# Presentation: 401141

## Circuit Quantum Electrodynamics: Single Microwave Photons on DemandSuperconducting qubits to flying qubitsR.J. Schoelkopf, M.H. Devoret and S.M. Girvin; Yale UniversityDMR 0653377 and DMR 0603369

- Cavity QED:an atom trapped between mirrors

- Integrated circuit in which a superconducting “qubit,” or artificial atom, is coupled to microwave photons trapped in a super-conducting transmission line “cavity,” in direct analogy with the field of cavity quantum electrodynamics (cavity QED) in quantum optics.

- First observation of single microwave photons on demand.  The control pulse Rabi flops the qubit and if the qubit ends up in the excited state, it decays, emitting exactly one microwave photon.

- cavity

- qubit

**=**

**Measured ****Control pulse**

**Measured ****Fluorescence**

**Notes:**
This work reports the first experimental demonstration of the single microwave photons on demand as well as the first transfer of quantum information from superconducting qubit to a ‘flying qubit’ (i.e. a coherent superposition of zero and one photon).
Circuit QED is a new field at the interface between quantum optics and condensed matter physics. It represents a new paradigm we are developing in which quantum optics experiments can be performed in an integrated electrical circuit, using microwaves instead of visible photons from lasers.  This system is a new testing ground for quantum mechanics of (relatively) large objects.  The microwaves have a wavelength of about 1 cm and yet act like particles (photons).  The artificial ‘atom’ actually consists of roughly 1 trillion aluminum atoms acting in concert like a single atom.  In addition to its fundamental interest, this new system has many desirable features as an architecture for developing quantum computers.
Classically light consists of waves, but quantum mechanically its energy is carried by particles called photons that have peculiar wave-like properties.  We have just celebrated the 100th anniversary of Einstein’s famous 1905 paper in which the existence of the photon was postulated to explain the photoelectric effect (which is the basis of digital cameras for example).  
If an atom in free space is placed in an excited quantum state it will eventually spontaneously emit a photon and fall down to a lower energy state.  This process is irreversible because the photon flies away from the atom at the speed of light never to return.  After several decades of dedicated effort, atomic physicists have learned to alter this situation dramatically by placing the atom inside a cavity (constructed from highly reflecting mirrors) that traps the photon and allows it to be repeatedly absorbed and reemitted by the atom may times before it escapes the cavity.   In this so-called ‘strong-coupling’ regime, the quantum state of the system becomes a peculiar coherent superposition of two simultaneous possibilities:  the energy is either an excitation of the atom or is a photon.  Quantum mechanics does not permit us to know which possibility actually occurs without destroying this delicate state.
The photons of visible light commonly emitted by lasers and light bulbs have extremely feeble energy (a small night light emits torrent of some 1015 photons per second but this still amounts to only about 1 watt of power).  Nevertheless the energy of an individual photon is large enough to cause chemical reactions (such as those that allow your eye to detect light) and to trigger special detectors based on the photoelectric effect known as photomultipliers.  Microwaves are a form of light but these waves oscillate at frequencies much lower than visible light.  The microwaves used in the Yale experiment are in a similar frequency range to those used in cellular telephones and oscillate a ‘mere’ six billion times per second, approximately 100,000 times slower than visible light.  The energy of the microwave photons is correspondingly 100,000 times lower and hence these pusillanimous objects are extremely difficult to detect on an individual basis.  The energy of a single one of these photons is roughly equivalent in magnitude to the sound energy impinging on your ear if you are standing in Los Angeles and someone in New York drops a pin.
There are two key features that made the Yale experiment possible.  First, we used an especially designed and micro-fabricated superconducting electrical circuit as a quasi-one-dimensional resonator (cavity) whose volume is more than one million times smaller than the usual three-dimensional cavities.  This confines the feeble energy of the photon to an extremely small space so that the photon will collide very frequently with the artificial atom (superconducting qubit) placed inside the cavity.  In fact, the rate of collision that is achieved (100 million per second) is within a factor of 2 of the theoretical maximum set by one of the fundamental constants of nature (the fine-structure constant).  
The qubit is a newly designed type (dubbed the ‘transmon’) and consists of two small pieces of aluminum separated by a thin barrier (Josephson tunnel junction) plus a relatively large antenna like structure (dubbed the ‘Bullwinkle’) about 0.3 millimeter in size which serves to strongly enhance the interaction between the qubit and the microwaves.  The qubit is quite large compared to a single atom, and yet the special quantum coherence properties of the superconducting state allow the electrons to travel in pairs (Cooper pairs) and behave collectively in such a way that the qubit has a very simple quantum energy levels similar to those of an atom, but on a much lower energy scale matched to the microwaves.  The two lowest energy states of the qubit differ by the transfer of a few pairs of electrons between the two aluminum grains.  [More precisely, the energy states of the qubit are coherent quantum superpositions of different charge states formed as the Cooper pairs quantum mechanically tunnel back and forth through the barrier separating the two aluminum grains.]  The combination of large artificial ‘atom’ size and exceptionally small cavity volume means that the photon collides with the ‘atom’ very frequently, some 100 million times per second.   [Another advantage of the artificial atom is that it is permanently mounted on the insulating substrate of the micro-chip exactly at the optimal location to couple to the photon and it cannot escape, a problem that atomic physicists routinely face.]
 It is not enough however to merely attenuate the microwaves intentionally being sent into the cavity in order to have a small photon number in the cavity.  One has to take care to cool the cavity and the wires leading down to it to extremely low temperatures because warm objects emit so-called black-body radiation.  The heat you feel on your face from infra-red radiation coming from a fireplace is an example of such thermally produced radiation.  At room temperature a human being typically radiates about 100 watts of power, but fortunately absorbs nearly the same amount of radiation emitted by the walls of the room.  The coldest parts of space, the inter-galactic vacuum regions of space are bathed in blackbody radiation with a temperature just a few degrees above absolute zero left over from the blazingly high temperatures of the big bang at the beginning of the universe and cooled by the subsequent expansion of the universe.  However the energy of the photons used in this experiment are smaller still and the entire device must be cooled to a temperature nearly 100 times colder than the coldest regions of outer space in order to keep blackbody radiation out.  Based on the results of the prior experiments we estimate that the average number of thermally produced photons in the cavity is well below 0.1.
Other solid state analogs of cavity QED have been envisaged by many authors in superconducting, semiconducting and even micro-mechanical systems. Our experiments constitute the first experimental realization of these ideas. This new regime of matter light interaction in a circuit, which we refer to as circuit quantum electrodynamics, will open new  possibilities to perform quantum optics experiments in solids.  The team at Yale intends to continue developing and refining their approach and is now applying it to multiple qubits in a cavity.  The techniques being developed could allow in the future for the production and detection of individual microwave photons with high efficiency and might someday form the basis for a new architecture for quantum computation and secure quantum communication using ‘flying qubits’.

## Flying QubitsR.J. Schoelkopf, M.H. Devoret and S.M. Girvin; Yale UniversityDMR 0653377 and DMR 0603369

**Postdoctoral Fellows:**
- 	Andrew Houck, Hannes Majer,  Jay Gambetta, Andreas Wallraff
**Graduate Students:** David Schuster, Joe Schreier, Blake Johnson, Jerry Chow, Lev Bishop, Terri Yu, Emily Chan
**Undergraduates:**
- 	This project has formed parts of six senior theses at Yale by Sameer Kumar ’04, Clifford Cheung ’04, Will Braff ’05, Joe Chen ’06, Jared Schwede ’07, Daniel Rubin ‘07.
**Publications:**
- ‘Resolving Single Photon States in a Superconducting Circuit,’ D.I. Schuster, A.A. Houck,  et al., Nature 445, 515 (2007).
- ‘Generating Single Microwave Photons in a Circuit,’ A. A. Houck, D. I. Schuster, et al., *Nature* (in press); cond-mat/0702648.
- ‘Coupling Superconducting Qubits via a Cavity Bus,’ J. Majer, J.M. Chow, et al., *Nature* (in press).

**Flying Qubits**R.J. Schoelkopf, M.H. Devoret and S.M. Girvin;* Yale University**DMR 0653377 and DMR 0603369*

_**Impact:**__  _This experiment and the new paradigm of “circuit quantum electrodynamics” creates many possibilities for doing quantum optics with circuits in an ultra-strong coupling regime not accessible with traditional atomic physics methods.

- ‘Fluorescence tomography’ showing that the quantum state of the qubit has been mapped onto the quantum state of the microwave field.