Recent News

Three Group Members Successfully Defend Theses

July 1, 2013 — Congratulations to the freshly minted Drs. Adam Sears, Matthew Reed, and Andreas Fragner. We wish them well on their respective journeys.

Observation of quantum state collapse and revival due to the single-photon Kerr effect

March 14, 2013 — Our paper, describing the realization of an artificial Kerr medium using a three-dimensional circuit quantum electrodynamic architecture, appeared in Nature today. We observe the single-photon Kerr effect acting on a coherent state which illustrates the quantum nature of this electromagnetic field and results in the creation of exotic quantum states (including the so-called Schrödinger cat state).

More Information is available at:

Citation: Nature 495, 205-209, (2013) doi:10.1038/nature11902

Reprint: arXiv

Realization of three-qubit quantum error correction with superconducting circuits

February 16, 2012 — Our paper describing the realization of quantum error correction in a superconducting circuit appeared in Nature today. Using a novel three-qubit gate, we have demonstrated the three-qubit phase-flip and bit-flip quantum error correction codes.

More information is available at:

Citation: Nature 482, 382-385, (2012) doi:10.1038/nature10786

Reprint: arXiv

New paper on long coherence times in transmon qubits!

December 7, 2011 — We have a new paper on the 3D implementation of a transmon qubit in a cavity, which has now appeared in Physical Review Letters. These new devices represent a significant breakthrough in the coherence of superconducting qubits, improving the state of the art by a factor of about 30. The results also show remarkable frequency stability, and indicate that the intrinsic coherence of Josephson junctions (still not the limit in these experiments!) is much better than expected. Future versions of these qubits could approach the error-correction threshold, and the results are very encouraging for the future of superconducting quantum computing.

There is also a Viewpoint article by Matthias Steffen discussing the paper here

Citation: Phys. Rev. Lett. 107, 240501 (2011)

Quantum Computing Featured in New York Times

November 8, 2010 — Quantum computing was featured in the New York Times today. The article discusses recent developments in the field of superconducting qubit research.

High-Fidelity Readout in Circuit Quantum Electrodynamics Using the Janyes-Cummings Nonlinearity

October 19, 2010 — We have a paper published today demonstrating high-fidelity readout of superconducting qubits using the Janyes-Cummings nonlinearity. We observe measurement fidelities to single qubit states as high as 87% and can measure joint qubit correlations with fidelity of at least 60%. This new protocol does not require any chance in sample design or extra hardware, and seems to work well over a wide range of sample parameters.

Citation: Phys. Rev. Lett. 105, 173601

Preparation and measurement of three-qubit entanglement in a superconducting circuit

transmonsOctober 19, 2010 — Our recent demonstration of three-qubit entanglement in a superconducting circuit has been published in Nature. More information is available at:

Citation: Nature 467, 574-578 (2010)

Detecting Photons in Cavities

September 12, 2010 New Paper appeared this month in Nature Physics demonstrating a quantum non-demolition method to detect photons in a cavity in the circuit QED architecture. Repeated measurements of a single photon demonstrate that the demolition is less than 10%.

Citation: Nature Physics 6, 663-667 (2010)

Positions available in Schoelkopf Lab

September 7th, 2010 — The Schoelkopf Lab is currently looking for several Postdoctoral Associates and graduate students. If interested in joining as a Postdoctoral Associate please submit your CV along with two letters of recommendation to Robert [dot] Schoelkopf [at] yale [dot] edu. If applying to the Graduate Program please see the Admissions Website.

Fast Reset and Suppressing Spontaneous Emission of a Superconducting Qubit

May 21, 2010New paper published today in Applied Physics Letters demonstrating a filter to protect superconducting qubits from spontaneous decay due to coupling to a cavity. This allows for the use of low-Q cavities for fast qubit readout and reset without adversely affecting normal qubit operations.

Citation: Applied Physics Letters 96, 203110 (2010)

Demonstration of Two-Qubit Algorithms Using a Superconducting Quantum Processor

Flux Bias Chip Nature

Optical image of our superconducting quantum processor. A transmission line cavity mediates the interaction between two transmon qubits located at opposite ends of the cavity. (image by Blake Johnson)

June 28, 2009 — Our paper reporting the implementation of simple quantum algorithms using a circuit QED architecture has been published online in Nature. News of this result is also being published in the online press. To read more, please see:

Citation: Nature 460, 240-244, (2009) doi:10.1038/nature08121

Randomized Benchmarking and Process Tomography for Gate Errors in a Solid-State Qubit

March 6, 2009 — Paper published today in Phys. Rev. Lett. demonstrating three different measurements of single-qubit gate errors on transmon qubits. One of the error measurement protocols, called randomized benchmarking, is particularly useful because it probes the response of the system to a large number of operations. Furthermore, it is a protocol which will continue to be useful as quantum processors grow beyond just a few qubits. In this work, we find a minimal average gate error of 1.1%, consistent with the other more well-known error metrics: pi-pi and quantum process tomography.

Citation: Phys. Rev. Lett. 102, 090502 (2009).

Nonlinear response of the vacuum Rabi resonance

Vaccuum Rabi Nature Physics Cover

February 2, 2009 — New paper published today in Nature Physics on the nonlinear response of a strongly driven qubit-cavity system. In the usual strong coupling regime of a qubit resonant with a cavity, a pair of vacuum Rabi peaks are observed. However, when the drive power of the qubit-cavity system is increased, we observe nonlinear response of the vacuum Rabi peaks. Supersplitting of each vacuum Rabi peak occurs and extra peaks with the characteristic sqrt{n} Jaynes-Cummings ladder spacing are seen.

Citation: Nature Physics 5, 105 - 109 (2009).

Rob Schoelkopf Wins Keithley Award for Advances in Measurement Science

October 3, 2008 — Rob Schoelkopf has been awarded the 2009 Joseph F. Keithley Award for Advances in Measurement Science, "for outstanding advances in measurement science or products that impact the physics community by providing better measurements." See the Yale press release.

Controlling the Spontaneous Emission of a Transmon Qubit

August 21, 2008 — New paper published today in Physical Review Letters on the spontaneous emission times (T1) of transmon qubits. This paper demonstrates the importance of carefully engineering the impedance of the environment seen by the qubit.

Citation: Phys. Rev. Lett. 101, 080502 (2008).

Latest Information on Transmon Qubits

June 20, 2008 — Two new papers on characterization of transmons are now available on the Arxiv.

Wiring Up Quantum Circuits

Feb 7, 2008. — Steve Girvin and Robert Schoelkopf provide a review of circuit quantum electrodynamics in the february issue of Nature.

Citation: Nature 451, 664-669 (7 February 2008)

Two Major Steps in Advancement of Quantum Computing

Quantum Bus Cover Nature

Sept 26, 2007 — Two major steps toward putting quantum computers into real practice — sending a photon signal on demand from a qubit onto wires and transmitting the signal to a second, distant qubit — have been brought about by a team of scientists at Yale. The accomplishments are reported in sequential issues of Nature on September 20 and September 27, on which it is highlighted as the cover along with complementary work from a group at the National Institute of Standards and Technologies.

Over the past several years, the research team of Professors Robert Schoelkopf in applied physics and Steven Girvin in physics has explored the use of solid-state devices resembling microchips as the basic building blocks in the design of a quantum computer. Now, for the first time, they report that superconducting qubits, or artificial atoms, have been able to communicate information not only to their nearest neighbor, but also to a distant qubit on the chip.

This research now moves quantum computing from “having information” to “communicating information.” In the past information had only been transferred directly from qubit to qubit in a superconducting system. Schoelkopf and Girvin’s team has engineered a superconducting communication ‘bus’ to store and transfer information between distant quantum bits, or qubits, on a chip. This work, according to Schoelkopf, is the first step to making the fundamentals of quantum computing useful.

The first breakthrough reported is the ability to produce on demand — and control — single, discrete microwave photons as the carriers of encoded quantum information. While microwave energy is used in cell phones and ovens, their sources do not produce just one photon. This new system creates a certainty of producing individual photons. “It is not very difficult to generate signals with one photon on average, but, it is quite difficult to generate exactly one photon each time. To encode quantum information on photons, you want there to be exactly one,” according to postdoctoral associates Andrew Houck and David Schuster who are lead co-authors on the first paper.

“We are reporting the first such source for producing discrete microwave photons, and the first source to generate and guide photons entirely within an electrical circuit,” said Schoelkopf. In order to successfully perform these experiments, the researchers had to control electrical signals corresponding to one single photon. In comparison, a cell phone emits about 10^23 (100,000,000,000,000,000,000,000) photons per second. The extremely low energy of microwave photons mandates the use of detectors and experiment temperatures just above absolute zero.

Two Transmon Resonator

Qubits, the building blocks of a future quantum computer, become useful when quantum communication between them can be established. In our experiment, this job is done by photons in a cavity on a microchip. This way, quantum information is successfully shuttled back and forth between two superconducting qubits. Click for a larger image.

“In this work we demonstrate only the first half of quantum communication on a chip — quantum information efficiently transferred from a stationary quantum bit to a photon or ‘flying qubit,’” says Schoelkopf. “However, for on-chip quantum communication to become a reality, we need to be able to transfer information from the photon back to a qubit.”

This is exactly what the researchers go on to report in the second breakthrough. Postdoctoral associate Johannes Majer and graduate student Jerry Chow, lead co-authors of the second paper, added a second qubit and used the photon to transfer a quantum state from one qubit to another. This was possible because the microwave photon could be guided on wires — similarly to the way fiber optics can guide visible light — and carried directly to the target qubit. “A novel feature of this experiment is that the photon used is only virtual,” said Majer and Chow, “winking into existence for only the briefest instant before disappearing.”

To allow the crucial communication between the many elements of a conventional computer, engineers wire them all together to form a data “bus,” which is a key element of any computing scheme. Together the new Yale research constitutes the first demonstration of a “quantum bus” for a solid-state electronic system. This approach can in principle be extended to multiple qubits, and to connecting the parts of a future, more complex quantum computer.

However, Schoelkopf likened the current stage of development of quantum computing to conventional computing in the 1950’s, when individual transistors were first being built. Standard computer microprocessors are now made up of a billion transistors, but first it took decades for physicists and engineers to develop integrated circuits with transistors that could be mass produced.

Schoelkopf and Girvin are members of the newly formed Yale Institute for Nanoscience and Quantum Engineering (YINQE), a broad interdisciplinary activity among faculty and students from across the university. Further information and FAQs about qubits and quantum computing are available on the cQED project page.

Other Yale authors involved in the research are J.M. Gambetta, J.A. Schreier, J. Koch, B.R. Johnson, L. Frunzio, A. Wallraff, A. Blais and Michel Devoret. Funding for the research was from the National Security Agency under the Army Research Office, the National Science Foundation and Yale University.

Citation: Nature 449, 328-331 (20 September 2007) doi:10.1038/nature06126 &
Nature 449, 443-447 (27 September 2007) doi:10.1038/nature06184

Generating single microwave photons in a circuit

Sing Phot Marestails

Sept 20, 2007 — New paper in today's issue of Nature. The paper is titled “Generating single microwave photons in a circuit.” Authors on the paper are A. A. Houck, D. I. Schuster, J. M. Gambetta, J. A. Schreier, B. R. Johnson, J. M. Chow, L. Frunzio, J. Majer, M. H. Devoret, S. M. Girvin and R. J. Schoelkopf.

The radiation from a microwave oven or cellphone does not seem to have much in common with light, but like its visible counterpart, microwaves are made of individual photons. However, conventional sources of microwave light, like the oven or phone, cannot produce just one photon. Even a very weak source has inherent noise, meaning that there is always some chance of having two or more photons. This work introduces a new source of microwaves in a circuit, capable of generating exactly one photon with no fluctuations. A single photon can travel along a transmission line for the equivalent of ten kilometers. Such single photon sources are capable of transmitting classical bits of information completely securely, preventing eavesdroppers from intercepting a private message. These sources can not only transmit classical information, but also a full quantum bit (qubit) of information. These “flying qubits” could connect distant bits in a quantum computer, allowing distant parts of the computer to communicate directly.

Frequently asked questions about the project are available here.

Citation: Nature 449, 328-331 (20 September 2007) doi:10.1038/nature06126

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