QFarm

QFarm Quantum Seminar Series

Q-Farm Quantum Seminar Series

Topic: 
TBA
Abstract / Description: 

Anna will give an overview of the process we use to model and design large-scale superconducting quantum circuits at Rigetti. I will introduce the different stages of the iterative design flow focusing on our modeling approach which combines classical lumped-element circuit models with circuit quantization to calculate the lossy eigenmodes of the system. Using this circuit theoretic technique, we studied multiplexed qubit readout schemes for scalable circuit architectures, for example coupling multiple resonators to a single Purcell filter to protect the qubit from radiative decay channels. I will present our findings on loss and mode hybridization as a function of the Purcell filter geometry and coupling capacitances.

Date and Time: 
Wednesday, September 25, 2019 - 12:00pm
Venue: 
Physics/Astrophysics (Varian II) Building, Room 102/103

Q-Farm Quantum Seminar Series presents TWO TOPICS

Topic: 
A quantum annealer with fully programmable all-to-all coupling via Floquet engineering -and- Spin squeezing in free-space atomic sensors
Abstract / Description: 

A quantum annealer with fully programmable all-to-all coupling via Floquet engineering
Peter McMahon

Quantum annealing is a promising approach to heuristically solving difficult combinatorial optimization problems. However, the connectivity limitations in current devices lead to an exponential degradation of performance on general problems. We propose an architecture for a quantum annealer that achieves full connectivity and full programmability while using a number of physical resources only linear in the number of spins. We do so by application of carefully engineered periodic modulations of oscillator-based qubits, resulting in a Floquet Hamiltonian in which all the interactions are tunable; this flexibility comes at a cost of the coupling strengths between spins being smaller than they would be had the spins been directly coupled. Our proposal is well-suited to implementation with superconducting circuits, and we give analytical and numerical evidence that fully-connected, fully-programmable quantum annealers with 1000 qubits could be constructed with Josephson parametric oscillators having coherence times of 500 microseconds, and other system-parameter values that are routinely achieved with current technology. Our approach could also have impact beyond quantum annealing, since it readily extends to bosonic quantum simulators and would allow the study of models with arbitrary connectivity between lattice sites. Describes work performed jointly with Tatsuhiro Onodera and Edwin Ng.


Spin squeezing in free-space atomic sensors
Zheng Cui

The compatibility of cavity-generated spin-squeezed atomic states with atom-interferometric sensors that require freely falling atoms is demonstrated. An ensemble of hundreds of thousands of spin-squeezed atoms in a high-finesse optical cavity with near-uniform atom-cavity coupling is prepared, released into free space, and measured using one of two different methods. The first method consists of recapturing the atoms in the cavity and probing them with the same QND measurement used to generate the initial entanglement among the atoms. Up to 9.8^{+0.5}_{-0.4} dB of metrologically-relevant squeezing is retrieved for few-hundred microseconds free-fall times, and decaying levels of squeezing are mapped out up to 3 milliseconds free-fall times. This protocol suffers of atom loss and atom-cavity coupling inhomogeneity after recapture. Fluorescence population spectroscopy is an alternative method when longer free-falls times are required. This method allows for the atom ensemble to free fall for up to 4 milliseconds without significant loss of squeezing nor quantum coherence. When operating as a microwave atomic clock with a 3.6 millisecond Ramsey time, a single-shot fractional frequency stability of 8.4(0.2)x10^{-12} is reported, 4.1(0.2) decibels below the quantum projection limit. The ability of the clock to utilize the maximum squeezing available is limited by microwave amplitude and phase noise, and external magnetic field fluctuations in the system. Free fall times of up to 8 milliseconds are also achieved, but at a loss of state coherence.

Date and Time: 
Wednesday, June 19, 2019 - 12:00pm
Venue: 
Y2E2 300

Q-Farm Quantum Seminar Series presents Entanglement Wedge Reconstruction and the Information Paradox - & - Large Momentum Transfer Clock Atom Interferometry

Topic: 
Entanglement Wedge Reconstruction and the Information Paradox | Large Momentum Transfer Clock Atom Interferometry
Abstract / Description: 

The Q-FARM seminar series is designed to bring together the various groups in the university interested in quantum science and engineering. Seminars take place every other Wednesday at lunch time, with two speakers per session giving short talks on their research. The primary goal of these seminars is to strengthen the community and increase collaboration. Theoretical and experimental talks are balanced so that the whole community may participate.


Entanglement Wedge Reconstruction and the Information Paradox, Geoff Penington

I will review the ideas of entanglement wedge reconstruction and holographic quantum error correction in AdS/CFT and show how they can be used to resolve the black hole information paradox.

Large Momentum Transfer Clock Atom Interferometry, TJ Wilkason

Atom interferometry is a promising candidate for future precision sensors with a diverse science impact, ranging from laboratory tests of general relativity and the equivalence principle, to searches for dark matter and detection of gravitational waves. I will present efforts towards increasing the sensitivity of atom interferometers through the use of narrow-line clock transitions in alkaline earth atoms such as strontium. These narrow-line transitions promise to circumvent existing constraints encountered in conventional atom interferometers, enabling increased space-time area through large momentum transfer (LMT) atom optics and reducing the sensitivity to laser phase noise. I will show recent results on LMT interferometry on the 689nm intercombination line of strontium, which demonstrate for the first time an LMT interferometer based on sequential single-photon transitions, a critical requirement for gravitational wave detection using clock atoms. We study spontaneous emission and other losses at large pulse area for this new type of atom interferometer and characterize the rejection of common laser phase noise in a gradiometer configuration.

Date and Time: 
Wednesday, May 22, 2019 - 12:00pm
Venue: 
Physics/Astrophysics (Varian II) Building, Room 102/103

Q-Farm Quantum Seminar Series presents Two Talks

Topic: 
Noise-resilient quantum circuits - AND - Spin squeezing in free-space atomic sensors
Abstract / Description: 

A noisy quantum computer can simulate any noiseless quantum computation with an overhead that scales very modestly with the size of the computation, in the asymptotic limit in which the size of the computation becomes large. However, the overhead in practice is still too large even for state-of-the-art quantum computing devices. In order to circumvent this problem, we propose a specialized algorithm that remains robust in the presence of error even without error correction. Even though the size of the circuit increases with the problem size, the accumulated error on the answer is stabilized at a level comparable to the physical noise rate. This is possible because the errors introduced at earlier times are judiciously diluted more by the design of the circuit. This algorithm can dramatically speed up the existing (classical) computational methods to study strongly interacting quantum many-body systems. One may thus optimistically hope to accurately predict the properties of such systems with a noisy quantum computer, provided that the noise rate in these devices continues to get lower.

- and - 

The compatibility of cavity-generated spin-squeezed atomic states with atom-interferometric sensors that require freely falling atoms is demonstrated. An ensemble of hundreds of thousands of spin-squeezed atoms in a high-finesse optical cavity with near-uniform atom-cavity coupling is prepared, released into free space, and measured using one of two different methods. The first method consists of recapturing the atoms in the cavity and probing them with the same QND measurement used to generate the initial entanglement among the atoms. Up to 9.8^{+0.5}_{-0.4} dB of metrologically-relevant squeezing is retrieved for few-hundred microseconds free-fall times, and decaying levels of squeezing are mapped out up to 3 milliseconds free-fall times. This protocol suffers of atom loss and atom-cavity coupling inhomogeneity after recapture. Fluorescence population spectroscopy is an alternative method when longer free-falls times are required. This method allows for the atom ensemble to free fall for up to 4 milliseconds without significant loss of squeezing nor quantum coherence. When operating as a microwave atomic clock with a 3.6 millisecond Ramsey time, a single-shot fractional frequency stability of 8.4(0.2)x10^{-12} is reported, 4.1(0.2) decibels below the quantum projection limit. The ability of the clock to utilize the maximum squeezing available is limited by microwave amplitude and phase noise, and external magnetic field fluctuations in the system. Free fall times of up to 8 milliseconds are also achieved, but at a loss of state coherence.

Date and Time: 
Wednesday, May 8, 2019 - 12:00pm
Venue: 
Physics/Astrophysics (Varian II) Building, Room 102/103

#StanfordToo: A Conversation about Sexual Harassment in Our Academic Spaces

Topic: 
#StanfordToo: A Conversation about Sexual Harassment in Our Academic Spaces
Abstract / Description: 

Individuals of all genders invited to be a part of:
#StanfordToo: A Conversation about Sexual Harassment in Our Academic Spaces, where we will feature real stories of harassment at Stanford academic STEM in a conversation with Provost Drell, Dean Minor (SoM), and Dean Graham (SE3). We will have plenty of time for audience discussion on how we can take concrete action to dismantle this culture and actively work towards a more inclusive Stanford for everyone. While our emphasis is on STEM fields, we welcome and encourage participation from students, postdocs, staff, and faculty of all academic disciplines and backgrounds.

Date and Time: 
Friday, April 19, 2019 - 3:30pm
Venue: 
STLC 111

QFarm Quantum Seminar Series

Topic: 
QFarm presents "Synthetic dimensions for photons: realizing nonreciprocity, artificial magnetic fields and the quantum Hall effect" & "Quantum dynamics of a few-photon parametric oscillator"
Abstract / Description: 

Synthetic dimensions for photons: realizing nonreciprocity, artificial magnetic fields and the quantum Hall effect
Avik Dutt, Stanford University

The dimensionality of a system strongly determines its properties, both in the classical and quantum regime. Recently, a lot of work has focused on using "synthetic dimensions" to probe higher-dimensional phenomena and topological physics on lower-dimensional systems, drastically reducing experimental complexity. These synthetic dimensions correspond to internal degrees of freedom of ultracold atoms or photons, such as the spin or orbital angular momentum. In this talk, I will discuss our work on realizing synthetic dimensions for photons – specifically using the frequency degree of freedom – in a photonic cavity. I will describe a technique we introduced which enables us to experimentally read-out the band structure of a system in this synthetic space. Next, we extend this technique to perform band structure spectroscopy of a system with multiple synthetic dimensions within a single cavity. This allows the observation of synthetic magnetic fields even for neutral particles like photons, thus breaking reciprocity and time-reversal symmetry. Finally, using this simple structure consisting of a single photonic cavity, we demonstrate how chiral one-way edge states – the hallmark of topological physics and the quantum Hall effect – can be seen directly in the measured band structure.


Quantum dynamics of a few-photon parametric oscillator
Zhaoyou Wang, Stanford University

Modulating the frequency of a harmonic oscillator at nearly twice its natural frequency leads to amplification and self-oscillation. Above the oscillation threshold, the field settles into a coherent oscillating state with a well-defined phase of either 0 or π. We demonstrate a quantum parametric oscillator operating at microwave frequencies and drive it into oscillating states containing only a few photons. The small number of photons present in the system and the coherent nature of the nonlinearity prevents the environment from learning the randomly chosen phase of the oscillator. This allows the system to oscillate briefly in a quantum superposition of both phases at once - effectively generating a nonclassical Schrödinger's cat state. We characterize the dynamics and states of the system by analyzing the output field emitted by the oscillator and implementing quantum state tomography suited for nonlinear resonators. By demonstrating a quantum parametric oscillator and the requisite techniques for characterizing its quantum state, we set the groundwork for new schemes of quantum and classical information processing and extend the reach of these ubiquitous devices deep into the quantum regime.


 

Date and Time: 
Wednesday, April 10, 2019 - 12:00pm
Venue: 
Physics/Astrophysics (Varian II) Building, Room 102/103
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