hybrid system composed of a superconducting circuit coupled to a spin ensemble. We show that spin squeezing can be generated by two different mechanisms: one-axis twisting and driven collective relaxation.

Quantum metrology including state preparation and readout times

There is growing belief that the next decade will see the emergence of sensing devices based on the laws of quantum physics that outperform some of our current sensing devices. For example, in frequency estimation, using a probe prepared in an entangled state can, in principle, lead to a precision gain compared to a probe prepared in a separable state. Even in the presence of some forms of decoherence, it has been shown that the precision gain can increase with the number of probe particles

Generating non-classical states from spin coherent states via interaction with ancillary spins

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Collapse and Revival and Cat States with an N Spin System

. Usually, however, the entangled and separable state preparation and readout times are assumed to be negligible. We find that a probe in a maximally entangled (GHZ) state can give an advantage over a separable state only if the entangled state preparation and readout times are lower than a certain threshold. When the probe system suffers dephasing, this threshold is much lower (and more difficult to attain) than it is for an isolated probe. Further, we find that in realistic situations the maximally entangled probe gives a precision advantage only up to some finite number of probe particles

Fractional revivals, multiple-Schrödinger-cat states, and quantum carpets in the interaction of a qubit withNqubits

N_\text{cutoff}

Making the most of time in quantum metrology: concurrent state preparation and sensing

that is lower for a dephasing probe than it is for an isolated probe.